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Fourier Transform Infrared Study on the State of Water Sorbed to Poly(ethylene glycol) Films† Hiromi Kitano,*,‡ Ken Ichikawa,‡ Makoto Ide,‡ Mitsuhiro Fukuda,§ and Wataru Mizuno| Department of Chemical and Biochemical Engineering, Toyama University, Toyama 930-8555, Japan, Textile Materials Science Laboratory, Hyogo University of Teacher Education, Yashiro-cho, Hyogo 673-1494, Japan, and Toyama Industrial Technology Center, Takaoka, Toyama 933-0981, Japan Received June 27, 2000. In Final Form: October 31, 2000 The state of sorbed water and sorption processes of water in poly(ethylene glycol) (PEG) films were studied by Fourier transform infrared. From the assignment and time evolution of the components of the O-H stretching band, the adsorption and penetration of water into the films are considered to proceed as follows: (1) At the polymer surface, a water molecule binds to the oxygen atoms of PEG with one of its hydrogen atoms (“binding water”). The same phenomenon more gradually occurs in the polymer matrix too. (2) The water binding to the oxygen atoms of PEG molecules with both of its hydrogen atoms (“bridging water”) is formed from the binding water, while a dimeric water is gradually formed by the association of the free water molecule with the binding water. (3) The amount of monomeric species (both binding water and bridging water) is equilibrated. (4) The water dimer is further formed by the association of the free water molecule with the binding water, and finally the amount of the dimer is equilibrated. When the molecular weight of PEG was small, a new band at 3400 cm-1 gradually appeared. This band could be attributed to the O-H stretching of water molecules attaching to hydroxyl groups at the end of PEG molecules. An ab initio molecular orbital calculation and hybrid density functional method supported the assignment of the peaks.
Introduction Deterioration of polymeric materials is very often caused by chemical and physical transformation of the polymers initiated by the sorbed water, and the water sorption to the polymers is intensely related to the interaction between the water molecule and the polymers.1 Previous studies have been mainly focused on the equilibrated state of water inside the solid polymers by using differential scanning calorimetry,2 solid-state nuclear magnetic resonance,3 etc. Only a limited number of researches have been made concerning the time dependence of water sorption to polymeric materials or distinction of the water at the airpolymer interfaces from that inside the polymers.1 Poly(ethylene glycol) (PEG) is a very important material in both industrial fields (detergent) and scientific fields (fusion of cells in the presence of PEG,4 evasion of endocytosis of PEG-coated liposomes,5 etc.). Previously, the state of water at the interface of an air-poly(ethylene † Presented at the 48th regional meeting of the Society of Polymer Science, Japan, at Toyama, Japan, in Oct 1998. * To whom correspondence should be addressed. Tel: +81-76-445-6868. Fax: +81-76-445-6703. E-mail: kitano@ eng.toyama-u.ac.jp. ‡ Toyama University. § Hyogo University of Teacher Education. | Toyama Industrial Technology Center.
(1) Kawagoe, M.; Takeshima, M.; Nomiya, M.; Qiu, J.; Morita, M.; Mizuno, W.; Kitano, H. Polymer 1999, 40, 1373. (2) Hey, M. J.; Ilett, S. M.; Mortimer, M.; Osates, G. J. Chem. Soc., Faraday Trans. 1990, 86, 2673. (3) (a) Li, S. Z.; Chen, R. S.; Greenboum, S. G. J. Polym. Sci., Polym. Phys. Ed. 1995, 33, 403. (b) Rueda, D. R.; Varkails, A. J. Polym. Sci., Polym. Phys. Ed. 1995, 33, 2263. (4) (a) Zschornig, O.; Arnold, K.; Ohki, S. Biochim. Biophys. Acta 1993, 1148, 1. (b) Lackney, V. K.; Spanswick, R. M.; Hirasuna, T. J. Plant Cell, Tissue Organ Cult. 1990, 23, 107. (c) Okayasu, R.; Cheong, N.; Illiakis, G. Int. J. Radiat. Biol. 1993, 64, 689. (5) Klibanov, A. L.; Maruyama, K.; Torchilin, V. P.; Huang, L. FEBS Lett. 1990, 268, 235.
