We directly measured the local composition profiles in phase-separating photocurable thin films using confocal Raman spectroscopy. To avoid light scattering at phase interfaces, we developed a novel technique to replace the solvent with a monomer to match the reflective indices in the cured films. The results indicated that the concentration distribution of the polymer was uniform in solvent-free monomer/initiator binary solutions, while it was spatially nonuniform when the solvent-based films were thermodynamically unstable and promoted reaction-induced phase separation upon UV irradiation on the top surface. In the latter case, the film exhibited a dual-layer structure, in which the polymer concentration was almost uniform near the top surface, while concentration gradients developed near the bottom surface. The thickness of the top layer with a uniform concentration profile increased with the increase in the UV light intensity. These results implied that the propagation of the reaction front and the resulting light-driven transport of the solvent toward the bottom coating layer played key roles in the formation of nonuniform concentration profiles in photocuring solution coatings.
Fabrication of ultraviolet (UV)-curable coatings with well-controlled porosities has attracted considerable research attention for bulk heterojunction solar cells,1,2 separators for lithium-ion secondary batteries,3 and precision filters for semiconductors.4,5,–6 Particularly, photo-induced phase separation in the presence of a volatile solvent provides a cost-effective, light-tunable route to submicron-scale porosity. Irradiation of UV light onto a photoreactive monomer solution film triggers polymerization and a spontaneous phase separation between polymer-rich and solvent-rich phases. Subsequent solvent evaporation from the latter phase results in the formation of discrete or bi-continuous pores in the polymer matrix. It is of practical importance to understand the evolutions of concentration distributions in curing coatings to fabricate porous structures with desirable optical, mechanical, and permeation properties.
Extensive studies have been performed to measure local compositions across the coating thickness. Wu et al.7 analyzed concentration profiles in phase-separating poly(vinylidene fluoride) (PVDF)/poly(styrene-co-maleic anhydride) (SMA) thin films using X-ray photoelectron spectroscopy. Luo et al.8 used cryogenic scanning electron microscopy to obtain cross-sectional images of drying aqueous dispersions of ceramic/latex composite films. However, these experimental techniques are not applicable for quantifying the degree of polymerization in photocurable systems. Among various methods used to determine composition distributions in reacting liquid films, confocal Raman spectroscopy has significant advantages over infrared spectroscopy,9 micro-magnetic resonance imaging (MRI),10 and scanning transmission X-ray microscopy (STXM)11 because of its high spatial resolution, suitability for in situ monitoring, and easy handling under ambient pressure. Raman spectroscopy has been widely used to analyze structural changes in polyaniline nanofibers in thermal annealing,12 biodegradability of poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate),13 and concentration profiles in Nafion,14 ZnO,15,16 TiO2,17 and diamond-like-carbon (DLC)18 thin films. More recently, inverse-micro-Raman spectroscopy19,20,21,–22 has been systematically used to investigate the evolution of local compositions in drying solution thin films. Despite the considerable recent progress in the development of confocal Raman spectroscopy, there have been few reports on the concentration profiles in phase separating polymer films because light scattering at the phase interface significantly reduces the Raman signals in the film, making the determination of local compositions by spectral analysis difficult.
In this study, we have developed a novel experimental technique to directly measure local concentration profiles in phase-separating photocurable thin films by replacing the solvent or the void by a reactive monomer to match the local reflective indices in the cured film.
Extensive research has been conducted to fill the voids in porous medium. Curtis et al.23 have demonstrated that porous silicon films exhibit different photoluminescent properties by replacing pores with hexane or heptane. Seo et al.24 fabricated dye-sensitized solar cells with long-term stability by filling the pores with a ternary solution of valeronitrile, dimethyl sulfoxide, and dimethylacetamide. Chen et al.25 examined the enhanced electrochemical properties of Al2O3 ceramic membranes by filling the voids with a mixture of 1-methyl-3-propylimidazolium iodide, lithium iodite hydrate, 4-tert-butylpyridine, and acetonitrile. However, most of these studies have been conducted to change the bulk properties of the films by pore-filling. To the best of our knowledge, few experimental approaches have been reported that utilize the pore-filling technique to improve the operability of confocal Raman spectroscopy in porous films.
