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Bound states of water in poly(vinyl alcohol) hydrogel prepared by repeated freezing and melting method
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Journal of Molecular Structure 875 (2008) 282–287 www.elsevier.com/locate/molstruc
Bound states of water in poly(vinyl alcohol) hydrogel prepared by repeated freezing and melting method Takahiko Nakaoki *, Hiroyuki Yamashita Department of Materials Chemistry, Innovative Materials and Processing Research Center, Ryukoku University, Seta, Otsu 520-2194, Japan Received 17 November 2006; received in revised form 28 April 2007; accepted 30 April 2007 Available online 8 May 2007
Abstract Bound state of water in poly(vinyl alcohol) (PVA) hydrogel produced by repeated freezing and melting method was investigated by thermal analysis and Raman spectroscopy. Water frozen in PVA hydrogel showed the lowering of the melting temperature depending on PVA concentration. This water can be treated as a supercooled water, of which the melting enthalpy is different from that of normal water frozen at 0 °C. Accurate melting enthalpies of supercooled water estimated by the difference of thermal capacity of supercooled water and ice allowed us to determine the weight content of supercooled bound water. The weight content of supercooled water decreased with increasing PVA concentration, and alternatively that of non-freezable water increased. The maximum content of nonfreezable water was 31.4 wt% for the 50 wt% PVA gel. The pore size filled with water was successfully estimated by thermodynamical equation. The diameter is 30.2 nm for 10 wt% gel, which is comparable to that derived from other experimental methods. Raman spectroscopy was used to investigate the hydrogen bond of water. With increasing concentration, the rate of water without hydrogen bond increased. This might be interpreted by the increment of surface area of water as the pore size is reduced. This also supports that the PVA hydrogel takes the pore model filled with water. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Poly(vinyl alcohol); Gel; Raman spectroscopy; Thermal analysis
1. Introduction Poly(vinyl alcohol) (PVA) is a representative water soluble polymer. PVA aqueous solution can form both chemical and physical gels depending on the preparation condition. Chemical gel is formed by cross-linking agent, radiation of c or electron rays, and so on, whereas physical gel is formed by crystal, ion bonding and so on. As for the molecular structure of PVA physical gel, Kaji et al. carried out the neutron scattering measurements for the PVA gel in mixed solvents of dimethylsulfoxide (DMSO) and water [1–7]. They showed that the small crystallites were constructed in the gel at the cross linking point. The average size and the distance between crystallites were estimated about 7 nm and 15–20 nm, respectively. *
Corresponding author. Tel.: +81 77 543 7661; fax: +81 77 543 7483. E-mail address: [email protected] (T. Nakaoki).
0022-2860/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2007.04.040
One of the important methods to make PVA physical gel is a repeated freezing and melting method, which is first reported by Peppas [8]. This hydrogel is featured by good mechanical property, elasticity and so on. Willcox et al. reported the microstructure of PVA hydrogel produced by this cycling method [9]. A low degree of crystallinity was induced during the first cycle with 3–8 nm of crystallite which is separated by mesh structure with the averaging space of 30 nm. The cryogenic transmission electron microscopy (cryo-TEM) provided the direct observation on pore structure of PVA hydrogel. The photograph showed rounded pore morphology with a diameter of 30 nm. Ricciardi et al. reported a quantitative result on phase structure [10–12]. They showed that the degree of crystallinity is in the order of 2–6% depending on the repeating time of freezing and melting. From the X-ray diffraction profile, the phase structure of PVA hydrogel was classified into three components of free water, crystalline
T. Nakaoki, H. Yamashita / Journal of Molecular Structure 875 (2008) 282–287
phase of PVA, and swollen amorphous phase including water. They concluded that the PVA hydrogel takes porous structure mainly occupied by water. In order to investigate the bound state of water, differential scanning calorimetry (DSC) is a powerful tool. Analysis on freezing and melting behavior of water in gel has been carried out for lots of hydrogels such as PVA [12–16], polysaccharides [17], and so on. The supercooled water frozen in PVA gel was investigated by Higuchi and Iijima [14]. They compared the difference of freezing and melting enthalpies during cooling and heating processes, respectively. Raman spectroscopy is a suitable technique for studying hydrogen bond of water. Numerous investigations have been reported on the structural properties of water and aqueous solutions. Recently Rull focused on the OH/OD vibrations of water in mixtures of D2O/H2O by Raman spectroscopy [18]. The O–D stretching mode is composed of three bands at 2520, 2587, and 2666 cm1 assumed to linear, bifurcated, and almost-free O–D modes, respectively. An additional band was observed at 2370 cm1 assignable to a mixture of three different modes as well as Fermi resonance effects. These results would be applied to analyze the bound state of water in PVA hydrogel. In this study, we apply the thermodynamical equation to the melting behavior of water frozen in PVA hydrogel. The weight content of free, freezable bound, and non-freezable water and the domain size of water in the PVA network were estimated by the thermodynamical equations. In addition the investigation on hydrogen bond was carried out by Raman spectroscopy. 2. Experimental section 2.1. Material
