Analysis of Molecular Surface/Interfacial Layer by Sum-Frequency Generation (SFG) Spectroscopy

Abstract

This chapter reviews recent progress in polymer surfaces and interface studies using sum-frequency generation (SFG) vibrational spectroscopy. SFG is a surface-specific vibrational spectroscopic technique that has spread on a worldwide scale since it was first reported in 1987. The SFG principles, instruments, techniques, and experiments are presented in detail, and recent results on interfacial physics and chemistry at jointed interfaces are described. It focuses on SFG studies of the surfaces and buried interfaces of polymeric materials, such as modification of polymer surfaces, polymer-water, polymer-metal, and polymer–polymer interfaces. This review demonstrates that SFG is a powerful technique for nondestructive, in situ measurement of molecular level understanding at complex polymer surfaces and interfaces.

1 Introduction

Surface/interfacial properties play dominating roles in many applications, such as biomedical implants, coatings, packaging materials, adhesives, lubricants, etc. Among them, adhesion is extremely important in many industrial applications, from the automotive and aircraft industries to the housing and the microelectronics industry. In the microelectronics industry, for example, epoxy polymeric underfills are commonly used to enhance the long-term drive reliability of microelectronics chip assemblies, and the buried interface structure of the underfill is critical to the lifetime of the device. Adhesion failures at this buried metal/epoxy interface can lead to premature and unexpected device failures. Therefore, understanding the relationship between adherent and adhesive interfacial structure and property is critical to improving microelectronics device performance.

Generally, polymer surface properties are believed to be determined by molecular surface structures. Therefore, it is crucial to characterize polymer surface structures at the molecular level for developing polymer surfaces with desired properties. In the last few decades, a variety of surface-sensitive analytical techniques have been developed. However, most such techniques often require a high vacuum to operate and the conductivity of the samples, cannot probe molecular level surface structure, or lack desired surface specificity. Therefore, probing polymer surface structures at the molecular level in situ in real-time is still not easy. Furthermore, particularly concerning the adhesion, it is expected that the structural changes of the adhesives, which are in a liquid state in the initial stages of adhesion and then turn into a solid state at the interface with curing, significantly affect the adhesive strength. However, it is not easy to nondestructively investigate the molecular structural changes at interfaces between liquids and solids, or buried interfaces of solids.

Recently sum-frequency generation (SFG) vibrational spectroscopy has been developed into a powerful and unique technique to investigate surface structures of various kinds of material surfaces. SFG is a second-order nonlinear optical effect, which requires high-energy pulses to generate enough signal for detection. SFG vibrational spectroscopy first appeared in publication in 1987, developed by Shen et al. at the University of California, Berkeley [1, 2]. In addition to the surface studies in the air such as polymeric materials surfaces [3, 4], block copolymer surfaces [5], resin [6], and polymer blends [7], buried interfaces such as polymers in water [8], polymer–polymer interfaces [9], and polymer-solid interfaces [10,11,12] have also been examined using SFG. Furthermore, SFG has been used to investigate molecular interactions between polymers and adhesion promoters and the structural changes at the interfaces to understand the molecular mechanisms of the adhesion of polymers.

The primary advantage of SFG spectroscopy is that vibrational resonances can be label-free probes. Because this technique is inherently a probe of broken centrosymmetry, surface specificity is not dependent on the shallow penetration of the infrared (IR) beam, as is the case with near-field wave probes such as in the cases of the grazing angle experiments or attenuated total reflection (ATR) IR spectroscopy. As a result, a potential application is where the molecule of interest at surfaces is the same as the molecule in the adjacent bulk. For example, at the polymer-water interface, SFG can probe the polymer chains at the surface on their own, excluding the contribution of the bulk polymers. Water molecules at the interface can be also studied in the same experiments without the contribution of the bulk water phase.

Currently, many books [13,14,15] and review articles [16,17,18,19,20,21,22,23,24] have been published covering the basic principles of vibrational SFG spectroscopy. Here, we present a schematic overview of SFG spectroscopy at the interface of materials, especially at polymeric surfaces and interfaces, as well as physical and molecular scientific approaches to reveal the true nature of the interfaces by combining SFG and other analytical techniques toward the target of the buried adhesive interface. With the purpose of this session, basic issues about SFG will be explained.

Before reviewing applications of SFG spectroscopy to study buried adhesion interfaces, we summarize in brief the basic theory and the experimental implementation of SFG as a methodology.

2 Basic Theory for Surface/Interface Sum-Frequency Generation

• (A) Nonlinear optics

SFG vibrational spectroscopy has developed based on advancements in nonlinear optics and vibrational spectroscopy, and the early work by Y. R. Shen and his collaborators established the theoretical and the experimental basis for the development of the SFG field of research [13, 15].

In nonlinear optics, the optical induced polarization $P(t)$ of a material is nonlinearly dependent on the input electric field strength, which can be expressed as

$$P\left(t\right)={\upchi }^{(1)}E\left(t\right)+{\upchi }^{(2)}{E}^{2}\left(t\right)+{\upchi }^{(3)}{E}^{3}\left(t\right)\cdots$$

(1)

Here, $E\left(t\right)$ is the input optical field, and ${\chi }^{(1)}$, ${\chi }^{(2)}$, and ${\chi }^{(3)}$ are, respectively, the linear susceptibility, the second-order, and the third-order nonlinear susceptibly of the material of interest. The generation of SFG signal arises from ${P}^{(2)}={\upchi }^{(2)}{E}^{2}\left(t\right)$, which is the second-order nonlinear polarization of the second term in Eq. (1), and it involves two distinct input optical frequency components. If we assume that the second-order nonlinear polarization is proportional to the square of the optical field,

$${P}^{(2)}(t)\propto {\left[E(t)\right]}^{2}$$

(2)

If the light electric field is constituted by two frequency components, the nonlinear polarization can be written as follows.

$$E\left(t\right)={E}_{1}\mathrm{cos}{\omega }_{1}t+{E}_{2}\mathrm{cos}{\omega }_{2}t$$

(3)

Then the nonlinear polarization is,

$$\begin{aligned} & P(t) \propto E_{1}^{2} {\text{cos}}^{2} \omega_{1} t + E_{2}^{2} {\text{cos}}^{2} \omega_{2} t + 2E_{1} E_{2} {\text{cos}}\,\omega_{1} t\,{\text{cos}}\,\omega_{2} t \\ & \,\,\,\, = \frac{{E_{1}^{2} }}{2}(1 + {\text{cos}}\,2\omega_{1} t) + \frac{{E_{2}^{2} }}{2}(1 + {\text{cos}}\,2\omega_{2} t) + 2E_{1} E_{2} \left[ {{\text{cos}}\left( {\omega_{1} + \omega_{2} } \right)t} \right. \\ & \,\,\,\,\,\,\,\,\,\,\left. { + \cos \left( {\omega_{1} - \omega_{2} } \right)t} \right] \\ \end{aligned}$$

(4)

From Eq. (4), second-order nonlinear optical effects enable the frequency conversion. This equation also indicates that the frequencies of the converted lights are the frequency doubles, the sum-frequency, and the difference frequency of the two input light frequencies, respectively.

• (B) Symmetry argument and the selection rule

A schematic of the energy diagram of the SFG is shown in Fig. 1 (left). A frequency fixed visible (${\upomega }_{1}$) beam and a frequency-tunable infrared (${\upomega }_{2}$) beam temporally and spatially overlap at a surface or interface to generate a sum-frequency ($\upomega = {\upomega }_{1}+{\upomega }_{2}$) signal which can be measured by a detection system (e.g., monochromator/photomultiplier tube (PMT)). When the IR frequency is tuned across a vibrational transition of the surface/interfacial molecules, the sum-frequency signal intensity is resonantly enhanced. An SFG spectrum can be acquired by plotting the SFG signal intensity as a function of the IR frequency. Under the electric dipole approximation, this process can occur only in the absence of inversion symmetry. When the inversion operation is applied to the isotropic material, nonlinear susceptibility ${\upchi }^{(n)}$ does not change even if $r$ is set to $-r$ due to the existence of an inversion center. On the other hand, the polarization $P$ becomes $-P$ and the electric field $E$ becomes $-E$. Therefore, ${\upchi }^{(n)}={\left(-1\right)}^{(n-1)} {\upchi }^{(n)}$ holds, and the nonlinear susceptibilities ${\upchi }^{(2)}, {\upchi }^{(4)}, \ldots$ of even-order become zero in a medium with inversion symmetry. This means that no SFG is generated from the isotropically oriented bulk. However, surfaces and interfaces necessarily lack inversion symmetry with respect to the normal direction, which allows ${\upchi }^{(2)}$ to have a certain value under dipole approximation. In other words, if we can rule out significant quadrupolar contributions to the induced polarization, the second-order susceptibility ${\chi }^{(2)}$ is nonzero when there is no inversion symmetry. This is why second-order nonlinear spectroscopy such as SFG is a surface- and interface-specific technique. The intensity ${I}_{SFG}$ detected at the sum-frequency is then given by

$${I}_{SFG}\propto {\left| {\chi }^{(2)}\right|}^{2}{I}_{VIS}{I}_{IR}$$

(5)

where ${I}_{VIS}$ and ${I}_{IR}$ are the intensities of the incident visible and infrared beams, respectively. As shown, SFG signal intensity is proportional to the square of a property, second-order nonlinear susceptibility ${\chi }^{(2)}$, of the material. Here, the effective susceptibility ${\chi }_{eff}^{(2)}$, which is related to the actual susceptibility, can be obtained by considering the electric field from the laser at the point where the SFG is generated. These local fields are what govern the SFG process. These local electric fields may differ from the fields of the incident electromagnetic waves, such as transmission or reflection at the surface, or propagation through the phase to reach the buried interface. In general, one can write

$${\chi }_{ijk,eff}^{(2)}={l}_{ii}{l}_{jj}{l}_{kk}{\chi }_{ijk}^{(2)}$$

(6)

where $l$ are the so-called local field correction factors and the indices i, j, and k refer to any of the laboratory frame Cartesian coordinates x, y, or z. These are merely the macroscopic part of the local field correction, a complete consideration of the local fields near the molecules also requires microscopic factors. In the coordinate system used here, P-polarized light has its electric field in the xz-plane, so the components of the field along x and z depend on the angle of incidence, θ. S-polarized light then has an electric field along y. A local field correction factor such as ${l}_{yy}$, for S-polarized light, is the ratio between the y-component of the field at the point of interest and the y-component of the incident light infinitely far from the surface.

SFG is equivalent to the second term in Eq. (1) as a second-order nonlinear optical process. The energy level diagram of the SFG process is illustrated in Fig. 1 (left). SFG resonance is a combination of an IR absorption and an anti-Stokes Raman scattering process. Thus the transition polarizability describing an SFG process, termed as hyperpolarizability or second-order polarizability, can be expressed by the infrared transition dipole moment ($\mu$) and Raman polarizability tensor ($\alpha$) as follows,

$${\beta }_{abc}^{(2)}=-\frac{1}{2{\varepsilon }_{0}{\omega }_{q}}\frac{\partial {\alpha }_{ab}^{*}}{\partial {Q}_{q}}\frac{\partial {\mu }_{c}}{\partial {Q}_{q}}$$

(7)

where ${\varepsilon }_{0}$ is the vacuum permittivity, ${\omega }_{q}$ is the angular frequency of the q-th vibrational mode, and ${Q}_{q}$ is the normal mode coordinate of the molecular vibration. The last two terms are the derivatives of the complex conjugate of Raman polarizability and infrared transition dipole moment components with respect to the normal coordinate of the q-th vibration mode, respectively. The “abc” in the equation indicates a molecular-fixed coordinate system. Experimentally, all of the surface/interfacial molecules probed by SFG can participate in this optical process.

Fig. 1

The energy diagrams for SFG, IR, Stokes and anti-Stokes Raman scattering

Figure 1 also shows the energy level diagrams of the IR, Raman, and SFG processes. Clearly, an SFG process is a combination of an IR process and anti-Stokes Raman scattering. In fact, the selection rule for the vibrational modes observed in SFG is both infrared- and Raman-active modes.

To distinguish it from the optical process of the sum-frequency generation commonly used in nonlinear optics, this SFG spectroscopy is often referred to as infrared–visible sum-frequency generation spectroscopy or vibrational sum-frequency generation.

• (C) Component of SFG intensity and the light polarization

Analogous with FT-IR spectroscopy, SFG can provide the molecular state at the surface/interface. In addition, the structural information can be evaluated quantitatively. Equation (8) shows the SFG output intensity in the reflection geometry. Here, ${n}_{i}({\upomega }_{i})$ represents the refractive index of the incident medium at frequency ${\upomega }_{i}$, and $\omega$ and $\theta$ are the frequency and reflection angle of the output beam, respectively. The angle $\theta$ is deduced from the conservation of momentum of the input and output photons; ${I}_{1}\left({\omega }_{1}\right)$ and ${I}_{2}\left({\omega }_{2}\right)$ are the intensities of the two input beams (visible and infrared beams used in SFG) at frequencies ${\omega }_{1}$ and ${\omega }_{2}$. ${\chi }_{eff}^{(2)}$ is the effective second-order nonlinear optical susceptibility measured for a particular polarization combination of the input and output beams in the SFG experiment.

$$I\left(\omega \right)\propto \frac{8{\pi }^{3}{\omega }^{2}{\mathrm{sec}}^{2}\theta }{{c}^{3}{n}_{1}({\upomega }_{1}){n}_{1}({\upomega }_{2}){n}_{1}(\upomega )}{\left|{\chi }_{eff}^{(2)}\right|}^{2}{I}_{1}\left({\omega }_{1}\right){I}_{2}\left({\omega }_{2}\right)$$

(8)

Equation (3.8) can be used to describe SFG signal intensity generated by surface/interface molecules in SFG. Note that the actual coefficient is not important in most cases since usually only the relative SFG intensity is compared.

If the IR frequency (${\omega }_{2}$) is near vibrational resonances, we can write

$${\chi }_{eff}^{(2)}={\chi }_{NR}^{(2)}+{\chi }_{R}^{(2)}={\chi }_{NR}^{(2)}{e}^{i\varphi }+\sum_{q}\frac{{A}_{q}}{{\omega }_{2}-{\omega }_{q}+i{\Gamma }_{q}}$$

(9)

where ${A}_{q}$, ${\omega }_{q}$, ${\Gamma }_{q}$ are the peak amplitude, the resonant vibrational frequencies, and the damping constants, respectively. ${\chi }_{NR}^{(2)}$ and $\varphi$ describes the non-resonant contributions and the phase difference between resonant and non-resonant terms, respectively. In general, Eq. (9) is used to fit all the measured spectra with ${A}_{q}$, ${\omega }_{q}$, ${\Gamma }_{q}$, and ${\chi }_{NR}^{(2)}$ as adjustable parameters. Usually, ${\chi }_{R}^{(2)}$ is the vibrationally resonant term which has a Lorentzian line shape.

Reversing the dipole direction changes the sign (or phase) of the amplitude ${A}_{q}$. This indicates that the absolute spatial orientation of the molecule at the interface can be determined by SFG. In other words, it can be determined whether the functional groups are oriented toward the upper medium or the opposite direction with respect to the interface. Unfortunately, since the conventional SFG measures the square of the absolute value of ${\chi }_{R}^{(2)}$ as shown in Eq. (9), information about the absolute dipole orientations of this dipole is missing. Such absolute orientation can be measured from the phase-sensitive SFG. More information on phase-sensitive SFGs can be found in review papers [25,26,27].

