Structure of Water Sorbed into Poly (MEA-co-HEMA) Films As

Sapporo 060-0812, Japan; Research & Development Center, Terumo Corporation,. Nakai-Machi, Kanagawa 259-0151, Japan; Toyama Industrial Technology ...
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Langmuir 2003, 19, 429-435

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Structure of Water Sorbed into Poly(MEA-co-HEMA) Films As Examined by ATR-IR Spectroscopy Makoto Ide,† Takayuki Mori,† Ken Ichikawa,† Hiromi Kitano,*,† Masaru Tanaka,‡ Akira Mochizuki,§ Hiroaki Oshiyama,§ and Wataru Mizuno| Department of Chemical and Biochemical Engineering, Toyama University, Toyama 930-8555, Japan; Research Institute for Electronic Science, Hokkaido University, and Japan Science and Technology Corporation, Kita 12 Nishi 6, Kita-ku, Sapporo 060-0812, Japan; Research & Development Center, Terumo Corporation, Nakai-Machi, Kanagawa 259-0151, Japan; Toyama Industrial Technology Center, Takaoka, Toyama 933-0981, Japan Received July 8, 2002. In Final Form: September 16, 2002 The structure of water sorbed into poly(2-methoxyethyl acrylate) (PMEA), poly(2-hydroxyethyl methacrylate) (PHEMA), and their copolymers (p(MEA/HEMA)) was investigated by attenuated total reflection infrared (ATR-IR) spectroscopy. The extinction coefficient of the OH stretching band of sorbed water (OH) was calculated from the band area obtained by IR measurement and the amount of sorbed water obtained by thermogravimetric analysis. When the polymers contacted with water vapor (relative humidity ) ∼55%), the OH values were quite similar in all polymers. On the other hand, when the polymers contacted with liquid water, the OH values were drastically changed by the content of 2-methoxyethyl acrylate (MEA). When the MEA content of the polymers was low (1 s) than the relaxation time of the rotational rearrangement of water molecules in a liquid state (τR ) 10-11 to 10-12 s).5 Therefore, it is expected that the study of the cold-crystallizable water using vibrational spectroscopy with a shorter observation time (10-13 to 10-14 s) than the τR of liquid water will clarify the structure of the cold-crystallizable water.6 In this paper, the state of water in polymer films was investigated using attenuated total reflection infrared spectroscopy (ATR-IR), which detects the water inside of (2) For example: (a) Vargas, R. A.; Zapata, V. H.; Matallana, E.; Vargas, M. A. Electrochim. Acta 2001, 46, 1699. (b) Qu, X.; Wirsen, A.; Albertsson, A.-C. Polymer 2000, 41, 4589. (c) Liu, W. G.; Yao, K. D. Polymer 2001, 42, 3943. (d) Lee, K. Y.; Ha, W. S. Polymer 1999, 40, 4131. (3) (a) Feldstein, M. M.; Shandryuk, G. A.; Kuptsov, S. A.; Plate, N. A. Polymer 2000, 41, 5327. (b) Graham, N. B.; Zulfiqar, M.; Nwachuku, N. E.; Rashid, A. Polymer 1989, 30, 528. (c) Ratto, J.; Hatakeyama, T.; Blumstein, R. B. Polymer 1995, 36, 2915. (d) Nishinari, K.; Watase, M.; Hatakeyama, T. Colloid Polym. Sci. 1997, 275, 1078. (e) Kirsh, Y. E.; Yanul, N. A.; Kalninsh, K. K. Eur. Polym. J. 1999, 35, 305. (4) (a) Ling G. N.; Zhang, Z. L. Physiol. Chem. Phys. 1983, 15, 407. (b) Hatakeyama, H.; Hatakeyama, T. Thermochim. Acta 1995, 308, 3. (5) (a) Eisenberg, D.; Kauzmann, W. The Structure and Properties of Water; Clarendon Press: London, 1969; Chapter 4. (b) Frank, E., Ed. WatersA Comprehesive Treatise; Plenum Press: New York, 1971. (c) Fukuda, M. JCPE J. 2001, 13, 105.