283
min heating rate under a flowing N2 atmosphere. Temperature and melting enthalpy were calibrated by indium and chloroform. Gel was closely sealed in an aluminum pan to prevent the evaporation of water. 2.4. Raman measurement Raman spectrum was observed by a JASCO NRS-2100 Laser Raman spectrometer. The measurements were carried out at room temperature. The 514.5 nm of argon ion laser was used for measurements. The output laser power was 100 mW. 3. Results and discussion 3.1. Melting behavior of water frozen in PVA gel Fig. 1 shows the DSC chart of the melting behavior of water frozen in PVA hydrogel condensed from 10 wt% to 30–90 wt%. A main endothermic peak shifted to lower temperature depending on concentration. In addition, a small peak was observed almost the same temperature around 0 °C. In Fig. 2 plotted the onset temperature of endothermic peak as a function of concentration. Since the temperature of the small peak is irrespective to polymer concentration and identified with that of pure water, this peak corresponds to the free water in the gel. The main endothermic peak which shifts to low temperature is regarded as freezable bound water. Following with the previous reports [9–12], we can postulate the pore model filled with water for PVA hydrogel. Therefore the low temperature shift can be interpreted by the supercooling effect based on thermodynamics. The enthalpy DH of supercooled water with melting temperature T is provided by the following equation:
PVA was purchased from Wako pure chemical Co. Ltd. The degree of polymerization is about 2000 and the degree of hydrolysis is 98%. The stereoregularity was determined to be mm = 0.20, mr = 0.51, and rr = 0.29 by 1H NMR measurement. 90wt%
2.2. Preparation of PVA hydrogel
2.3. DSC measurement DSC measurement was performed by using a Rigaku Thermo Flex TAS 300 DSC 8230D with scans at 5 °C/
Endo.
80wt%
PVA/water gel was prepared by the following method proposed by Peppas [8]. First PVA was dissolved in water at 120 °C for 2 h in autoclave. This homogeneous solution was quenched into the thermostat at 10 °C and kept it for 20 h. Then it was held in thermostat at 25 °C for 4 h. This process was repeated twice in order to promote gelation. Condensed gels were prepared by evaporating water at room temperature. The initial concentration was 10 wt%, and then concentrated to 30–90 wt%.
75wt% 70wt% 60wt% 50wt% 40wt% 30wt%
-50
-25
0
25
50
Temperature / oC Fig. 1. DSC curves of melting behavior of water frozen in PVA hydrogel condensed from 10 wt% to desired concentration.
284
T. Nakaoki, H. Yamashita / Journal of Molecular Structure 875 (2008) 282–287
cooled water makes it possible to determine the content of freezable bound water. In Fig. 3 plotted the experimentally obtained DH, which is much smaller than the theoretical one, because the experimental value is divided by the total weight of water including both freezable and nonfreezable water. So the weight content of freezable bound water (wB) can be estimated by the following equation:
5 0
o
Temperature / C
-5 -10 -15
DH obs B ¼ wB DH ðT Þ
-20
where DH obs B is an experimental value of supercooled bound water. Similarly the weight content of free water (wF) is also estimated by the following equation:
-25 -30 -35
ð3Þ
DH obs F ¼ wF DH ð273Þ 0
20
40
60
80
100
Concentration / wt% Fig. 2. Concentration dependence of melting temperature of water frozen in the PVA hydrogel. h, normal water; and s, supercooled water.
DH ðT Þ ¼ DH ð273Þ
Z
273
ð1Þ
DC p dT T
where DH(273) is a melting enthalpy of normal water, and DCp is the difference of heat capacity between supercooled water and ice [19]. This DH(T) (J/g) is approximately expressed as the equation [20,21] DH ðT Þ ¼ 334:1 þ 2:119 DT 0:00783 DT 2
ð4Þ
DH obs F
ð2Þ
where DT is the temperature difference between melting temperature of supercooled water (T) and freezing point of normal water (T273). Fig. 3 shows the DH(T) of supercooled water calculated by Eq. (2). The DH(T) was reduced with lowering the supercooled temperature. For example the DH(T) is reduced around 20% when the supercooled temperature is 20 °C. The accurate DH(T) for super-
where is an experimental value of free water. The rest of water can be assumed to the weight content of non-freezable water (wN) as follows: wN ¼ 1 wS wF :
ð5Þ
Fig. 4 plotted each water contents against the whole water as a function of polymer concentration. The relative ratio of freezable bound water decreased with increasing concentration. Alternatively, the relative ratio of non-freezable water became larger, and it became a major component more than 40 wt%. The critical concentration to be only non-freezable water was 86.4 wt%. Hatakeyama et al. reported that the content of non-freezable water for the 9.1 wt% PVA hydrogel is around 23% against the whole water [17]. In our case, it took 14.6% which is almost identical with their result. If we do not use the supercooled DH, that value would be much smaller. So the use of supercooled DH provided more accurate result. In order to make clear the amount of water in one gram of gel, these water contents were re-calculated against the whole gel and plotted in Fig. 5. For low concentration gel, the content of non-freezable water is as low as 13.1 wt%. Perhaps the small amount of polymer chain is
350 100
Content in the whole water / %
300
Enthalpy / J/g
250 200 150 100 50
80
60
40
20
0 0
10
20
30
40
ΔT/K Fig. 3. Melting enthalpy of supercooled water as a function of supercooled temperature. Closed circles and open squares showed the calculated value by Eq. (2) and the experimental value of freezable bound water in the PVA hydrogel, respectively.