glycol) dimethyl ether (PEG-DME) film was distinguished from that inside the polymer film by using the transmission and the attenuated total reflection (ATR) infrared spectroscopies.6 In this paper, the O-H stretching peaks for water sorbed to the solid film of poly(ethylene glycol) (PEG) of various molecular weights were analyzed to clarify the effect of OH end groups of PEG on the interaction with water molecules. An ab initio molecular orbital (MO) calculation method and hybrid density functional method were adopted to support the assignment of the O-H stretching peaks. By the in situ difference infrared spectroscopic method, the dynamics of water sorption to the film at an early stage (within 830 s after onset of the sorption of water) were directly observed. Experimental Section Materials. PEGs [viscosity-averaged molecular weight (Mv) ) 1.1 × 104 (11K), 1.5 × 104 (15K), 2.8 × 104 (28K), 4.9 × 104 (49K), and 6 × 104 (60K)] and PEG-DME [Mw ) 2.1 × 103 (2.1K)] from Aldrich, Milwaukee, WI, were purified by precipitation in ethanol-diethyl ether. Poly(methyl methacrylate) (PMMA; atactic, Mv ) 1.23 × 105) from Wako Pure Chemicals, Osaka, Japan, was used without further purification. Poly(2-hydroxyethyl methacrylate) (PHEMA; Mv ) 6.4 × 104) was prepared by the conventional radical polymerization of 2-hydroxyethyl methacrylate (HEMA; purified by distillation in vacuo) in methanol at 65 °C for 24 h using 2,2′-azobis(isobutyronitrile) (AIBN) as the initiator (molar ratio of HEMA and AIBN, 100:1) and subsequent precipitation from diethyl ether. Polyethylene film (PE; low-density polyethylene; thickness, 10 µm) from Ohji Special Carton Co., Tokyo, Japan, was used without further purification. The Mv values of PEG, PMMA, and PHEMA were determined by using an Ubbelohde dilution-type viscometer (type 0B; Kusano, Tokyo, Japan). The K and R values used in the Mark-Houwink(6) Ide, M.; Yoshikawa, D.; Maeda, Y.; Kitano, H. Langmuir 1999, 15, 926.
10.1021/la0008986 CCC: $20.00 © 2001 American Chemical Society Published on Web 02/13/2001
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Sakurada equation ([η] ) KMvR) were 12.5 × 10-3 mL/g and 0.78 in water at 30 °C,7 4.8 × 10-3 mL/g and 0.80 in chloroform at 25 °C,8 and 52.4 × 10-3 mL/g and 0.51 in methanol at 30 °C9 for PEG, PMMA, and PHEMA, respectively. Other reagents were commercially available. Distilled water was further purified by a Milli-Q-labo (Millipore). FTIR Measurements. For transmission measurements, PEG films were prepared on CaF2 plates (φ ) 20 mm, thickness ) 2 mm) from a chloroform solution (5 mg/L) under a dry N2 gas and dried at 25 °C under vacuum for 3 h. The thickness of the film was determined to be 8 µm from the diffraction of visible light using a fiber-optic multichannel photodetection system (MCPD1100; Otsuka Electronics, Hirakata, Japan) coupled with an optical microscope (Optiphot; Nikon, Tokyo, Japan). The infrared spectra of the films were recorded on a system 2000 FTIR (PerkinElmer) with a mercury cadmium telluride (MCT) detector. All spectra between 2500 and 4000 cm-1 were collected with a resolution of 4 cm-1 and 32 scans. To observe the O-H stretching band of sorbed water to the PEG films, a homemade sample holder, which was tightly sealed except the portals (i.d. 0.8 mm) for the inlet and outlet of dry N2 and water vapor of a constant relative humidity (RH), was used. The water vapor at about 50% RH (50 ( 5% RH) was prepared by passing the air at 1.8 mL‚s-1 through a saturated aqueous NaBr solution. The RH value was continuously monitored by a model SU-610 humidity meter, Test, Yokohama, Japan. Two CaF2 plates, on which the PEG films had been coated beforehand, were facing each other at a distance of 3 mm in the sample holder, and the IR background was repeatedly measured while the dry N2 gas flowed through the portals into the holder for about 1 h. When the background was equilibrated, the IR difference spectra of the sorbed water to the PEG films were collected at appropriate time intervals after onset of the flowing of water vapor (time evolution method). A ZnSe element (80 × 10 × 5 mm; trough plate ZnSe 45°; Pike Technologies, Madison, WI) was used for the ATR-IR measurements. PEG films with a thickness of 20 µm were prepared on the ZnSe element from a chloroform solution (50 mg‚L-1) under dry N2 and dried at 25 °C under vacuum for 3 h. An ATR cell was tightly sealed except the portals for the inlet and outlet of the gas. Differential Scanning Calorimetry. Enthalpies for melting of the polymer samples (∆Hmelt) were analyzed