Cellulose

, Volume 20, Issue 1, pp 67–81

Study on the thermoresponsive two phase transition processes of hydroxypropyl cellulose concentrated aqueous solution: from a microscopic perspective

Authors

  • Ying Jing
    • State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, and Laboratory of Advanced MaterialsFudan University
    • State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, and Laboratory of Advanced MaterialsFudan University
Original Paper

DOI: 10.1007/s10570-012-9816-z

Cite this article as:
Jing, Y. & Wu, P. Cellulose (2013) 20: 67. doi:10.1007/s10570-012-9816-z

Abstract

In this paper, it was discovered that during the heating process from 35 to 63 °C, hydroxypropyl cellulose (HPC) concentrated aqueous solution (20 wt%) would first go through coil-to-globule transition and then sol–gel transition with temperature elevation. The microdynamic mechanisms of the two phase transitions were thoroughly illustrated using mid and near infrared spectroscopy in combination with two-dimensional correlation spectroscopy (2Dcos) and perturbation correlation moving window (PCMW) technique. Mid infrared spectroscopy is an effective way to study the hydrophobic interactions in HPC molecules. And near infrared spectroscopy is a potent method to study hydrogen bonds between HPC molecules and water molecules. Boltzmann fitting and PCMW could help determine the exact transition temperatures of each involving functional groups in the two processes. Moreover, 2Dcos was used to discern the sequential moving orders of the functional groups during the two phase transitions. Depending on the structure of HPC and the thermodynamic conditions, the dominating associative elements in either process might vary. During the coil-to-globule transition, HPC molecules precipitated to form an opaque system with mobility.It was discovered that the driving force of the coil-to-globule transition process in microdynamics could only be the dehydration and hydrophobic interactions of C–H groups. However, in the sol–gel transition, the system crosslinked to form a physical network with no mobility. The driving force of this process in microdynamics was primarily the self-assembly behavior of O–H groups in HPC “active molecules”.

Keywords

Hydroxypropyl celluloseTwo-dimensional correlation spectroscopyCoil-to-globule transitionSol–gel transitionHydrophobic interactionsHydrogen bonding

Introduction

Large interests have been aroused in associative water soluble polymers during the past decades (Clasen and Kulicke 2001). Such polymers usually contain hydrophilic and hydrophobic moieties (block or pendent groups) which self-associate in aqueous media. These peculiar systems can be used in a wide range of applications such as food, pharmaceutical, biomedical and cosmetic industries (Klouda and Mikos 2008). In the context of these applications, the conception of smart system responding strongly under slight and precise stimulus is a great challenge (Kobayashi et al. 1999). Temperature is one of the most important stimuli for such applications, and polymers having response to temperature are often named in the literature with the generic term “thermosensitive polymers” (Carotenuto and Grizzuti 2006). These thermosensitive polymers tend to self-associate above the lower critical solution temperature (LCST) and/or below the upper critical solution temperature (UCST) via intra and/or intermolecular interactions (Mori et al. 2010). Most of the water soluble thermosensitive polymers are from synthetic origin, such as poly(N-isopropylacrylamide) (PNIPAM) (Wei et al. 2009) or poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock copolymers (Alexandridis and Hatton 1995) or poly(ethylene glycol)-biodegradable polyester copolymers (Du et al. 2007). However, for biocompatibility, environmental friendliness, and renewability, development of systems based on natural or semi-natural polymers, mainly polysaccharides, is in expansion.