0 0
20
40
60
80
100
PVA concentration / wt% Fig. 4. Concentration dependence on the weight content of water against the whole water. s, free water (wF); h, freezable bound water (wB); and n, non-freezable water (wN).
80
16
60
12
Pore radius / nm
Content the whole water /%
T. Nakaoki, H. Yamashita / Journal of Molecular Structure 875 (2008) 282–287
40
285
8
4 20
0 0 0
0 20
40
60
80
PVA concentration / wt%
not enough to trap water molecules to construct non-freezable water. The maximum content of non-freezable water was 31.4 wt% for 50 wt% gel, and then it decreased with polymer concentration because the total amount of water is decreasing. Ricciardi et al. reported the quantitative phase structure by X-ray diffraction pattern [10,11]. The PVA hydrogel consists of three components; crystalline, pure water, and intermediate phase consisting of swollen PVA chains and water. Perhaps water in the intermediate phase would correspond to non-freezable water because water molecules are located in the entangled PVA chains. They estimated that this phase takes 13.4% for 11.98 wt% gel. This is in good agreement with our result that the non-freezable water includes 13.1 wt% for the 10 wt% PVA gel. It was concluded that more accurate water content can be estimated by taking into consideration of the reduced DH(T) due to supercooling effect. 3.2. Relationship between domain size of water and pore size of PVA gel A theory to determine the pore size in porous material has been derived from a thermodynamical relationship during freezing and melting processes of water. Ishikiriyama et al. reported the relationship between pore radius in silica gel and supercooled temperature of water [21,22]. The pore radius of freezable water r is expressed as aðT Þ DT
40
60
80
100
Fig. 6. Pore radius of water calculated by Eq. (6).
Fig. 5. Concentration dependence on the weight content of water against the whole gel. s, free water (wF); h, freezable bound water (wB); and n, non-freezable water (wN).
r¼
20
Concentration / wt%
100
ð6Þ
where a(T) is a function of pore shape and supercooled temperature. Assuming a spherical pore shape, a(T) was estimated to be 33.41–0.0959DT for heating process [21]. In Fig. 6 plotted the pore radius r for spherical models as
a function of DT. Following to the structural model that the PVA gel has a porous structure filled with water, this relationship can be applied to the PVA hydrogel. The pore radius of 10 wt% PVA gel takes 15.1 nm. For higher concentration gel, the pore radius became as small as a few nanometers. Willcox et al. reported the quantitative result on pore size of PVA hydrogel by cryo-TEM and X-ray diffraction measurements. The cryo-TEM measurement showed the mesh structure with rounded about 30 nm, and the distance between crystallites was estimated to be around 20–35 nm by X-ray diffraction measurement [9]. These values correspond to the pore size filled with water in PVA hydrogel. In our result, the diameter of 10 wt% gel is around 30.2 nm. This is comparable with the result estimated by other techniques. Therefore our results support the porous model filled with water. Since the supercooled water is occupied in the porous structure, there is little interaction with PVA chain except surface area. This allowed us to deal as coagulated water in the closed space. 3.3. O–H stretching mode by Raman spectra The O–H stretching mode in Raman spectrum is sensitive to hydrogen bond. In case of PVA hydrogel, the O– H stretching modes due to PVA and water are observed in the same region. In order to distinguish these vibrational modes, deuterium oxide (D2O) was used instead of normal water. As a result, the O–H stretching mode for PVA is observed around 3500 cm1, whereas the O–D mode for D2O is observed around 2500 cm1. Fig. 7 showed Raman spectra of O–D stretching mode of D2O in PVA/D2O gel concentrated from 10 wt% gel. For higher concentration gel, the band at 2710 cm1 was clearly isolated from the main peak. The O–D mode of water consists of four components depending on hydrogen bond [18]. The O–D modes were assignable to a multiple hydrogen bond, a linear hydrogen bond in a tetrahedral-like configuration, a bifurcated hydrogen bond, and very weak or non-hydrogen
286
T. Nakaoki, H. Yamashita / Journal of Molecular Structure 875 (2008) 282–287 90 90wt% 80
Intensity ratio, %
80wt%
Int.
70wt% 60wt% 50wt%
70
20
40wt% 10 30wt%
2900
2700
2500
Wavenumber /
2300
0
2100
Int.
2600
2400
20
40
60
80
100
PVA concentration, wt%
Fig. 7. Raman spectra of O–D stretching mode for D2O.
