Temperature Dependence of the Structure of Bound Water in Dried

Feb 19, 2010 - The rate changes at around 310 K, while that for the bulk water is constant in the temperature range studied. The rates of change of th...
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J. Phys. Chem. B 2010, 114, 3419–3425

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Temperature Dependence of the Structure of Bound Water in Dried Glassy Poly-N,N,-dimethylacrylamide Yurina Sekine* and Tomoko Ikeda-Fukazawa Department of Applied Chemistry, Meiji UniVersity, Kawasaki 214-8571, Japan ReceiVed: July 19, 2009; ReVised Manuscript ReceiVed: February 06, 2010

Raman spectroscopy was used to investigate the temperature dependence of structural changes of bound water in dried glassy poly-N,N,-dimethylacrylamide in the temperature range 286.1-329.7 K. The results show that the frequency of the O-H stretching mode of the bound water that is present in the dried glassy polymer shifts to the higher side with increasing temperature. The rate changes at around 310 K, while that for the bulk water is constant in the temperature range studied. The rates of change of the frequencies for the CdO stretching mode and CH3 rocking mode also change at around 310 K. These results indicate a significant change in the interaction between the bound water and polymer chains at 310 K. Temperature dependence of the local structure of the bound water was analyzed by applying a structural model of bulk water to the spectra of the O-H stretching region. The result shows that the density of a tetragonal water structure consisting of four hydrogen bonds increases with increasing temperature below 310 K and begins to decrease at temperatures above 310 K. Further, estimates of the water content indicate that the evaporation rate of the bound water significantly changes at around 310 K. These results suggest that the bound water present in the dried glassy polymer can be classified as being in two states. At temperatures below 310 K, the water that forms a shell layer around the polymer chains evaporates, while at temperatures above 310 K the water that is bound to polar groups of polymer chains begins to evaporate. The structural changes of bound water might have important implications for the interpretation of properties of hydrated polymer systems, including both biological and synthetic polymers. I. Introduction Water plays a crucial role in the behavior of hydrated polymer systems. The properties of water in hydrated polymer systems and the associated phenomena are important subjects in areas as diverse as material science, biology, and chemical physics. For instance, it is known that water can contribute favorably to the conformational stability of biopolymers, including proteins, DNA, or enzymes, and is essential for their biological functions.1,2 Therefore, properties of water in hydrated polymer systems have been investigated using various experimental techniques such as differential scanning calorimetry (DSC),3-6 nuclear magnetic resonance (NMR),1,7-11 infrared spectroscopy,12,13 Raman spectroscopy,14-18 dielectric spectroscopy,19-21 X-ray diffraction,2 and simulation.22-24 The structure and properties of water in hydrated polymer systems differ from those of bulk water. In general, water in hydrated polymer systems has been classified into three types: bound water, intermediate water, and free water.25 Free water consists of water molecules that have a structure similar to that of bulk water, and their interaction with the polymer chains is negligible. Bound water is usually associated with water molecules that form strong hydrogen bonds with polar polymers or that interact strongly with ionic groups in the polymer chains. Intermediate water lies between the bound and free water and interacts weakly with the polymer chains. The relative amounts of the three types of water depend on the water content of the gel, among other factors.3-9,13,14,16,18,19 * To whom correspondence should be addressed. Present address: Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, 2-3-10 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-0062, Japan. E-mail: [email protected].