10.1021/la020617p CCC: $25.00 © 2003 American Chemical Society Published on Web 12/20/2002

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Table 1. Characteristics of the Polymers Used

polymer PHEMA PMEA50 PMEA60 PMEA70 PMEA80 PMEA90 PMEA

feed composn copolym composna [MEA content [MEA/ HEMA (mol %)] (mol /mol)] 0:100 50:50 60:40 70:30 80:20 90:10 100:0

0 49 59 69 82 89 100

mol wt 6.4 × 103 b 3.8 × 103 c (1.36)d 5.6 × 103 c (1.30)d 5.5 × 103 c (1.38)d 6.5 × 103 c (1.55)d 6.5 × 103 c (2.09)d 8.6 × 103 c (1.63)d

a Determined by 1H NMR. b Determined by viscometery; viscosity-averaged molecular weight. c Determined by GPC; weightaveraged molecular weight. d Dispersion of polymer.

the polymer matrix.7 Polymers used in this work are poly(2-methoxyethyl acrylate) (PMEA), poly(2-hydroxyethyl methacrylate) (PHEMA), and their copolymers. Both PMEA and PHEMA have been used as biocompatible materials.8 It has been reported by Tanaka et al. that the cold-crystallizable water exists in polymers with a higher MEA content, whereas it does not in the polymers with a lower MEA content.8a We noted this point and investigated the structural properties and the generation of the cold-crystallizable water in the matrix of copolymers. Experiments A. Materials. Poly(2-hydroxyethyl methacrylate) (PHEMA) was prepared by radical polymerization using 2,2′-azobisisobutyronitrile (AIBN) as an initiator in MeOH at 70 °C for 12 h. After evaporation, the product polymer was precipitated in diethyl ether. Poly(2-methoxyethyl acrylate) (PMEA) and 2-methoxyethyl acrylate-2-hydroxyethyl methacrylate copolymers (p(MEA/ HEMA)) with five different compositions were prepared by radical polymerization using AIBN in 1,4-dioxane at 70 °C for 12 h, and the precipitation of the polymer was carried out in hexane. The feed compositions of the monomers for the copolymerization were MEA/HEMA ) 90:10, 80:20, 70:30, 60:40, and 50:50 (mol/mol). The compositions of the copolymers were determined by 1H NMR (CDCl3, DX-400, JEOL, Tokyo, Japan). The molecular weights of PMEA and the copolymers were evaluated by gel permeation chromatography (Waters Model 440; column, GPC K-804L, Shodex; mobile phase, CHCl3; standard samples, polystyrene, Mw ) 1.7 × 103 to 60 × 103, Showa Denko). The molecular weight of PHEMA was evaluated by viscometry (Ubbelohde dilution type viscometer, Kusano, Tokyo, Japan) at 30 °C with MeOH as solvent because PHEMA is not soluble in CHCl3. The MarkHouwink-Sakurada parameters used for the evaluation of the molecular weight of PHEMA were R ) 0.51 and K ) 52.4 × 103 (mL/g) in MeOH at 30 °C.9 The compositions and molecular (6) (a) Maeda, Y.; Kitano, H. Spectrochim. Acta 1995, A51, 2433. (b) Maeda, Y.; Kitano, H. Trends Phys. Chem. 1997, 6, 269. (c) Maeda, Y.; Ide, M.; Kitano, H. J. Mol. Liq. 1999, 80, 149. (d) Ide, M.; Yoshikawa, D.; Maeda, Y.; Kitano, H. Langmuir 1999, 15, 926. (e) Kitano, H.; Ichikawa, K.; Ide, M.; Fukuda, M.; Mizuno, W. Langmuir 2001, 17, 1889. (f) Ichikawa, K.; Mori, T.; Kitano, H.; Fukuda, M.; Mochizuki, A.; Tanaka, M. J. Polym. Sci. B: Polym. Phys. 2001, 39, 2175. (g) Kitano, H.; Ichikawa, K.; Fukuda, M.; Mochizuki, A.; Tanaka, M. J. Colloid Interface Sci. 2001, 242, 133. (7) (a) Thouvenin, M.; Linossier, I.; Sire, O.; Pe´ron, J.-J.; Valle´eRe´hel, K. Macromolecules 2002, 35, 489. (b) Muta, H.; Ishida, K.; Tamaki, E.; Satoh, M. Polymer 2002, 43, 103. (c) Mura, C.; Yarwood, J.; Swart, R.; Hodge, D. Polymer 2001, 42, 4141. (8) (a) Tanaka, M.; Mochizuki, A.; Ishii, N.; Motomura, T.; Hatakeyama, T. Biomacromolecules 2002, 3, 36. (b) Tanaka, M.; Motomura, T.; Ishii, N.; Shimura, K.; Onishi, M.; Mochizuki, A.; Hatakeyama, T. Polym. Int. 2000, 49, 1709. (c) Tanaka, M.; Motomura, T.; Kawada, M.; Anzai, T.; Kasori, Y.; Shiroya, T.; Shimura, K.; Onishi, M.; Mochizuki, A. Biomaterials 2000, 21, 1471. (d) Tanaka, M.; Mochizuki, A.; Motomura, T.; Shimura, K.; Onishi, M.; Okahata, Y. Colloids Surf., A: Physicochem. Eng. Aspects 2001, 193, 145. (e) Tanaka, M.; Mochizuki, A.; Shiroya, T.; Motomura, T.; Shimura, K.; Onishi, M.; Okahata, Y. Colloids Surf., A: Physicochem. Eng. Aspects 2002, 203, 195.