2800
0
cm-1
2200
Wavenumber / cm -1 Fig. 8. Curve fitting analysis of O–D stretching mode of D2O in 30 wt% PVA/D2O gel. The band at 2400, 2505, 2600, and 2710 cm1 are assignable to the O–D stretching mode with multi, linear, bifurcated, and non-hydrogen bond, respectively.
bond. The typical curve fitting analysis for the spectrum of 30 wt% gel is shown in Fig. 8. Following the previous report, the observed O–D mode was separated to the peaks at 2400, 2505, 2600, and 2710 cm1 by curve fitting analysis that are assignable to multiple, linear, bifurcated, and nonhydrogen bond, respectively. The intensity ratio was plotted in Fig. 9. The main band due to a linear hydrogen bond at 2505 cm1 and the band for multiple hydrogen bond at 2400 cm1 decreased in intensity, while the band due to very weak or non-hydrogen bonded mode at 2710 cm1 increased. Judging from the result by DSC measurements, the spectral change of O–D stretching mode would be arisen from the increment of non-freezable water or the reduced size of freezable bound water. Since non-freezable water would be located in the swollen PVA network, some interactions are predicted between water and PVA. The interaction with PVA chain is difficult to interpret because of complex hydrogen bond, which includes not only hydro-
Fig. 9. Intensity ratio of almost-free (,), linear (h), bifurcated (n), and multiple (s) modes for D2O.
gen bond between water and PVA but also inter- and intrachain hydrogen bond due to PVA crystal. Since these interactions correspond to make the hydrogen bond stronger, the increment of non-hydrogen bond for D2O is impossible to be explained by these effects. The second possibility is the reduced size of freezable bound water. A small pore makes the pore surface larger. Perhaps the water molecules at pore surface would be comparatively isolated from other water molecules. Therefore the increment of non-hydrogen bond for D2O might be assumed to the water molecules on the pore surface. This also support the pore model filled with water.
4. Conclusions The supercooled bound water frozen in PVA hydrogel was investigated by thermal analysis, and hydrogen bond of water was studied by Raman spectroscopy. The melting temperature of water frozen in the PVA gel was reduced with increasing PVA concentration. This low temperature shift can be interpreted by the melting of supercooled water. The melting enthalpy of supercooled water is reduced by thermodynamical effect based on the difference of heat capacity between ice and supercooled water. The weight content of freezable bound water was estimated by DH(T) of supercooled water. With increasing PVA concentration, the content of freezable bound water decreased whereas that of non-freezable water increased. Over 86.4 wt%, there exists only non-freezable water. From the view of the weight content against whole gel, the content of non-freezable water provided the maximum at 31.4 wt% for 50 wt% PVA gel. The pore size filled with water in PVA gel was estimated by following the thermodynamical equation derived by Ishikiriyama et al. The pore diameter is 30.2 nm for 10 wt% gel, and decreased with PVA concentration. This pore size is in good agreement
T. Nakaoki, H. Yamashita / Journal of Molecular Structure 875 (2008) 282–287
with the value obtained by other experimental methods such as cryo-TEM and X-ray diffraction measurements. The hydroxyl group of water is sensitive to hydrogen bond, which is detectable by Raman spectroscopy. In order to distinguish the water and PVA, D2O was used as a solvent. The O–D stretching mode of D2O with non-hydrogen bond mode is increased with increasing concentration. Since the pore size decreased with concentration as shown by DSC measurement, the surface area of water in the pore increased. This allowed the content of D2O without hydrogen bond larger. These DSC and Raman measurements supported that the PVA hydrogel takes the pore model filled with water. Acknowledgements This work was partially supported by a grant from High-Tech Research Center Program for private universities from the Japan Ministry of Education, Culture, Sports, Science and Technology. References [1] T. Kanaya, M. Ohkura, K. Kaji, M. Furusaka, M. Misawa, Macromolecules 5609 (1994) 27. [2] H. Takeshita, T. Kanaya, K. Nishida, K. Kaji, Macromolecules 7815 (1999) 32. [3] T. Kanaya, M. Ohkura, H. Takeshita, K. Kaji, M. Furusaka, H. Yamaoka, G.D. Wignall, Macromolecules 3168 (1995) 28.