Figure 2 shows the definition of light polarization used in SFG. SFG spectroscopy treats two incident lights and one output light. Each light can adopt a polarization parallel to the plane of incidence (p-polarization) and a polarization perpendicular to the plane of incidence (s-polarization). This combination of incident and output polarization is important for analyzing molecular orientation. As indicated before, the SFG signal consists of a second-order nonlinear susceptibility, which is composed of third-order tensor components. For example, on an isotropically oriented surface, only four out of the 27 tensor components can have independent values from the symmetry requirement. These are ${\chi }_{xxz}={\chi }_{yyz}, { \chi }_{xzx}={\chi }_{yzy}, {\chi }_{zxx}={\chi }_{zyy},$ and ${\chi }_{zzz}$. . In the polarization combination of SSP (SFG is S-polarized, visible light is S-polarized, and infrared light is P-polarized), for example, yyz element of these four independent tensor components is observed. Similarly, in the SPS polarization combination, the yzy component will be observed, and the four components xxz, xzx, zxx, and zzz, will constitute the peaks of the spectrum in the PPP polarization combination.

Fig. 2

Schematics of the light polarization in SFG

• (D) Determination of the molecular orientation

Molecular orientation information of surface molecules and functional groups can be determined from SFG spectra collected with different combinations of polarizations such as SSP, PPP, and SPS [28,29,30,31]. Such orientation information can be evaluated by the intensity ratio of different vibrational modes of the functional group in the same spectrum or the intensity ratio of the same peak in SFG spectra collected with different polarization combinations.

In the case of SFG, the hyperpolarization tensor of the molecule is projected onto the lab coordinate system when the experiments are performed with laser light of a certain polarization combination. From this projection, the orientation of the molecule can be obtained, as described below.

As mentioned above, SFG probes the second-order nonlinear susceptibility ${\upchi }^{(2)}$, which is a third-rank tensor. In SFG experiments, different combinations of polarization S or P of the input and output laser beams can probe different tensor components of the second-order nonlinear susceptibility.

$$\begin{aligned} \chi_{eff,SSP}^{(2)} & = L_{yy} \left( \Omega \right)L_{yy} \left( {\omega_{1} } \right)L_{zz} \left( {\omega_{2} } \right)\sin\theta_{2} \chi_{yyz} \\ \chi_{eff,SPS}^{(2)} & = L_{yy} \left( \Omega \right)L_{zz} \left( {\omega_{1} } \right)L_{yy} \left( {\omega_{2} } \right)\sin\theta_{1} \chi_{yzy} \\ \chi_{eff,PSS}^{(2)} & = L_{zz} \left( \Omega \right)L_{yy} \left( {\omega_{1} } \right)L_{yy} \left( {\omega_{2} } \right)\sin \theta \chi_{zyy} \\ \chi_{eff,PPP}^{(2)} & = - L_{xx} \left( \Omega \right)L_{xx} \left( {\omega_{1} } \right)L_{zz} \left( {\omega_{2} } \right)\cos \theta \,\cos \theta_{1} \,\sin \theta_{2} \chi_{xxz} \\ & \,\,\,\,\,\, - L_{xx} \left( \Omega \right)L_{zz} \left( {\omega_{1} } \right)L_{xx} \left( {\omega_{2} } \right)\cos \theta \,\sin \theta_{1} \,\cos \theta_{2} \chi_{xzx} \\ & \,\,\,\,\,\, + L_{zz} \left( \Omega \right)L_{xx} \left( {\omega_{1} } \right)L_{xx} \left( {\omega_{2} } \right)\sin \theta \,\cos \theta_{1} \,\cos \theta_{2} \chi_{zxx} \\ & \,\,\,\,\,\, + L_{zz} \left( \Omega \right)L_{zz} \left( {\omega_{1} } \right)L_{zz} \left( {\omega_{2} } \right)\sin \theta \,\sin \theta_{1} \,\sin \theta_{2} \chi_{zzz} \\ \end{aligned}$$

(10)

where ${L}_{ij}$ denotes Fresnel factors, and $\theta$, ${\theta }_{1}$, ${\theta }_{2}$ are the angles between the surface normal and the SFG signal, input visible, and input IR beams, respectively. The second-order nonlinear susceptibility tensor ${\upchi }^{(2)}$ in the lab-fixed (x, y, z) coordinate system is proportional to the response of the molecule as described by the molecular hyperpolarizability tensor, ${\beta }_{ijk}$. The nonlinear susceptibility ${\chi }_{ijk}^{(2)}$ is related to the molecular hyperpolarizability ${\beta }_{ijk}$ by a coordinate transformation, which can be regarded as a projection:

$$\chi _{{ijk,q}}^{{(2)}} = N\sum\limits_{{l,m,n}} {\left\langle {\left( {\hat{i} \cdot \hat{l}} \right)\left( {\hat{j} \cdot \hat{m}} \right)\left( {\hat{k} \cdot \hat{n}} \right)} \right\rangle } \beta _{{lmn,q}}$$

(11)

Thus, SFG measurements with a certain combination of polarized input and output beams can be related to the orientation angle of the molecule (or functional group) with respect to the laboratory coordinate system. For example, for the symmetric stretching mode of a methyl group with ${C}_{3v}$ symmetry, we can obtain;

$$\begin{aligned} \chi _{{xxz,s}} = & \chi _{{yyz,s}} = \frac{1}{2}N_{s} \beta _{{ccc}} [{\text{cos}}{\mkern 1mu} \theta (1 + r) - {\text{cos}}^{3} {\mkern 1mu} \theta (1 - r)] \\ \chi _{{xzx,s}} = & \chi _{{yzy,s}} = \chi _{{zxx,s}} = \chi _{{zyy,s}} = \frac{1}{2}N_{s} \beta _{{ccc}} [{\text{cos}}{\mkern 1mu} \theta - {\text{cos}}^{3} {\mkern 1mu} \theta (1 - r)](1 - r) \\ \chi _{{zzz,s}} = & N_{s} \beta _{{ccc}} [r{\mkern 1mu} {\text{cos}}{\mkern 1mu} \theta + cos^{3} {\mkern 1mu} \theta (1 - r)] \\ \end{aligned}$$

(12)

Here, $r={\beta }_{aac}/{\beta }_{ccc}$, and $\theta$ is defined as the angle between the methyl principal axis and the z-axis along the surface normal. The number density, ${N}_{s}$ is a constant property of the surface or interface and does not change when different polarization combinations of the input and output beams are used in an SFG experiment. Therefore, important information on molecular orientation may be extracted from experimental measurements as a ratio of the signal strength measured in two polarization combinations. This approach does not require knowledge of the surface coverage, because it cancels when the signal strength ratio is taken.

• (E) Purpose of the fitting of the SFG spectra and notes

By fitting the SFG spectra using Eq. (9), not only do we obtain the parameters for determining the molecular orientation, but also the obtained parameter is very useful to learn the state of the molecules at the interfaces. As described in the preceding section, ${\chi }_{R}^{(2)}$ is the vibrationally resonant term that has a Lorentzian line shape. ${A}_{q}$, ${\omega }_{q}$, ${\Gamma }_{q}$ are the peak amplitude, the resonant vibrational frequencies, and the damping constants, respectively. Let us consider an isotropic surface. For a certain vibrational mode, for example, the ${A}_{q}$ obtained for SSP polarization combination corresponds to the tensor component ${\chi }_{yyz}$ of the nonlinear susceptibility associated with the SSP polarization. Thus, for the same vibrational mode, ${\Gamma }_{q}$ and ${\omega }_{q}$ should be the same, even for different polarization combinations.

The peak positions of the vibrational modes ${\omega }_{q}$ observed in SFG are important for assignment, as in IR absorption. The peak position changes depending on the environmental conditions to which the molecule (or functional group) is exposed. For example, OH and C=O groups change their peak positions depending on hydrogen bonding strength, which is useful for studying interactions at interfaces.

Regarding the peak width, wider peak widths indicate more inhomogeneity, while narrower peak widths indicate molecules at the interface are under specific conditions. The higher the surface crystallinity, the narrower the peak width. Peak width is also influenced by environmental conditions as well as the peak position. However, the peak width is ordinarily not smaller than the resolution of the spectrometer and the line width of the lasers, so the overall resolution of the instrument should be considered in the fitting process.

3 Experimental Equipment

3.1 General Description of the Experimental Equipment

Although the experimental setup for SFG is rather simple as compared with other nonlinear optical processes, its signal level is quite weak, due to the technique of acquiring only the signal coming from the sample surfaces. A schematic diagram for the SFG experimental arrangement in the co-propagation mode is depicted in Fig. 3. In general, frequency fixed visible (${\omega }_{1}$) beam and a frequency-tunable IR (${\omega }_{2}$) beam are overlapped temporally and spatially at the sample surface or interface to generate a sum-frequency ($\omega ={\omega }_{1}+{\omega }_{2}$) beam which can be detected by the photodetectors. When the IR frequency is tuned across a vibrational transition of the surface/interfacial molecules, the SFG signal intensity is resonantly enhanced. An SFG spectrum can be acquired by plotting the SFG signal intensity as a function of the IR frequency.

Fig. 3

Schematic of the experimental arrangement of the co-propagation geometry and the energy diagram of SFG

Currently, SFG spectrometers are generally classified into broadband and narrowband systems. This section describes the configuration of each equipment and its features.

3.2 SFG Spectroscopy with Narrowband Input

A scanning-type SFG spectrometer usually utilizes picosecond (ps) pulses generated from a high-energy pulse laser system to pump frequency conversion systems to generate a visible beam with a fixed wavelength and a wavelength tunable mid-IR beam.

Figure 4 depicts the schematics of the optical layout of the ps-SFG system. Generation of high-energy mid-IR beam is relatively more difficult than generating a visible or near-IR beam. For example, in a ps-SFG spectrometer, a mode-locked Nd:YAG laser beam with 1064 nm output is first frequency-doubled to 532 nm. Part of the 532 nm beam is used directly as the visible pump beam for the SFG. Other part of the 532 nm beam and a portion of the 1064 nm beam is then used for optical parametric generation (OPG) and amplification (OPA) to produce a near-IR beam, which is used in difference frequency generation (DFG) to produce the mid-IR beam. By adjusting the angle of the nonlinear optical crystal (β-BaB₂O₄) in the OPG/OPA generation and the angle of the grating for wavelength selection can sweep the mid-IR beam step by step to obtain an SFG spectrum. For the signal acquisition, a monochromator and a highly sensitive photomultiplier tube are usually used. A ps-SFG system is slow to acquire one spectrum. This is because, at each tuning step, which usually takes seconds to complete, only one frequency in the spectrum can be measured. On the other hand, ps-SFG systems usually have relatively high spectral resolution (below 5 cm^–1), determined by the frequency bandwidth of a ps pulse. Since the SFG signal intensity depends on the incident light intensity, the intensity of each wavelength of visible light and infrared light is measured simultaneously with an IR intensity monitor and a photodetector, respectively, and the SFG spectrum can be obtained later by dividing the SFG intensity by these values. Another disadvantage of ps-SFG system is that the sample damages due to the irradiation of highly intense laser pulses are unavoidable. To avoid the sample damages, the laser pulse intensities per unit area must be reduced, which simultaneously leads to a lower SN of the SFG spectra and a further increase in the measurement times.

Fig. 4

Schematic of the optical setup of the ps-SFG spectrometer

Mode-locked Nd:YAG lasers provide 10–30 ps pulses at about 50 mJ/pulse with a repetition rate of 10–50 Hz. These pulses can generate the visible pump pulses used in SFG, as well as a wide range of wavelength tunable infrared pulses. Figure 5 illustrates the schematics of the typical wavelength tunable system. Such systems are now commercially available and have been adopted by many laboratories for SFG spectroscopic researches. Although these systems are fairly reliable and relatively easy to maintain, the disadvantage of this ps-wavelength tuning system is that the pulse repetition rate is too low and the pulse width too wide, resulting in a low signal collection rate and slow recording of a spectrum [15].

Fig. 5

Schematic of the wavelength tunable system based on an optical parametric generation/amplification (OPG/OPA) system commonly used in the ps-SFG spectrometer

In this ps-SFG system, it is possible to use near-infrared light at 1064 nm or ultraviolet light at 355 nm instead of the visible excitation light at 532 nm for the SFG measurements. However, in these cases, mirrors, filters, polarizers, half-wave plates, and lenses used in the optical path of the visible lights as well as the grating of the spectrometer and the photomultiplier tube of the detector need to be changed to the corresponding SFG light wavelengths emitted from the sample surface. In particular, when used at longer wavelengths, a CCD detector or other detector that is compatible with longer wavelengths must be selected.

3.3 SFG Spectroscopy with a Broadband Input

A broadband SFG system usually requires a broadband high-repetition femtosecond (fs) laser source, e.g., a Ti:sapphire mode-locked laser system with an amplifier system to amplify the pulse energy. Figure 6 depicts the schematics of the optical layout of the fs-SFG system. High-energy broadband mid-IR pulses are also generated by the OPG/OPA-DFG method. Visible light pulses are usually narrowed by the bandpass filter or the monochromator. The overlapping of the narrowband visible beam and the broadband mid-IR beam simultaneously excites multiple vibrational bands at an interface. The broadband SFG spectrum is then acquired by the spectrometer equipped with a charge-coupled device (CCD) array for signal detection. A fs broadband SFG system has higher spectral collection speed, up to milliseconds. However, due to the relative wide bandwidth of the visible beam, the spectral resolution is typically lower than a ps-SFG system. However, broadband SFG with high resolution of less than 1 cm^–1 has been recently developed [32, 33]. In recent years, the development of the heterodyne detected SFG technique for determining the absolute molecular orientation at the interface, and in particular, the broadband SFG technique, with its high repetition rate lasers, has outstanding advantages for this method [34, 35]. Furthermore, recent improvements in laser technologies have also enabled the development of high-repetition broadband SFG systems, and the mainstream of SFG is rapidly shifting from the picosecond systems to the fs broadband SFG systems. These innovative SFG system enable to obtain hyper-spectral images at the interfaces [36,37,38].

Fig. 6

Schematic of the optical setup of the fs-SFG spectrometer

3.4 Doubly-Resonant Sum-Frequency Generation Spectrometer

Doubly-resonant SFG measurements are powerful techniques to clarify the interfacial structure of the organic materials used in organic electronic and optoelectronic devices. The signal enhancement is expected only for species that have an electronic absorption at the photon energy of the SFG, as shown in Fig. 7 [39]. Therefore, doubly-resonant effect offers a kind of molecular specificity to SFG. Furthermore, an electronic excitation spectrum of the interfacial chemical species for each vibrational band can be obtained. Thus, the SFG electronic excitation profiles are valuable for studying interfacial layers that exhibit complex vibrational spectra due to the coexistence of multiple chemical species, since the vibrational bands can be classified with reference to corresponding electronic spectra. The measurement of SFG excitation profiles can be also an effective way to obtain electronic spectra of the molecules, particularly at the interface on opaque substrates where electronic absorption spectrum measurement is difficult. Additionally, it is also possible to measure the interface of the OLED materials that emit very strong photoluminescent light in visible region, since the output SFG emerges on the Anti-Stokes side of the excitation wavelength.