The properties of bound water significantly differ from those of free and intermediate water. Bound water does not show a melting transition, even below 173 K, whereas the melting temperatures for the free and intermediate water are about 273 and 248 K, respectively.3,4 Using X-ray diffraction, Nakasako2 studied the state of water around an enzyme (trypsin) and found that water networks of various shapes cover the entire trypsin molecule. This suggests that the structure of the bound water is an important factor for determining its unique properties. In our previous study, using Raman spectroscopy, we investigated structural changes of water in a poly-N,N,-dimethylacrylamide (PDMAA) hydrogel during dehydration.18 This study showed that most of the residual water in the dried glassy polymer exists in a two-dimensional hydrogen-bonded network. This result demonstrates that the structure of the bound water significantly differs from that of bulk water. The structure of bound water changes with temperature. Kuwabara et al.20 employed dielectric spectroscopy to study the relaxation process of bound water around DNA and found that the relaxation process associated with bound water exhibits a dielectric transition of the order-disorder type at the denaturation temperature Tm (around 343-353 K). At that temperature, bulk water exhibits no transition behavior. Based on these results, the authors concluded that the cluster of bound water molecules assumes an ordered structure on the surface of B-form DNA below Tm, while the cluster is thermally unstable above Tm. Using NMR spectroscopy, Ogiwara and Kubota7 showed that the state of bound water present in cellulose with low water content (5.4-7.4 wt %) changes at around 323-343 K, which is the glass transition temperature of dry cellulose. These results suggest that the structure of bound water is sensitive to

10.1021/jp906826q  2010 American Chemical Society Published on Web 02/19/2010

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Figure 1. Chemical structure of PDMAA.

temperature, and that the mechanism whereby the structure changes with temperature differs from that of bulk water. To understand the function of hydrated polymer systems, including both biological and synthetic polymers, the structural changes of bound water need to be further investigated. However, there are limited methods available for studying these structures: the amount of bound water relative to the total amount of the water in the polymer system is too small for direct detection. We previously proposed that a dried glassy polymer would be a good indicator of structural changes of bound water in a hydrated polymer system.18 In the present study, we measure Raman spectra of a dried PDMAA hydrogel to investigate the temperature dependence of the bound water structure. Figure 1 shows the chemical structure of PDMAA. Because PDMAA itself is water-soluble and does not show a lower critical solution temperature (LCST) in aqueous solutions,26 it is suitable for investigating the structural changes of the bound water only. Because the O-H stretching modes of water molecules provide direct information about the water structure, we used dried glassy PDMAA in the present study to investigate the structural changes of bound water. II. Experimental Section N,N-Dimethylacrylamide (DMAA) monomer (Kohjin Co., Ltd.) was purified by filtering through activated alumina. N,N′Methylenebis(acrylamide) (BIS) was used as the cross-link. Potassium persulfate (KPS) was used as the initiator, and N,N,N′,N′-tetramethylenediamine (TEMED) as the catalyst. The PDMAA hydrogel was synthesized by mixing water (30 mL), BIS (0.046 g), DMAA (2.97 g), TEMED (24 µL), and KPS (0.03 g). The detailed preparation of PDMAA hydrogel has been described previously.18 To dehydrate the sample, it was stored in a desiccator (AS ONE Co., Ltd.) for 1872 h at 298 ( 3 K and humidity of 45 ( 15%. The volume and operating humidity limit of the desiccator are 330 × 345 × 525 mm3 and 15%, respectively. The thickness of the dried sample was 4 mm, and its water content was 1.73 wt %. The water content of the dried sample Wd is defined as

Wd ) [(wd - wfd)/wint] × 100

(1)

where wd is the weight of the dried sample, wfd is the weight of the fully dehydrated sample after heating at 423 K for 1 h, and wint is the weight of the sample before dehydration. The value of wd was 1.40; wfd, 1.17; and wint, 13.4 g. A series of Raman spectra was measured using the JASCO NRS-3100 instrument. The excitation radiation for Raman emission was produced using a YAG laser in single-mode operation at 532 nm, with an output power of 100 mW. The spectra were measured over the frequency range 100-4000 cm-1. The laser beam was focused onto the surface of the gel using an optical system to a spot having a diameter of 5 µm. During the Raman measurement, the temperature of the dried

Figure 2. Raman spectra of (a) dried glassy PDMAA and (b) bulk water in the frequency region of O-H stretching modes of water in the temperature range 286.1-329.7 K.