with a differential scanning calorimetry (DSC; model TAS-200, Rigaku, Tokyo, Japan). The melting thermograms were recorded at a heating rate of 10 °C/min under an argon flow. The crystallinity of the polymer sample was estimated by the ratio of the ∆Hmelt value experimentally obtained and that reported for the crystalline polymer (4.10 and 9.41 kJ‚mol-1 for PE10 and PEG,11 respectively). Calculation of Vibrational Frequencies Using ab Initio MO and Density Functional Methods. As the model compounds, various combinations of water, dimethyl ether, ethanol, 2-methoxyethanol, and 1,2-dimethoxyethane were used. Molecular geometries of the model compounds were optimized using Møller-Plesset perturbation theory of the second-order (MP2) level with a 6-31+G(d,p) basis set.12,13 Their theoretical frequencies of O-H stretching were then calculated for the optimized structures. Because the MP2/6-31+G(d,p) calculation for some model compounds required so much computational time, geometries of all of the model compounds were also optimized and followed by the frequency calculation using a hybrid density functional method, Becke3LYP (B3LYP) 14,15 with a 6-31++G(d,p) basis set. Recently, the B3LYP exchange-correla(7) Bailey, F. E., Jr.; Kucera, J. L.; Imhof, L. G. J. Polym. Sci. 1958, 32, 517. (8) Bischoff, J.; Desreux, V. Bull Soc. Chim. Belg. 1952, 61, 10. (9) Fort, R. J.; Polyzoidis, T. M. Eur. Polym. J. 1976, 12, 685. (10) Wunderlich, B.; Czornyj, G. Macromolecules 1977, 10, 906. (11) Miller, R. L. In Polymer Handbook, 3rd ed.; Brandrup, J., Immergut, E. H., Eds.; John Wiley & Sons: New York, 1989; p VI-72. (12) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; John Wiley & Sons: New York, 1986; Chapter 6. (13) Del Bene, J. E. J. Chem. Phys. 1987, 86, 2110. (14) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 1372, 5648. (b) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (15) Lee, C.; Yamg, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
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Figure 1. Time evolution of IR spectra for the O-H stretching band of sorbed water in a PEG 60K film (transmission mode).
Figure 2. O-H stretching band of sorbed water in PEG, PE, and PEG-DME films by transmission mode (830 s after onset of the sorption): - - -, PE. tion method has been widely used to obtain the frequency of a number of molecular systems.16 All of the calculations were carried out on a SGI Origin 2000 computer using a Gaussian 98 program package.17 The deviation from the experimental frequency was generally found even for a single water molecule when we employed larger basis sets and more accurate correlation. Our object is, however, the assignment of the experimental O-H stretching with the help of the theoretical frequency of the model compounds. Therefore, theoretical frequencies from the MP2/6-31+G(d,p) and B3LYP/6-31++G(d,p) calculations using the shift parameter to the experimental ones were used in the present study. The frequencies of single water in vapor were obtained by tightly optimizing the structure, and the shift parameter was determined as 0.9401 and 0.9584 for MP2 and B3LYP, respectively.
Results and Discussion A. Transmission IR Difference Spectra of Water Sorbed to PEG Films at 50% RH. The IR absorption band of O-H stretching for water molecules sorbed to PEG films increased with the contact time with the air of 50% RH and leveled off as exemplified in Figure 1. The O-H stretching bands of water (transmission mode) in five kinds of PEG films and PEG-DME film 830 s after onset of the contact with the air (50% RH) are shown in Figure 2, where all of the spectra for the films (thickness, 8 µm) were normalized by the absorbance at 3550 cm-1. The difference spectrum for the PE film corrected by the film thickness (10 µm) was also shown for comparison. The spectra for PEG with small molecular weight had a broad shoulder around 3500-3300 cm-1, and with the increase in molecular weight, the spectra approached that of PEG-DME. The absorbance of OH end groups of dry films (thickness 8 µm) was much smaller than that of the films 830 s after onset of the sorption measurements (absorbance of PEG 6K and 60K; 0.0066 and 0.0043 at 3450 cm-1 f 0.0554 and 0.0442 at 3500 cm-1, respectively), which means that the effect of absorbance corresponding (16) (a) Reimers, J. R.; Hall, L. E. J. Am. Chem. Soc. 1999, 121, 3730. (b) Stevens, P. J.; Devlin, F. J.; Chablowski, C. F.; Frisch, M. J. Submitted for publication in J. Chem. Phys. (17) Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998.