weights of the polymers are compiled in Table 1. Other reagents were commercially available. The quality of water used in all experiments was Milli-Q grade (resistivity > 17 MΩ‚cm). B. Measurements. (a) IR Spectroscopy. Infrared spectra were recorded on a Perkin-Elmer System 2000 FT-IR (Perkin-Elmer Instruments). All spectra were collected in the 4500-2500 cm-1 region with a resolution of 4 cm-1, with 32 scans, and at 25 °C. To observe the water sorbed into the interior of the polymer films, the attenuated total reflection (ATR) method with a ZnSe element (80 mm × 10 mm × 5 mm) was used. The transmission IR provides information on water molecules located both inside and at the surface of the polymer film. On the other hand, the ATR method can exclusively analyze the water existing inside the polymer matrix, if the film is satisfactorily thick in comparison with the penetration depth of the evanescent IR.7 The ATR-IR measurements were carried out according to the following procedure: (1) The polymer films were cast on the ZnSe element from polymer solutions (solvent: CHCl3 for PMEA and the copolymers, and MeOH for PHEMA) under dry N2 and dried for 24 h under vacuum. (2) After the ATR cell was tightly sealed except for the inlet and outlet of dry N2 gas and water vapor or liquid water, the background was repeatedly measured while the dry N2 gas flowed into the cell until the background was equilibrated. (3) The water vapor with a constant relative humidity (RH ) 50 ( 5%), which was prepared by passing air at 1.8 mL/s through a saturated aqueous NaBr solution, was flowed into the ATR cell by an air compressor. The liquid water was injected into the ATR cell with a syringe. Hereafter, the former and the latter methods will be called the “water vapor method” and the “liquid water method”, respectively. (4) The IR spectra of water sorbed into the polymer films were collected at appropriate time intervals after onset of the flowing of water vapor or the injection of liquid water. (5) All of the collected spectra were calibrated by eq 1, because the penetration depth (dp) of evanescent IR depends on the wavelength of the incident light:

absν ) absνmdp(ν)/dp(4500)

(1)

where ν is the wavenumber, absν and absνm are the corrected and measured absorbances at ν, respectively, and dp(ν) and dp(4500) are the penetration depths at ν and 4500 cm-1, respectively. A detailed description of the penetration depth is in the Appendix. The thickness of the polymer films was determined to be ∼10 µm from the diffraction of visible light (MCPD-1100, Otsuka Electronics, Hirakata, Japan). (b) Thermogravimetric Analysis. The number of sorbed water molecules per monomer unit of polymer (Nwater) at equilibrium was measured by a thermogravimetric analyzer (Pyris 1 TGA, Perkin-Elmer Instruments). The TGA measurements were carried out according to the following procedures: (1) The polymer film (∼15 µg) was prepared by casting a polymer solution (solvent: CHCl3 for PMEA and the coplymers, and MeOH for PHEMA) onto an aluminum pan under dry N2. (2) The polymer film was annealed for several hours at 80 °C and then dried for 24 h under vacuum at room temperature. (3) The exact weight of the dried polymer film was evaluated from the difference in weight between the bare aluminum pan and the polymer-coated one. (4) The dried polymer film was allowed to hydrate in a constant relative humidity chamber (RH ) ∼55%) for several hours or by contact with liquid water for 1 h at 25 °C. (5) The mass change of the hydrated polymer film (∆m) by heating the film from 25 °C to 180 °C (heating rate ) 10 °C/min) was recorded on the thermogravimetric analyzer. The number of sorbed water molecules per monomer unit of the polymer (Nwater) was estimated from ∆m, the weight of the dry polymer, and the formula weight of monomer residue in the polymer (M). The concentration of water sorbed into the polymer film (c) was calculated from Nwater and the density of the polymer. The Nwater and c values were compiled in Table 2. (9) Reihanian, H.; Yu, T. L.; Jamieson, A. M. Polym. Prepr. 1981, 20, 78.