sample was controlled between 286.1 and 329.7 K using a Peltier device (Japan High Tech Co., Ltd.). The change in thermal properties of the dried glassy PDMAA with increasing temperatures was measured with a differential scanning calorimeter (DSC) analysis system (SII Seiko Instruments, SSC/5200). The temperature was increased at a rate of 10 K/min during the DSC measurement. III. Results Figure 2 shows the Raman spectra of the dried glassy PDMAA and the bulk water in the frequency region of the O-H stretching modes in the temperature range 286.1-329.7 K. The intensity was normalized using the intensity of the isosbestic point at 3425 cm-1.27 As shown in Figure 2, the spectra of the dried glassy PDMAA significantly differ from that of the bulk water. For both spectra, however, the relative intensity and the frequency of the O-H stretching modes vary with temperature. A similar trend for bulk water was observed in previous studies.27,28 Several peaks exist in the spectra of the frequency region 2600-4000 cm-1 (Figure 2a). These peaks are assigned to the six C-H vibrational modes of the polymer chain and the five O-H stretching modes of the water molecule.18 To analyze the frequency and the relative intensity of the O-H stretching modes for the dried glassy PDMAA, we decomposed the spectra of the frequency region 2600-4000 cm-1 by fitting the data to 11 modes. Lorenzian functions were used for the six C-H vibrational modes, and Gaussian functions were used for the five O-H stretching modes.18 Although there are two N-H stretching modes for BIS (i.e., around 3258 and 3308 cm-1) in the frequency region, these modes were not used in the analysis. The estimates using the spectra of dried glassy PDMAA and a powder of BIS show that the intensity of the N-H stretching modes for BIS is about 0.05 times lower than the O-H stretching mode at 3290 cm-1. Thus, these peaks are negligible in comparison with the 11 modes. Figure 3a-c shows the fitting results for the O-H stretching modes of the dried glassy PDMAA at 286.5, 303.4, and 329.3 K. For comparison, we also decomposed the spectrum of bulk water in the frequency region 2800-4000 cm-1 by fitting the data to five O-H stretching modes. Figure 3d-f shows the fitting results of the spectra of bulk water at 286.1, 303.2, and 329.7 K.

Structure of Bound Water in Dried Glassy PDMAA

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Figure 4. Temperature dependence of frequency of the out-of-phase mode of O-H stretching for (a) dried glassy PDMAA and (b) bulk water. The dashed line represents the fitting lines. The solid line indicates the position of 310 K. Figure 3. Fitting curves in the frequency region of O-H stretching modes for the dried glassy PDMAA at (a) 286.5 K, (b) 303.4 K, and (c) 329.3 K and bulk water at (d) 286.1 K, (e) 303.2 K, and (f) 329.7 K. Solid circles are the experimental data. The dashed and solid lines represent the fitting curves and sum of the fitting curves, respectively.

In our investigation of the temperature dependence of the water spectra, we followed a bulk water model.13,17,27-36 Although there has been much discussion about how to assign modes, five modes of bulk water were assigned as follows. Figure 3d shows the spectrum of bulk water at 286.1 K. The dashed lines show the five modes obtained by fitting the data using Gaussian functions. The peaks are observed at 3050, 3224, 3396, 3513, and 3632 cm-1. According to the assignment in the literature,13,17,27-35 the peaks are classified as two types of modes: (1) water molecules with four hydrogen bonds, that is, two protons and two lone electron pairs are involved in hydrogen bonding (the peaks at 3050, 3224, and 3396 cm-1); (2) weakly or non-hydrogen-bonded water molecules in which the hydrogen bonds of the water molecules have been broken, in part or entirely (the peaks at 3513 and 3632 cm-1). Within the band corresponding to the four hydrogen-bonded molecules, the peak at 3224 cm-1 is associated with the collective in-phase vibrations of all molecules in the aggregate, whereas the 3396 cm-1 peak is associated with the vibration that is not in-phase between the first and higher shell of neighboring molecules (i.e., the outof-phase mode of O-H stretching).13,17,31,34,35 Hereinafter, we designate these peaks as νip (HB) and νoop (HB). The peaks at 3513 and 3632 cm-1 correspond to the symmetric and asymmetric stretching of the weakly hydrogen-bonded water molecules, respectively.13,17,29,31,34-36 We designate these peaks as νs (NHB) and νas (NHB). The peak at 3050 cm-1 is believed to arise from the Fermi resonance between the overtone of the bending mode (i.e., the 1644 cm-1 peak) and νip (HB) (i.e., the 3224 cm-1 peak).28 The peak at 3050 cm-1 is designated as νf (HB). Figure 4 shows the temperature dependence of the frequency of νoop (HB) (i.e., the peak at 3396 cm-1 in Figure 3d) for the dried glassy PDMAA and bulk water. The peak for both the dried glassy PDMAA and the bulk water shifts to a higher frequency with increasing temperature. However, a clear difference is observed in the rate of frequency change with