Fig. 7

Reprinted with permission from Ref. [39]. Copyright 2002 Elsevier, All Rights Reserved

Visible wavelength dependent SFG spectra of a molecular monolayer of Rhodamine 6G molecules on fused quartz in SSP polarization combination normalized against the signal from a z-cut quartz.

Detail of the experimental setup of the doubly-resonant SFG measurements is shown in the previous publications [40,41,42]. In addition to the frequency-tunable IR laser beam, a frequency-tunable visible laser beam was generated by the other optical parametric generators/amplifiers (OPG/OPA) pumped by a mode-locked Nd:YAG laser at 1064 nm. The visible beam, tunable from 420 to 640 nm, was generated in a LiB₃O₅ (LBO) crystal mounted in OPG/OPA pumped by the 355 nm beam. The spectral resolution of the tunable visible beam can be calibrated with the Hg lines. To eliminate the scattered visible light and the photoluminescent light from the samples, the sum-frequency output signal in the reflected direction was filtered with short-wave-pass filters, prism monochromator, and grating monochromator.

Because doubly-resonant SFG is associated with the electronic excitation of the molecules, the observed molecular vibrations are more strongly affected by the vibrations directly related to the electronic excitation, and the intensity of observed molecular vibrations is more strongly affected by the vibrations directly related to electronic excitation, such as stretching vibrations of phenyl groups, rather than those of the functional groups on the side chains, which are not directly related to the electronic excitation. Therefore, the doubly-resonant effect is very useful for SFG measurements of molecules with π-conjugated systems, especially for the investigation of the molecular fingerprint region from 1000 to 2000 cm⁻¹. However, this wavenumber region is also strongly affected by the water vapor present in the optical path, making it difficult to measure. Therefore, the surrounding area of the optical path, including the sample stage, must be purged with N₂ gas or dry air to prevent the effect of water vapor.

3.5 Experimental Conditions for Polymeric Material Surfaces and Adhesive Interfaces

Figure 8 shows a photograph of the sample box of the SFG spectrometer. If the measurement is performed in air, the solid sample surfaces can be measured directly by setting the sample on the sample stage and adjusting the height of the sample surface to the height where the SFG light is generated.

Fig. 8

Photograph of the sample box of the phase-sensitive SFG spectrometer

In the practical sample measurements, if the substrate or sample is transparent, it is rather difficult to find the sample position because SFG light is hardly generated at IR wavelengths in the region where there is no SFG peak. In the case of samples with absorption in the visible light range or on metal substrates, it is necessary to adjust the laser beam intensity considering the effects of laser-induced damages to the samples. The surface of the sample should preferably be as smooth as possible, considering the loss of SFG signal, but depending on the sample, powder samples can also be measured. Note that the effect of the multiple reflections of the lights must be considered when using very thin substrates. Since SFG is a laser-based technique, it can easily interfere and produce interference peaks on the spectra.

To measure the interface between liquid and solid samples, it is necessary to prepare the liquid cell on the sample stage, such as shown in Fig. 9. The solid sample specimen is attached to the top of the piston unit that moves up and down and is placed in the close position to the window (e.g., transparent fused quartz, sapphire, or CaF₂).

Fig. 9

a Photograph of the SFG liquid cell for solid/liquid interface measurement, and b Schematic of the SFG liquid cell

The SFG light intensities generated from liquid and solid interfaces are often very weak because of the Fresnel coefficients at the interfaces and molecular orientation. To enhance this SFG intensity, the total internal reflection SFG technique using a prism is often used. This technique uses the total internal reflection that occurs at the interface between the prism with a high refractive index and the organic material with a lower refractive index, and is an effective technique to enhance the weak SFG signal from the interfaces. This TIR-SFG is extremely powerful at the interface between solid (refractive index ~ 1.5) and water (refractive index ~ 1.33), for example. Furthermore, by rotating the solution cell with a cylindrical prism and tuning the angle of incidence of the lights, it is possible to selectively detect SFG signals from multiple organic layer interfaces on the substrate, as illustrated in Fig. 10 [43]. However, prisms that are transparent in both the infrared and visible wavelength regions are limited to a few materials such as sapphire (refractive index, n = 1.75). The refractive indices of the infrared prisms made by CaF₂ or fused quartz are lower than those of general organic materials, and the laser light penetrates the sample without causing total reflection. If the film thickness of the sample in contact with the prism is too thin, the multiple reflections mentioned above may occur. If the target material is transparent polymer, it is not enough to make the thickness of the film thinner; it is necessary to place the opposite side of the polymer in contact with the prism with a rough surface or in contact with light absorbing materials. This is a very important procedure because SFG uses coherent laser lights, so it is necessary to prevent the interference fringes caused by multiple reflections to overlap in the spectra.

Fig. 10

Reprinted with permission from Ref. [43]. Copyright 2018 American Chemical Society, All Rights Reserved

Schematics of the hemicylindrical prism functionalized with OTS and then coated with PDMS pressed into a liquid cell. Two sets of beam angles are illustrated.

4 Applications of SFG Spectroscopy to Study Polymeric Materials Surfaces and Interfaces

The structure and orientation of molecules at polymer surfaces have a great impact on adhesion. Good adhesion is given by the work of adhesion at the interface between two different materials. As per the Young–Dupré equation, the work of adhesion is calculated:

$${W}_{SL}={\gamma }_{1}+{\gamma }_{2}-{\gamma }_{12}$$

(13)

The interfacial free energy (${\gamma }_{12}$) is obtained from the surface free energy of solids 1 and 2 as follows:

$${\gamma }_{12}={\gamma }_{1}+{\gamma }_{2}-2{\left({\gamma }_{1}\times {\gamma }_{2}\right)}^{2}={\left(\sqrt{{\gamma }_{1}}-\sqrt{{\gamma }_{2}}\right)}^{2}$$

(14)

According to Eqs. (13) and (14), ${\gamma }_{12}$ of the interface decreases when the value of ${\gamma }_{2}$ is closer to that of ${\gamma }_{1}$ and the work of adhesion ${W}_{SL}$ gets closer to ${\gamma }_{1}$ and ${\gamma }_{2}$. Thus, the interfacial adhesion should improve when the surface free energy of the surface of solid 1 is expected to be close to that of the surface of solid 2.

In general, polymeric materials have low surface free energies, and many of them show poor adhesion properties. The surface free energies of these polymers are often governed by the orientation and segregation of the surface functional groups, and understanding the orientation of the functional groups on the polymer surfaces are important issues for adhesion.

4.1 Chemical Structure of Adherent Surfaces

4.1.1 Characterization of the Surface Modified Polymer Surfaces for Adhesion Improvement

Polymers are key materials for reducing car body weight. Reducing the car body weight allows for lower carbon dioxide emissions and reduced fuel consumption [44]. Though high adhesive properties are an essential issue for the polymers used in the automotive components, some polymer materials, such as polypropylene used for automobile bumpers, have low adhesive properties without surface treatment. Generally, surface treatments such as plasma treatment, flame treatment, deep UV/excimer laser irradiation, and chemical treatment are often used to modify the surface properties of polymers [45,46,47,48,49,50,51,52,53,54]. Adhesion improvement by the surface treatment is believed to affect the adhesion of polymers not only through the introduction of the functional groups on the polymer surfaces, but also through several other factors, including changes in surface morphology and wettability. The adhesive mechanism has been often discussed based on the studies using X-ray photoelectron spectroscopy (XPS) and Attenuated total reflection infrared spectroscopy (ATR-IR); these techniques, however, can observe the near-surface region, but not the outermost layer surface or the buried interface. In addition, it is extremely difficult to obtain information on the molecular orientation using XPS and ATR-IR. Information on molecular structure, such as the orientation and orientation distribution of functional groups at the outermost surface of polymeric materials, is extremely important for elucidating the adhesion mechanism. Therefore, it is necessary to directly observe the outermost surface both before and after surface treatments to understand the adhesion mechanisms at the surface treated polymers. In this section, we have investigated the effect of N₂ gas plasma treatment from the outermost surface to the bulk region of the polypropylene using SFG and ATR-IR measurements. Furthermore, we investigated the variations in crystallinity of the surface treated polypropylene surfaces by Raman micro-spectroscopy [55].

• (A) N₂ Plasma treatment for the polypropylene resin surfaces

A commercially available 3 mm thick polypropylene board was used as the sample. Atmospheric pressure plasma treatment was performed by irradiating N₂ gas plasma at atmospheric conditions by moving the sample stage in one direction at 5 mm/s. The N₂ gas flow rate was set to 15 L/min. One reciprocation was counted as one treatment, up to 8 reciprocations in this experiment. After the N₂ plasma treatment, SFG measurements were performed immediately to avoid the influence of adsorption of water molecules in the air and changes over time. The ATR-IR and confocal Raman scattering measurements were performed under ambient conditions.

Figure 11 shows the ATR-IR spectra for the N₂ plasma-treated polypropylene samples using Ge prisms. To discuss the changes in the functional groups by plasma treatments, ATR-IR spectra are subtracted from the plasma treated polypropylene to that of the untreated one. As shown in Fig. 11, the intensity of the C=O and O–H bands increased while repeating the N₂ plasma irradiation. The C=O band split into two bands at 1685 and 1720 cm⁻¹, which correspond to the amide and carbonyl groups, respectively. A simple model for the mechanism of introduction of C=O and O–H groups by plasma irradiation has been proposed in Ref. [50], in which a simple model of insertion of polar functional groups is proposed as one possibility, but the real situation at the plasma-treated surface is expected to be more complex.

Fig. 11

Reprinted with permission from Ref. [55]. Copyright 2018 Elsevier, All Rights Reserved

The subtracted IR spectra of the pristine and N₂ plasma-treated polypropylenes in the a C=O and b O–H stretching regions collected using the Ge prism.

Figure 12 shows the SFG spectra of the pristine and N₂ plasma treated polypropylene surfaces in the C-H stretching region. The peaks derived from the methylene (CH₂) functional groups were observed at 2838 and 2915 cm⁻¹, corresponding to the CH₂ symmetric (d⁺) and asymmetric (d⁻) stretching vibrations, respectively. The three peaks derived from the methyl (CH₃) group were observed at 2872, 2932, and 2955 cm⁻¹. The peaks at 2872 and 2955 cm⁻¹ were assigned to the symmetric (r⁺) and asymmetric (r⁻) stretching vibrational modes. The peak at 2932 cm⁻¹ corresponds to the Fermi resonance between the CH₃ symmetric stretching and bending overtone [3, 56]. As illustrated in Fig. 12a, SFG spectra were changed significantly with increasing number of surface treatments, while the SFG peak intensities after N₂ gas plasma treatment in the C-H stretching region became weaker than that of pristine sample up to the fourth treatments. This indicates that the initial stage of the N₂ gas plasma induces the disordering of the polymer chains at the surfaces. The SFG spectrum of the eight-reciprocation sample was observed to be remarkably different than that of pristine polypropylene surface. Further, additional peaks appeared at 2850 and 2906 cm⁻¹. This remarkable spectral change suggests that the polymer chains at the polypropylene surfaces have been cleaved and degraded by the excessive plasma exposure.

Fig. 12

Reprinted with permission from Ref. [55]. Copyright 2018 Elsevier, All Rights Reserved

a SFG spectra of the pristine and N₂ gas plasma-treated polypropylene surfaces in the SSP polarization combination in C-H stretching region. b Changes in the d⁺/r⁺ ratio of the plasma-treated polypropylene surfaces as a function of the N₂ plasma treatment cycles.

In Fig. 12b, we show the change in the d⁺/r⁺ ratio as a function of the number of surface treatments. It is known that the d⁺/r⁺ ratio is a good indicator of the surface disorder in terms of the orientation of the long alkyl chains. On the other hand, the d⁺/r⁺ ratio is also associated with the surface free energy. As can be seen in Fig. 12b, the d⁺/r⁺ ratio increased with increasing plasma treatment cycles, i.e., the ratio of CH₂ groups increased and CH₃ decreased at the topmost surface. It is known that the surface free energy of the CH₂ groups is larger than that of the CH₃ groups [57]. Thus, this result implies that the surface free energy of polypropylene increased after conducting the N₂ gas plasma treatment. This interpretation is also consistent with previous reports, which state that the water static contact angle of plasma-treated polypropylene decreased after the plasma treatment [50].

The fact that plasma treatment improves the hydrophilicity of the polymer surfaces implies the presence of functional groups such as C=O and O–H on the surfaces. The presence of these functional groups has been previously reported by ATR-IR in Fig. 11 and XPS [58]. Depth analysis of ATR-IR experiments of the plasma treated samples in Fig. 11 reveals the introduction of both hydrophilic C=O and O–H groups in the polypropylene surface region. The intensities of these bands increase with increasing the number of the N₂ plasma treatment cycles. This indicates that the amount of the hydrophilic groups increases with increasing the cycles of N₂ gas plasma treatments. In the case of O–H bands observed for the ZnSe prism indicates that the O–H groups are distributed at least ca. 600 nm from the topmost surface toward the bulk.

Figure 13 presents the SFG spectra of the O–H and C=O stretching regions shown in the ATR-IR in Fig. 11. In the N₂ plasma treated samples, a slight and broad O–H band was observed, which was not detected before the plasma treatment. After the eight repetitions of plasma treated samples, the O–H band was clearly observed in the SFG spectra, but interestingly, its intensity gradually decreased with time. On the other hand, the C=O stretching peak was not observed until six repetitions of the plasma exposure (Fig. 13b). The over-treated sample exhibited barely discernible C=O stretching peak, although this peak disappeared immediately during the SFG measurements. The disappearance of these C=O and O–H peaks is possibly related to the glass transition temperature. According to previous SFG reports for polystyrene, plasma exposure introduces the C=O groups on the surface [59]. For polystyrene, the glass transition temperature is known much higher than room temperature. In contrast, since the glass transition temperature of the polypropylene is below room temperature, the main chain easily rotates during the measurements and the molecular structure of the surface is easily changed. This change in molecular orientation on the surface of hydrophobic materials is called hydrophobic recovery [60], and by measuring the surface with SFG continuously from immediately after the surface treatments, such dynamic molecular movements can be understood.

Fig. 13

Reprinted with permission from Ref. [55]. Copyright 2018 Elsevier, All Rights Reserved

The SFG spectra of the pristine and N₂ gas plasma treated polypropylene surfaces collected by SSP polarization combination. a O–H stretching and b C=O stretching regions.

In the case of Raman scattering, it has been reported that the intensity ratio of the peaks at 811 and 843 cm⁻¹ are related to the degree of crystallinity for polypropylene. Figure 14b shows the variations in peak intensity ratio I₈₄₃/I₈₁₁ from the Raman data of the N₂ plasma treated polypropylene in Fig. 14a, and it is found that the peak intensity ratio increased with increasing treatment cycles. Since the peaks at 811 and 843 cm⁻¹ are respectively related to the crystalline and amorphous phase of polypropylene, the Raman observations indicate that a thin amorphous layer is formed after plasma irradiation at the surface region.