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Table 2. Concentration (c), Area of νOH Band, and Extinction Coefficient (EOH) of Water Sorbed into Various Polymer Films at Equilibrium water vapor

liquid water

polymer

Nwatera (unit-1)

103c b (mol/cm3)

area of νOH (cm-1)

10-7OH c (cm/mol)

Nwatera (unit-1)

103c b (mol/cm3)

area of νOH (cm-1)

10-7OH c (cm/mol)

PHEMA PMEA50 PMEA60 PMEA70 PMEA80 PMEA90 PMEA

0.392 0.243 0.218 0.182 0.153 0.153 0.047

3.58 2.22 1.99 1.66 1.40 1.40 0.43

66.6 37.7 32.5 29.7 20.6 16.5 11.4

5.3 4.9 4.7 5.1 4.2 3.4 7.6

3.20 2.28 1.39 0.928 0.621 0.537 0.218

29.2 20.8 12.7 8.47 5.67 4.90 1.99

309 334 316 320 297 311 373

3.0 4.6 7.1 11 15 18 54

a Determined by TGA measurement. b Calculated from the N water value, the averaged formula weight (M) of the monomer residue in polymer, and the density of polymer (Dpolymer): c ) DpolyNwater/M. c Evaluated by eq 2.

Figure 1. IR spectra obtained by the water vapor method for the water sorbed into (a) PHEMA and (b) PMEA films as a function of time.

Results and Discussion A. Water Vapor Sorbed into Polymer Films. (a) Homopolymers. When PHEMA and PMEA were in contact with water vapor (RH ) ∼55%), the time evolutions of infrared spectra for the water molecules sorbed into these polymers were obtained as shown in Figure 1. The intensity of the band, which could be attributed to the OH stretching (νOH) of water within the polymer matrix, increased and leveled off at 830 s or above in both polymers, which shows that the water sorptions into both PHEMA and PMEA were equilibrated at t > 830 s. The figure also shows that there was a large difference between the profiles of the spectra in the lower frequency region (3400-3000 cm-1), which must be caused by the chemical composition of the polymers. Therefore, the bands centered at around 3400 and 3200 cm-1 could be assigned to the water hydrating to the hydroxyl group in the 2-hydroxyethyl methacrylate (HEMA) residue. The other components in the 3600-3500 cm-1 region were observed in both polymers and have also been observed in the IR spectra of the water sorbed into other polymers with carbonyl and/or ether groups such as poly(ethylene oxide) (PEO), poly(methyl methacrylate), poly(2-methoxyethyl methacrylate), and so forth.6d-g Therefore, the bands in the higher frequency region could be assigned to the water hydrating to the oxygen atom of the carbonyl and/or ether groups. In addition, a weak broad band was also observed in the lower frequency region for PMEA. This band might correspond to the water molecules bound to the oxygen atoms of MEA with both of their H atoms (bridging water). In the previous report, the bridging water was also observed in the PEO film.6d These assignments do not conflict with the theoretical frequency, which was calculated with a GAUSSIAN 98 program package, of νOH for

Figure 2. (top) IR spectra obtained by the water vapor method for the water sorbed into the copolymers of MEA/HEMA at equilibrium (t ) 830 s). (bottom) Correlation of HEMA content with the OH (b), c (O), and area of the νOH band (×) at equilibrium. The regression coefficient for the plot of the HEMA content vs band area is 0.999.

the water hydrating to the oxygen atom of model compounds (methyl ether, ethanol, and methyl acetate, etc).6e-g (b) Copolymers. The spectra of water sorbed into the random copolymers with various compositions were collected. In all cases, the νOH band changed in the same manner as that of the homopolymer films (the time evolution of the spectra was not shown). The IR spectra at equilibrium (t ) 830 s) are shown in Figure 2. The profile of these spectra could be explained by a combination of the spectra for PHEMA and PMEA, and a linear relationship existed between the νOH band area and the HEMA content. (A regression coefficient for the plot of the HEMA content vs band area is 0.999.) These results suggest that water monomolecularly binds to HEMA and MEA residues, which was supported by the extinction coefficient of the sorbed water (OH). It has been generally found that the OH of water molecules depends on their hydrogen-bonding environments: (1) The OH is increased by both the formation of water clusters and the hydration but is diminished by their disruption. (2) The OH of the water cluster is 5 and 10 times or more larger than those of the ordinary hydrating water and monomeric water (gaseous state), respectively.5 The OH of the water sorbed into the polymer film was evaluated by eq 2:

∫

OH ) ( absν dν)/(cde)

(2)

where ν is the wavenumber, absν is the absorbance at ν, c is the concentration of water in the polymer matrix, and de is the effective path length. The detailed description of

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Figure 3. (top) IR spectra obtained by the liquid water method (solid line) for the water sorbed into PHEMA and PMEA and (broken line) for pure liquid water. (bottom) Time evolution of spectra obtained by the liquid water method for the water sorbed into PMEA and PHEMA.