In this study, the pore-filling technique enabled us to directly determine the local composition profiles in liquid thin films accompanying the reaction-induced phase separation (RIPS). Generally, the time evolutions of concentration distributions become complex in RIPS processing because the solvent drying, photo-induced polymerization reactions, and the phase separation proceed simultaneously in the photocuring liquid films. Although previous studies on RIPS have shown that monomer conversion26,27,–28 and phase morphology29,30 depend on the photoirradiation and/or the solvent drying conditions, no quantitative data is currently available on the local composition profiles in photoreactive coatings. Herein, we provide the first evidence that nonuniform concentration distributions of the polymer develop under certain photoirradiation conditions. In the experimental section, a new pore-filling approach is presented. In the results section, we first report the concentration profiles before and after photocuring in thermodynamically stable, solvent-free binary systems. Next, we discuss the evolutions of concentration gradient in phase-separating systems in the presence of solvent. The results revealed that the polymer concentration distribution becomes uniform across the critical thickness of the film surface, whereas a concentration gradient develops near the bottom.
We used polyester acrylate (M9050, Toagosei Mw = 1000 − 1500) as the photoreactive monomer and 1, 3 α-alkyl amino phenone (Irgacure 379 EG, BASF) as the photoinitiator. Methyl isobutyl ketone (MIBK, Wako) was chosen as the solvent, which is immiscible with the reactant of the polymerization reaction. The monomer-to-initiator mass ratio was fixed at 95:5 (w/w), and the initial solvent-to-nonvolatile mass ratio ranged between 0:10 and 6:4 (w/w).
The solution was dropped on a glass plate (Fig. 1a) and sandwiched between two glass plates to form 100-μm-thick solution films. UV-LED light of wavelength 365 ± 5 nm (8332A-AC8365, CCS) was irradiated on the top of the film to induce radical polymerization reactions (Fig. 1b). Our preliminary experiments for the initiator-free monomer solution showed that the UV-LED irradiation of 880 mJ/cm2 in intensity resulted in an increase in the sample temperature of less than 0.5 K, indicating that the heat conduction from the light source as well as the heat generation by light absorption were negligible. After a curing time of less than 10 s, we conducted confocal Raman spectroscopy (DXR2 Raman Microscope, Thermo Scientific) to determine local compositions across the thickness (Fig. 1c). In the presence of solvent, we conducted the following pretreatment of the sample. First, the film was peeled off from one side of the glass (Fig. 1d). Second, a certain amount of MIBK/M9050 binary solution of ratio 6:4 (w/w) was dropped onto the sample surface to replace the solvent in cured films. The solvent was removed for 10 min by heating the sample on a hot stage at 40°C (Fig. 1f). The amount of residual solvent after the solvent replacement procedure was less than 0.6 wt%. Finally, a new glass plate was carefully attached to the sample surface to ensure a film thickness of 100 ± 2 μm. The local compositions in the film after solvent replacement were measured by confocal Raman spectroscopy (Fig. 1g). The porous structures on the top and bottom surfaces of dried films were characterized by scanning electron microscopy (SEM, SU3500, Hitachi High Technologies).
Confocal Raman spectroscopy was performed by irradiating the liquid sample with a 532-nm excitation laser beam with a spot diameter of 1 μm through an objective lens (numerical aperture: 0.75, magnification: 50×). The laser focus was scanned across the sample thickness, with a spatial resolution of 1.3 µm, using a mechanical stage with positioning accuracy of 0.2 µm. The measurement positions in the sample were determined based on the predetermined position of the surface of the mechanical stage, the thickness of the glass substrate, and the moving distance between the substrate surface and the focal position of the laser beam. The Raman signals, after passing through a Rayleigh-light-cut filter, were dispersed by a grating and detected by a charge-coupled-device (CCD). To identify the local composition from the Raman spectrum, we used the three distinct peaks at 600, 1600, and 1640 cm−1, associated with the C–C–C bonds in MIBK, the aromatic ring in Irg379, and C=C double bonds in M9050, respectively. These peaks are sharp, distinct, and of high intensity, enabling us to identify the amount of each component. Following the method reported by Schabel et al.,29,31 we built calibration curves for the intensity ratios of the Raman peaks at different mass ratios32 and used them to calculate the compositions of the three different components in the Raman spectra. To calculate the polymer concentration, we measured the reduction in peak height at 1640 cm−1 after UV irradiation. The detailed experimental procedures are given elsewhere.32
Results and discussion
We first examined the depth profiles of the monomer and the polymer in solvent-free binary systems. The monomer distribution was uniform across the thickness before UV irradiation (Fig. 2a). The measured monomer mass fraction was ~ 0.95, which agreed with the initial monomer mass fraction in the coating fluids. UV irradiation onto the coating leads to a uniform concentration profile of the polymer with a mass fraction of 0.6 (Fig. 2b). The SEM images showed no phase-separated structures on the cured film surface (not shown here).