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Table 1. Center of the Peak Determined by Fourier Self-Deconvolutiona polymer PEG-DME PEG 60K PEG 49K PEG 28K PEG 15K PEG 11K a
Chart 1. Susceptible Structure of Hydration of a PEG Molecule
peak 1 peak 2 peak 3 peak X peak 4 peak 5 3589 3581 3582 3573 3579 3578
3519 3518 3518 3518 3520 3523
3441 3452 3454 3452 3443 3441
3405 3398 3390
3275 3277 3276 3277 3278 3275
3067 3069 3069 3069 3069 3069
In reciprocal centimeters.
Figure 3. IR spectral analysis for the O-H stretching band of sorbed water in (a) PEG 60K and (b) PEG 11K films (830 s after onset of the sorption).
to the O-H ends on the difference spectrum of sorbed water is not significant. The fixed peak positions were determined by a Fourier self-deconvolution of the original O-H stretching bands in Figure 2 for the curve fitting. In the case of two kinds of PEG (49K and 60K) and PEG-DME films, the O-H stretching band was decomposed into five Gaussian components with fitting parameters given in Table 1 (Figure 3 a). As for the O-H stretching band of PEG with smaller molecular weight (11K, 15K, and 28K), on the contrary, the band could be decomposed into six Gaussian peaks. The additional peak (peak X) for the PEG with smaller molecular weight appeared at around 3400 cm-1 (Figure 3b). The stretching band of water sorbed to a PE film was negligibly small in comparison with those of PEGs and PEG-DME (Figure 2). The crystallinity of the PE film was evaluated to be 80% and that of PEG was around 70% irrespective of its molecular weight (PEG 11K of powder form, 81%; 28K, 63%; 49K, 74%) by the DSC measurements. Because gaseous water molecules diffuse and adsorb mostly to the amorphous region in the polymeric materials,18 the DSC data indicated that at least 20% of PE chains in the film were ready to sorb water molecules with a help of van der Waals forces, for example. The very small peak intensity of the O-H stretching in the PE film which had a comparable crystallinity to the PEG films, however, suggests that the amount of water sorbed by the van der Waals force would be very small, and water molecules in the PEG films dominantly sorb to the ether oxygen atom of the PEG chains. This is because the binding energy of the van der Waals interaction (about 9.3 kJ mol-1)19 is much smaller than that of hydrogen bonding (about 20 kJ mol-1).19 The influence of the van der Waals (18) (a) Klute, C. H. J. Polym. Sci. 1959, 41, 307. (b) Pace, R. J.; Datyner, A. Polym. Eng. Sci. 1980, 20, 51. (19) Voet, D.; Voet, J. D. Biochemistry, 2nd ed.; John Wiley & Sons: New York, 1995.