Figure 4. (top) IR spectra obtained by the liquid water method (solid line) for the water sorbed into the copolymers at equilibrium (t ) 35 min) and (broken line) for pure liquid water. (bottom) Correlation of HEMA content with the OH (b), c (O), and area of the νOH band (×) at equilibrium.

the OH value is in the Appendix. The c values, which are compiled in Table 2, increased with the decrease in the MEA content, whereas the OH values did not change significantly (Figure 2). Following the general findings described above, the results in the table indicate that the hydrogen-bonding environment of the sorbed water vapor should be the same in all polymers. After all, when PMEA, PHEMA, and their copolymer films are in contact with water vapor, the water sorbed into the films locates around the hydrogen bonding sites (i.e., the oxygen atoms of ether, carbonyl, and hydroxyl groups) of the polymers without forming a water cluster. B. Liquid Water Sorbed into Polymer Films. Next, the state of water sorbed into the polymer films in contact with liquid water was examined. The IR spectra of the water sorbed into the homopolymers (PHEMA and PMEA) and their copolymers at equilibrium (t ) 35 min) are shown in Figures 3 and 4, respectively, and the c and OH values are compiled in Table 2. Figure 3 showed that the

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equilibration of the water sorption in both PHEMA and PMEA was attained at 2100 s or above. (a) Homopolymers. 1. PHEMA. In PHEMA, there was no significant difference in the νOH bands of the sorbed water obtained by the water vapor (Figure 1) and liquid water (Figure 3) methods. Moreover, the OH values at equilibrium were nearly equal to each other (Table 2), which suggests that the state of water sorbed into PHEMA does not depend on the state (gaseous or liquid) of the water contacting with the polymer. As described in the Introduction, the water in the polymer matrix is categorized into three types, “free”, “intermediate”, and “nonfreezable” waters.1-4 Tanaka et al. examined the state of water sorbed into PHEMA which was in contact with liquid water by using differential scanning calorimetry (DSC), and they reported that the water in PHEMA could be categorized into two types: free (40%) and nonfreezable (60%) waters.8a The concentration of the water sorbed into PHEMA at equilibrium was almost the same in both their (30 × 10-3 mol/cm3) and our (29.2 × 10-3 mol/cm3) experiments. Therefore, the IR spectra obtained in this work should be attributed to the free and the nonfreezable waters. The latter is categorized into the water molecules attracted strongly to the polymer chains, and therefore, the assignment that the obtained spectrum can be attributed to the nonfreezable water is easily acceptable and does not conflict with our interpretation. On the other hand, the existence of “free water”, which is generally categorized into bulklike water, was not detected spectrophotometrically. Some possible explanations for the discrepancy between the results by thermodynamic and vibrational spectroscopic analyses are as follows: (1) Water molecules other than the primary hydrating water (nonfreezable water) disperse monomolecularly in the polymer matrix at room temperature, but on cooling, the water molecules, which are close to each other, gather together to form a small ice.2 Such water may be detected by DSC as “free (bulklike) water”. (2) The water hydrated weakly to the polymer chain must be detected by IR spectroscopy as hydration water, whereas such water may be detected as bulklike water by DSC. These discrepancies are conceivable, taking into account that the observation time of DSC (>1 s) is much longer than that of IR (10-13 to 10-14 s): The relaxation time of rotational rearrangement of water in the liquid phase (τR ) 10-11 to 10-12 s) is longer than the observation time of IR and much shorter than that of DSC. As a result, DSC analysis can detect the change in diffusional motion of water, whereas it cannot detect the change in the hydrogen-bonded network structure of water. On the other hand, IR analysis (vibrational spectroscopy) can detect the change in the hydrogenbonded network structure of water. For this reason, the “free water” detected by DSC is not completely equal to that detected by “IR”. Therefore, from the standpoint of the hydrogen-bonding network structure of water, it could be said at least that the free water is lacking in the PHEMA matrix under our experimental conditions, regardless of whether PHEMA contacts with liquid water or water vapor. In other words, most of the water sorbed into PHEMA is attracted to the polymer chain, and the effective hydration region around the HEMA residue should be much larger than the primary hydration region. 2. PMEA. In PMEA, drastic changes in both the spectral profile at equilibrium (see Figures 1 and 3) and the OH value (Table 2) for the sorbed water obtained by the two