Cellulose derivatives exhibiting a clear thermosensitive behavior in aqueous solution greatly satisfy the urgent demand mentioned above. Hydroxypropyl cellulose (HPC), one of the water soluble cellulose derivatives, has been approved by the united states food and drug administration in biomedical and pharmaceutical applications for its nontoxicity, biocompatibility and biodegradability. It is soluble in water and a range of organic solvents, convenient for structural modification. In addition, it undergoes phase transitions upon changes of both temperature and concentration (Suto and Suzuki 1997; Mustafa et al. 1993). It has been well studied in many works that HPC will precipitate from its aqueous solution above its LCST (~45 °C) (Lu et al. 2002), which is known as the coil-to-globule transition. This phenomenon has been studied in dilute HPC solutions using various experimental techniques like turbidimetry, viscometry, rheology, laser light scattering and differential scanning calorimetry (DSC) from a macroscopic scale (Bumbu et al. 2004; Carotenuto and Grizzuti 2006; Gao et al. 2001). However, few works have concentrated on this coil-to-globule transition in molecular level. In addition to the coil-to-globule transition, Carotenuto etal. found that concentrated HPC solution (>20 wt%) would subject to another sol–gel transition at even higher temperature using rheological method (Carotenuto and Grizzuti 2006). The phenomenon of transition from a solution to a gel is commonly referred to as the sol–gel transition and hydrogel is thus formed. Thermoresponsive hydrogels especially from natural origin, for example cellulose derivatives, chitosan, dextran, xyloglucan, and gelatin, are of particular interest in the fields of injectable drug delivery and tissue engineering recently. In cellulose derivatives, the most extensively investigated one for biomedical applications is methylcellulose because of its remarkable spontaneous gelation behavior simply due to hydrophobic interactions between 60 and 80 °C in a wide concentration range (Li et al. 2002). However, the sol–gel transition temperature of HPC is much lower than that of methylcellulose and its transition mechanisms are more complicated. In this paper, both the coil-to-globule transition and the sol–gel transition at different temperatures in the same HPC solution (20 wt%) were well studied using both middle (MIR) and near infrared (NIR) spectroscopy. Near infrared spectroscopy has long been employed as a unique tool for investigating hydrogen bonding and hydration of self-associated molecules (Adachi et al. 2002). In this work, NIR spectroscopy can trace the spectral variations of both HPC and water in the dynamic phase transition processes. Additionally, two-dimentional correlation infrared spectroscopy (2Dcos) in combination with perturbation correlation moving window (PCMW) technique is used to further illustrate the microdynamic mechanisms of the two processes.

Two-dimentional correlation infrared spectroscopy is a mathematical method whose basic principles were first proposed by Noda in 1986. So far, 2Dcos has been widely used to study the spectral variations of different chemical species under various external perturbations (e.g.Temperature, pressure, concentration, time, pH, etc.) (Noda 2008). By spreading peaks along a second dimension, 2Dcos can sort out complex or overlapped spectral features and get an enhanced spectral resolution. Due to the different responses of different species to external variables, additional information about molecular motions or conformational changes can be obtained which can not be extracted directly from one-dimensional spectra. Further, 2Dcos can provide the specific order occurring under a certain physical variable.

Perturbation correlation moving window is a technique proposed by Morita in 2006 to give much wider applicability through introducing the perturbation variable into the correlation equation (Morita et al. 2006; Thomas and Richardson 2000). Perturbation correlation moving window has the ability in determining transition points along with monitoring complicated spectral variations along the perturbation direction.

Experimental section

Materials

Hydroxypropyl cellulose was purchased from Sigma Aldrich (average Mw ~80,000, average Mn ~10,000, MS 3–3.5, DS 2.2–2.8. Fourier transform infrared spectroscopy spectrum is shown in Fig. S1. 1HNMR spectrum is shown in Fig. S2. The calculated DS is 2.1). 20 wt% HPC D2O and H2O solutions were prepared and kept for one week before experiments in order to ensure the complete dissolution of HPC.

Instruments and measurements

Differential scanning calorimetry

Calorimetric measurements for the prepared samples (20 wt% HPC H2O and D2O solutions) were performed on a Mettler-Toledo differential scanning calorimeter thermal analyzer. All the experiments were carried out in a nitrogen atmosphere. The temperature range was 25–65 °C, and the heating rate was 10 °C min−1.

Fourier transform infrared spectroscopy

The sample of 20 wt% HPC D2O solution for MIR detection was prepared by being sealed between two pieces of microscope CaF2 windows which have no absorption bands in MIR region. The sample of 20 wt% HPC H2O solution for NIR detection was prepared by being sealed in the sample cell (quartz glass, 1 mm width) which has no absorption bands in NIR region. The FTIR measurements were performed on a Nicolet Nexus 470 spectrometer with a spectral resolution of 4 cm−1 and 32 scans were accumulated to obtain an acceptable signal-to-noise ratio. Variable-temperature spectra were collected between 35 and 63 °C for MIR, and between 35 and 60 °C for NIR with an increment of 0.5 °C (accuracy: 0.1 °C). Manual method was used to change the temperature, and IR spectrum was collected for each temperature point. The baseline correct processing was performed by the software of OMNIC 8.0. To guarantee data reproducibility, both MIR and NIR experiments were taken for more than three times for a valid result.

Investigation methods

Two-dimensional correlation analysis (2Dcos)

Mear infrared and NIR spectra recorded at an interval of 0.5 °C were selected in certain wavenumber ranges and 2D correlation analysis wascarried out using the software 2D Shige ver. 1.3 (©ShigeakiMorita, Kwansei-Gakuin University, Japan, 2004–2005), andwas further plotted into the contour maps by the Originprogram ver. 8.0. In the contour maps, red colors are definedas positive intensities, while blue colors as negative ones.