force on the sorption processes, therefore, was not considered in this work. Gravimetric measurements indicated that the amount of water sorbed to the PEG films was 0.020 g/g of polymer in 30 min at the most. This value is in good agreement with that reported previously.20 The DSC data (crystallinity of PEG films, ca. 70%) showed that the maximum amount of water sorbed was evaluated to be about 0.16 water molecule/monomer residue (-CH2CH2O-) in the amorphous region. This value indicates that water molecules sorb to the PEG films mostly monomolecularly, and only water dimers partly exist in the film at the most. In other words, the formation of a cluster (trimer, tetramer, etc.) of water molecules in the PEG films in contact with the air for 30 min is not highly probable. The amount of sorbed water necessary for the complete formation of a water cluster inside the film is calculated to be 0.82 g of water/g of PEG at the least, because the number of hydrating water molecules was estimated to be 2-6/monomer unit of PEG in concentrated aqueous solutions.21 In addition, it was suggested that, in concentrated aqueous PEG solutions, each of two hydrogen atoms of the water molecule partly bind to the oxygen atom of the polymer chains22 (cross-linking of PEG chains via the water molecule; “bridging water”). B. Assignment of the O-H Stretching Peaks. Taking account of the amount of water sorbed to the PEG films, peak 1 in Figure 3a can be assigned to the O-H stretching band for a monomeric water binding to the ether oxygen atom of the polymer (“binding water”; Chart 1). Peak 2 corresponds to the stretching band for a water molecule which is hydrogen-bonded to another water molecule binding to PEG (water molecules are dimeric; Chart 1). Peak 3 existing at the lower wavenumber region corresponds to the stretching of a water molecule hydrogen-bonded to both a water molecule and the ether oxygen of PEG (water molecules are dimeric). Peaks 4 and 5 existed at a lower wavenumber region than peaks 1-3. The O-H stretching band of the water molecule, in general, is shifted to the lower frequency region by the formation of a hydrogen bond.23 Therefore, the absorption peak for the “bridging water” should exist at a lower wavenumber region than peaks 1-3, because of the association with two ether oxygen atoms which are polar and act as hydrogen acceptors as described above.22,24 (20) Smith, K. L.; Cleve, R. V. Ind. Eng. Chem. 1958, 50, 8. (21) (a) Hager, S. L.; MacRury, T. B. J. Appl. Polym. Sci. 1980, 25, 1559. (b) de Vringer, T.; Joosten, J. G. H.; Junginger, H. E. Colloid Polym. Sci. 1986, 264, 623. (c) Tilcock, C. P. S.; Fisher, D. Biochim. Biophys. Acta 1982, 688, 645. (22) Lu¨sse, S.; Arnold, K. Macromolecules 1996, 29, 4251. (23) Eisenberg, D.; Kauzmann, W. The Structure and Properties of Water; Clarendon Press: London, 1969; Chapter 4. (24) Bieze, T. W. N.; Barnes, A. C.; Huige, C. J. M.; Euderby, J. E.; Leyte, J. C. J. Phys. Chem. 1994, 98, 6568.
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Table 2. Curve-Fitting Parameters of the Pure Water and Sorbed Water in the PEG 60K Film peak water pure water water sorbed to PEG 60K
center of the peak (cm-1) relative area to peak 1 center of the peak (cm-1) relative area to peak 1
Consequently, peak 4 can be assigned to the O-H stretching band for a water molecule hydrogen-bonded to two PEG chains (“bridging water”). It should be mentioned here that the possibility of the presence of a water molecule hydrogen-bonded to neighboring ether oxygen atoms in the same PEG chain (Chart 1, peak 4b, which can also be categorized in “bridging water”) cannot be fully excluded. As mentioned in section A, the amount of sorbed water (1 water molecule/6 monomer units in the amorphous region) indicates that water molecules bind to the PEG mostly monomolecularly, and the dimeric hydration might be probable at the most. Therefore, the coupling of the O-H stretching in hydrogen-bonded water clusters would be quite seldom. Peak 5 was obscure because the lower frequency region of this component overlapped with the difference band for the C-H stretching of the polymer. Moreover, the relative intensity of peak 4 to peak 1 was smaller than unity even after the equilibration (see Table 2), which shows that the amount of monomeric water in the PEG film is much larger than that of bridging water, because the absorptivity of O-H oscillators in lesshydrogen-bonded water molecules is