287
[4] T. Kanaya, H. Takeshita, Y. Nishikoji, M. Ohkura, K. Nishida, K. Kaji, Supramol. Sci. 215 (1998) 5. [5] H. Takeshita, T. Kanaya, K. Nishida, K. Kaji, T. Takahashi, M. Hashimoto, Phys. Rev. E 2125 (2000) 61. [6] H. Takeshita, T. Kanaya, K. Nishida, K. Kaji, Macromolecules 7894 (2001) 34. [7] H. Takeshita, T. Kanaya, K. Nishida, K. Kaji, Phys. B 78 (2002) 311. [8] N.A. Peppas, Makromol. Chem. 3433 (1975) 176. [9] P.J. Willcox, D.W. Howie, K. Shimidt-Rohr, D.A. Hoagland, S.P. Gido, S. Pudjijanto, L.W. Kleiner, S. Venkatraman, J. Polym. Sci.: Part B: Polym. Phys. 3438 (1999) 37. [10] R. Ricciardi, F. Auriemma, C. De Rosa, F. Laupretre, Macromolecules 1921 (2004) 37. [11] R. Ricciardi, F. Auriemma, C. Gaillet, C. De Rosa, F. Laupretre, Macromolecules 9510 (2004) 37. [12] R. Ricciardi, G. Mangiapia, F. Lo Celso, L. Paduano, R. Triolo, F. Auriemma, C. De Rosa, F. Laupretre, Chem. Mater. 1183 (2005) 17. [13] T. Hatakeyama, A. Yamauchi, H. Hatakeyama, Eur. Polym. J. 61 (1984) 20. [14] A. Higuchi, T. Iijima, Polymer 1207 (1985) 26. [15] T. Hatakeyama, A. Yamauchi, H. Hatakeyama, Eur. Polym. J. 361 (1987) 23. [16] W. Li, F. Xue, R. Cheug, Polymer 12026 (2005) 46. [17] T. Hatakeyama, F.X. Quinn, H. Hatakeyama, Carbohydr. Polym. 155 (1996) 30. [18] F. Rull, Pure Appl. Chem. 1859 (2002) 74. [19] C.A. Angell, J. Shuppert, C. Tucker, J. Phys. Chem. 3092 (1973) 77. [20] D.A. Randall, International Critical Tables V–VII, McGraw-Hill, New York, 1930. [21] K. Ishikiriyama, M. Todoki, K. Motomura, J. Colloid Interface Sci. 92 (1995) 171. [22] K. Ishikiriyama, M. Todoki, J. Colloid Interface Sci. 103 (1995) 171.
Journal of Molecular Structure 875 (2008) 282–287 www.elsevier.com/locate/molstruc
Bound states of water in poly(vinyl alcohol) hydrogel prepared by repeated freezing and melting method Takahiko Nakaoki *, Hiroyuki Yamashita Department of Materials Chemistry, Innovative Materials and Processing Research Center, Ryukoku University, Seta, Otsu 520-2194, Japan Received 17 November 2006; received in revised form 28 April 2007; accepted 30 April 2007 Available online 8 May 2007
Abstract Bound state of water in poly(vinyl alcohol) (PVA) hydrogel produced by repeated freezing and melting method was investigated by thermal analysis and Raman spectroscopy. Water frozen in PVA hydrogel showed the lowering of the melting temperature depending on PVA concentration. This water can be treated as a supercooled water, of which the melting enthalpy is different from that of normal water frozen at 0 °C. Accurate melting enthalpies of supercooled water estimated by the difference of thermal capacity of supercooled water and ice allowed us to determine the weight content of supercooled bound water. The weight content of supercooled water decreased with increasing PVA concentration, and alternatively that of non-freezable water increased. The maximum content of nonfreezable water was 31.4 wt% for the 50 wt% PVA gel. The pore size filled with water was successfully estimated by thermodynamical equation. The diameter is 30.2 nm for 10 wt% gel, which is comparable to that derived from other experimental methods. Raman spectroscopy was used to investigate the hydrogen bond of water. With increasing concentration, the rate of water without hydrogen bond increased. This might be interpreted by the increment of surface area of water as the pore size is reduced. This also supports that the PVA hydrogel takes the pore model filled with water. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Poly(vinyl alcohol); Gel; Raman spectroscopy; Thermal analysis
1. Introduction Poly(vinyl alcohol) (PVA) is a representative water soluble polymer. PVA aqueous solution can form both chemical and physical gels depending on the preparation condition. Chemical gel is formed by cross-linking agent, radiation of c or electron rays, and so on, whereas physical gel is formed by crystal, ion bonding and so on. As for the molecular structure of PVA physical gel, Kaji et al. carried out the neutron scattering measurements for the PVA gel in mixed solvents of dimethylsulfoxide (DMSO) and water [1–7]. They showed that the small crystallites were constructed in the gel at the cross linking point. The average size and the distance between crystallites were estimated about 7 nm and 15–20 nm, respectively. *
Corresponding author. Tel.: +81 77 543 7661; fax: +81 77 543 7483. E-mail address: [email protected] (T. Nakaoki).