Perturbation correlation moving window (PCMW)

Fourier transform infrared spectroscopy spectraused for 2D correlation analysis were also used to perform PCMW analysis. Primarydata processing was carried out with the method Moritaprovided and further correlation calculation was performedusing the same software 2D Shige ver. 1.3 (©ShigeakiMorita,Kwansei-Gakuin University, Japan, 2004–2005). Similarly, the final contour maps were plotted by the Origin programver. 8.0, with the red colors defined as positive intensities and blue colors as negative ones. An appropriate window size (2m + 1 = 11) was chosen to generate PCMW spectra with good quality.

Results and discussion

The chemical structure of HPC is demonstrated in Scheme 1 and tentative band assignments are listed in Table 1. The reason of assignments are in the supporting information. The digital images of the two phase transitions of 20 wt% HPC H2O solution during heating are shown in Fig. 1. At room temperature, the solution was transparent with mobility. When the temperature reached 45 °C, precipitation occurred and the system became opaque but still with mobility. However, at 60 °C, the system had gone through sol–gel transition with no mobility. Expected reversibility is seen during the cooling process.
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Scheme 1

Chemical structure of hydroxypropyl cellulose (HPC). C1′−3′ belong to the hydroxyl group of HPC and C1−6 belong to the cellulose backbone

Table 1

Tentative band assignments according to 2Dcos during the heating process and phase transition temperatures of certain functional groups in HPC determined by Boltzmann Fitting

Wavenumber

(cm−1)

Assignments

Transition temperature

(°C)

2,990

νas (CH in C1′H3) strong hydrationa

51.0

2,965

νas (CH in C1′H3) weak hydration

50.2

2,945

νas (CH in C3′H2)a

51.6

2,922

ν (CH in C2′H)

c

2,904

ν (CH in C1−6H)

2,985

νas (CH in C1′H3) strong hydrationb

60.2

2,938

νas (CH in C3′H2)b

60.3

7,270, 7,189

2ν (OH in water) weak hydrogen bond with HPC

7,080

2ν (OH in water) strong hydrogen bond with HPC

6,760, 6,720

2ν (OH in HPC) weak hydrogen bond with watera

45.7

6,470

2ν (OH in HPC) strong hydrogen bond with water

6,890

2ν (OH in HPC) self-assembled

6,800

2ν (OH in HPC) weak hydrogen bond with waterb

53.8

aIn the first phase transition; b in the second phase transition; c not obtained

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Fig. 1

Digital images of 20 wt% HPC H2O solution at different temperatures (a) room temperature, (b) 45 °C, (c) 60 °C, (d) 45 °C cooling from 60 °C and (e) room temperature cooling from 60 °C

In this work, MIR and NIR spectroscopy were combined to better illustrate the spectroscopic vibrations of the hydroxypropyl groups and cellulose backbones of HPC, together with the solvent molecules, during the two phase transition processes. In MIR analysis, D2O rather than H2O, was used here as the solvent to eliminate the overlap of the O–H stretching vibrations (υ(OH)) of water at about 2,900–3,300 cm−1 with the C–H stretching vibrations (υ(CH)) of HPC at about 2,800–3,000 cm−1 (Sun et al. 2008). In NIR analysis, H2O was directly used as the solvent, and the υ(OH) first overtones of both H2O and HPC could be obtained simultaneously.

The isotope effect between HPC 20 wt% H2O and D2O solution has influence on the thermal behavior of HPC according to the DSC results shown in Fig. 2. It can be seen that the peak temperature in the phase transition of HPC in D2O (50.6 °C) is a little lower than that in H2O (52.8 °C). The magnitudes of the hysteresis of the transition temperatures (the difference between the transition temperatures in the heating process) are only weakly affected by isotopic substitutions. The endothermic peak (ΔH > 0) in DSC curves can be explained using Gibbs free energy theory. The two phase transitions are both hydrophobic interactions in nature, and the entropy of the system tends to increase (ΔS > 0). To guarantee a successful progressing of phase transitions (ΔG < 0), ΔH has to be positive and smaller than TΔS. Besides, in either of the DSC curve only one endothermic peak can be observed, indicating that DSC is not sensitive enough to be utilized to analyze the two phase transitions. In this context, FTIR spectroscopy together with its two-dimensional correlation analysis well tackles the problem and can explain the phase transition mechanisms in molecular level.
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Fig. 2

DSC curves of 20 wt% HPC H2O solution and 20 wt% HPC D2O solution in 25–65 °C temperature range