extremely smaller than that of fully hydrogen-bonded water.25 In bulk water systems such as aqueous solutions, the characteristics of water have been determined by the analysis of the band for a water cluster.24-30 For example, Walrafen decomposed the O-H stretching Raman bands of pure water into five Gaussian components and assigned the components at 3620, 3500, 3390, and 3215 cm-1 to a free water, water molecules whose hydrogen bonds are partly broken, fully hydrogen-bonded water molecules uncorrelated (not in-phase), and fully hydrogen-bonded water molecules correlated (in-phase, both donation of the protons and acceptance of the protons with both of the electron lone pairs).26,27 The same assignments were made by Walrafen for the IR spectra of pure water too.27a In the solid polymer systems containing a very small amount of sorbed water, however, the monomeric and dimeric bands are very important for analyses of the state and sorption processes of water to the polymers. The additional peak around 3400 cm-1 for PEG 11K, 15K, and 28K could be attributed to the hydrogen-bonded water molecules at the end of the polymer (Chart 2 part a or b). This is because the peak intensity increased with a (25) (a) Van Thiel, M.; Becker, E. D.; Pimentel, G. C. J. Chem. Phys. 1957, 27, 486. (b) Swenson C. A. Spectrochim. Acta 1965, 21, 987. (26) Walrafen, G. E. In Structure of Water and Aqueous Solutions; Luck, W. A. P., Ed.; Verlag Chemie: Weinheim, Germany, 1974. (27) (a) Walrafen, G. E. J. Chem. Phys. 1967, 47, 114. (b) Walrafen, G. E. J. Chem. Phys. 1984, 81, 669. (28) (a) Terada, T.; Maeda, Y.; Kitano, H. J. Phys. Chem. 1993, 97, 3619. (b) Maeda, Y.; Tsukida, N.; Kitano, H.; Terada, T.; Yamanaka, J. J. Phys. Chem. 1993, 97, 13903. (c) Terada, T.; Inaba, T.; Kitano, H.; Maeda, Y.; Tsukida, N. Macromol. Chem. Phys. 1994, 195, 3261. (d) Tsukida, N.; Maeda, Y.; Kitano, H. Macromol. Chem. Phys. 1996, 197, 1681. (e) Tsukida, N.; Muranaka, H.; Ide, M.; Maeda, Y.; Kitano, H. J. Phys. Chem. B 1997, 101, 6676. (29) (a) Maeda, Y.; Kakinoki, K.; Kitano, H. J. Raman Spectrosc. 1996, 27, 425. (b) Ide, M.; Maeda, Y.; Kitano, H. J. Phys. Chem. B 1997, 101, 7022. (30) (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.
transmission ATR
1
2
3
4
3620 1 3581 1 1
3520 2.3 3518 0.41 0.33
3411 12 3452 1.32 1.12
3260 7.3 3277 0.47 0.93
Figure 4. Effect of molecular weight of PEG on the peak areas for water molecules at various states (transmission mode). The peak area for peaks 1-4 was normalized by the number of monomer residues in the film, and that of peak X was normalized by the number of end OH groups of PEG in the film: O, peak 1; b, peak 2; 4, peak 3; 2, peak 4; 9, peak X. Chart 2. Susceptible Structure of Hydration at the End of a PEG Molecule (Peak X)
decrease in the molecular weight, which increases the number of hydroxyl end groups (Figure 4). C. Comparison of Vibrational Frequencies Obtained by Experiments and Calculations. These assignments for the O-H stretching components are qualitatively in accordance with that predicted by the ab initio MO calculation and hybrid density functional method (Table 3).12 The calculated intensity of the symmetric stretching vibration for hydrating water was much larger than that of the antisymmetric one in many cases. For example, the intensity ratio of νas to νs for the monomeric water (“binding water”) predicted by the density functional method was 1:5.4. Therefore, we assumed that peak 1 around 3580 cm-1 corresponds to the symmetric stretching vibration, and a small negligible antisymmetric vibration peak exists at a slightly higher frequency region (around 3670 cm-1). Other peaks were also attributed to the relatively larger peaks for each hydration mode, and the smaller peaks were considered to be hidden behind the neighboring large peaks of other hydration modes. As for peaks 4 and X which were assigned to the hydration modes 4a and 4b in Chart 1 and a and b in Chart 2, respectively, the wavenumbers calculated were
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Table 3. Calculated Wavenumbers for the O-H Stretching of Water Molecules Hydrating Model Compoundsa
a Values obtained by multiplying the shift parameter, 0.9401 for MP2 and 0.9584 for B3LYP. The frequencies used for the discussion are indicated by the boldface numbers.
Figure 5. IR spectra of the O-H stretching band of sorbed water in PEG 11K and 60K, PMMA, and PHEMA films (830 s after onset of the sorption).