Fig. 14

Reprinted with permission from Ref. [55]. Copyright 2018 Elsevier, All Rights Reserved

a Raman spectra of the pristine and N₂ plasma treated polypropylene near the surface region. b The peak intensity ratio, I₈₄₃/I₈₁₁, is plotted as a function of the number of plasma treatments.

The annular dark field (ADF) STEM image shown in Fig. 15 also reveals the existence of the amorphous layer with a thickness of ca. 50 nm formed at the 4-times N₂ plasma-treated polypropylene/adhesive interface bonded after the plasma surface treatment. This STEM image clearly indicates that the plasma treatment forms the roughness of the polypropylene surface with complicated nanometer features and the adhesive can be penetrated such small features. In polypropylene, we note that the amorphous part can be selectively stained, which enables us to see the polypropylene lamellar structure. As can be seen in the ADF-STEM image in Fig. 15, the polypropylene lamellar is formed just below the surface of polypropylene, indicating that the plasma treatment can selectively modify the surface region (ca. 50 nm thick) of the polypropylene without the damage of the bulk polypropylene. In the ADF-STEM image, the bright part indicates to the RuO₄-stained area. Therefore, it is clearly recognized that thin amorphous layer is formed at the interface even after the forming of the joint interface. The thin amorphous layer on the surface contains polar groups such as hydroxyl and carbonyl groups, and we believe that the formation of such thin amorphous layers greatly contributes to the improvement of the adhesive properties of the poor adhesive polypropylene.

Fig. 15

Reprinted with permission from Ref. [55]. Copyright 2018 Elsevier, All Rights Reserved

STEM-ADF image of the interfacial region between the 4-times N₂ gas plasma treated polypropylene (upper part) and the adhesive (lower part).

• (B) Flame treatment for the polypropylene resin surfaces

As in the case of the plasma treatment process, flame treatment is also used to improve the adhesion strength of polypropylene. Next, we discuss how the surface structure induced by the flame treatments on the polypropylene differs from that induced by that of the plasma treatment [61]. The flame treatment of the samples was performed by a commercial gas burner and the polypropylene plates were moved at a constant speed in one direction at 200 mm/s using automatic moving stage. The distance between the polypropylene surface and the burner's nozzle was 3 cm. In each surface treatment, the samples were moved back and forth once as a one-time treatment, up to 8 times on each surface. After these surface treatments, the adhesive strengths of the polypropylene surfaces were apparently improved compared to the untreated surfaces. In the sample treated with N₂ plasma four times, the adhesive strength was improved by more than 8.0 MPa. In addition, the adhesive strengths increased up to 5.7 MPa in the case of four times flame-treated samples. These results indicate that both treatments are effective techniques for improving the adhesion properties of polypropylene surfaces.

In Fig. 16, the subtracted ATR-IR spectra of the flame-treated polypropylenes are shown. As with the plasma-treated samples, the IR band intensities of both C=O and O–H bands increased with increasing treatment cycles. The IR band intensity of the amide group, which is observed at 1680 cm⁻¹, but not the carbonyl groups, increased with repeating N₂ plasma exposure, while the intensity of the C=O band corresponding to the carbonyl group, rather than the amide group, increased with repeating flame treatment. This trend of the flame treatment was opposite to that of the N₂ gas plasma treatment case.

Fig. 16

Reprinted with permission from Ref. [61].Copyright 2018 Elsevier, All Rights Reserved

Subtracted ATR-IR spectra of flame-treated polypropylenes collected using Ge prism. a C=O stretching region and b O–H stretching region.

To examine the impact of the treatments on the bulk region of polypropylene, the depth profiles of the O–H and C=O groups were investigated using two different ATR-IR prisms, Ge and ZnSe, which have different refractive indices. The depth analyzes for the N₂ plasma-treated and flame-treated polypropylenes are shown in Fig. 17. In both surface treatments, the corresponding IR band intensities of the hydrophilic functional groups increase with increasing number of treatment cycles. These results suggest that the amount of the introduced functional groups increases with the number of treatment cycles. Although the trends of increase in the number of the hydrophilic functional groups after both treatments are almost the same, the observed IR band intensities of the introduced functional groups are quite different. The IR intensities of the O–H bands of the N₂ plasma-treated polypropylene samples are larger than those of the flame-treated samples. In addition, the IR intensities of the O–H bands of the N₂ plasma-treated samples observed with the Ge prism are larger than those observed with the ZnSe prism. This result indicates that the O–H moieties introduced by N₂ plasma irradiation are distributed unevenly near the polypropylene surface region, rather than in the bulk direction, since the IR probing depth of the O–H band region is estimated to be approximately 200 nm and 600 nm for the Ge and ZnSe prisms, respectively. On the other hand, no significant difference in the IR intensity of the C=O band is observed for both N₂ plasma- and flame-treated polypropylenes depending on the prism, indicating that the introduced carbonyl functional groups are almost uniformly distributed from the surface to the bulk region.

Fig. 17

Depth profile analysis of the O–H and C=O groups evaluated from ATR-IR as a function of the treatment cycles. (left) N₂ plasma treatment. (right) Flame treatment. Reprinted with permission from Ref. [61]. Copyright 2018 Elsevier, All Rights Reserved

For the investigation of the molecular structure at the topmost surfaces of polypropylenes before and after surface treatments, SFG measurements were performed for the pristine, 4-times N₂ plasma-treated, and 4-times flame-treated polypropylene surfaces under atmospheric conditions. Figure 18 shows the SFG spectra of each sample taken with the polarization combinations of SSP and PPP in the C-H stretching region. The SFG spectra of the N₂ plasma-treated and flame-treated polypropylenes surfaces show a decrease in the SFG signal intensities and a shift in the peak position is observed with some peaks for each treated sample, indicating that the conformations of the treated polypropylene surfaces are changed by both the N₂ gas plasma treatment or flame treatment. To confirm the changes in the orientations of the surface functional groups before and after surface treatments, the polar tilt angles of the CH₃ group of the polypropylene are estimated. Before the surface treatments, the methyl groups on the polypropylene surfaces are inclined at about 66° from the surface normal. After the N₂ gas plasma and flame treatments, the orientations of CH₃ change to 56° and 85° from the surface normal, respectively. These results indicate that the molecular conformations of the treated polypropylene surfaces have been modified by the N₂ plasma irradiation or burning with flame, resulting in the rearrangement of the molecular orientation. For the N₂ plasma treated sample, a new broad feature is found at 2855 cm⁻¹ [55] probably because decomposed species are created by excessive N₂ plasma irradiation.

Fig. 18

Reprinted with permission from Ref. [61]. Copyright 2018 Elsevier, All Rights Reserved

SFG spectra in C-H stretching region of the pristine, N₂ plasma-treated and flame-treated polypropylenes acquired with the polarization combinations of a SSP and b PPP. The solid curves are the fitting curves. The SFG spectra were normalized by the peak intensities of the 2880 cm⁻¹ of the pristine polypropylenes collected by the SSP polarization combination.

Contrary to the case of the plasma-treated samples, no SFG bands derived from both C=O and O–H bands are observed for the flame-treated samples. Such a result suggests that the functional groups introduced by plasma irradiation or flame treatment are not exposed on the air side of the uppermost surfaces. Previous works have also suggested that the introduced polar functional groups are flipped toward the bulk side [58]. Because the glass transition temperature of polypropylene is lower than room temperature, the molecular orientation can be easily changed to migrate polar groups toward the bulk side, where the surface free energy becomes lower.

The question is what is formed on the polypropylene surface by these two surface treatments? Fig. 19 shows the IR spectra of polypropylene treated with plasma and flame treatment and what happens when the treated polypropylene is rinsed by water. After surface treatments of the polypropylenes, hydroxyl and C=O bands are observed in both surface treatments, but after rinsing by water, all of these bands decrease in intensity in the IR. This indicates that the molecules created by these surface treatments are relatively low molecular weights and contain large numbers of polar groups such as hydroxyl and C=O groups. Note that O–H and C=O almost disappeared after rinsing with water in the plasma treated polypropylenes, while they still remained after rinsing in the flame treated one.

Fig. 19

ATR-IR spectra of a plasma-treated and b flame-treated polypropylenes before and the after rinsed by water

The existence of such low molecular weight molecules is also indicated by the change in the contact angle before and after rinsing by water. Figure 20 illustrates the changes in static water contact angles before and after rinsing by water. The contact angle of plasma-treated polypropylene changes significantly after rinsing by water, and it shows water repellency is rather higher than that of the untreated one. Surprisingly, it is noteworthy that even when the low molecular weight molecules on the surface are almost completely removed by such rinsing, there is little difference in adhesive strength. In other words, the low molecular weight components containing large numbers of polar groups, which are generated near the surface by the surface treatments, do not significantly affect the adhesion mechanisms.

Fig. 20

Changes in the static water contact angles of the surface treated polypropylenes before and after rinsing by water

4.1.2 Characterization of Primer Modified Polymer Surfaces

Adhesive promoters for the chemically inert materials have often been used for improvement of the poor adhesion. Such adhesion promoters are believed to connect directly to the surfaces to be bonded and thereby modify the surface properties. However, direct evidence of chemical reactions occurring at the bonding interface has not been presented for most of the material combinations used to date. This is probably due to the fact that it is technically difficult to characterize the nature of chemical bonds at the buried interface between an adherent and an adhesive at the molecular level. Therefore, we have used an indirect indicator, such as the adhesion strength, to evaluate the impact of the surface pre-treatment on the process. However, adhesion strength is rarely dependent on a single factor, making it difficult to clarify the promotion of chemical bond formation only from this parameter.

Here we show the molecular reaction behavior at the interface between an epoxy polymer and a primer with isocyanate functional groups [62]. Isocyanates are well-known to react with hydroxyl groups to form urethane bonds, and this chemical reaction is popularly used in polyurethane polymerization processes. Therefore, infrared spectroscopy can be used to directly monitor the progress of the polymerization reaction in the bulk from the disappearance of isocyanate bands and the formation of C=O bands. However, since the formation of the urethane bonds at the interface between the isocyanate and the epoxy polymers has not been directly observed so far, we wished to observe this chemical reaction at the interface in situ.

A mixture of the precursors of the epoxy polymer, bisphenol A diglycidyl ether and triethylenetetramine, were diluted with chloroform and then spin-coated on the transparent calcium fluoride substrates coated with a 100 nm thick silica film. After spin-coating, the polymer films were cured at 80 °C for 18 h. 5 wt% butyl acetate or chloroform solutions of 4,4-methylene diphenyl diisocyanate (MDI) were prepared as a model isocyanate primer compound, since MDI is the main component of commercial isocyanate primers. The chemical structures of the precursors of the epoxy polymer and MDI are illustrated in Fig. 21.

Fig. 21

Reprinted with permission from Ref. [62]. Copyright 2018 Royal Society of Chemistry, All Rights reserved

Chemical structures of the materials used in this study.

Figure 22a, d show the SFG spectra in the C=O stretching region of the epoxy polymer when in contact with MDI butyl acetate and chloroform solutions, respectively. When urethane bonds are formed between the hydroxyl groups of the epoxy polymer and the isocyanate groups of MDI molecules, a C=O band derived from the urethane bond appears around 1730 cm⁻¹ [63]. Before the contact with the MDI solution, no C=O peak is seen at the epoxy surface, as shown in the red curve in Fig. 22a. However, a C=O stretching band appeared when the epoxy polymers come into contact with the MDI solutions at room temperature. In the case of the SFG results obtained from the interface between the epoxy polymer and the MDI butyl acetate solution, we have to consider the contribution from the C=O stretching band of butyl acetate as in the case of the ATR-IR. However, the appearance of a C=O stretching peak was also observed in the SFG spectrum taken from the interface between the epoxy polymers and the chloroform solution of MDI, as shown in Fig. 22d. Therefore, we can exclude the contribution from the C=O stretching band of butyl acetate and we conclude that the SFG peak at 1730 cm⁻¹ originates from the C=O stretching of urethane bonds formed at the epoxy surfaces. The SFG spectra of the hydroxyl groups of the epoxy polymer should change as they react with the isocyanates of MDI, since the urethane bonds are formed by the reaction between the isocyanate groups of the MDI molecule and the surface hydroxyl groups of the epoxy polymer. Figure 22c, f show the SFG spectra of the epoxy polymer interfaces before and after being in contact with the butyl acetate and chloroform solutions of MDI in the O–H stretching region. The hydrogen-bonded O–H bands are clearly visible in the SFG spectra of the epoxy polymer before contact with the MDI solutions, but they disappear completely after contacting the MDI solution. On the other hand, O–H bands remain in the SFG spectra of the epoxy/butyl acetate solvent interface as shown in Fig. 22c. The fact that the O–H bands completely disappear by contacting with MDI-butyl acetate solution and do not disappear only by contacting with butyl acetate solvent without MDI may support that the O–H group on the epoxy surface was spend for the reaction with MDI. Even if not, there is certainly an interaction between MDI and O–H in epoxy polymer. Note that the intensity of the O–H stretching band became weak in the SFG spectrum in contact with liquids, as shown in Fig. 22c. This is due to the Fresnel factor difference between solid/air and solid liquid interfaces. Although the SFG peak intensity does not only represent the amount of the surface functional groups, this result suggests that the hydroxyl groups on the epoxy polymer surfaces have reacted with isocyanate groups on the MDI. For comparison, in Figs. 22d, f, we also demonstrate the SFG measurements for the polypropylene/MDI chloroform solutions. Since polypropylene does not have a hydroxyl group, it can be found that no C=O stretching band is appeared at the interface, even when it contacts with the MDI solution. It can be seen that the peaks attributed to hydroxyl groups on the epoxy polymer surface may originate not only from the O–H groups on the polymer surface, but also from water molecules adsorbed on the epoxy polymer surface.

Fig. 22

Reprinted with permission from Ref. [62]. Copyright 2018 Royal Society of Chemistry, All Rights reserved

a–c SFG spectra of the epoxy polymer surface (red open square), epoxy/butyl acetate solvent interface (green open square), and epoxy/MDI butyl acetate solution interfaces (black filled square). The SFG spectra in the O–H region c of the butyl acetate/epoxy interface with and without MDI were multiplied by 5 for ease of comparison with that of the air/epoxy interface. d–f Polypropylene/MDI chloroform solution interface (blue filled circle) and epoxy/MDI chloroform solution interface (black filled circle).

Figure 22b, e illustrate the SFG spectra in the NCO stretching region at the interfaces between the epoxy polymer and the MDI solutions. It is interesting to note that the asymmetric stretching mode of the isocyanate groups in the MDI molecule is observed at 2270 cm⁻¹ for both the butyl acetate and chloroform solutions of MDI epoxy interfaces. This peak can be assigned to the unreacted isocyanates groups of MDIs. As shown in Fig. 22e, the unreacted NCO peak is also observed in the SFG spectrum of polypropylene/MDI solution interface. To remove the influence of the unreacted MDI molecules adsorbed at the epoxy polymer surfaces completely, the epoxy polymers are rinsed with acetone after being in contact with the MDI–butyl acetate solution. Furthermore, the samples are dried by heating at 80 °C to eliminate the influence of the residual butyl acetate and acetone. Even after rinsing and heating, peaks derived from both C=O and NCO stretching can be observed at the epoxy polymer surface in the air atmosphere, as shown in Fig. 23.