0022-2860/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2007.04.040
One of the important methods to make PVA physical gel is a repeated freezing and melting method, which is first reported by Peppas [8]. This hydrogel is featured by good mechanical property, elasticity and so on. Willcox et al. reported the microstructure of PVA hydrogel produced by this cycling method [9]. A low degree of crystallinity was induced during the first cycle with 3–8 nm of crystallite which is separated by mesh structure with the averaging space of 30 nm. The cryogenic transmission electron microscopy (cryo-TEM) provided the direct observation on pore structure of PVA hydrogel. The photograph showed rounded pore morphology with a diameter of 30 nm. Ricciardi et al. reported a quantitative result on phase structure [10–12]. They showed that the degree of crystallinity is in the order of 2–6% depending on the repeating time of freezing and melting. From the X-ray diffraction profile, the phase structure of PVA hydrogel was classified into three components of free water, crystalline
T. Nakaoki, H. Yamashita / Journal of Molecular Structure 875 (2008) 282–287
phase of PVA, and swollen amorphous phase including water. They concluded that the PVA hydrogel takes porous structure mainly occupied by water. In order to investigate the bound state of water, differential scanning calorimetry (DSC) is a powerful tool. Analysis on freezing and melting behavior of water in gel has been carried out for lots of hydrogels such as PVA [12–16], polysaccharides [17], and so on. The supercooled water frozen in PVA gel was investigated by Higuchi and Iijima [14]. They compared the difference of freezing and melting enthalpies during cooling and heating processes, respectively. Raman spectroscopy is a suitable technique for studying hydrogen bond of water. Numerous investigations have been reported on the structural properties of water and aqueous solutions. Recently Rull focused on the OH/OD vibrations of water in mixtures of D2O/H2O by Raman spectroscopy [18]. The O–D stretching mode is composed of three bands at 2520, 2587, and 2666 cm1 assumed to linear, bifurcated, and almost-free O–D modes, respectively. An additional band was observed at 2370 cm1 assignable to a mixture of three different modes as well as Fermi resonance effects. These results would be applied to analyze the bound state of water in PVA hydrogel. In this study, we apply the thermodynamical equation to the melting behavior of water frozen in PVA hydrogel. The weight content of free, freezable bound, and non-freezable water and the domain size of water in the PVA network were estimated by the thermodynamical equations. In addition the investigation on hydrogen bond was carried out by Raman spectroscopy. 2. Experimental section 2.1. Material
283
min heating rate under a flowing N2 atmosphere. Temperature and melting enthalpy were calibrated by indium and chloroform. Gel was closely sealed in an aluminum pan to prevent the evaporation of water. 2.4. Raman measurement Raman spectrum was observed by a JASCO NRS-2100 Laser Raman spectrometer. The measurements were carried out at room temperature. The 514.5 nm of argon ion laser was used for measurements. The output laser power was 100 mW. 3. Results and discussion 3.1. Melting behavior of water frozen in PVA gel Fig. 1 shows the DSC chart of the melting behavior of water frozen in PVA hydrogel condensed from 10 wt% to 30–90 wt%. A main endothermic peak shifted to lower temperature depending on concentration. In addition, a small peak was observed almost the same temperature around 0 °C. In Fig. 2 plotted the onset temperature of endothermic peak as a function of concentration. Since the temperature of the small peak is irrespective to polymer concentration and identified with that of pure water, this peak corresponds to the free water in the gel. The main endothermic peak which shifts to low temperature is regarded as freezable bound water. Following with the previous reports [9–12], we can postulate the pore model filled with water for PVA hydrogel. Therefore the low temperature shift can be interpreted by the supercooling effect based on thermodynamics. The enthalpy DH of supercooled water with melting temperature T is provided by the following equation:
PVA was purchased from Wako pure chemical Co. Ltd. The degree of polymerization is about 2000 and the degree of hydrolysis is 98%. The stereoregularity was determined to be mm = 0.20, mr = 0.51, and rr = 0.29 by 1H NMR measurement. 90wt%
2.2. Preparation of PVA hydrogel
2.3. DSC measurement DSC measurement was performed by using a Rigaku Thermo Flex TAS 300 DSC 8230D with scans at 5 °C/
Endo.
80wt%
PVA/water gel was prepared by the following method proposed by Peppas [8]. First PVA was dissolved in water at 120 °C for 2 h in autoclave. This homogeneous solution was quenched into the thermostat at 10 °C and kept it for 20 h. Then it was held in thermostat at 25 °C for 4 h. This process was repeated twice in order to promote gelation. Condensed gels were prepared by evaporating water at room temperature. The initial concentration was 10 wt%, and then concentrated to 30–90 wt%.
75wt% 70wt% 60wt% 50wt% 40wt% 30wt%
-50
-25
0
25
50
Temperature / oC Fig. 1. DSC curves of melting behavior of water frozen in PVA hydrogel condensed from 10 wt% to desired concentration.
284
T. Nakaoki, H. Yamashita / Journal of Molecular Structure 875 (2008) 282–287
cooled water makes it possible to determine the content of freezable bound water. In Fig. 3 plotted the experimentally obtained DH, which is much smaller than the theoretical one, because the experimental value is divided by the total weight of water including both freezable and nonfreezable water. So the weight content of freezable bound water (wB) can be estimated by the following equation:
5 0
o
Temperature / C
-5 -10 -15
DH obs B ¼ wB DH ðT Þ
-20
where DH obs B is an experimental value of supercooled bound water. Similarly the weight content of free water (wF) is also estimated by the following equation:
-25 -30 -35
ð3Þ
DH obs F ¼ wF DH ð273Þ 0
20
40
60
80
100
Concentration / wt% Fig. 2. Concentration dependence of melting temperature of water frozen in the PVA hydrogel. h, normal water; and s, supercooled water.