Next we examined the photocuring of coatings in the presence of solvent. The SEM images of the dried film surfaces showed distinct pores of diameters less than 100 nm (Fig. 3). Formation of the porous structure results from the photo-induced phase separation. The solvent chosen was miscible with the monomer, but immiscible with the product of photopolymerization, i.e., the polymer. The photocurable solutions become thermodynamically unstable as the reactions proceed upon UV light irradiation, promoting spontaneous phase separation. Subsequently, the volume occupied by the solvent was replaced by air when the solvent-rich phases were dried, resulting in a certain film porosity. In contrast to conventional spin coating techniques for polymer blend films,33 this process does not require multi-step rinsing procedures to remove either phase in a selective solvent, providing a promising method for roll-to-roll high-speed fabrications.
To examine how phase separation influences the Raman spectra, we recorded the Raman spectra at different positions in the thickness direction. Figures 4a–4d show typical Raman spectra when solvents remain in the phase-separating coating. The Raman spectrum of the inside of the top glass plate exhibited a broad peak in the range 250–1200 cm−1, which can be attributed to SiO2 in the glass (Fig. 4a). Scanning the focal position toward a depth of 5 μm from the glass–sample interface revealed a sharp peak at 2960 cm−1, corresponding to the C–H bonds, along with the distinct peaks at 600, 1600, and 1640 cm−1 (Fig. 4b). These Raman signals significantly decay in intensity as the measurement depth increases to 80 μm (Fig. 4c) and 120 μm (Fig. 4d) from the sample surface. We obtained no broad peak at the wave numbers of 250–1200 cm−1 when the focal position was located inside the bottom glass plate (Fig. 4d). These facts imply that the laser beam scatters locally between the solvent-rich and polymer-rich interfaces, resulting in decayed beam intensity across the thickness. Indeed, preliminary measurements of the reflective index (n) showed that n = 1.396 and 1.520 for the solvent and the polymer, respectively, implying a large difference in n between the separating phases. To match the reflective indices across the phase interfaces, we replaced the solvent with the solvent/monomer binary solutions and removed the solvent by evaporation. Figures 4e and 4h show the Raman spectra after solvent replacement. The focal positions in the thickness direction in Figs. 4e–4h are the same as those in Figs. 4a–4d, respectively. The distinct Raman peaks were observed even at great depths in the coating (Figs. 4g and 4h), suggesting that solvent replacement allows us to avoid interfacial beam scattering, and therefore, obtain local compositions inside the phase-separating coatings. In addition, Fig. 4g showed no peak at 600 cm−1, associated with the C–C–C bonds in MIBK, indicating complete solvent replacement near the bottom surface of the coating. Herein, we note that determination of the monomer concentration profiles before solvent replacement is not straightforward. The intensity of the peak at 1640 cm−1 (Fig. 4b), associated with the C=C double bonds, represents the sum of the peak intensities of the unreacted monomer (Iu) and the monomer that replaces the solvent (Ir). Since the Raman spectral analysis described here does not provide a quantitative way to distinguish between Iu and Ir, our new technique is inherently limited to the measurements of concentration profiles of the polymer and the photoinitiator.
To investigate the effect of initial composition on the concentration distribution in the cured coating, we measured the composition profiles before and after photoirradiation for three different initial solvent concentrations. The light intensity was fixed at 880 mJ/cm2. The measured concentration profiles of the monomer and polymer across the thickness are depicted in Figs. 5a and 5b, respectively. The monomer concentrations before the light irradiation were uniform, regardless of the initial concentrations of the solvent. However, the concentration distributions of the polymer became nonuniform; the polymer composition was almost uniform in a certain thickness range near the top surface, whereas it decreased with an increase in depth. The polymer concentration increased with a decrease in the initial solvent concentration at the same thickness position.
In the second series of experiments, the UV light intensity was varied, while the initial composition was kept constant at a solvent/monomer/initiator ratio of 60:38:2 (w/w). The measured polymer concentrations were negligible at any depth in the absence of photoirradiation (Fig. 6). Upon UV irradiation, the polymer concentrations near the top surface were uniform, whereas concentration gradients developed near the bottom surface, indicating a particular dual-layer structure. The thickness of the top layer increased with increasing UV intensity. The increase in light intensity lead to an increase in the polymer concentration, which eventually became a constant (of ~ 0.3) near the top surface. In contrast, the polymer concentrations near the bottom surface showed a weaker dependence on the light intensity as compared to those near the top surface, thus giving rise to show an inflection point in the concentration profile at a certain thickness. On the other hand, the local concentrations of the photoinitiator remained uniform across the thickness within an uncertainty of ± 3×10−3 at any given light intensity (not shown here).