Fig. 23

Reprinted with permission from Ref. [62]. Copyright 2018 Royal Society of Chemistry, All Rights reserved

SFG spectra for the C=O stretching region and NCO asymmetric stretching region of the epoxy polymer surface treated with an MDI–butyl acetate solution, after rinsing the surface with acetone and heating at 80 °C. The inset shows the SFG spectra of the epoxy polymer surface before and after treating with MDI–butyl acetate solution for the OH stretching region taken in the air atmosphere.

Based on these observations, we postulate the chemical reaction processes that can occur at the interface between the epoxy polymer and the MDI molecule, as shown in Fig. 24. Upon contact of the epoxy polymer surface with the MDI solution, the isocyanate moieties on one side of the MDI molecule react rapidly with the hydroxyl groups on the epoxy polymer surface. On the other hand, the isocyanate groups on the other side of the MDI molecule remain in the unreacted state. Since the isocyanate group reacts highly with polyols, which are components of polyurethane adhesives, the residual isocyanate function of MDI can react with the polyol groups in the adhesive. In fact, in the SFG spectra, signal of residual isocyanate completely disappears when MDI-treated epoxy surfaces are contacted with polyethylene glycol. Consequently, a chemical bridge is formed between the epoxy polymer and the adhesive by the MDI promoter. This is expected to improve the bond strength between the epoxy polymer and the polyurethane adhesive. Indeed, primers containing molecules with isocyanate groups have been used for several decades to promote adhesion between clear epoxy coatings and polyurethane adhesives.

Fig. 24

Possible chemical reaction between MDI and the epoxy surface

5 Investigation of Buried Polymer/polymer Interfaces

SFG spectroscopy has been extensively applied to probe buried polymer interfaces, including those with liquids, organic solids, inorganic solids, other polymers, and metals. Dhinojwala group studied the interface between polystyrene (PS) and a comb polymer poly(vinyl N-octadecyl carbamate-co-vinyl acetate) (PVDC) using SFG spectroscopy [64].

PVDC has long alkyl side chains. The long, hydrophobic alkyl side chains segregate to the air interface and provide a nonstick surface for a good release. Therefore, the adhesion and wetting behavior is strongly influenced by the bulk side chain melting temperature. In their work, a total internal reflection (TIR) SFG geometry shown in Fig. 25 was applied based on a sapphire prism. From the SFG observation using TIR geometry, both the methyl and methylene groups from the comb polymer were ordered at the polymer/polymer interface. As shown in Fig. 26, the presence of the methylene signal at the PS/PVDC interface indicated the gauche defects on PVDC alkyl side chains at the interface. The phenyl groups from PS were also ordered at the interface, while the phenyl rings are tilted with respect to the interface normal [64]. Dhinojwala group also studied the molecular orientation of the polymer interfaces and their temperature variations of two release agents, poly(vinyl octadecylcarbamate-co-vinyl acetate) (PVNODC) and poly(octadecyl acrylate) (PA-18) [65]. On heating, an abrupt drop in SFG intensity at 43 °C for the PS/PA-18 interface coincides with the bulk transition temperatures of PA-18, also at 43 °C. The drop in SFG intensity for the PVNODC/PS interface is gradual and matches with the broad transitions observed in bulk PVNODC. These observations indicate that the disordering is correlated with the bulk structure and transition temperatures [65].

Fig. 25

Reprinted with permission from Ref. [64]. Copyright 2002 American Chemical Society, All Rights reserved

Schematic illustration of the bilayer sample of the sapphire prism and experimental geometry. The bilayer consisted of a film of poly(vinyl-N-octadecyl carbamate-co-vinyl acetate) on top of a dPS or hPS film on the equilateral triangle sapphire prism. A visible beam (800 nm) and tunable IR beam (3.1–3.8 μm) were internally reflected at the Comb/dPS (or hPS) interface.

Fig. 26

Reprinted with permission from Ref. [64]. Copyright 2002 American Chemical Society, All Rights reserved

SFG spectra in SSP (S-polarized SFG beam, S-polarized visible beam, P-polarized IR beam) polarization combination. a dPS/Comb interface of a bilayer sample at 8°. b hPS/Comb interface of a bilayer sample at 8°. c Air/Comb interface of a solitary Comb film at 42°. d Comb/sapphire interface of a solitary Comb film at 8°

More practical adhesive/release agent interfaces are those used in pressure-sensitive adhesive (PSA) tapes [66]. Poly(ethylene-co-vinyl-n-octadecyl carbamate) (PEVODC) belongs to the general class of long alkyl side chain polymers, which are widely used as release coatings for the commercial PSA tapes. As shown in Fig. 27, there are significant differences in the peak intensities corresponding to the as-cast and annealed films. By comparing each r⁺ peak intensity in the SSP and PPP modes, the net orientation of the polar tilt angles of the methyl groups was evaluated to be around 46° (non-annealed, as-cast) and 51° (annealed after casting) from the surface normal, respectively. Even when angle distribution is taken into account, the calculated angles are similar for both as-cast and annealed films. If comparable values for the polar tilt angle and orientation distribution of the methyl groups of the side chains are assumed, the surface density of oriented methyl groups is calculated to be 1.7 times higher in the annealed film than in the as-cast one, according to a previously reported equation. This observation clearly indicates that the highly-ordered crystalline-like domains of the octadecyl chains are formed by the annealing at the PEVODC surfaces. The peel force of the as-cast film is higher than that of the annealed one, suggesting that disordered alkyl chains between the crystalline domains can potentially increase the peel force. Therefore, the improvement in the crystallinity of the alkyl side chain suppresses the intercalation of the side chains of the adhesive at the adhesive/release agent interface and results in a reduction in the peel force. Thus, highly-ordered crystalline domain structures play important roles in reducing the peel force for the PSA tapes, which is attributed to the suppression of polymer-network entanglement.

Fig. 27

Reprinted with permission from Ref. [66]. Copyright 2018 Elsevier, All Rights reserved

SFG spectra of PEVODC films on the PET substrates. a as-cast film dried under ambient conditions (scaled) and b annealed at 100 °C. SSP (black circles) and PPP (white circles) polarization combinations.

Figure 28 presents the changes in the SSP-polarized SFG spectra of the interface between a typical acrylic adhesive and PEVODC by varying the annealing temperatures [66]. Two peaks corresponding to the methyl groups of the side chains are observed at 2880 cm⁻¹ (r⁺) and 2940 cm⁻¹. Note that almost no sum-frequency signal is generated from the adhesive surface, as shown in Fig. 28. Hence, we can be confident that this peak is due to the methyl groups of the alkyl side chains of PEVODC, suggesting that the orientation of the alkyl side chains of PEVODC is maintained after contact with the adhesive. After heating the adhesive/PEVODC layer at 40 °C for 90 min, the intensity of the r⁺ peak remains unchanged states, as shown in the blue curve in Fig. 28. When the films were heated at 70 °C, however, the intensity of the r⁺ peak decreased drastically. In the case of the 70 °C heat-treated films, the peel force increased to 0.33 N. A curing temperature of 100 °C further decreased the r⁺ peak intensity and an increase in the peel force were observed. The phase transition of the alkyl side chains occurs at 54 °C, followed by the phase transition of the main chain at 76 °C. Therefore, the intercalation of polymer chains at the adhesive/PEVODC interface is promoted by the curing process. These observations suggest that the randomization of PEVODC side chains at the interface and the intercalation of these polymer side chains by curing lead to an increase in the peel force values.

Fig. 28

Reprinted with permission from Ref. [66]. Copyright 2018 Elsevier, All Rights reserved

SFG spectra of adhesive/PEVODC interfaces in SSP polarization combination: untreated (black) and after heated to 40 °C (blue), 70 °C (green), and 100 °C (red). The SFG spectrum of the adhesive surface is also shown for comparison (open circles). Heating time of each sample was 90 min.

6 Probing Adhesive Interfaces

When adhesives are used in the assembling of buildings, houses, and other constructions, there are many advantages expected, such as easier assembly and weight reduction. On the other hand, it is extremely difficult to predict and evaluate the reliability of adhesion and joining. This is because adhesion between two objects occurs at a “buried interface,” and it is difficult to observe this interface directly. However, SFG spectroscopy enables to approach these interfaces by preparing the conditions that allow the light to reach the interfaces.

6.1 Polyurethane Adhesives

Polyurethane compounds and sealants are widely used as encapsulants and protective barriers. Despite the utility of these polyurethanes, the basic mechanisms of adhesion are not well understood due to the difficulty of direct observation of buried interfaces. Interfacial properties such as adhesion are determined by the molecular structure of the interface. In this study, the curing behavior of polyurethane was observed in situ using SFG spectroscopy [67].

Polyurethane adhesives were prepared by mixing compounds with isocyanate groups (Millionate-NM, Tosoh) and main agents of branched aliphatic compounds having hydroxyl groups (Fig. 29), defoaming by vacuum evacuation, and stirring. This polyurethane adhesive is completely cured by heating at 120 ºC for approximately 5 h after being coated on the substrates. Schematic of the heating sample stage used in the SFG measurements is shown in Fig. 30a. The temperature is monitored by the thermocouple connected to the sample stage and regulated by a digital temperature controller with proportional-integral-derivative (PID) control.

Fig. 29

Chemical structures of the compounds in polyurethane adhesive

Fig. 30

a Schematic of the heating sample stage used in the SFG measurements. b Temperature dependent SFG spectra collected from aluminum oxide/polyurethane adhesive interface in SSP mode

Figure 30b shows the SFG spectra of the polyurethane adhesive immediately after the preparation, which was applied to thin aluminum oxide-coated CaF₂ substrates and then heated sequentially from room temperature up to 150 °C. Around room temperature, the SFG peaks observed in the C-H stretching region gradually change with increasing temperature.

To investigate the temperature dependence of the SFG spectra, two-dimensional correlations are taken as shown in Fig. 31. Two-dimensional correlation spectroscopy, which is a well-known technique used in NMR, IR, and near-infrared spectroscopy, is a visualization method that can display the changes in a system when stimuli with certain directions (e.g., electric field, mechanical stress, temperature, or temporal changes) are applied [68,69,70]. By applying this two-dimensional correlation technique to the vibrational spectroscopy, it is possible to observe changes in the orientation distribution of dipole moments in a system under external stimuli. In the two-dimensional correlation spectra, there are two two-dimensional spectra that can be acquired: the synchronous and the asynchronous correlation spectra. Peaks that appear on the diagonal line in the synchronous spectra are called autocorrelation peaks, indicating that these peaks are responsive to a certain external stimulus. Peaks that appear off-diagonal are called cross-peaks and are useful for studying interactions between functional groups. If the sign of the cross-peaks of two wavenumbers (${\upnu }_{1}$ and ${\upnu }_{2}$) is positive in synchronous correlation, it indicates that the ${\upnu }_{1}$ and ${\upnu }_{2}$ bands are oriented in the same direction to each other, i.e., they have the same reorientation in response to the external stimuli.

Fig. 31

(Left) Synchronous and (right) asynchronous two-dimensional correlation spectra of heat curing behavior of aluminum oxide/polyurethane adhesive interface

On the other hand, in the asynchronous correlation spectra, no autocorrelation peaks appear on the diagonal line, resulting in a two-dimensional spectrum that is asymmetric with respect to the diagonal line. The sign of the synchronous correlation represents the relative direction of the spatial reorientations, whereas the sign of the asynchronous correlation represents the temporal relationship of the reorientations, i.e., the relationship between the time before and after the reorientation changes. When the peak sign is positive, it indicates that the functional group corresponding to the wavenumber ${\upnu }_{1}$ on the horizontal axis is reoriented before the functional group corresponding to the wavenumber ${\upnu }_{2}$ on the vertical axis. In the case of a negative cross-peak in the synchronous correlation spectrum, the temporal relationship in the asynchronous correlation corresponds to the opposite of the previous relationship [68,69,70].

In the SFG spectra of the polyurethane adhesives in Fig. 30, the peaks at 2850, 2880, 2940, and 3320 cm⁻¹ are attributed to symmetric CH₂, symmetric CH₃, CH₃ Fermi resonance, and NH stretching vibrations, respectively. The NH stretching derived from the urethane bond is observed immediately after application, indicating that the polymerization curing reaction is not in progress in the bulk, however, the urethane bond is already formed at the adhesive interface and the NH is already created. It is believed that adhesive strength between metal and epoxy adhesives is expressed by the interaction between the oxide film and the adhesive molecules. Functional groups such as amines and amides may act as Brønsted or Lewis bases because the oxygen and nitrogen atoms have a lone electron pair. Also, the aluminum oxide surface coated on the substrate exhibits OH groups, which can act as Brønsted acid or Lewis acid owing to the coordinatively unsaturated metals. The observation of NH groups in the SFG spectra suggests that Lewis acid–base interactions at the interface with the aluminum oxide have taken place immediately after the application of the adhesive. Such interactions may have attracted NH groups in the urethane bonds, and NH groups were observed at the interface of the polyurethane adhesive before curing. In fact, it has been reported recently that amine containing in polymers segregate at the metal–metal interfaces [71, 72]. On the other hand, the CH₂ and CH₃ of the alkyl chains are not clearly visible immediately after the application of the adhesive, and it is considered that the alkyl chains are nearly in a randomly oriented conformation.

Two-dimensional correlation spectra of the heating behavior in Fig. 31 show that the peaks of CH₂ and CH₃ have a positive sign with synchronous correlation. This indicates that immediately after the application of the adhesive, the NH groups of the urethane bond gather at the aluminum oxide interface, and CH₂ and CH₃ are in almost random orientation at room temperature. However, with the progress of the curing by heating, the SFG peaks of CH₂ and CH₃ become distinct at the interface. The cross-peaks at 2850 and 2940 cm⁻¹ in the asynchronous correlation spectrum in Fig. 31 show a negative sign, indicating that the CH₃ changes occur prior to the changes in CH₂. The signs of the cross-peaks at 2850, 2880, and 3320 cm⁻¹ are both negative in the synchronous correlation spectrum and negative in the asynchronous correlations. This indicates that NH groups decrease at the interface as the curing proceeds, and that the time behavior is preceded by the changes in CH₃ and CH₂. In general, it is known that urethane adhesives do not adhere well to the aluminum. During the curing process, the alkyl main chains are aligned and the NH groups that have been segregated at the interface due to their interaction disappear. This may be one of the reasons for the loss of adhesiveness against aluminum adherents after the curing of the polyurethane adhesives.

6.2 Silyl-Terminated Polyether Adhesives

Silyl-terminated polyester liquid polymer reacts with moisture in the air at room temperature when combined with a curing catalyst. The elastic sealants using this polymer are known as modified silicone-based sealants, and they have excellent durability, heat and cold resistances, and good economic performance. Elastic adhesives using this silyl-terminated polyester polymer also have high resistance to external stresses such as mechanical impacts and vibrations, as well as temperature changes, and are used in architectural applications.

In this study, the adhesive materials, silyl-terminated polyester (MS-polymer™), and adhesives were provided by Cemedine Co., Ltd. SFG measurements at the interfaces during curing were performed for each of the Silyl-terminated polyester and aminosilane agent blended with the silyl-terminated polyester. The Chemical structures of the materials used in this study are shown in Fig. 32 and their compositions are summarized in Table 1 and are referred to as MS-1 and MS-3, respectively. 3-aminopropyltrimethoxysilane coupling agents are added to silyl-terminated polyester to accelerate the curing reaction, as indicated in Table 1.