Figure 3 shows the MIR and NIR spectroscopic analysis of the whole heating process. The MIR spectra of υ(CH) region from 35 to 63 °C is shown in Fig. 3a. Perturbation correlation moving window is an available method for determining transition points. The 2D synchronous spectrum of PCMW performed in the region 3,020–2,830 cm−1 is presented in Fig. 3b. From the spectrum, two phase transitions are clearly observed with the first transition occurring at approximately 50 °C and the second at approximately 60 °C. The NIR spectra of υ(OH) first overtone region from 35 to 60 °C is shown in Fig. 3c. It is noted that the absorption intensity at one typical band 6,780 cm−1 between 42 and 60 °C appears to exhibit a “double anti-sigmoidal-shaped” decreasing according to Boltzmann fitting(using the Origin program) (Popescu et al. 2006) (Fig. 3d), which indicates two first order transtions for this band occur during the heating process. The first transition temperature is T1 = 45.6 °C corresponding to the coil-to-globule transition, and the second is T2 = 53.7 °C corresponding to the sol–gel transition. It can be seen that C–H and O–H groups have different responses to temperature change during transition processes. The detailed spectroscopic analysis of the two transitions will be thoroughly discussed in the following paper.
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Fig. 3

a MIR spectra of υ(CH) region of 20 wt % HPC D2O solution with temperature increase from 35 to 63 °C (the large arrow indicates the direction of spectral shift with temperature), b 2D synchronous spectrum of PCMW performed in the temperature range 35–63 °C in the region 3,020–2,830 cm−1. Red colors are defined as positive intensities, while blue colors as negative ones, c NIR spectra of υ(OH) first overtone region of 20 wt% HPC H2O solution during heating from 35 to 60 °C (the large arrow indicates the direction of spectral shift with temperature), and d the absorption intensity variation as a function of temperature at 6,780 cm−1 between 42 and 60 °C. The solid line was obtained from the Boltzmann fitting method.All temperature interval is 0.5 °C

Analysis of the first transition: coil-to-globule transition

The C–H region

1D MIR spectra of the stretching vibration of various C–H groups between 35 and 56 °C are demonstrated in Fig. 4a. All bands present an apparent red shift with temperature rise, showing an intense dehydration process of the C–H groups (Maeda 2001). The absorption intensity variations of 2,990, 2,965, 2,945 cm−1 bands appearing in 2D synchronous PCMW spectrum between 45 and 56 °C are shown in Fig. 4b–d. The absorption intensity variations of 2,990 and 2,945 cm−1 take on an “anti-sigmoidal-shaped” decreasing, while that of 2,965 cm−1 exhibits an “sigmoidal-shaped” increasing. The phase transition temperatures of the three C–H species are listed in Table 1, and they are in good accordance with the transition temperatures determined by 2D synchronous PCMW. Both 2D synchronous PCMW spectrum and the nonlinear Boltzmann fitted curves show a coil-to-globule transition at ~50 °C for C–H groups in HPC.
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Fig. 4

MIR spectra of υ(CH) region of 20 wt% HPC D2O solution with temperature increase from 35 to 56 °C with an interval of 0.5 °C (the large arrow indicates the direction of spectral shift with temperature) (a), the absorption intensity variations as a function of temperature between 45 and 56 °C at 2,990 cm−1 (b), 2,965 cm−1 (c), 2,945 cm−1 (d), with an interval of 0.5 °C. The solid lines were obtained from the Boltzmann fitting method

As has been mentioned above, 2Dcos can not only enhance spectral resolution, but also discern the specific order taking place under external perturbations. The 2D-IR correlation spectra are charaterized by two independent wavenumber axes (υ12) and a correlation intensity axis. Two types of spectra, 2D synchronous and asynchronous spectra are obtained in general. The 2D synchronous spectra are symmetric with respect to the diagonal line in the correlation map. Some peaks appearing along the diagonal are called the autopeaks, and the symbols of them are always positive, as autopeaks represent the degree of autocorrelation of perturbation-induced molecular vibrations. Where the autopeak appears, the peak at this wavenumber would change greatly under environmental perturbation. Off-diagonal peaks, named cross-peaks (Φ(ν1, ν2)), can be positive or negative. Positive cross-peaks demonstrate the intensity variations of the two peaks are taking place in the same direction (both increase or both decrease) under the environmental perturbation; while the negative cross-peaks help to infer that the intensities of the two peaks change in opposite directions under perturbation. The 2D asynchronous spectra are asymmetric with respect to the diagonal line in the correlation map. Unlike synchronous spectra, only off-diagonal cross-peaks would appear in asynchronous spectra, and these cross-peaks can be either positive or negative. The intensity of the asynchronous spectrum (ψ(ν1, ν2)) represents sequential or successive changes of spectral intensities observed at ν1 and ν2. With the cross-peaks both in synchronous and asynchronous maps, we can get the specific order of the spectral intensity changes taking place while the sample is subjected to an environmental perturbation. According to Noda’s rule (Noda 1993, 2000; Noda et al. 1999), if the cross-peaks (ν1, ν2, and assume ν1 > ν2) in synchronous and asynchronous spectra have the same sign, the change at ν1 occurs prior to that of ν2, and vice versa. If the intensity of the cross-peak in synchronous spectrum is zero, ν1 has no relationship with ν2. If the intensity of the cross-peak in asynchronous spectrum is zero, ν1 and ν2 change simultaneously.