Figure 5. Raman spectra of the dried glassy PDMAA in the frequency region of vibrational modes of the polymer chain at 286.5, 305.5, and 329.3 K.

temperature. The rate associated with the dried glassy PDMAA changes at around 310 K, although that of the bulk water is constant in this temperature range. For the dried glassy PDMAA, the increasing rates of the frequency below and above 310 K are 0.769 and 0.415 cm-1 K-1, respectively. The increasing rate of the frequency of bulk water is 0.348 cm-1 K-1. To investigate the structural change of the polymer chain, we analyzed the CdO stretching mode and CH3 rocking mode. Figure 5 shows the Raman spectra of the dried glassy PDMAA in the frequency region of the polymer modes in the temperature range 286.5-329.3 K. The peaks at 1151 and 1614 cm-1 are assigned to the CH3 rocking mode and CdO stretching mode, respectively. The assignments for modes other than these have been described previously.18 To determine the frequency of the CH3 rocking mode and CdO stretching mode, we analyzed the spectrum of the frequency region 1100-1800 cm-1 by fitting the data to 10 modes. Lorenzian functions were used for all components. Although there are 13 modes in the frequency region, the spectrum was decomposed into 10 modes, as previously mentioned, because the polymer skeletal vibration modes (the peak around 1183 cm-1), NH bending mode for BIS (the peak around 1560 cm-1), and O-H bending mode (the peak around 1669 cm-1) are too weak for decomposition. Figure 6a-c shows the temperature dependence of the frequencies of νoop (HB), the CdO stretching mode, and the CH3 rocking mode. The frequency for the CdO stretching mode increases with increasing temperature, whereas that for the CH3

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Figure 6. Temperature dependence of frequency of (a) out-of-phase mode of O-H stretching, (b) CdO stretching, and (c) CH3 rocking modes. The solid line indicates the position of 310 K.

rocking mode decreases. The rates of change with temperature for these modes also shift at around 310 K. The increasing rates for the CdO stretching mode below and above 310 K are 0.241 and 0.104 cm-1 K-1, respectively. The decreasing rates for the CH3 rocking mode below and above 310 K are -0.138 and -0.0792 cm-1 K-1, respectively. IV. Discussion A. Water Evaporation with Heating. The bound water is expected to evaporate with heating. To investigate the effects of the evaporation on the structural change of the bound water, we estimated the changes in water content with temperature. We used the integrated intensity ratios of the O-H stretching modes (i.e., the peaks at 3050, 3285, 3442, 3542, and 3655 cm-1 at 286.5 K), IO-H, to the C-H stretching mode (i.e., the peak at 2936 cm-1 at 286.5 K), IC-H, of the Raman spectra for the dried glassy PDMAA as a measure of water content. Figure 7(a) shows temperature dependence of IO-H/IC-H. The IO-H/ IC-H value decreases as temperature increases and reaches 0.129 at 329.3 K. Under the assumption that the polarizability of water and polymer molecules remains constant, the intensity ratio, IO-H/IC-H, should be proportional to the water content in the dehydrated sample, as shown in Figure 7b. The rate of change IO-H/IC-H with temperature decreases also at around 310 K;

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Figure 7. (a) Temperature dependence of the ratio of integrated intensities of the O-H stretching modes to the C-H stretching mode, IO-H/IC-H, in the dried glassy PDMAA. (b) Dependence of water content on IO-H/IC-H obtained from the spectral measurements during the drying process at 298 K. (The solid circles indicate the data obtained by the previous study.18 The open circle indicates the data of the initial sample in the present study.) (c) Temperature dependence of water content in the dried glassy PDMAA. The solid line indicates the position of 310 K.