DH ðT Þ ¼ DH ð273Þ
Z
273
ð1Þ
DC p dT T
where DH(273) is a melting enthalpy of normal water, and DCp is the difference of heat capacity between supercooled water and ice [19]. This DH(T) (J/g) is approximately expressed as the equation [20,21] DH ðT Þ ¼ 334:1 þ 2:119 DT 0:00783 DT 2
ð4Þ
DH obs F
ð2Þ
where DT is the temperature difference between melting temperature of supercooled water (T) and freezing point of normal water (T273). Fig. 3 shows the DH(T) of supercooled water calculated by Eq. (2). The DH(T) was reduced with lowering the supercooled temperature. For example the DH(T) is reduced around 20% when the supercooled temperature is 20 °C. The accurate DH(T) for super-
where is an experimental value of free water. The rest of water can be assumed to the weight content of non-freezable water (wN) as follows: wN ¼ 1 wS wF :
ð5Þ
Fig. 4 plotted each water contents against the whole water as a function of polymer concentration. The relative ratio of freezable bound water decreased with increasing concentration. Alternatively, the relative ratio of non-freezable water became larger, and it became a major component more than 40 wt%. The critical concentration to be only non-freezable water was 86.4 wt%. Hatakeyama et al. reported that the content of non-freezable water for the 9.1 wt% PVA hydrogel is around 23% against the whole water [17]. In our case, it took 14.6% which is almost identical with their result. If we do not use the supercooled DH, that value would be much smaller. So the use of supercooled DH provided more accurate result. In order to make clear the amount of water in one gram of gel, these water contents were re-calculated against the whole gel and plotted in Fig. 5. For low concentration gel, the content of non-freezable water is as low as 13.1 wt%. Perhaps the small amount of polymer chain is
350 100
Content in the whole water / %
300
Enthalpy / J/g
250 200 150 100 50
80
60
40
20
0 0
10
20
30
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ΔT/K Fig. 3. Melting enthalpy of supercooled water as a function of supercooled temperature. Closed circles and open squares showed the calculated value by Eq. (2) and the experimental value of freezable bound water in the PVA hydrogel, respectively.
0 0
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40
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PVA concentration / wt% Fig. 4. Concentration dependence on the weight content of water against the whole water. s, free water (wF); h, freezable bound water (wB); and n, non-freezable water (wN).
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not enough to trap water molecules to construct non-freezable water. The maximum content of non-freezable water was 31.4 wt% for 50 wt% gel, and then it decreased with polymer concentration because the total amount of water is decreasing. Ricciardi et al. reported the quantitative phase structure by X-ray diffraction pattern [10,11]. The PVA hydrogel consists of three components; crystalline, pure water, and intermediate phase consisting of swollen PVA chains and water. Perhaps water in the intermediate phase would correspond to non-freezable water because water molecules are located in the entangled PVA chains. They estimated that this phase takes 13.4% for 11.98 wt% gel. This is in good agreement with our result that the non-freezable water includes 13.1 wt% for the 10 wt% PVA gel. It was concluded that more accurate water content can be estimated by taking into consideration of the reduced DH(T) due to supercooling effect. 3.2. Relationship between domain size of water and pore size of PVA gel A theory to determine the pore size in porous material has been derived from a thermodynamical relationship during freezing and melting processes of water. Ishikiriyama et al. reported the relationship between pore radius in silica gel and supercooled temperature of water [21,22]. The pore radius of freezable water r is expressed as aðT Þ DT
40
60
80
100
Fig. 6. Pore radius of water calculated by Eq. (6).
Fig. 5. Concentration dependence on the weight content of water against the whole gel. s, free water (wF); h, freezable bound water (wB); and n, non-freezable water (wN).
r¼
20
Concentration / wt%
100
ð6Þ
where a(T) is a function of pore shape and supercooled temperature. Assuming a spherical pore shape, a(T) was estimated to be 33.41–0.0959DT for heating process [21]. In Fig. 6 plotted the pore radius r for spherical models as
a function of DT. Following to the structural model that the PVA gel has a porous structure filled with water, this relationship can be applied to the PVA hydrogel. The pore radius of 10 wt% PVA gel takes 15.1 nm. For higher concentration gel, the pore radius became as small as a few nanometers. Willcox et al. reported the quantitative result on pore size of PVA hydrogel by cryo-TEM and X-ray diffraction measurements. The cryo-TEM measurement showed the mesh structure with rounded about 30 nm, and the distance between crystallites was estimated to be around 20–35 nm by X-ray diffraction measurement [9]. These values correspond to the pore size filled with water in PVA hydrogel. In our result, the diameter of 10 wt% gel is around 30.2 nm. This is comparable with the result estimated by other techniques. Therefore our results support the porous model filled with water. Since the supercooled water is occupied in the porous structure, there is little interaction with PVA chain except surface area. This allowed us to deal as coagulated water in the closed space. 3.3. O–H stretching mode by Raman spectra The O–H stretching mode in Raman spectrum is sensitive to hydrogen bond. In case of PVA hydrogel, the O– H stretching modes due to PVA and water are observed in the same region. In order to distinguish these vibrational modes, deuterium oxide (D2O) was used instead of normal water. As a result, the O–H stretching mode for PVA is observed around 3500 cm1, whereas the O–D mode for D2O is observed around 2500 cm1. Fig. 7 showed Raman spectra of O–D stretching mode of D2O in PVA/D2O gel concentrated from 10 wt% gel. For higher concentration gel, the band at 2710 cm1 was clearly isolated from the main peak. The O–D mode of water consists of four components depending on hydrogen bond [18]. The O–D modes were assignable to a multiple hydrogen bond, a linear hydrogen bond in a tetrahedral-like configuration, a bifurcated hydrogen bond, and very weak or non-hydrogen
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Intensity ratio, %
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Int.