Here, all the MIR spectra in 3,020–2,830 cm−1 region during the coil-to-globule transition process between 35 and 56 °C were used to perform 2D correlation analysis as shown in Fig. 5. In the synchronous spectrum (Fig. 5a), there are four autopeaks developing at 2,990, 2,965, 2,945, and 2,865 cm−1, in accordance with the 2D synchronous PCMW specrum, which point out the prominent changes of these four peaks with temperature elevation. The negative synchronous cross-peak at (2,990, 2,965) cm−1 indicates that the heat-induced intensity variations of the above two peaks take place in different directions. The positive synchronous cross-peak at (2,990 and 2,945) cm−1 shows that the intensities of these two peaks vary in the same direction. Analysis of the asymetric stretching vibrations of the C–H groups is adequate, so the band at low wavenumber 2,865 cm−1 which is attributed to the symmetric stretching vibration of C–H group will not be included in the discussion.
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Fig. 5

2D synchronous (a) and asynchronous (b) spectra of 20 wt% HPC D2O solution generated from all MIR spectra in 3,020–2,830 cm−1 region between 35 and 56 °C with an interval of 0.5 °C

2D asynchronous spectra can significantly enhance the spectral resolution. In Fig. 5b, two new bands, 2,922 and 2,904 cm−1 have been identified. The symbols of the cross-peaks at (2,990, 2,922) cm−1 and (2,945, 2,922) cm−1 are positive in the synchronous and negative in the asynchronous spectra. This infers that when heated, the intensity of the 2,922 cm−1 band varies prior to those of the 2,990 and 2,945 cm−1 bands. The symbols of the cross-peak at (2,965, 2,922) cm−1 are negative in the synchronous and positive in the asynchronous spectra, which also infers that the 2,922 cm−1 band varies prior to the 2,965 cm−1 band. However, the symbols of the cross-peaks at (2,990, 2,904) cm−1 and (2,945, 2,904) cm−1 are all negative in the synchronous and asynchronous specra, indicating that 2,990  and 2,945 cm−1 change before 2,904 cm−1. The cross-peaks at (2,965, 2,904) cm−1 are both positive in the synchronous and asynchronous spectra, also indicating that 2,965 cm−1 changes before 2,904 cm−1. The cross-peaks in the 2D spectra finally help to conclude the changing sequence of the five bands observed in the spectra as 2,922 > 2,990, 2,945 > 2,965 > 2,904 cm−1. (The symbol “>” means change prior to, and the detailed determination process is in supporting information).

The stretching vibration of methyne groups (C1−6H) in the cellulose backbone lies at 2,904 cm−1 (Maeda 2001). However, due to the hydrogen bond acceptor effect of hydroxyl groups, the stretching vibration of methyne groups (C2′H) will shift to higher wavenumber at 2,922 cm−1 (Gruenloh et al. 1999).The 2,990 cm−1 band with the highest wavenumber is attributed to the asymmetric stretching vibration of methyl groups (C1′H3) interacting with a greater number of D2O molecules (Maeda et al. 2000; Sun et al. 2007). The 2,945 cm−1 band belongs to the asymmetric stretching vibration of methylene groups (C3′H2) (Maeda et al. 2000; Sun et al. 2007). During the heating process, the methyl groups (2,990 cm−1) go through dehydration, and the band moves to lower wavenumber as shown in the 1D spectrum (Tamai et al. 1996).The band of the vibration of methyl groups without hydration interaction with water exists at 2,965 cm−1 (Sun et al. 2007). Detailed band assignments concerning all the bands appearing in 2Dcos in this paper are listed in Table 1.