Water Sorbed into Poly(MEA-co-HEMA) Films

analysis methods were observed. The obtained spectrum by the liquid water method was very similar to that of pure liquid water.10 Furthermore, the OH value of the sorbed water by the liquid water method was about 8 times larger than that by the water vapor method, suggesting that bulklike water exists in the PMEA matrix. Moreover, from the following experimental results, it is considered that the water sorbed into PMEA does not disperse monomolecularly in the polymer matrix after completion of the primary water sorption but preferentially forms a water cluster. Figure 3 (bottom), which shows the time evolution of the IR spectra of the water sorbed into PHEMA and PMEA by the liquid water method, indicates that the water sorption into PHEMA is very fast, whereas that into PMEA is slow. This slow increase in the νOH band indicates that the water sorption into PMEA accompanies relaxation of the polymer chain. In other words, the water pools are formed in the polymer matrix by thrusting the polymer chains aside during the water sorption process. This phenomenon probably results from the much lower glass transition temperature of PMEA (∼ -50 °C in a dry state)8 and the poor affinity of water for the MEA residue. A noteworthy point is that the water, which is spectroscopically similar to pure water, was observed in the PMEA matrix. This is contradictory to the fact that the amount of sorbed water (c ) 1.99 × 10-3 mol/cm3 ) 0.22 water molecules/monomer unit of PMEA) seems to be extremely small to form the bulklike water, taking into account that 22%11 of the sorbed water or more was consumed in the primary hydration. This result indicates that the effective hydration region around the MEA residue is vanishingly small. Namely, the interaction between the primarily hydrating water and the water surrounding it is very weak. Consequently, in view of the vibrational spectroscopy, it could be said that the water entrapped in the PMEA film behaves like bulk water. However, one is driven to consider that the water in the pools within the polymer matrix is not the so-called bulk water, because the water caged in a small space very often shows an anomalous thermodynamic property.6a-c,12 Tanaka et al. thermodynamically evaluated the water sorbed into PMEA in contact with liquid water and reported that anomalous water, which freezes at around -50 °C upon heating and is called “cold-crystallizable water”, other than the intermediate and nonfreezable waters, exists in the PMEA matrix. Furthermore, they reported that there was no free water in the PMEA matrix at the water content ) 3.0 wt % (Nwater ) 0.217, c ) 1.98 × 10-3 mol/cm3).8b The water content was almost the same in both their and our experiments. Probably, the water with the large OH value observed in the PMEA film would be the cold-crystallizable water. Our consideration is based on a hypothesis that the OH values of intermediate and nonfreezable waters obtained by the liquid water method have to be equal to those obtained by the water vapor method. Mention is made here of the reports on the cold crystallization of water. The cold crystallization of water (10) The IR spectrum for pure liquid water was measured by the transmission method with a path length ) ∼5 µm and CaF2 windows. (11) This value was calculated from the c values for PMEA obtained by the water vapor (c ) 0.43) and the liquid water (c ) 1.99) methods. (12) (a) Katayama, S.; Fujiwara, S. J. Am. Chem. Soc. 1979, 101, 4485. (b) Mallamace, T.; Migliardo, P.; Vasi, C.; Wanderlingh, F. Phys. Chem. Liq. 1981, 11, 47. (c) Murase, N.; Gonda, K.; Watanabe, T. J. Phys. Chem. 1986, 90, 5420. (d) Kubota, K.; Fujishige, S.; Ando, I. J. Phys. Chem. 1990, 94, 5154. (e) Terada, T.; Inaba, T.; Kitano, H.; Maeda, Y.; Tsukida, N. Macromol. Chem. Phys. 1994, 195, 3261.