the slopes below and above 310 K are -0.0103 and -0.00317 cm-1 K-1, respectively. In our previous study, we measured the Raman spectra during the dehydration process of the PDMAA hydrogel.18 This study showed that most of the water in the dehydrated gel with water content below 3.45 wt % exists as bound water. Using the data of the dehydrated gel with water content below 3.45 wt %, the relation between IO-H/IC-H and water content for bound water is obtained as shown in Figure 7b. The open circle in Figure 7b indicates the data of the initial sample in the present study. This value is consistent with the trend observed in the previous study. The linear relationship between water content and intensity ratio is given by 5.28 × (IO-H/IC-H) - 0.666. The water content in the dried glassy PDMAA can be estimated using this equation. Figure 7c shows the relationship between the calculated water content and temperature. This calculation indicates that about 84% of the bound water evaporates by the time the temperature reaches 310 K. This sudden change in evaporation rate suggests that a significant structural change occurs in the polymer chain and the bound water. The structural changes are discussed in the following sections. B. Structural Change of the Polymer Network. The frequencies of the CdO stretching and CH3 rocking modes vary with temperature. As shown in Figure 6b, the frequency of the CdO stretching mode shifts to the higher side with increasing temperature. The frequency of the CdO stretching mode is associated with the CdO bond length.12,13,22,37 Using ab initio

Structure of Bound Water in Dried Glassy PDMAA molecular orbital calculations, Torii et al.22 showed that the CdO bond length is strongly affected by both the dielectric effect of solvents and the formation of amide-solvent hydrogen bonds. According to these calculations, the higher frequency shift of the CdO stretching mode observed in this study means that the CdO bond shrinks because the number density of the hydrogen bonds between the amide group and the bound water decreases as a result of evaporation of the bound water. The thermal effect is also one of the causes of the higher frequency shift for the CdO stretching mode. As shown in Figure 6c, the frequency of the CH3 rocking mode shifts to the lower side with increasing temperature. The frequency of the CH3 rocking mode is associated with the repulsive interaction between water and the adjacent methyl groups. The lower frequency shift of the CH3 rocking mode means that the rocking motion of the methyl group is amplified due to the decreased hydrophobic interaction with the surrounding water, that is, the decrease in the density of the residual bound water. If the density of the water is constant, the frequency should shift to the higher side with increasing temperature due to the thermal effect. However, the result shows that the lower frequency shifts as the temperature increases. Therefore, the effect of evaporation is the dominant factor in the frequency change observed in the CH3 rocking motion. From Figure 6b and c, it can be observed that the slopes of the frequency of the CdO stretching and CH3 rocking modes change abruptly at around 310 K, indicating a change in the interaction between the water molecules and the polymer chains at around 310 K. In addition, at that temperature, the dependence of water content on temperature changes (Figure 7c). These results can be attributed to one or both of the following possible mechanisms: (1) the structure of the residual bound water changes and/or (2) the structure of the polymer network changes with temperature. To confirm the structural changes of the polymer network with temperature, we analyzed the C-N-C asymmetric stretching mode. Figure 6d shows the temperature dependence of the frequencies of the C-N-C asymmetric stretching mode and that the asymmetric stretching mode shifts slightly to the lower side with temperature. The rate change cannot be observed at around 310 K in C-N-C asymmetric stretching mode. (The decreasing rate for the C-N-C asymmetric stretching mode is -0.0209 cm-1 K-1.) Because the C-N-C asymmetric stretching mode is not affected by a change in the interaction between the water molecules and the polymer chains, the results suggest that the polymer network does not change at around 310 K. The C-N stretching mode and C-C stretching mode also do not abruptly shift at around 310 K. This result suggests that the variations observed in the changing rates of the frequencies and water content at around 310 K resulted from structural changes of the bound water. To confirm the glass transition temperature of the dried glassy PDMAA, we measured the entropy of the sample using DSC. Figure 8 shows the DSC curve of the dried glassy PDMAA with increasing temperature; the glass transition of the dried glassy PDMAA is observed at 325 K. This result also suggests that the variations observed in the changing rates of the energies and water content at around 310 K resulted from structural changes in the bound water. To confirm the homogeneity of the sample, we measured the spatial dependence of the frequency for the CdO stretching mode at 298 K. Figure 9 shows the frequency map of the CdO stretching mode in a region of 200 × 200 µm2. The frequency