The O–H region

For better investigation of the coil-to-globule transition of HPC concentrated solution, the first overtones of O–H groups (2υ(OH)) are also studied using NIR spectroscopy. O–H bands from HPC and water molecules may be well separated in NIR spectroscopy, thus making it an effective method in the analysis of O–H groups both in HPC and water during the phase transition (Takeuchi et al. 2005). Figure 6 shows the 1D NIR spectra of 2υ(OH) in 7,340–6,000 cm−1 region between 35 and 48 °C. The whole spectra take on a blue shift tendency during the heating process because of the weakening of O–H hydrogen bonds (Czarnik-Matusewicz et al. 2005). When weak hydrogen bond forms, the force constant k of O–H covalent bond becomes larger according to the wavenumer function (shown in Equation S1), for the interaction between hydrogen bond acceptor and hydrogen bond donor becomes weak and the interaction of O–H covalent bond itself becomes strong. The absorption intensity variation of the 6,720 cm−1 band takes on an “anti-sigmoidal-shaped” decreasing trend between 42 and 48 °C with the coil-to-globule transition temperature at 45.7 °C.
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Fig. 6

1D NIR spectra of 2υ(OH) in 7,340–6,000 cm−1 region between 35 and 48 °C with a temperature interval of 0.5 °C (the large arrow indicates the direction of spectral shift with temperature) (a), the absorption intensity variation as a function of temperature between 42 and 48 °C at 6,720 cm−1 with an interval of 0.5 °C (b). The solid line was obtained from the Boltzmann fitting method

2D correlation NIR analysis was performed to investigate the microdynamics of O–H groups in HPC and water molecules during the first transition process, and the synchronous and asynchronous spectra are shown in Fig. 7. The strong autopeak at 6,720 cm−1 in the synchronous spetrum indicates that this band suffers most in the phase transition process. The appearance of some weak autopeaksabove 7,000 cm−1 (7,270, 7,189, 7,080 cm−1), which may be attributed to the 2υ(OH) of water molecules, indicates water participation in the phase transition process. In the asynchronous spectrum, a new band at 6,760 cm−1 appears, which may belong to the same species with 6,720 cm−1. The symbols of the cross-peaks at (7,270, 6,720) cm−1, (7,189, 6,720) cm−1 and (7,080, 6,720) cm−1 in synchronous and asynchronous spectra are all negative, showing that the intensities of 7,270, 7,189 cm−1, 7,080 cm−1 change prior to that of 6,720 cm−1. The situation is the same with the 6,760 cm−1 band. However, the intensities of the cross-peaks at (7,270, 7,189) cm−1, (7,270, 7,080) cm−1 and (7,189, 7,080) cm−1 in the asynchronous spectrum are close to zero, which means the intensities of 7,270, 7,189 and 7,080 cm−1 bands vary at the same time, confirming the fact that they come from the same species—water. The intensity of the cross-peak at (6,760, 6,720) cm−1 in the asynchronous spectrum is also zero, demonstrating that the intensities of the two bands also vary at the same time and comfirm the fact that they both belong to HPC molecules (Sun and Wu 2011). Generally, the changing sequence of the bands observed in the 2D NIR spectra can be summed up as 7,270, 7,189, 7,080 > 6,720, 6,760 cm−1 (detailed determination process is demonstrated in supporting information). This declares that in 35–48 °C temperature region, OH groups of the water molecules (7,270, 7,189, 7,080 cm−1) hydrogen bonded with HPC change first, and then the OH groups of HPC (6,720, 6,760 cm−1) start to move. The OH groups of HPC are mostly hydrogen bonded with water, and the intensities of the two bands (6,720, 6,760 cm−1) decrease with temperature, which indicates a dehydration process occurs.
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Fig. 7

2D synchronous (a) and asynchronous (b) spectra of 20 wt% HPC H2O solution generated from all NIR spectra in 7,340–6,000 cm−1 region between 35 and 48 °C

The transition temperature of the 6,720 cm−1 band (45.6 °C) determined by Boltzmann fitting is lower than those of the 2,990  , 2,966  and 2,945 cm−1 bands (~50 °C), indicaiting that O–H groups change prior to C–H groups in the coil-to-globule transition. The total transition mechanism can be illustrated in Scheme 2. In the first step, the D2O or H2O molecules hydrogen bonded with HPC hydroxyl groups first dissociate the hydrogen bond with the rising temperature, resulting in the dehydration of the OH groups in HPC. The methyne groups bonded to OH groups (C2′H, 2,922 cm−1) then starts to move, followed by the simultaneous dehydration process of the methyl (C1′H3, 2,990 cm−1) and methylene (C3′H2, 2,945 cm−1) groups. In the second step, the dehydrated methyl groups (C1′H3, 2,965 cm−1) interact with each other due to the hydrophobic effect, thus causing the coil-to-globule transition of HPC. In the third step, the adjacent inter or intramolecular structural units of HPC backbone (C1−6H, 2,904 cm−1) diffuse and aggregate to complete the coil-to-globule transition. It is worth noting that the driving force of this process is the hydrophobic interactions of C–H groups. The role of O–H groups in HPC is to initiate the motion of the hydrophobic groups. Besides, the hydrophilicity of the O–H groups in HPC also shields the strong intermolecular interactions of the hydrophobic groups, which prevent them from directly forming physical hydrogel like methylcellulose(Chevillard and Axelos 1997). This behavior is similar to protein-like copolymers, in which the hydrophilic polar shells prevent the interactions of the hydrophobic cores (Wahlund et al. 2002; Lozinsky et al. 2003, 2006).
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Scheme 2