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in many polymer and gel systems has been investigated thermodynamically using DSC.3 Following the literature, the number of water molecules per monomer unit of the cc ), where the cold crystallization appears, polymers (N water depends on the kind of polymer, and the solubility of polymers in water does not seem to be an important factor cc values in the cold crystallization. For example, the N water 3d for water-soluble gelatin and poly(N-vinylcaprolactam) (PVLC)3e were reported to be in the ranges 2.9-6.2 and 8.0-10, respectively. As for water-insoluble polymers, the cc N water values for PMEA8a examined in the present work and chitosan3c were reported to be 0.22 and in the range 7.9-11.7, respectively. Taking into account that there is cc for each polymer except for an upper limit of N water PMEA, the size of the space caging the water molecules must be closely related to the cold crystallization of water. Namely, the cold crystallization is generated by caging water in a small space. Moreover, the fact that the lower cc is very small indicates that the interaction limit of N water between the primarily hydrating water and the water surrounding it should be very weak. In other words, the cold-crystallizable water molecules hardly interact with the polymer. This is because all of the water interacting with the polymer, irrespective of the way, must be detected by DSC measurement as nonfreezable and/or intermediate waters. The state of water in the polymer matrix observed in the thermodynamic analysis of the cold-crystallization phenomena was in agreement with that in the spectroscopic analysis of the water sorbed into PMEA in this work. Therefore, as mentioned above, it was concluded that the sorbed water with a larger OH value in the PMEA film is cold-crystallizable water. (b) Copolymers. The spectra and the values of OH and c for the sorbed water obtained by the liquid water method are given in Figure 4 and Table 2, respectively. The c values for these polymers were proportional to the HEMA contents in a way similar to that of the water vapor method, whereas the OH values were not (Figure 4, bottom). The OH value obtained by the liquid water method in the HEMA content region 40-100 mol % was nearly equal to that by the water vapor method. In the HEMA content region 0-30 mol %, on the other hand, the OH values by the liquid water method were much larger than that by the water vapor method, and this tendency was more pronounced with a decrease in the HEMA content. Assuming that there is no microdomain with phase separation in the copolymer films, this result is compatible with the finding that the effective hydration region around the HEMA residue was larger than that around the MEA residue. Though there is no experimental evidence, the microdomain would probably be absent. This is because, if phase separation between the MEA and HEMA moieties occurs, the OH value should be proportional to the HEMA content. Mention should be made here concerning the size of the hydration area around the polymers. At this moment, quantitative evaluation of the hydration area by the ATRIR method adopted in this work is not possible. To solve this problem, spectroscopy, which can exclusively detect the structure of water in the vicinity of polymer-water interfaces, seems to be highly useful. An attenuated total reflection Raman spectroscopy,13 for example, would be one candidate for that purpose. (13) Nickolov, Z. S.; Earnshaw, J. C.; McGarvey, J. J. J. Raman Spectrosc. 1993, 24, 411.

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Figure 5. (top) Relationship between the content of HEMA and the OH values obtained by the water vapor (hashed bar) and liquid water (solid bar) methods. (bottom) Relationship between number of adhered platelets onto the surface of polymers and the content of HEMA. The results of blood compatibility were from ref 8a.

C. Correlation of the State of Water in the Polymer Film with Blood Compatibility of the Polymer. Many researchers have pointed out that the generation of blood compatibility of materials is related to the water structure on the material surfaces.8,14 In this work, the structure of water on the surface of the polymer film could not be observed directly. However, as mentioned in Sections A and B, on the analogy of the OH value for the sorbed water molecule, the water structure on the polymer surfaces could be estimated. Figure 5 (bottom) shows a correlation of the blood compatibility8a of the MEA-containing copolymers examined in this work with the OH values of the sorbed water. The blood compatibility reported by Tanaka et al. was evaluated by the number of platelets adhered onto the polymer surface.8a As shown in the figure, the OH value of the water sorbed into the polymer in contact with liquid water was inversely proportional to the number of adhered platelets; the number of adhered platelets increased with a decrease in the OH value. The generation of blood compatibility, of course, is also strongly related to the surface free energy and surface morphology15 of the materials. From the results in Figure 5, however, it can be said that one important factor in the generation of blood compatibility is a small effective hydration region around the polymer chain; in other words, the effect of the polymer chain on the water structure is weak. Why is a small hydration region an important factor for the generation of blood compatibility? Probably, this would be strongly related to the hydrating water, which is considered to stabilize the structure of proteins and cells,14 (14) For example: (a) Uedaira, H. Water and Metal Cations in Biological Systems; Pullman, B., Yagi, K., Eds.; Japan Scientific Societies Press: Tokyo, 1980; p 47. (b) Tanzawa, H. Jpn. J. Artif. Organs 1986, 15, 16. (c) Yamada, N. A.; Tanzawa, H. J. Appl. Polym. Sci. 1991, 43, 1165. (d) Israelachivili, J.; Wennerstrom, H. Nature 1996, 379, 219. (e) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibins, P. E. J. Phys. Chem. B 1998, 102, 426. (f) Kikuchi, A.; Karasawa, M.; Tsuruta, T.; Kataoka, K. J. Colloid Interface Sci. 1993, 158, 10. (g) Kataoka, K.; Ito, H.; Amano, H.; Nagasaki, Y.; Kato, M.; Tsuruta, T.; Suzuki, K.; Okano, T.; Sakurai, Y. J. Biomater. Sci., Polym. Ed. 1998, 9, 111. (h) Iwasaki, Y.; Fujiike, A.; Kurita, K.; Ishihara, K.; Nakabayashi, N. J. Biomater. Sci., Polym. Ed. 1996, 8, 91. (i) Ishihara, K.; Nomura, H.; Mahara, T.; Kurita, K.; Iwasaki, Y.; Nakabayashi, M. J. Biomed. Mater. Res. 1998, 39, 323. (15) For example: (a) Pignataro, B.; Conte, E.; Scandurra, A.; Marletta, G. Biomaterials 1997, 18, 1641. (b) Holly, F. J. Colloids Surf. 1984, 10, 343. (c) Wang, P.; Tan, K. L.; Kang, E. T.; Neoh, K. G. J. Membr. Sci. 2002, 195, 103.