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Figure 8. DSC curve of dried glassy PDMAA with increasing temperatures. The dashed line represents the fitting lines.

Figure 9. Frequency map of the CdO stretching mode for the dried glassy PDMAA in a region of 200 × 200 µm2 measured at 298 K.

is 1615.8 ( 0.2 cm-1 and does not depend on the spatial. This result suggests that there is no domain structure in the sample. C. Structural Change of Bound Water. As shown in Figure 4, the vibrational energies of νoop (HB) for the dried glassy PDMAA are different from those of the bulk water. This difference in the frequency is attributed to the structural difference between the bound and bulk water. The frequency of the O-H stretching modes depends on the length of the O-H bond. For instance, significant differences are observed in the frequency between bulk water, amorphous ice, and crystalline ice.38,39 From the difference in νoop (HB), the structural changes of water can be described as follows. The frequencies of νoop (HB) in the dried glassy PDMAA are higher than those of bulk water over the temperature range (Figure 4). The higher frequency shift indicates that the O-H bond of bound water in the dried glassy PDMAA is shorter than that in bulk water, thus indicating that the mean strength of the hydrogen bonds of the water in the dried glassy PDMAA is weaker than those in the bulk water. This result is in agreement with our previous work.18 From the Raman spectra measurements, we showed that most of the residual water in the dried glassy PDMAA exists as bound water and it has a more extended networked structure consisting of weak hydrogen bonds in comparison with that in bulk water.18 The frequencies of νoop (HB) for both the bulk water and the dried glassy PDMAA shift to the higher side with increasing temperature (Figure 4). The higher frequency shifts result from a decrease in the strength and the number of hydrogen bonds due to thermal effects. For the dried glassy PDMAA, evaporation of the residual bound water may also be one of the reasons for the higher frequency shift. When water evaporates from the dried glassy PDMAA, the O-H bond length shrinks as a result of the decrease in attractive interaction with the surrounding water. To investigate the details of the structural changes of the bound water, we analyzed the intensity ratio of the O-H stretching modes (i.e., νf (HB), νip (HB), νoop (HB), νs (NHB), and νas (NHB)). The water structures can be analyzed by the intensity ratio of the five peaks.17,27,35,40 Under the assumption that the polarizability of water molecules does not change, the intensity

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Figure 10. Temperature dependence of the ratio of integrated intensities of vibrational modes from hydrogen-bonded water, IHB, and those from non-hydrogen-bonded water, INHB, (IHB/INHB). The solid and open circles represent the value for dried PDMAA and bulk water, respectively. The solid line indicates the position of 310 K.