Microdynamics of the HPC molecules in 20 wt% HPC D2O solution in the coil-to-globule transition process. (D2O can be substituted by H2O)

Analysis of the second transition: sol–gel transition

Apart from the coil-to-globule transition, concentrated HPC solution will go through another sol–gel transition at higher temperature, and its microdynamic mechanism is eagerly studied.

The C–H region

1D MIR spectra of the stretching vibrations of various C–H groups between 56.5 and 63 °C are demonstrated in Fig. 8a. The band positions only have a quite subtle red shift in this temperature range but all band intensities take on a decreasing trend, indicating that this process only occurs to some “active molecules”(Carotenuto and Grizzuti 2006), and is not intense. The absorption intensity variations of 2,985 and 2,938 cm−1 bands appearing in 2D synchronous PCMW spectrum between 56.5 and 63 °C are shown in Fig. 8b and c. The absorption intensity variations of 2,985 and 2,938 cm−1 both take on an “anti-sigmoidal-shaped” decreasing trend. The phase transition temperatures of the two C–H species are also listed in Table 1, and they are in good accordance with the transition temperatures determined by 2D synchronous PCMW.
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Fig. 8

MIR spectra in 3,020–2,830 cm−1region of 20 wt% HPC D2O solution with temperature increase from 56.5 to 63 °C with an interval of 0.5 °C (the large arrow indicates the direction of spectral shift with temperature) (a), the absorption intensity variations as a function of temperature between 57 and 63 °C at 2,985 cm−1 (b) 2,938 cm−1 (c). The solid lines were obtained from the Boltzmann fitting method

2D correlation analysis of the MIR spectra in 3,020–2,830 cm−1 region between 56.5 and 63 °C were performed to investigate the microdynamics of the sol–gel transition process as shown in Fig. 9. The 2D synchronous spectrum shows three strong autopeaks 2,985, 2,938 and 2,890 cm−1. Their cross-peaks are all positive, indicating that they have similar response of spectral intensities to temperature perturbation–all decrease during the sol–gel transition process as has been proved in 1D spectra. The 2D asynchronous spectrum shows three new peaks at 2,965, 2,904, and 2,865 cm−1. The 2,890 and 2,865 cm−1 bands which are attributed to the symmetric vibrations of the C–H groups also will not be included in the discussion. As space is limited, the determination details of sequential orders have been presented in supporting information, and only the final specific order is given as follows: 2,985 > 2,938 > 2,904 > 2,965 cm−1.
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Fig. 9

2D synchronous a and asynchronous b spectra of 20 wt% HPC D2O solution generated from all MIR spectra in 3,020–2,830 cm−1 region between 56.5 and 63 °C

After the coil-to-globule transition process, most HPC chains or segments precipitate, leaving behind a dilute solution of coil-like HPC chains or segments, which are designated as “active molecules” (Carotenuto and Grizzuti 2006). The 2,985 and 2,938 cm−1 bands belong to the methyl (C1H3) and methylene (C3′H2) groups of the “active molecules”. There is a red shift in the two bands of the “active molecules” compared with those of the HPC molecules before the coil-to-globule transition (2,990 cm−1 for C1′H3 and 2,945 cm−1 for C3′H2 respectively). It is the less hydration of the “active molecules” at higher temperature that causes the red shift. During the sol–gel transition, the methyl groups of the remaining “active molecules” (2,985 cm−1) dehydrate first, and then the methylene groups of the HPC molecules (2,938 cm−1) experience the process of dehydration. Because of the dilution of “active molecules”, HPC backbones (2,904 cm−1) have to move in order to shorten the distances of structure units far apart, thus allowing the hydrophobic interactions of dehydrated methyl groups (2,965 cm−1).

