Anal. Chem. 2007, 79, 3455-3461
Infrared and Near-Infrared Spectral Evidence for Water Clustering in Highly Hydrated Poly(methyl Methacrylate) Reikichi Iwamoto*,† and Toshihiko Matsuda‡
NIRS Institute of Water, Yuyamadai 2-7-10, Kawanishi, Hyogo 666-0137, Japan, and KRI, Inc. Chudojiminamimachi 134, Shimogyoku, Kyoto 600-8813, Japan
First, we developed quantitative analytical methods of water in poly(methyl methacrylate) (PMMA) in various hydrated states by utilizing the first combination and OH stretching bands of water at about 5240 and 3630 cm-1, respectively. Next, we investigated how the state of water depended on its quantity or the mole ratio of water to the CO (denoted as the H2O/CO ratio), which only interacts with water in PMMA, mainly on the basis of the band feature of the OH stretching bands. Below the H2O/CO ratio of 0.032, the contained water, which shows two clear bands at about 3630 and 3550 cm-1, is hydrogen-bonded to two CdO groups as CdO::H-O-H::OdC to form “the hydration core”. The spectrum of the water that exceeds the ratio in question shows one broader band only, the frequency of which shifts downward with the increasing hydration. From detailed analysis of the behavior of the OH stretching and combination bands in relation to the H2O/CO ratio, we have concluded that the water that exceeds the hydration ratio becomes mobile to aggregate or “cluster” around hydrated sites rather than nonhydrated ones in the PMMA matrix, although the latter is much larger in population. Interactions of water in polymers have long been the subject of interest for polymer scientists.1-29 It was proposed from the * To whom correspondence should be addressed. E-mail: iwamoto@ kjb.biglobe.ne.jp. † NIRS Institute of Water. ‡ KRI, Inc. (1) Pauling, L. J. Am. Chem. Soc. 1945, 67, 555-557. (2) Dole, M.; Faller, I. L. J. Am. Chem. Soc. 1950, 72, 414-419. (3) Zimm, B. H.; Lundberg J. L. J. Phys. Chem. 1956, 60, 425-428. (4) Barrie, J. A.; Platt, B. Polymer 1963, 4, 303-313. (5) Barrie, J. A.; Machin, D. Trans. Faraday Soc. 1971, 67, 2970-2978. (6) Turner, D. T.; Abell, A. K. Polymer 1987, 28, 297-302. (7) Kalachandra, S.; Kusy, R. P. Polymer 1991, 32, 2428-2434. (8) Errede, L. A. Adv. Polym. Sci. 1991, (99), 1-93. (9) Lee, S.-B.; Rockett, T. J.; Hoffman, R. D. Polymer 1992, 33, 3691-3697. (10) Schult, K. A.; Paul, D. R. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 2805-2817. (11) Marais, S.; Nguyen, Q. T.; Devallencourt, C; Metayer, M.; Nguyen, T. U., Schaetzel, P. J. Poly. Sci., Part B: Polym. Phys. 2000, 38, 1998-2008. (12) Bashir, Z.; Church, S. P.; Waldron, D. Polymer 1994, 35, 967-976. (13) Kusanagi, H.; Yukawa, S. Polymer 1994, 35, 5637-5640. (14) Rueda, D. R.; Valkalis, A.; Viksne, A.; Calleja, F. J. B.; Zachmann, H. G. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 1653-1661. (15) Rueda, D. R.; Varkalis, A. J. Polym. Sci., Part B: Polym. Phys. 1995, 33, 2263-2268 10.1021/ac0621928 CCC: $37.00 Published on Web 03/31/2007
© 2007 American Chemical Society
studies of transportation properties of water in polymers that dissolved water formed clusters therein.3,4 Several studies were made afterward to explain transportation properties of water in polymers in terms of its clustering.5,8,10 Infrared spectroscopy should give important information about how water exists in polymers. Many scientists applied infrared spectroscopy to study intermolecular interactions of water in polymers,12-23 but their results were not always clear about how water exists therein. We also have studied interactions of water in various polymers by infrared and near-infrared spectroscopy.24-29 In the polymers, which possess a functional group of an acceptor type such as an ester group as in poly(ethylene-co-vinyl acetate),27 water is hydrogen-bonded through the OH to one or two CdO, depending on whether the COO group is rarely or densely distributed in the matrix, respectively. The