still larger than the experimental values. This is probably because the water molecule hydrogen-bonded to the OH groups of the PEG molecule might be quickly bound by another water molecule or by the ether group of another PEG chain. The wavenumbers calculated for these species (parts d and e of Chart 2) were not inconsistent with the experimental values (peak X). On the contrary, the theoretical value for a cyclic hydration at the end of the PEG chain (Chart 2c) did not agree with the experimental one at all. In the case of “bridging water” (peaks 4a and 4b in Chart 1), the potential energy minimum might not fit well with the structure in which the three components (CH3-O-CH3, H-O-H, and CH3-O-CH3) are in the propinquity of each other, resulting in a larger calculated frequency number than the experimental value. The IR difference spectra of water adsorbed to PEG 11K and 60K, PHEMA, and PMMA films in 830 s are shown in Figure 5 for comparison. The large broad band around 3200-3300 cm-1 for PHEMA can be attributed to the water molecules which are hydrogen-bonded to the hydroxyl group in the side chain of the polymer, because peak X for PEG also existed in the same region, and the
intensity of the peak for PEG 11K was more significant than that for PEG 60K as mentioned above. As for PMMA, on the other hand, the stretching band was much smaller and relatively sharper than that of PEG 60K. Furthermore, the peaks for PMMA existed in a higher wavenumber region than PEG 60K. This is because water molecules are more weakly bound to an ester group of the polymer than to an ether group. The wavenumbers for water molecules hydrating ester and ether moieties were predicted by the ab initio MO of the MP2/6-31+G(d,p) level (νas and νs for water molecule hydrating dimethyl ether before correction with the shift parameter, 3965 and 3707 cm-1; those hydrating methyl acetate, 3976 and 3794 cm-1; those for a free water, 4014 and 3867 cm-1). A similar tendency was previously reported by Kusanagi and Matsuda using the ab initio MO of the HF/4-31G level (νas and νs for water molecule hydrating dimethyl ether, 4048 and 3878 cm-1; that hydrating methyl acetate, 4090 and 3943 cm-1; that for a free water, 4124 and 3969 cm-1).31 The calculated values supported the difference between the wavenumbers for the O-H stretching vibration of free water and water hydrating ester and ether moieties. However, the wavenumbers obtained experimentally were always much smaller than the calculated ones by Kusanagi and Matsuda, which was attributed to the condensed systems at their actual hydration.31 D. Time Dependence of the Peak Intensity. Furthermore, we examined the dynamics of water sorption to the polymer matrix by the time-resolved IR measurements. The intensity of the difference spectra for water sorbed in the PEG films saturated by 830 s after onset of the flow of water vapor (50% RH) as shown in Figure 1. The difference spectra observed for PEG 49K and 60K (31) Kusanagi, H.; Matsuda, T. Kobunshi Ronbunshu 1996, 53, 716.
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Figure 6. Time evolution of peak areas for the O-H stretching band of water sorbed on PEG 60K film measured by transmission mode: O, peak 1; b, peak 2; 4, peak 3; 2, peak 4. Table 4. Relaxation Times for an Increase in the Absorbance of Water at Various States in and on PEG Films relaxation time τ (s) assignment peak 1 peak 2 peak 3 peak 4 peak X peak 1 peak 2 peak 3 peak 4 peak X
PEG-DME
60K
49K
28K
15K
11K
59 80 75 69 70
55 70 66 63 69
50 74 69 64 59
87 100 99 78 100
103 122 119 90 105
118 138 134 96 111
(a) Transmission Method 53 51 60 85 69 74 81 63 70 60 61 62
115 123 120 82
(b) ATR Method 116 98 120 114 116 110 91 74
and PEG-DME at any time could be decomposed into five Gaussian components of the same peak positions as those compiled in Table 1. Figure 6 shows the relationship between the area of each component (peaks 1-4) and the flowing time of the water vapor (50% RH) for PEG 60K. All data points could be fitted with single-exponential curves, and the relaxation time for each component was determined by using
A ) A0[1 - exp(-t/τ)]
(1)
where A, A0, and τ are the area for each component, the maximal area, and the relaxation time, respectively. The τ values obtained by the transmission mode (τtrans) are compiled in Table 4. In the case of PEG with smaller molecular weight, the broad band could be decomposed into six peaks, and the τ value for peak X was evaluated too (Table 4). E. ATR-IR Difference Spectra of Water Sorbed to PEG Films at 50% RH. Being different from the transmission IR band which provides the information of water locating both inside and at the surface of PEG films, 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 (dp ) 0.54 µm)32 of the evanescent IR. Therefore, the ATR-IR difference spectra for the PEG films with a thickness of 20 µm were not disturbed by the water at the outer surface of the films. To confirm this, the spectrum of sorbed water in the PEG 60K film with various thicknesses (4, 8, 16, and 25 µm) obtained by the transmission method was compared (32) dp is determined by using the following equation: dp ) λ1/{2π(sin2 θ - nPEG/ZnSe2)0.5}, where θ, nPEG/ZnSe, and λ1 are the incident angle of light on the surface of the ZnSe element, the ratio of refractive index of PEG to that of the ZnSe element (the refractive indices of PEG and ZnSe: 1.45 and 2.4, respectively), and the ratio of the wavelength (λ) of incident light to the refractive index of the ZnSe element, respectively. When λ of incident light is 3 µm (wavenumber ) 3333 cm-1), the value of dp is 0.54 µm.