Table 1 Compositions of the silyl-terminated polyester adhesives (by weight)

As in the case of the polyurethane adhesives in the preceding section, SFG measurements at the adhesive interfaces were performed for aluminum oxide-coated CaF₂ substrates immediately after the coating, during curing, and after enough time had passed for the adhesive to become hardened.

Fig. 32

Chemical structures in the silyl-terminated polyester adhesives

Figure 33 shows the time evolution of the SFG spectra at the interface between MS-3 and the aluminum oxide thin layer from immediately after the application up to 22 h. Characteristic peaks at 2840 and 2940 cm⁻¹ are observed immediately after the coating of MS-3 mixed with aminosilane. The peaks are assigned to the methyl symmetric stretching vibration derived from the –OCH₃ of aminosilane and the Fermi resonance of the –OCH₃, respectively [73]. At the MS-3 interface in contact with the aluminum oxide film, these silane-derived peaks gradually decrease through time and, though it depends on the surrounding conditions, almost disappear completely after approximately 2.5–4 h. Instead of this decrease in intensities of the peaks, other new peaks appear at 2850 and 2882 cm⁻¹. These new peaks correspond to the symmetric stretching vibrations of methyl and methylene groups originating from the polyester polymer, as shown later, while the Fermi resonance at 2940 cm⁻¹ is indistinguishable because of the overlap between the methoxymethyl group origin and the methyl origin. The time dependence of these SFG spectra strongly suggests that the functional groups present in the polymer main chain are not observed immediately after application, probably due to the random orientation of the polymer chains and the segregation of the silane coupling agent at the interfaces.

Fig. 33

SSP-polarized SFG spectra following the time evolution of the MS-3 and aluminum oxide interface after application

The –OCH₃ bonded to the Si atom of these aminosilanes and the SiOCH₂CH₃ group at the end of the MS-polymer decompose upon reaction with moisture. To observe the reaction behavior with water at the interface, SFG spectra measured from the C-H to the O–H band regions are shown in Fig. 34. Since the substrate was coated with aluminum oxide that had been cleaned by ozone treatments, broad O–H band was observed before the polymer coating, probably due to the O–H groups on the aluminum oxide and the water adsorbed on the surface. These hydrogen-bonded O–H bands are still observed immediately after adhesive application on the aluminum oxide layer, however, the water bands observed at the interface decrease in intensity as the adhesive cures. This suggests that the adsorbed water molecules present at the interface are consumed by the Si-OCH₃ decomposition reaction. The tack-free time of MS-3 is ca. 6 min on the adhesive surface. However, at the interface with aluminum oxide, the decreases of Si-OCH₃ and O–H groups are found to occur over a period of a few hours. This is probably because molecular mobilities are suppressed at the interface and there are less occasions for reaction with water than at the surface, resulting that the active silane coupling agent takes some time to react with the water molecules.

Fig. 34

SSP-polarized SFG spectra from CH to OH stretching region following the time evolution of the MS-3 and aluminum oxide interface after application

In the SFG spectrum of MS-1/AlO_x interface shown in Fig. 35 exhibits 2850, 2882, and 2940 cm⁻¹ peaks, derived from the main chain of the silyl-terminated polyester are observed. As shown in Fig. 35, the SFG spectrum of the MS-3 at the interface after completely cured is almost identical to that of MS-1 interface, which does not contain any silane coupling agents. This result suggests that the main chain of the modified silicone adhesives has almost the same structure with respect to molecular orientation at the interface after curing. However, as shown in Table 1, these adhesives differ significantly in curing time, suggesting that their curing reaction behavior in the bulk is quite different. Since the adhesion strength of MS-3 is much stronger than that of MS-1, therefore we conclude that MS-3 has different curing behavior due to the segregation of silane coupling agents at the interface, and that this different reaction behavior at the interface leads to the different adhesion strength.

Fig. 35

SSP-polarized SFG spectra of the MS-3/AlO_x and MS-1/AlO_x interfaces after completely cured, respectively

7 Metal/polymer Interfaces

Polymers are frequently used in contact with metals, such as during metal coating and adhesion. Preventing delamination or adhesion failure at the interface between metal and polymer or adhesive is an extremely important issue in electronics, automobiles, and other industrial applications. The adhesion should be contributed by such molecular level physical and chemical interactions and appeared as the detectable macroscopic adhesion strength. Since SFG can be applied to any interface that light can pass through and reach, it is particularly adept in elucidating chemical interactions at heterogeneous interfaces. However, metals are opaque in both infrared and visible lights, the polymer-metal interface is usually analyzed by applying thin organic materials on to the metal, which light can pass through, and analyzing the polymer/metal interfaces through this very thin film.

X. Lu et al. studied the buried PMMA/Ag interface from the analysis of the SFG spectra using multiple reflections [74]. In this analysis, SFG signals were measured from a series of polymer films of different thicknesses. The different thicknesses produced different interference patterns in the overall SFG spectrum (Fig. 36), which could be used to extract the SFG spectral pattern from the buried interface. Such a method requires the preparation of polymer films of different thicknesses on top of the metal surface and accurate measurement of the film thickness. By using the SFG spectra from several samples with different film thicknesses and the interference model considering multiple reflections for each film thickness, the SFG spectral pattern contributed from the PMMA/Ag buried interface can be extrapolated. This SFG spectral analysis indicates that the structure of PMMA at the PMMA/Ag interface is different from that of the PMMA surface in air. On the PMMA surfaces in air, ester methyl groups are dominant and are regularly oriented towards the surface normal direction. On the other hand, at the PMMA/Ag interface, in addition to ester methyl groups, methylene and α-methyl groups are aligned toward the surface normal direction. The presence of methylene and α-methyl groups at the interface suggests that the ester methyl groups at the PMMA/Ag interface are more tilted away from the Ag substrate with a larger tilt angle [74].

Fig. 36

Reprinted with permission from Ref. [74]. Copyright 2008 American Chemical Society, All Rights Reserved

The SFG spectra of PMMA films with different thicknesses on silver substrates acquired in SSP polarization combination.

In another method, where thin polymer film is sandwiched between metal and quartz window, the polymer/metal interfacial SFG spectra are obtained in a single measurement, as shown in Fig. 37 [75, 76]. This is because the SFG signal generated from the polymer/silica (or CaF₂) interface is usually negligible compared to the signal generated at the polymer/metal interface. Consequently, it was found that the SFG spectra measured with PPP polarization combination from thin film deposited on metal surfaces are mainly from the buried interface. In PS/silver interface using the “sandwiched” geometry, it was found that the PS phenyl rings adopted order at the interface. Molecular orientation analysis concluded that the phenyl rings at the interface tilted away from the silver surface [76].

Fig. 37

Reprinted with permission from Ref. [75]. Copyright 2009 American Chemical Society, All Rights Reserved

a Schematics of the sample geometry and the relation between the surface (or interface, XYZ) and molecular (abc) coordinate systems. b SSP and PPP SFG spectra collected using the “sandwiched” sample geometry to probe the PS/Ag interface.

Recently, X. Lu et al., analyzed epoxy/steel interface using sandwiched configuration [77], they concluded that the DGEBA/steel interface (Fig. 38) was dominated by the hydrophobic groups such as the CH₃ and CH₂ groups. Meanwhile, the hydrophilic groups, for example, the OH groups of DGEBA and absorbed water molecules only had very low order [77]. It should be noted that the analysis of buried interfaces of organic thin films on metals by SFG is rather more difficult and complicated than expected because of the interference effect, which requires smooth surfaces, difficulties in controlling the thickness of the polymer thin films, and the fact that the molecular orientation of both the polymer surfaces and interfaces is loose and less variable than would be expected.

Fig. 38

SSP (black empty squares) and PPP (red empty circles) SFG spectra for the DGEBA/steel interface, with the polymer film thickness of ∼155 nm. a C-H stretching region. b O–H stretching region. Reprinted with permission from Ref. [77]. Copyright 2022 American Chemical Society, All Rights Reserved

By the way, if the metal is very thin, then it is possible for it to transmit both visible and infrared lights. This configuration is effective when the interaction between the metals and the polymers is to be investigated. Here describes the study of two kinds of epoxy polymers with the chemical structures shown in Fig. 39 deposited on a fused silica substrate with 2 nm Cu thin film by sputter coating, to see how the molecular orientations change at the interface due to the interaction between the polymers and the Cu. As shown in Fig. 40, the Cu sputtered thin film has a very smooth surface and is nearly unoxidized, as confirmed by AFM and Auger spectroscopy.

Fig. 39

Chemical structures of the epoxy polymers used in the SFG in Fig. 41

Fig. 40

AFM topographic images of the a fused quarts and b 2 nm thick Cu sputtered fused quarts surfaces. Auger spectrum of the Cu sputter thin film

Figure 41 shows the SFG spectra of the diglycidyl ether Bisphenol A (DGEBA) based and 3,4-Epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate (EC) based epoxy polymers in C-H stretching region. For comparison, Fig. 41 also displays the SFG spectra of the fused quartz interfaces without the Cu sputter films. In the case of the SFG spectra of DGEBA, C-H modes derived from the phenyl ring are not observed, suggesting that the phenyl ring is randomly oriented at the interfaces. At the interface between fused quartz and epoxy, CH₂ stretching vibrations originating from the cyclohexane ring are observed at 2845 cm⁻¹ for both polymers, but the intensities of the peaks become smaller when the Cu layers are inserted in between. Since this change is the same for both the DGEBA and the EC polymers, it is thought that the presence of Cu has significantly modified the orientation of the stiffener derived moiety. The reduction in the intensity of the symmetric stretching vibration of the CH₂ of the cyclohexane ring suggests that aliphatic rings are oriented nearly parallel to the surface at the interface with the presence of Cu.

Fig. 41

SSP (black circles) and PPP (red circles) SFG spectra for the a quartz/DGEBA, b quartz/Cu/DGEBA, c quartz/EC, and d quartz/Cu/EC interfaces

From the analysis of the orientation angles of methyl groups at the DGEBA interfaces, the methyl groups are 62° from the surface normal at the fused silica interface without Cu, whereas this angle changes to 42° with the Cu layer in place. These results indicate that the presence of the Cu layer changes the orientation of the cyclohexane rings.

For the analysis of metal/polymer joint interfaces, the cast or spin-coated thin films on metals have been used in many cases, as we have already described. In real joints, however, molten polymeric materials to metal are not always in the same molecular state as the spin-coated thin films. Isotactic polypropylene (iPP) mixed with a small amount of maleic anhydride-grafted polypropylene (PPgMA) was found to induce a dramatic improvement in the strength of adhesion [78].

Confocal Raman spectroscopy is known to be able to characterize the local crystallinity of PP using the 808 and 840 cm⁻¹ peaks assigned to helical and short chains within the crystalline phase, respectively, and the peak at 830 cm^–1 assigned to the nonhelical amorphous phase [79]. Figure 42 presents the spectral changes in the region near the free surface exposed to the air and plots of the amorphous fractions as a function of the distance from the surface, respectively. A less crystalline iPP/PPgMA layer is formed near the Al/PP interface based on Raman observations, the thickness of which was estimated to be ca. 3 μm. In contrast, the region below the free surface exposed to the air, in contrast, is highly crystalline, and the crystallinity is independent of the distance from the surface. The interfacial crystallinity of iPP and pure PPgMA showed a low percentage of amorphous phase regardless of the distance from the interface. This observation suggests that blending of PPgMA to iPP leads to the formation of the amorphous-rich phase near the Al/PP interface during the hot-melt bonding process.

Fig. 42

a, c Cross-section Raman spectra of iPP/PPgMA 80/20 film around the interface and around the free surface face to the air, respectively. b, d Fractions of the amorphous phase around the interface and around the free surface face to the air, respectively. Reprinted with permission from Ref. [78]. Copyright 2021 American Chemical Society, All Rights Reserved

To gain a better understanding at the interface, the SSP and PPP SFG spectra of the C-H stretching region are collected from the fractured surfaces of the Al/(iPP/PPgMA 80/20 wt%) lamination. The upper and lower panels in Fig. 43 show the SFG spectra obtained from the iPP and Al fractured surfaces, respectively.

Fig. 43

SFG a ppp and b ssp spectra taken from the fracture surfaces of the PP (upper) and Al (lower) of the interface of Al/(iPP/PPgMA 80/20 wt%). The IR and visible beam paths and the SFG signal generation are depicted in the insets of a. Reprinted with permission from Ref. [78]. Copyright 2021 American Chemical Society, All Rights Reserved

As shown in Fig. 43a, relatively sharp peaks corresponding to the stretching bands of the segments containing the –CH₃ side group and the methylene groups in the main chain of iPP, were observed with high intensities from the PP surface, while the Al/iPP interface produced only a few broad peaks corresponding to the –CH₃ symmetric and asymmetric stretching bands with relatively low intensities compared to the SFG peaks from the iPP surface. It should be noted that the intensities of the SFG peaks did not represent the amounts of the corresponding segments at the surfaces. Instead, they represented the orientation and the orientation distribution of the functional groups at the surfaces. The asymmetric feature of the fracture surfaces observed by the TEM was associated not only with the topographic roughness but also with the orientation of the molecules at the fractured surfaces. The fracture surface of the iPP side maintained the high order of the iPP chain due to its crystalline structure which was evidenced by the strong SFG intensities. In contrast, the fact that the SFG spectra obtained from the iPP on the Al surface corresponded rather well with the previously reported spectra of iPP spin-cast films [3] indicated that the iPP/PPgMA was in a disordered and amorphous state. Thus, these SFG results suggest that the iPP/PPgMA molecules at the Al/PP interface are rather disordered and have random orientation. The Raman and SFG spectroscopies results suggested that the blending of iPP with PPgMA induces the formation of a relatively less crystalline layer in the interfacial region [78].

8 Bio-adhesive Interfaces

In nature, there are a number of bio-related adhesives used by various organisms in high-humidity and underwater environments to capture their prey, defend themselves, and build nests. Spiders often hunt in wet habitats and overcome this challenge using sticky aggregate glue droplets whose adhesion is resistant to interfacial failure under humid conditions. Dhinojwara’s group investigate the mechanism of aggregate glue adhesion by using SFG spectroscopy in conjunction with infrared spectroscopy [80]. As shown in Fig. 44, by putting on the silk glue on top of the sapphire prism and controlling the humidity, they found that glycoproteins act as primary binding agents at the interface. As humidity increases, reversible changes in the interfacial structure of glycoproteins are observed, as shown in Fig. 45. Interestingly, liquid-like water at the interface bands is not observed at the interface, even though liquid-like water increases inside the bulk with increasing humidity. They conclude that the hygroscopic compounds in aggregate glue sequester interfacial water. Using hygroscopic compounds to sequester interfacial water provides a novel design principle for developing water-resistant synthetic adhesives.

Fig. 44

Reprinted with permission from Ref. [80]. Copyright 2018 Nature, All Rights Reserved

Schematic illustration of the silk glue. Multiple silk beads-on-a string (BOAS) strands, comprised of aggregate glue droplets (beads) supported on an underlying flagelliform thread, are collected from 2–3 webs of Larinioides cornutus and deposited on a sapphire prism. The sapphire prism with BOAS strands is s depicted in the optical microscope image shown in c. Scale bar is 100 μm.