Upon UV irradiation, photoinitiators are activated to form radicals, which then attack monomers to induce sequential polymerization reactions. An increase in the initiator concentration results in a decay in the light intensity across the thickness34 because the initiator molecules preferentially absorb light, leading to local consumption of the monomer and thus conversion to polymers, in a certain exposed area under high light intensities. Thus, we expect that nonuniform polymer concentration profiles could develop at high initiator concentrations. However, the polymer concentration distributions were more uniform at the initiator concentration of 5 wt% (Fig. 2b) as compared to those at 2 wt% (Fig. 6). Hence, a physical model that takes into account the local distribution of the light intensity is not sufficient to explain the evolution of polymer distributions across the film.
To understand the mechanism underlying the evolution of the concentration profiles, we consider the light-driven multicomponent diffusion in the UV-curing coatings. As mentioned above, the decay in UV light intensity in the thickness direction leads to a higher concentration of active radicals near the top surface. When the radicals diffuse from the top toward the bottom, the polymerization reactions proceed up to a larger depth in the coating, leaving a reacted zone on top of the unreacted zone (Figs. 7a–7d). The reaction front between the two zones propagates toward the bottom and eventually reaches the bottom surface (Figs. 7b and 7e). Assuming that limited amounts of the radical are formed after short-time photoirradiation, the concentration of the polymers generated by the photoreactions can become constant even when all radicals are consumed for the polymerization, so that a uniform polymer distribution is observed near the top surface. This physical picture is consistent with the concentration profiles shown in Fig. 2.
On the other hand, light-driven diffusion of nonreactive components can occur in photocurable coatings. Upon exposure to spatially nonuniform light, preferential photoreactions, and hence, local film shrinkage, would result in exclusion of the nonreactive chemical spices from the exposed area toward the unexposed area.35 Because MIBK is a nonreactive solvent, light irradiation from the top surface leads to exclusion of the solvent from the top of the film, thus decreasing the concentrations of the monomer and photoinitiator near the bottom surface. As a result, the photoreactions are locally hindered, leading to lower concentrations of the polymer in the diluted area near the bottom surface (Figs. 7c and 7f). This physical model is qualitatively consistent with the nonuniform polymer concentration distribution shown in Figs. 5b and 6.
One might argue that the heating of the sample to remove the solvent may impact the curing state. To check this, we conducted the Raman spectroscopy in the absence of photoirradiation at 40 and 25°C and found no apparent difference in Raman spectra before and after the heating, indicating that the effect of thermal curing is negligible in our systems.
Our previous measurements revealed that the porosity at the bottom surface decreased with increasing light intensity.32 This is possibly because the faster polymerization reactions at higher light intensities solidify the microstructures in early phase-separating stages. However, it is not immediately clear how the formation of phase-separated microstructures influences the composition profiles of polymers in the thickness direction. Photo-induced phase separation is initiated when the polymer concentration reaches a thermodynamic threshold. The interface between the phase-separating and nonseparating zones propagates toward the bottom as does the reaction front. When photo-driven solvent transport occurs in the presence of bi-continuous porous structures that span the thickness direction, the pores may act as diffusion paths for the solvent, resulting in a higher mass flux of the solvent compared to that in the case of nonseparating films. Future research will be directed toward controlling the polymer composition profiles by tuning the phase-separated phase morphologies and pore size distributions of the photocured films.
We directly measured the local composition profiles in phase-separating photocurable thin films using confocal Raman spectroscopy. To avoid light scattering at phase interfaces, we developed a novel technique to replace the solvent with a monomer to match reflective indices in the cured films. The results revealed that the polymer concentration distribution was uniform in solvent-free monomer/initiator binary solutions, while it was spatially nonuniform when the solvent-based films were thermodynamically unstable and promoted reaction-induced phase separation upon UV irradiation on the top surface. In the latter case, the film exhibited a dual layer, in which the polymer concentration was almost uniform near the top surface, while concentration gradients developed near the bottom surface. The thickness of the top layer with a uniform concentration distribution increased with the increase in the UV light intensity. These results implied that the propagation of the reaction front and the resulting light-driven transport of the solvent toward the bottom coating layer played key roles in the formation of nonuniform concentration profiles in photocured solution coatings.
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Yoshihara, H., Yamamura, M. Concentration profiles in phase-separating photocuring coatings. J Coat Technol Res 16, 1629–1636 (2019) doi:10.1007/s11998-019-00216-3
- UV curing
- Reaction-induced phase separation
- Confocal Raman spectroscopy