ratio of the five peaks can be considered to be a measure of number density of two types of water structures: (1) the water molecules with four hydrogen bonds and (2) the weakly or nonhydrogen-bonded water molecules. Figure 10 shows the ratio of the integrated intensities of the vibrational modes from the hydrogen-bonded water (νf HB, νip HB, and νoop HB), IHB, and those from the non-hydrogen-bonded water (νs NHB and νas NHB), INHB, for the dried glassy PDMAA (solid circles) and bulk water (open circles). It can be observed that the IHB/INHB values of the dried glassy PDMAA are larger than those of the bulk water over the temperature range. This indicates that the bound water in the dried glassy PDMAA has a network structure more extensive than that of bulk water. Further, the rate of change of the IHB/INHB value with temperature for the dried glassy PDMAA is larger than that of the bulk water. This suggests that the structure of the bound water is more temperature sensitive than that of the bulk water. As shown in Figure 10, the IHB/INHB value of the bulk water decreases linearly with increasing temperatures above 310 K. The rate of change of the IHB/INHB value of the bulk water is -0.0159 cm-1 K-1. The components of IHB are associated with a tetragonal structure consisting of four hydrogen bonds. Therefore, the decrease in the IHB/INHB value indicates a decrease in the amount of tetragonal structure resulting from weakening or breaking of hydrogen bonds as the temperature increases. This result is in agreement with the result of previous studies.28 For the dried glassy PDMAA, the IHB/INHB value increases with increasing temperature below 310 K, and it begins to decrease at temperatures above 310 K. The changing rates of the IHB/INHB value of the dried glassy PDMAA below and above 310 K are 0.721 and -0.233 cm-1 K-1, respectively. This indicates that the temperature dependence of structural changes of the bound water significantly differs from that of bulk water. Using dielectric spectrum measurements, Miura et al.21 showed that bound water in an aqueous solution of albumin can be classified as being in two different states: one involving a shell layer around the polymer chains, and the other involving water directly attached to the polymer chain. According to this model, bound water can be described as illustrated in Figure 11. The components of IHB and INHB are associated with a tetragonal structure consisting of four hydrogen bonds (the type (a) structure in Figure 11) and nontetragonal structure consisting of weak hydrogen bonds (the type (b) structure in Figure 11), respectively. Thus, the increase in IHB/INHB shows that the rate of increase in the amount of type (a) structure in the residual water of the dried glassy PDMAA increases; that is, the water molecules that have type (b) structures evaporate. Above 310 K, the IHB/INHB value begins to decrease, indicating that the water molecules with type (a) structure begin to evaporate. As shown in Figure 7c, about 84% of the bound water evaporates

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Figure 11. Schematic illustration of structural changes of bound water in dried glassy PDMAA.

at temperatures below 310 K. Thus, we conclude that the relative amounts of the type (a) and (b) structures in the bound water change at around 310 K. The phenomena observed in the present study are similar to those of the LCST-type phase transition. Okada and Tanaka24 theoretically explained that the LSCT-type phase transition of PNIPA solution at around 306 K is induced by cooperative hydration or dehydration of water molecules that are bonded to amide groups of the PNIPA. Although the PDMAA does not undergo the LCST-type phase transition, a significant structural change of the bound water is observed in the present study at 310 K. These results suggest that the combination of the state of bound water and chemical structure of polymer might be important factors governing the function of hydrated polymer systems. V. Conclusions We used Raman spectroscopy to investigate the mechanism of structural changes of bound water in dried glassy PDMAA in the temperature range 286.1-329.7 K. In dried glassy PDMAA, most of the residual water exists as bound water. Therefore, the spectra observed in the present study are associated with structural changes of the bound water. The results showed that structure changes at around 310 K. Further, it was found that the evaporation rate of the bound water also changes at the same temperature. This suggests that two types of bound water exist in the dried glassy PDMAA. At temperatures below 310 K, the water that forms a shell layer around the polymer chains evaporates, while at temperatures above 310 K the water that is bound to the polar groups of polymer chains begins to evaporate. Although the PDMAA solution does not show any LCST behavior, structural change of the bound water is observed in the present study at 310 K. Alphonse et al.42 showed that bulk water has a minimum in the heat capacity at ca. 308 K. This indicates a weak continuous phase transition of water exists at this temperature. The temperature is close to 310 K. Therefore, the transition of water might have implications for understanding the structural change of the bound water. These results suggest that structural changes of the bound water and the chemical structure of the polymer must be considered to understand the nature of hydrated polymer systems. To further investigate this behavior in other kinds of polymers, experiments are planned on the structural change of bound water in dried glassy PNIPA. Details will be reported in a forthcoming paper. Acknowledgment. The authors would like to thank Naohiro Ikeda and Satoshi Morikubo for helpful assistance. References and Notes (1) Otting, G.; Liepinsh, E.; Wuthrich, K. Science 1991, 254, 974. (2) Nakasako, M. J. Mol. Biol. 1999, 289, 547.

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