Fig. 45

Reprinted with permission from Ref. [80]. Copyright 2018 Nature, All Rights Reserved

SFG spectra for pristine aggregate glue/sapphire at low (red empty squares) and high (blue filled squares) humidity collected in PPP a and SSP b polarization combinations. Similar spectra are collected for washed aggregate glue/sapphire interfaces in PPP c and SSP d polarization combinations at low (black empty circles) and high (blue filled circles) humidity.

A famous example of biologically related adhesion phenomena is the gecko's foot adhesion. Representative studies by Autumn et al. [81, 82] implied that van der Waals forces (vdW) dominate adhesion in geckos. Recently, the adhesive strength of gecko feet has been investigated by SFG spectroscopy by Dhinojwara et al. [83] In their experiments, gecko setae is placed in contact with a sapphire prism, and changes in the peak positions of hydroxyl groups that are not hydrogen bonded on the sapphire surface are monitored by SFG spectroscopy. In the sapphire-air interface, SFG spectrum shows a sharp peak at 3710 cm⁻¹ in the OH vibration region, which can be attributed to the free OH groups on the sapphire surface [84,85,86]. With the gecko setae in contact, this sapphire OH peak becomes broader and shifts to lower wave numbers (∼3600 cm⁻¹). This large shift can be due to acid–base interactions between sapphire OHs and the unbound lipid groups exposed on the surface of gecko setae since the vdW interactions shift the sapphire OH peak by only 20–30 cm⁻¹ [84,85,86]. When the gecko setae are again removed from contact, the SFG spectrum shows residue on the surface of the sapphire surface, confirming that a lipid footprint is left behind.

9 Molecular Conformation at the Liquid Interfaces

The adhesive is usually applied to the adherend in liquid state. In this case, adherend surfaces are likely to have different structures from those exposed to air. Thermo-responsive polymers can undergo soluble-insoluble transition when the temperature of the polymer solution reaches the lower critical solution temperature (LCST). Poly(N-isopropylacrylamide) (PNIPAM) exhibits a phase separation at around 32 °C. PNIPAM has hydrophilic groups as well as hydrophobic groups, thus both hydrophilic and hydrophobic interactions are an important role in the thermo-shrinking transition. When this transition occurs in solution, it is called a coil-to-globule transition and takes place over a narrow temperature range. PNIPAM has received attractive attention as an intelligent polymer in biotechnology, because of its thermal responsivity. When this polymer is grafted onto a solid substrate, the surface exhibits temperature-dependent properties, such as wettability and cell adhesion behavior [87, 88].

Figure 46 presents the SFG spectra of PNIPAM grafted on fused quartz substrate with the SSP, PPP, and SPS polarization combinations, measured in air at room temperature. The peaks at 2871 and 2975 cm⁻¹ are assigned to the symmetric and the asymmetric stretching of the CH₃ of isopropyl groups, respectively [89]. The peak at 2940 cm⁻¹ can be attributed from both the Fermi resonance of symmetric CH₃ and the methylene antisymmetric stretching modes [89, 90]. The shoulder at around 2850 cm⁻¹ is derived from the symmetric stretching of the CH₂ group of the main chain [89, 90]. The observation of the strong CH₃ stretching peak together with the weak CH₂ peak indicates the presence of the ordered isopropyl groups tilting toward the surface normally under ambient conditions. From the quantitative analysis of the peak strengths for the CH₃, net orientation of the polar tilt angle of the isopropyl groups is about 40 ± 3°, with the twist angles of about 10 ± 10° in air atmosphere. This indicates that the side chains of the PNIPAM are nearly upright at the air/PNIPAM interface. Since the end of the isopropyl termini are hydrophobic in nature, it is reasonable to assume that the isopropyl groups are pointing toward the air side.

Fig. 46

Reprinted with permission from Ref. [8]. Copyright 2007 American Chemical Society, All Rights Reserved

SSP, PPP, and SPS SFG spectra of the grafted-PNIPAM surface in air.

Fig. 47

Reprinted with permission from Ref. [8]. Copyright 2007 American Chemical Society, All Rights Reserved

SSP, PPP, and SPS SFG spectra of the D₂O/PNIPAM interfaces collected at the different temperatures.

At the PNIPAM/D₂O interface, as shown in Fig. 47, the SFG signal intensities derived from PNIPAM are changed depending on the water temperature, which is caused by the disordering of the polymer chain owing to the changes in the solubility of the PNIPAM. When the water temperature is increased, furthermore, red-shifts of the SFG peaks due to the dehydration of the alkyl group are observed. This result indicates that the C-H groups interact with water below LCST [91, 92]. From the quantitative analysis of the molecular orientation, restructuring of the main chain due to the dehydration at the water/PNIPAM interface is suggested [8].

The orientation of the surface functional groups when in contact with various types of liquids are important factors in determining surface properties. The conformation of the sol-gel-derived hybrid film containing decylsilyl groups (C₁₀H₂₁Si, C10-hybrid film) under dry (air/C10-hybrid film interface) and wet (probe liquids/C10-hybrid film interface) conditions were investigated by using SFG spectroscopy [93]. Figure 48 shows SSP-polarized SFG spectra of the hybrid films in the C-H stretching region (2800–3000 cm⁻¹) under both dry (in air) and wet (in contact with D₂O, DMF-d7, IPA-d8, and toluene-d8) conditions. When the hybrid film is dry, three peaks corresponding to CH₃ group were observed at 2880, 2960, and 2940 cm⁻¹, corresponding to the C-H symmetric stretching vibration (r⁺), the C-H asymmetric stretching vibration (r⁻), and the Fermi resonance (FR). Two peaks corresponding to methylene groups (CH₂) at 2850 (C-H symmetric stretching vibration (d⁺)) and 2920 cm⁻¹ (C-H asymmetric stretching vibration (d⁻)) were also detected in the spectrum. These two peaks are indicative of the presence of gauche defects in alkyl chains because CH₂ peaks would not be detected if the alkyl chains are in the all-trans conformation. In the present case, the surface-tethered alkyl groups in the uppermost region are judged to be loosely packed, allowing for readily reorientation against their environment.

Fig. 48

Reprinted with permission from Ref. [93]. Copyright 2014 American Chemical Society, All Rights Reserved

a SSP-polarized SFG spectra of our C10-hybrid films in C-H stretching region of alkyl groups at air/and four different liquids/film interfaces. (The spectra are normalized by the peak intensity of the methyl groups and are offset upward for clarity.) b Changes in the d⁺/r⁺ ratio and contact angle hysteresis (Δθ) as a function of the dielectric constants of four different probe liquids.

After contacting the C10-hybrid films with perdeuterated probe liquids (D₂O, DMF-d7, IPA-d8, and toluene-d8), the d⁺/r⁺ peak strength ratio in the SFG spectra is obviously changed, as shown in Fig. 48b. This d⁺/r⁺ peak strength ratio appears to correlate strongly to the ratio of CH₂ to CH₃ groups at the outermost surface (but does not indicate the absolute number of CH₃ and CH₂ groups at the interfaces) and can be employed to evaluate the relative conformational order of alkyl chains because the ratio decreases with the decreasing number of gauche defects at the liquid/solid interface.

When the surface was in contact with probe liquids having high dielectric constants, such as D₂O, DMF-d7, and IPA-d8, the d⁺/r⁺ ratio increased from 0.61 when dry to 0.89, 1.37, and 2.09 when in contact with IPA-d8, DMF-d7, and D₂O, respectively. The greater the dielectric constant of the probe liquid, the greater the increase in the d⁺/r⁺ ratio observed upon wetting as shown in Fig. 48b. Such marked increases in d⁺/r⁺ values indicate that the conformational ordering of alkyl chains at the liquid interface are collapsed when wetted with a liquid having a high dielectric constant. However, when a surface was in contact with toluene-d8, the d⁺/r⁺ ratio markedly decreased to 0.33, suggesting that alkyl chains assume a more extended conformation (enhanced conformational ordering) into the liquid disfavoring gauche defects. These observed changes in the d⁺/r⁺ ratio closely reflect the changes in contact angles as a drop of probe liquid advances and then recedes over an area of the surface (i.e., contact angle hysteresis), as shown in Fig. 48b.

10 Molecular Conformation at the Organic Device Interfaces

Polymer LEDs are one of the most promising applications given the current great interest in the development of ultrathin computer monitors and television, i.e., flat-panel displays and flexible displays. Recently, one of the conjugated polymers, the poly(9,9-dioctylfluorene) (PFO, chemical structure is depicted in the inset of Fig. 49) and fluorene-arylene copolymers have been intensively studied because of its applications in the LEDs due to their highly efficient blue photoluminescence [94]. In an organic device, the charge carriers must be injected through polymer/electrode interfaces. Since the electronic properties and energy level alignment of electrode/organic interface significantly affect the performance of the organic LEDs, it is extremely important to understand the interaction between the electrode and organic polymer materials and the electronic structures at the buried interfaces [42]. Doubly-resonant SFG spectroscopy can be applied to study the changes in the electronic structure at the interfaces as well as the orientation of the functional groups.

Fig. 49

Reprinted with permission from Ref. [42]. Copyright 2010 Royal Society of Chemistry, All Rights Reserved

SFG spectra of PFO/PEDOT:PSS/quartz surface with various visible wavelength. Spectra are offset for clarity.

Figure 49 shows the SFG vibrational spectra from the PFO/PEDOT:PSS/quartz surface collected with various visible wavelengths in an SSP polarization combination [42]. In the SFG measurements, z-cut quartz plate is used as the reference for the SFG signal. This is because z-cut quartz is transparent to visible light, so the SFG susceptibility of the quartz is expected to be approximately constant in the visible region. As shown in Fig. 49, strong vibrational band was observed at 1610 cm⁻¹ in all spectra, and the band intensity increased when the visible probe wavelength was changed from 550 to 435 nm. The vibrational band at 1610 cm⁻¹ is derived from the C=C symmetric stretching of the fluorene rings located at the PFO backbone. When the visible wavelength is near 435 nm, the electronic resonance enhancement of the SFG spectra is observed which produces an SFG wavelength near 407 nm with an IR beam of 1610 cm⁻¹.

Figure 50 presents the visible wavelength dependence of the SFG spectra from the buried Al/LiF/PFO interface in SSP polarization combination [42]. In this study, the LiF and the Al layers were directly deposited on the spin-coated PFO/PEDOT:PSS onto CaF₂ substrate, as illustrated in the inset in Fig. 50. As can be seen in Fig. 50, the vibrational band at 1610 cm⁻¹ is still observed in all SFG spectra and its intensity shows remarkably visible wavelength dependence. The peak position of the band does not change between the PFO surface and the PFO interface, indicating that the PFO is not degraded by the Al deposition. It should be noted that in the case of the DR-SFG spectra of the Al/LiF/PFO interface, the SFG peak presents a different shape from that observed in the air/PFO interface. This difference is attributed to the different interference phenomena with the SFG non-resonant contribution arising from the Al substrate.

Fig. 50

Reprinted with permission from Ref. [42]. Copyright 2010 Royal Society of Chemistry, All Rights Reserved

SFG spectra of buried electrode/PFO interface with various visible wavelength. Spectra are offset for clarity.

Generally, there are two types of optical processes in IR-visible SFG, as mentioned in the theoretical section. The first one is an electronic transition followed by a vibrational transition (VIS-IR SFG), and the other type begins with a vibrational transition followed by an electronic transition (IR-VIS SFG) [95]. Since the electronic relaxation times are generally quite short as compared to the vibrational relaxation times, the contribution of the VIS-IR SFG is expected to be generally negligible. If the VIS-IR SFG occurs, increase of the non-resonant background is expected because of the ultrafast dephasing dynamics of the S1 state [96]. Therefore, only the IR-VIS SFG will be considered in the following analysis.

The curves b and c in Fig. 51 show the changes in the Al of the 1610 cm⁻¹ peak extrapolated from the fitting of the DR-SFG spectra in Figs. 49 and 50 as a function of the photon energies of the output SFG light. Figure 51a also shows the optical absorption spectrum of the PFO spin-coated film. The broad optical absorption band originates from inhomogeneously broadened π → π* transitions of the glassy PFO. As shown in Fig. 51, the SFG electronic excitation spectrum obtained from the air/PFO interface is somewhat red-shifted with respect to an optical absorption of PFO film. The SFG excitation spectrum at the Al/LiF/PFO interface is also plotted in Fig. 51c, and it is further red-shifted with respect to that of the PFO surface. It should be noted that these red-shifts are not caused by the visible variations of the Fresnel factors. We should note that the wavelength changes in the Fresnel factor ${F}_{yyz}$ both the air/PFO and the CaF₂/Al interfaces do not explain the changes of the SFG electronic excitation profiles of the air/PFO and the buried interface, as shown in Fig. 51.

Fig. 51

Reprinted with permission from Ref. [42]. Copyright 2010 Royal Society of Chemistry, All Rights Reserved

a Optical absorption spectrum of the PFO. b The SFG electronic excitation profile of air/PFO interface and the Fresnel factor ${F}_{yyz}$ (blue curve) at air/PFO interface, and c the SFG electronic excitation profile of Al/LiF/PFO interface and the ${F}_{yyz}$ at CaF₂/Al interface (red curve). Solid black lines in a and b are guide to the eyes. d Schematics of the planar configuration at the interface.

The red-shifts of the SFG electronic excitation spectra can be interpreted by the stress-induced surface confinement effects of the polymer backbone, as in the case of the MEH-PPV interfaces [97]. As shown in Fig. 52, K. C. Chou’s group reported that the conjugation length of MEH-PPV at the solid interface is longer than that in the bulk, which is attributed to the stress-induced backbone flattening [97]. In general, the optical band gap of a conjugated polymer is closely correlated to the π-conjugation length. Conjugated polymer chains are composed of a series of linked segments, each of which has a different degree of π-electron delocalization. Although the extent of the conjugation is limited by the torsion of the polymer backbone, the longer the segment is, the smaller the optical band gap of the conjugated polymers due to the increasing average effective conjugation length. The restriction of the torsion angle between adjacent segments at the air/polymer and the solid/polymer interfaces induces a longer conjugation length.

Fig. 52

Reprinted with permission from Ref. [97]. Copyright 2008 American Chemical Society, All Rights Reserved

Optical absorption spectrum of bulk MEH-PPV film (solid line) and the surface SFG electronic spectra of MEH-PPV at MEH-PPV/solid (■) and air/MEH-PPV interfaces (●).

It is well-recognized that the bulk solid-state films of PFO exhibit complex phase behavior. Disordered PFO forms the glassy phase where the polymer backbones do not form specific conformation with long-range order. On the other hand, PFO in the so-called β-phase is an extended conformation of PFO chains and possesses lower energy, due to the backbone planarization [98]. Single molecule spectroscopy demonstrates that the β-phase features of PFO are the results of stress-induced backbone flattening of polymer chain [99]. The optical absorption spectra of β-phase PFO exhibit the characteristic shoulder band around 430 nm in comparison with the glassy PFO. The SFG electronic excitation profiles of the PFO interfaces have a maximum of around 410–420 nm, and this peak position is close to the shoulder absorption of the β-phase PFO, rather than that of the glassy PFO. Because of the restriction of the torsion angle between adjacent segments, the conformation of the polymer backbone is limited at the polymer interfaces. As a result, the effective conjugation length at the interface is increased. The proposed planar orientation of the PFO chains at the interfaces is shown in Fig. 51d.