Ide et al.

around their surfaces. If the hydration region around the polymer chain and the interaction between the primarily hydrating water and the water surrounding it are large, the hydrating water on the proteins or cell surfaces might be partly taken by the polymer chain, and subsequently, the denaturation and adhesion of the proteins or cells to the polymer surface would occur by the sharing of hydrating water between the polymer chain and the proteins or cell surfaces. Our hypothesis regarding the generation of blood compatibility seems to be acceptable intuitively, but there is no definitive evidence at this moment. More detailed investigations of the relationship between the generation of blood compatibility and the water structure in contact with polymer matrixes are now in progress.

Conclusion The effective hydration region around the polymer residues was examined by a comparison of the OH values for the water sorbed into p(MEA/HEMA) in contact with liquid water and water vapor. The OH value for the water sorbed into the polymer with a higher content of MEA in contact with liquid water was much larger than the value for that in contact with water vapor. This result seemed to indicate that the interaction between the primary hydration water around the MEA-containing polymer chain and the waters surrounding the primary hydration water is very weak; namely, the effective hydration area around the polymer chain is very small. The water with the larger OH value among all the waters sorbed into the MEA-containing polymers seems to be cold-crystallizable water, which has been observed by DSC as anomalous water other than intermediate and nonfreezable waters. Comparing the spectroscopic results obtained in this work with those thermodynamically obtained previously,3 the cold crystallization of water might be generated by caging the water molecules in a small space by the polymer chain with a small hydration area. From a correlation of the OH values of the water sorbed into the polymers investigated in this work with their blood compatibility, it was suggested that the sorbed water with a larger OH value is strongly related to the generation of biocompatibility of the materials.

Acknowledgment. This work was supported by the Grants-in-Aid (12450381, 13555260, 13022225) from the Ministry of Education, Science, Sports and Culture. The authors are indebted to Terumo Corporation, Tokyo, Japan, for financial support. M.I. wishes to thank the Saneyoshi Scholarship Foundation, Tokyo, Japan, for financial support.

Appendix The penetration depth (dp) of evanescent IR is determined using the following equation:

dp ) λ1/[2π(sin2 θ - npoly/ZnSe2)1/2]

(A1)

where θ is the incident angle of light on the surface of the ZnSe element, and npolymer/ZnSe is the ratio of the refractive index of the polymer to that of the ZnSe element (the refractive indices of the polymer and ZnSe are assumed

Water Sorbed into Poly(MEA-co-HEMA) Films

Langmuir, Vol. 19, No. 2, 2003 435

to be 1.49 and 2.4, respectively). For the ZnSe element(80 mm × 10 mm × 5 mm) used in the present work, θ is 45°. The λ1 value is the ratio of the wavelength (λ) of incident light to the refractive index of the ZnSe element. When the λ of the incident light is 3 µm (wavenumber ) 3333 cm-1), dp ) 0.54 µm.

Therefore, ν can be calculated by

ν ) absν/[(DpolyNwater/M)(dpNR/2)]

(A3)

Here

∫ν dν

(A4)

∫{absν/[(DpolyNwater/M)(dpNR/2)]} dν

(A5)

OH )

For an ATR-IR measurement, the absorbance at ν is described as

From eqs A3 and A4,

absν ) νcde

(A2)

ν is the extinction coefficient at ν; c is the water concentration; de is the effective path length and is obtained by the dp and the number of reflections (NR): de ) dp(NR/2). The dp is 0.437 µm because, as described in the Experimental Section, the collected spectra were calibrated on the basis of the dp at 4500 cm-1. The NR value was calculated to be 16 from the geometrical analysis. c is obtained by the number of water molecules per monomer unit of the polymer (Nwater), the density of the polymer (Dpolymer), and the averaged formula weight of the monomer residue in the polymer (M): c ) DpolyNwater/M.

OH )

where Dpoly, Nwater, M, dp, and NR values are constant. Therefore, eq A4 can be rewritten as follows:

∫ ) (∫absν dν)/(cde)

OH ) ( absν dν)/[(DpolyNwater/M)(dpNR/2)] (A6)

The dimension of the extinction coefficient evaluated here (OH) differs from that usually used, because the integrated value of absorbance is divided by the path length and concentration. LA020617P