70wt% 60wt% 50wt%
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2700
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Wavenumber /
2300
0
2100
Int.
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2400
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PVA concentration, wt%
Fig. 7. Raman spectra of O–D stretching mode for D2O.
2800
0
cm-1
2200
Wavenumber / cm -1 Fig. 8. Curve fitting analysis of O–D stretching mode of D2O in 30 wt% PVA/D2O gel. The band at 2400, 2505, 2600, and 2710 cm1 are assignable to the O–D stretching mode with multi, linear, bifurcated, and non-hydrogen bond, respectively.
bond. The typical curve fitting analysis for the spectrum of 30 wt% gel is shown in Fig. 8. Following the previous report, the observed O–D mode was separated to the peaks at 2400, 2505, 2600, and 2710 cm1 by curve fitting analysis that are assignable to multiple, linear, bifurcated, and nonhydrogen bond, respectively. The intensity ratio was plotted in Fig. 9. The main band due to a linear hydrogen bond at 2505 cm1 and the band for multiple hydrogen bond at 2400 cm1 decreased in intensity, while the band due to very weak or non-hydrogen bonded mode at 2710 cm1 increased. Judging from the result by DSC measurements, the spectral change of O–D stretching mode would be arisen from the increment of non-freezable water or the reduced size of freezable bound water. Since non-freezable water would be located in the swollen PVA network, some interactions are predicted between water and PVA. The interaction with PVA chain is difficult to interpret because of complex hydrogen bond, which includes not only hydro-
Fig. 9. Intensity ratio of almost-free (,), linear (h), bifurcated (n), and multiple (s) modes for D2O.
gen bond between water and PVA but also inter- and intrachain hydrogen bond due to PVA crystal. Since these interactions correspond to make the hydrogen bond stronger, the increment of non-hydrogen bond for D2O is impossible to be explained by these effects. The second possibility is the reduced size of freezable bound water. A small pore makes the pore surface larger. Perhaps the water molecules at pore surface would be comparatively isolated from other water molecules. Therefore the increment of non-hydrogen bond for D2O might be assumed to the water molecules on the pore surface. This also support the pore model filled with water.
4. Conclusions The supercooled bound water frozen in PVA hydrogel was investigated by thermal analysis, and hydrogen bond of water was studied by Raman spectroscopy. The melting temperature of water frozen in the PVA gel was reduced with increasing PVA concentration. This low temperature shift can be interpreted by the melting of supercooled water. The melting enthalpy of supercooled water is reduced by thermodynamical effect based on the difference of heat capacity between ice and supercooled water. The weight content of freezable bound water was estimated by DH(T) of supercooled water. With increasing PVA concentration, the content of freezable bound water decreased whereas that of non-freezable water increased. Over 86.4 wt%, there exists only non-freezable water. From the view of the weight content against whole gel, the content of non-freezable water provided the maximum at 31.4 wt% for 50 wt% PVA gel. The pore size filled with water in PVA gel was estimated by following the thermodynamical equation derived by Ishikiriyama et al. The pore diameter is 30.2 nm for 10 wt% gel, and decreased with PVA concentration. This pore size is in good agreement
T. Nakaoki, H. Yamashita / Journal of Molecular Structure 875 (2008) 282–287
with the value obtained by other experimental methods such as cryo-TEM and X-ray diffraction measurements. The hydroxyl group of water is sensitive to hydrogen bond, which is detectable by Raman spectroscopy. In order to distinguish the water and PVA, D2O was used as a solvent. The O–D stretching mode of D2O with non-hydrogen bond mode is increased with increasing concentration. Since the pore size decreased with concentration as shown by DSC measurement, the surface area of water in the pore increased. This allowed the content of D2O without hydrogen bond larger. These DSC and Raman measurements supported that the PVA hydrogel takes the pore model filled with water. Acknowledgements This work was partially supported by a grant from High-Tech Research Center Program for private universities from the Japan Ministry of Education, Culture, Sports, Science and Technology. References [1] T. Kanaya, M. Ohkura, K. Kaji, M. Furusaka, M. Misawa, Macromolecules 5609 (1994) 27. [2] H. Takeshita, T. Kanaya, K. Nishida, K. Kaji, Macromolecules 7815 (1999) 32. [3] T. Kanaya, M. Ohkura, H. Takeshita, K. Kaji, M. Furusaka, H. Yamaoka, G.D. Wignall, Macromolecules 3168 (1995) 28.
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