There is great interest on the study of the semiconductor/dielectric interface of organic field-effect transistors (OFETs), where a conducting channel is formed. SFG spectroscopy is known not only for studying the molecular orientation at the interfaces, it is also a valuable technique for obtaining the information about the charge accumulation at the interfaces. This is known as electric field-induced SFG, which utilizes phenomena in which the SFG signal intensities are modulated and increased by the electric field generated by the accumulated charges at the interface [37, 100, 101]. In SFG measurements of the OFETs that use P3HT as an organic semiconductor, Miranda's group has reported a voltage-dependent increase in the intensity of the SFG signal of the PMMA used as the insulating layer when a voltage is applied to the device, as shown in Fig. 53 [102]. SFG results revealed that in-plane electric field within the organic dielectric layer are formed by the charge injection.

Fig. 53

Reprinted with permission from Ref. [102]. Copyright 2018 American Chemical Society, All Rights Reserved

Schematics of the OFET and the SFG spectra in the C=O stretching mode for the Si/SiO₂/PMMA/P3HT OFET under different gate voltages.

11 Comprehensive Study of Adhesive Interfaces Combining SFG with Other Techniques

Elucidation of “how adhesives form their interfaces with adherends” is important for both the developer and the user of adhesives, as it helps determine the adhesive composition and surface treatment methods. Studying the interfacial phenomena will provide the information on chemical interactions, molecular orientation, and the formation of chemical bonds and electric dipoles. Since such details on molecular scale are important to design the adhesive, it is still being studied both experimentally and theoretically. From the late 1980s to the early 1990s, many studies on the surface of exfoliated specimens by XPS have been reported [103,104,105,106]. Among them, it was confirmed that the curing agent of epoxy adhesive is protonated by the hydroxyl groups on the surface of stainless steel and aluminum, and the existence of acid–base reaction has been clarified [103, 105,106,107]. In addition, since an electric double layer (positive on the molecular side and negative on the metal side) is formed at the interface by protonation, the presence or absence of protonation at the interface can be indirectly detected by measuring the contact potential difference using the Kelvin probe method (KP) [104]. In addition to surface-sensitive techniques such as XPS and KP, techniques capable of characterizing buried interfaces such as SFG and scanning transmission electron microscopy (STEM)-electron energy loss (EELS) are available at the adhesive interface. By using it for analysis, it is possible to grasp the actual state of the interface, and the understanding progresses further.

This section describes analytical research on the interface between aluminum, which is a representative lightweight material used for car bodies and aircraft, and adhesives. In particular, the chemical interactions at the interface between adhesives and aluminum oxide (AlO_x) are described. Here, AlO_x was selected as the adherend because the surface of metal aluminum is covered with a naturally oxidized film, and the surface of the oxide film is in direct contact with the adhesive. Figure 54 shows the molecular structures of each component in the epoxy and urethane adhesive described in this section. Epoxy adhesive is made by mixing bisphenol A type epoxy resin (DGEBA) with triethylenetetramine (TETA) as a curing agent and adding a small amount of 2,4,6-tris(dimethylaminomethyl)phenol (TDAMP) as a curing accelerator. rice field. On the other hand, as the urethane adhesive, two kinds of diphenylmethane diisocyanate (MDI) to which polypropylene glycol (PPG), 1,4-butanediol (BG), and trimethylolpropane (TMP) were added were used. These adhesives were cured at 100 ºC for 30 min. and 120 ºC for 5 h, respectively.

Fig. 54

Molecular structures of epoxy and urethane adhesives used in this study

11.1 Acid–Base Interaction at the Epoxy Adhesive/AlO_x Interface

At the interface between our epoxy adhesive and AlO_x, protonation of the curing agent amine was observed. Figure 55a shows the XPS spectra in the N 1s region of the obliquely polished epoxy adhesive/Al joint. Photoemission based on N–H and N–C is observed around 400 eV. In addition, a weak component was separated around 402 eV. Since the peak at this position is attributed to a nitrogen atom that has become a cation [108], it is suggested that the amine received a proton from the hydroxyl group on the aluminum surface at the interface upon contact with the adhesive and became N⁺. Previous reports have focused on the adhesive interface, but according to STEM-EELS measurements, the aluminum hydroxide on the aluminum surface changes to aluminum oxide as the epoxy adhesive cures, and the aluminum side [109]. Even when only TETA, which was used as a curing agent, was adsorbed on aluminum foil, two photoemission peaks were separated [Fig. 55a], and their positions are 399.0 and 400.5 eV. Similar to the adhesive interface, the high binding energy side is attributed to protonated TETA. Protonation at the interface should form an electric double layer at the interface, with the TETA side being positive and the aluminum side being negative. As a result, the electric potential on the aluminum surface should decrease, but when the contact potential difference (CPD) measured by KP is compared before and after the TETA coating, the contact potential difference is indeed negative, indicating a decrease in the work function. [Fig. 55b]. The results of KP measurements are consistent with those reported by Salgin et al. [104]. If protonation at the epoxy adhesive/aluminum interface can be predicted from KP measurements, it may be possible to easily infer the interaction at the interface. However, it should be noted that protonation is not the only source of change in CPD. Amines such as TETA are known to cause n-type doping [110], and the formation of an interfacial dipole layer resulting from electron transfer can also change the contact potential difference. In addition, TETA itself has a permanent dipole, and orientational polarization is also possible.

Fig. 55

a N 1s XPS spectra of epoxy/AlO_x interface (top) and TETA-treated Al sheet (bottom). b The values of CPD for a UV/ozone-cleaned Al sheet and TETA-treated Al

Such protonation has been reported also for curing agent amines such as amidoamine [106] and diaminodiphenylmethane (DDM) [103]. Other curing agents for epoxy adhesives include aromatic amines such as 4,4′-diaminodiphenylsulfone (DDS), acid anhydrides, mercaptans, and novolac-type phenolic resins. Therefore, it is necessary to clarify the details of the chemical reaction between each curing agent and AlO_x, and carefully investigate the relationship among the interfacial phenomena, fracture modes, and durability of adhesive joints. In addition, there is still the possibility that the epoxy group of the epoxy resin reacts with the hydroxyl group on the surface [105]. Thus, full understanding of the adhesive/adherend interface is not reached.

So far, we have discussed the interfaces between clean aluminum and adhesives. In the process of assembling a car body in an actual factory environment, the aluminum surface remains covered with antirust oil, press oil, etc. [111]. The adhesive should exhibit good oil adhesion despite the presence of oil on the surface. Hong et al. reported hardener amines on oiled aluminum (Al 2024-T3) as well as on clean plates [112]. This means that the amine diffuses through the oil layer and preferentially adsorbs to the aluminum surface. It has also been found that when adhering epoxy:amidoamine adhesives to oil-contaminated rigid plates, the addition of a silane coupling agent aids penetration of the curing agent and promotes protonation of the amine [113]. That is, the adhesive composition affects the acid–base reaction at the interface.

11.2 Formation of Covalent Bonds

Molecules used in adhesives contain reactive functional groups such as amino groups, epoxy groups, isocyanate groups, phosphate groups, carboxyl groups, alkoxysilanes, and so on. Upon the contact of an adhesive interface with aluminum surface, the formation of the covalent bond could be caused by the reaction of adhesives with hydroxyl groups at the adherend. That covalent bond should ensure the adhesion and thus its direct observation is desired. SFG is a powerful technique for directly observing chemical bonds between adherends. So far, research has been reported on the formation of urethane bonds at the interface between 4,4′-MDI (Fig. 54) and epoxy resin [62], and the formation of urethane bonds at the interface between primers having isocyanate groups and urethane resin [114]. We tried to observe the chemical bond formation on the MDI/AlO_x surface by SFG.

Figure 56 shows the SFG spectra taken at SSP polarization combination in the carbonyl region at the mixture of 4,4′-MDI and 2,4′-MDI/AlO_x interface. When the isocyanate group of MDIs reacts with the hydroxyl group on the AlO_x surface, urethane bonds are formed as shown in the chemical reaction formula below.

Fig. 56

SSP polarized SFG spectra of MDI/AlO_x interfaces and cured urethane/AlO_x interface

Scheme 1

Chemical reaction between isocyanate and hydroxyl groups

Therefore, the formation of urethane bonds can be confirmed by detecting N–H or C=O generated by this reaction. For the MDI mixture/AlO_x interface, multiple peaks were observed in the C=O region. The peaks observed in the range of 1700–1770 cm^–1 should be derived from urethane bonds. The result suggests that the above chemical reaction is certainly progressing on the substrate surface. After heat treatment (at 120 ºC for 5 h), the peak at 1720 cm^–1 remains, suggesting that the orientation of C=O becomes uniform due to the formation of hydrogen bonding. On the other hand, two peaks are also observed in the range of 1580–1630 cm^–1. These can be attributed to the formation of urea due to the reaction between water adsorbed on the surface of the aluminum substrate or water in the atmosphere and isocyanate [115, 116]. The reaction between water and isocyanate groups is as follows.

Scheme 2

Chemical reaction between urea and water

Carbonyl groups derived from urethane bonds were also observed in the SFG spectrum of the cured urethane adhesive (bottom in Fig. 56). This suggests that MDI in the urethane adhesive reacts with hydroxyl groups on the AlO_x surface. The presence of urethane bonds on the aluminum surface was also suggested by time-of-flight secondary ion mass spectrometry [116]. Direct observation of the chemical bonds, however, provides direct proof of chemical bond formation.

11.3 Ordering of Functional Group at AlO_x Interface

Various functional groups of the adhesive components orient as they interact with the substrate. Contact angle measurements for the clean AlO_x used in this study found that the surface energy of a clean AlO_x surface was about 45 mJ m^–2. One may expect that polar functional groups, such as amino and hydroxyl groups, preferentially orient at the interface. However, it is not always the case that only these functional groups are oriented. Figure 57 shows SFG spectra in the C-H region of cured epoxy adhesive and urethane adhesive. Distinct peaks were observed at 2850 and 2880 cm^–1 for both adhesives. These peaks can be assigned to symmetric stretching vibrations of methylene and methyl groups, respectively. That is, hydrocarbons of DGEBA and PPG exhibit molecular order at the AlO_x interface. The ordering of hydrocarbon moieties was also observed in other adhesive systems. Thus, this observation should be a general trend at the adhesive/AlO_x interface.

Fig. 57

SFG spectra of cured epoxy/AlO_x (top) and urethane/AlO_x interfaces taken at SSP polarization

The current interpretation is that the adhesive components are constrained at the AlO_x interface through acid–base reactions and chemical bond formation. In addition to the energy stabilization associated with these phenomena, the orientation of the hydrocarbon moieties and enhancement of the molecular order maximize the energy gain via van der Waals interactions at the adherend interface. Moreover, since the ordering of alkyl chains was generally observed at the AlO_x interface even with adhesives having completely different adhesive strengths, it is difficult to assume that the presence of hydrocarbons at the interface is one of the causes of the weaker adhesion.

11.4 Interaction Between Surface O–H Bonds and Adsorbates

If we can obtain physical parameters related to adhesive strength from SFG probing buried interfaces, we can obtain information on adsorption energy at the adhesive/adherend interface. Dhinojwala's group calculated the interaction enthalpy at the interface between sapphire and organic molecules using the Badger-Bauer equation, using the hydroxyl peak position on the sapphire surface as an index [117]. Basically, when an interaction including a dispersion force works, the hydroxyl group peak shifts to the lower wavenumber side. The greater the amount of shift, the stronger the interaction.

As an example, Fig. 58a shows an SFG spectrum in the O–H region for the interface between AlO_x and silicone oil. Silicone oil is used as a release agent and is a well-known source of contamination on aluminum surfaces [111]. The hydroxyl peak on the AlO_x surface is located at 3700 cm^–1 at the air interface, but a low wavenumber shift of about 10 cm^–1 was observed upon contact with oil. This is comparable to the amount of shift observed at the polyisobutylene/sapphire interface. Therefore, it appears that the silicone oil interface weakly interacts with the AlO_x surface via dispersion forces. However, a broad peak is clearly observed below 3600 cm^–1. When silicon oil was spin-coated on aluminum foil and the CPD was measured by KP, a decrease of less than 0.1 eV was observed [118]. This suggests that the dipole layer was formed as a result of regular arrangement of silicon and oxygen atoms due to the constrained conformation of dimethylsiloxane at the interface. The broad tailing feature toward the low wavenumber could originate from the stronger interaction between the oil molecules having dipoles and AlO_x surface than the dispersion force. In such cases, it is not possible to simply calculate the adsorption enthalpy change from the peak shift of O–H bonds.

Fig. 58

SFG spectra of various adsorbates/AlO_x interfaces. a silicone oil, b TETA, and c epoxy and urethane adhesives after curing. Polarization combination is ssp

On the other hand, when TETA or DGEBA with polar groups such as amino and hydroxyl groups, is brought into contact with the AlO_x surface, a broad SFG with a peak below 3650 cm^–1 is observed [Fig. 58b]. The N–H stretching vibration is observed at 3320 cm^–1 in the TETA spectrum. When urethane and epoxy adhesives are cured, the surface hydroxyl peaks almost disappear [Fig. 58c]. Since the refractive index of the resin is larger than that of air, the Fresnel coefficient decreases, and the O–H signal on the AlO_x surface is weakened at the adhesive interface compared to the air interface. However, even if this point is taken into account, the strength of the SF signal is significantly reduced. This is considered to be the result of the consumption of hydroxyl groups on the AlO_x surface due to chemical bonding and acid–base reactions at the adhesive interface [109].

In this section, the chemical reactions, formation of covalent bonds, and orientation of functional groups at the interface between epoxy and urethane adhesives and AlO_x are analyzed using surface science techniques to clarify the adhesion. In recent years, both experimental and theoretical analyzes of adhesive interfaces have progressed, but the interface study is still necessary to obtain a deep understanding of adhesion. There are many unclear points about the relationship between the molecular scale environment at the interface and the initial adhesive strength and durability. Evaluations of chemical interaction and interfacial adhesion strength for the same specimen will provide their correlation.

12 Summary and Outlook

In this chapter, we have described the molecular behavior at the interfaces of adhesion, mainly emphasizing the analysis using SFG spectroscopy. In the analysis of adhesive interfaces, we have shown that focusing on the interfaces has the potential to reveal new phenomena that had not been observed before. However, it is most important to examine the molecular behavior at the “interphase” of the adhesive interfaces at the same time, and in this respect, it is essential to investigate the chemical states, crystallinity, and segregation of the molecules in the vicinity of the interfaces by means of infrared spectroscopy and Raman scattering.

It should be noted that this chapter does not specifically mention techniques such as SHG and SHG spectroscopy, phase-sensitive SFG, which is often referred to as heterodyne detected SFG, SFG scattering, and SFG imaging, but it is needless to admit that these techniques will play a large contribution to the analysis of adhesive interfaces with the advancement of laser and measuring technologies.