The O–H region

The overtones of the stretching vibration of the O–H groups in HPC and water in the sol–gel transition are studied using NIR spectroscopy. Figure 10a shows the 1D NIR spectra of 2υ(OH) in 7,340–6,000 cm−1 region between 48.5 and 60 °C. A decreasing trend together with a subtle blue shift is observed from the spectra. The absorption intensity change of the 6,800 cm−1 band between 48.5 and 60 °C is demonstrated in Fig. 10b from which the transition temperature is determined to be 53.8 °C due to Boltzmann fitting method.
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Fig. 10

1D NIR spectra of 2υ(OH) in 7,340–6,000 cm−1 region between 48.5 and 60 °C (the large arrow indicates the direction of spectral shift with temperature) (a), the absorption intensity variation at 6,800 cm−1 as a function of temperature between 48.5 and 60 °C (b)

The microdynamics of the O–H groups during the sol–gel transition process are analyzed using 2D correlation NIR spectroscopy and the synchronous and asynchronous spectra are shown in Fig. 11. A strong autopeak at 6,800 cm−1 is observed in the synchronous spectrum, indicating that the 6,800 cm−1 band suffers most during the sol–gel transition. In the asynchronous spectrum, two new bands 6,890 and 6,470 cm−1 appear. The symbols of the cross-peaks at (6,890, 6,470) cm−1, (6,890, 6,800) cm−1 and (6,800, 6,470) cm−1 are all positive in the synchronous and negative in the asynchronous spectra. After detailed determination (see supporting information), the varying sequence of the bands is as follows: 6,470 > 6,800 > 6,890 cm−1.
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Fig. 11

Synchronous (a) and asynchronous (b) spectra of 20 wt% HPC H2O solution generated from all NIR spectra in 7,340–6,000 cm−1 region between 48.5 and 60 °C

The remaining coil-like HPC “active molecules” after the coil-to-globule transition result in the dilution of the solution where the O–H groups of HPC “active molecules” at low wavenumber 6,470 cm−1 may form strong hydrogen bonds with water molecules (Sun and Wu 2011). However, with the advent of the sol–gel transition, the strong hydrogen bonds turn to weak ones. The appearance of high wavenumber 6,800 cm−1 band indicates that the combination of HPC O–H groups with water molecules becomes weak—a dehydration process (Sun and Wu 2011). After the dehydration process, O–H groups of HPC “active molecules” may self-assemble to form a network, and the 6,890 cm−1 band is attributed to self-assembled O–H groups (Sun and Wu 2011).

The sol–gel transition process is vividly depicted in Scheme 3. In the first step, the strong hydrogen bonds of the O–H groups (6,470 cm−1) of HPC “active molecules” with water become weak. The dehydrated O–H groups self-assemble to form a network. In the second step, the methyl groups (C1′H3, 2,985 cm−1) of the “active molecules” go through dehydration, followed by the dehydration of methylene groups (C3′H2, 2,938 cm−1). In the third step, the distant HPC main chains diffuse and aggregate (C1−6 H, 2,904 cm−1), and crosslinking occurs due to self-assembly of O–H groups an intermolecular hydrophobic interactions. It is worth noting that the dominating effect in this process is the self-assembly behavior of the O–H groups in HPC “active molecules”. It is this driving force that shortens the distance of the interacting hydrophobic groups, thus forming a large network system.
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Scheme 3

Microdynamics of the active molecules in 20 wt% HPC D2O solution in the sol–gel transition process. (D2O can be substituted by H2O)

Conclusions

The coil-to-globule and sol–gel transition phenomena of concentrated HPC aqueous solution are observed during heating and their microdynamic mechanisms are well studied using FTIR spectroscopy in combination with 2Dcos and PCMW. Two regions involving C–H-related fundamental stretching vibrations and O–H-related overtones are focused on to trace nearly all the group motions of HPC molecules as well as the solvent molecules upon heating. In the first transition process, water dissociate with the O–H groups of HPC, and the dehydrated O–H groups of HPC initiate the motion of the methyne groups (C2′H). And then the methyl (C1′H3) and methylene (C3′H2) groups dehydrate simultaneously, followed by the hydrophobic interactions of the dehydrated methyl groups (C1′H3). The diffusion and aggregation of the adjacent inter or intramolecular structural units of HPC backbone result in the shrinkage of the coil. In the second transition process, the HPC “active molecules” play the most important role. The O–H groups of HPC “active molecules” dehydrate and self-assemble to form a network. And then the methyl (C1′H3) and methylene groups (C3′H2) of HPC “active molecules” dehydrate. The self-assembly of O–H groups and the hydrophobic interactions of C–H groups after the aggregating of the main chain lead to the crosslinking of the system. The dominating effect in each phase transition process is clearly revealed. However, it is worth noting that the two phase transitions are not clear-cut because of the overlap in the transition temperatures of certain functional groups.

Acknowledgments

This work was financially supported by National Science Foundation of China (NSFC) (No. 20934002, 51073043) and the National Basic Research Program of China (No.2009CB930000).

Supplementary material

10570_2012_9816_MOESM1_ESM.pdf (153 kb)
Supplementary material 1 (PDF 153 kb)

Copyright information

© Springer Science+Business Media Dordrecht 2012