Figure 7. O-H stretching band of sorbed water in PEG and PEG-DME films detected by ATR mode (830 s after onset of the sorption).
with that by the ATR method. With an increase in the thickness of the film, the absorbance in the low wavenumber region obtained by the former method became larger and approached that by the latter method (data not shown), which definitely indicates that the ATR spectra exclusively give the information within the film. The ATR-IR difference spectrum (Figure 7) of sorbed water in PEG films normalized at 3550 cm-1 was deconvoluted into five components with the same parameters as those for the transmission difference spectra. The relative intensity of peak 4 to peak 1 was 2 times larger than those obtained by the transmission method (Table 2), which shows that the percentage of bridging water at the surface is smaller than that inside the PEG film. It should be mentioned here that the influence of the surface layer considerably extends to the interior region, because the data obtained from the transmission and ATR methods were fairly different. In addition, the influence of the ZnSe crystal on the structure of the polymer film cannot be excluded in the ATR method, which is the best for the observation far from the air-polymer interface at this moment. The τ values determined by the ATR method for each component using eq 1 (τATR) are also shown in Table 4. The difference in the relaxation times obtained by the transmission and ATR methods would be mainly attributed to the difference in the regions observed: In the ATR measurements, the observation area existed at least 19 µm deep from the air-polymer film interface. Therefore, it took much longer time for the polymer film to be equilibrated with sorbed water than in the transmission measurements where the thickness of the film was 8 µm. F. Sorption Processes of Water to PEG Films. From the assignment and time evolution of the components of the O-H stretching IR band, the processes of sorption of water followed by the penetration into the polymer matrix might be as follows: (1) At the polymer surface, monomeric water binds to the oxygen atom of the polymer, because the τtrans value for peak 1 is the smallest and, subsequently, the bridging water is formed. (2) The formation of dimer is completed after the water sorption in the polymer matrix is equilibrated, because the τtrans values of peaks 2 and 3 are larger than those for peaks 1 and 4 and similar to each other. In the polymer matrix, the bridging water is equilibrated at first, and subsequently the binding water is equilibrated. Finally, the dimeric water is equilibrated by the attachment of a water molecule to the binding water. Previously, Sutandar et al. investigated the state of water inside of PMMA films by using the ATR-IR method and showed that the adsorption of water into the PMMA films sequentially proceeds from monomer, dimer, trimer, ..., small cluster, to large cluster.33 Their result was different from ours. Though Sutandar et al. also observed
Water Sorbed to Poly(ethylene glycol) Films
a prompt sorption of water to the PMMA film 10 min after the onset of the contact with water vapor, they mainly studied the process of water sorption in the long period (4 h), whereas we examined the early stage (0-830 s) of water sorption by using in situ difference spectroscopy. This would be the reason for the difference in the assignment of water molecules sorbed to the polymeric films by us from that by Sutandar et al. Conclusion It was proposed that five kinds of sorbed water molecules [binding water (peak 1), dimeric water (peaks 2 and 3), bridging water (peak 4), and hydrogen-bonded water to the end OH group (peak X)] exist in the PEG film. The penetration processes of these species could be presumed from the evolution time as follows: (1) At the polymer surface, a water molecule binds to the oxygen atoms of (33) Sutandar, P.; Ahn, D. J.; Franses, E. I. Macromolecules 1994, 27, 7316.
Langmuir, Vol. 17, No. 6, 2001 1895
PEG with one of its hydrogen atoms (“binding water”). The same phenomenon occurs more gradually in the polymer matrix too. (2) The water binding to the oxygen atoms of PEG molecules with both of its hydrogen atoms (“bridging water”) is formed from the binding water, while the water dimer is gradually formed by the association of the free water molecule with the binding water. (3) The amount of monomeric species (both binding water and bridging water) is equilibrated. (4) The water dimer is further formed by the binding of the free water molecule to the binding water, and finally the amount of dimer is equilibrated. Acknowledgment. This work was supported by Grants-in-Aid (11167236 and 12450381) from the Ministry of Education, Science, Sports and Culture. The authors are indebted to Rengo Co., Osaka, Japan, and Terumo Corp., Tokyo, Japan, for their financial support. LA0008986