results were negative about the clustering of water therein. In poly(ethylene-co-vinyl alcohol), which possesses an amphoteric functional group of alcoholic OH, the broad shape of the combination and OH stretching bands of the contained water seemed to indicate that water clusters in associated OH groups. However, consideration of the mole ratio of water per OH group led us to the conclusion that water does not cluster by itself but forms combined hydrogen-bonding networks with OH groups in the OH-associated domain, whereas water is one-to-one hydrogen-bonded as an acceptor to an OH in the OH-isolated domain.29 (16) Kusanagi, H. Chem. Lett. 1997, 683-684. (17) Kusanagi, H. Kobunshi Ronbunshu 1996, 53, 123-127. (18) Kusanagi, H.; Matsuda, T. Kobunshi Ronbunshu 1996, 53, 716-721. (19) Murphy, D.; de Pinho, M. N. J. Membr. Sci. 1995, 106, 243-257. (20) Marechal, Y. Faraday Discuss. 1996, 103, 349-361. (21) Pereira, M. R.; Yarwood, J. J. Chem. Soc., Faraday Trans. 1996, 92, 27312735. (22) Sammon, C.; Mura, C.; Yarwood, J.; Everall, N.; Swart, R.; Hodge, D. J. Phys. Chem. B 1998, 102, 3402-3411. (23) Sammon, C.; Deng, C.; Mura, C.; Yarwood, J. J. Mol. Liq. 2002, 101, 3554. (24) Iwamoto, R.; Oguro, K.; Sato, M.; Iseki, Y. J. Phys. Chem. B 2002, 106, 6973-6979. (25) Iwamoto, R.; Murase, H. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 17221729. (26) Iwamoto, R.; Matsuda, T.; Sasaki, T.; Kusanagi, H. J. Phys. Chem. B 2003, 107, 7976-7980. (27) Iwamoto, R.; Matsuda, T. J. Poly. Sci., Part B: Poly. Phys. 2005, 43, 777785. (28) Iwamoto, R.; Matsuda, T. Spectrochim Acta, Part A 2005, 62, 1016-1022. (29) Iwamoto, R.; Matsuda, T.; Amiya, S.; Yamamoto, T. J. Poly. Sci., Part B: Poly. Phys. 2006, 44, 2425-2437.
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Figure 1. A series of near-infrared and infrared spectra a-f for a PMMA sample of 1-mm thickness, which was fully hydrated in liquid water, during dehydration. The ordinate is common for all the spectra. The upper spectra in the region above 4800 cm-1 are expanded by 4.9 times against the given ordinate.
In the present report, we studied water in poly(methyl methacrylate) (PMMA), which is contained up to its saturation in immersion in water. We first established near-infrared and infrared methods for quantitative analysis of water in PMMA. Next we investigated how the spectrum of OH stretching bands of water depended on its content therein. The results strongly suggested that above a certain content water aggregates around hydrated CdO sites. EXPERIMENTAL SECTION Materials. The sample of PMMA was commercially obtained as pellets from General Scientific Corp. The sample of 0.15-mm thickness was prepared from the pellets by hot-press. The PMMA sample of 1-mm thickness was obtained from Mitsubishi Rayon Co. Ltd. Spectroscopic Measurements. We measured the nearinfrared and infrared spectra of a sample in the 11000-2100-cm-1 range by transmission at room temperature, using a Nicolet Magna 760 Fourier transform infrared spectrometer with a detector of DTGS, a beam splitter of CaF2, and white light, with the resolution of 4 cm-1 and 100 scans, the measurement time being 130 s. A sample was immersed in water at room temperature, so that it contained water to saturation. The sample of 1-mm thickness was immersed for more than 10 days for its saturation, but that of 0.15-mm thickness was immersed for several hours. The spectrum of a hydrated sample of 0.15-mm thickness, which had been immersed in water, was continuously measured through spontaneous dehydration in open air until it was dried to the level near the saturated content in the air, the time interval between measurements being ∼5 min at first and ∼25 min at last during the measurements. After dehydration in the open air, it was further dehydrated under a flow of dry air up to the most possible dehydration, the time interval between successive measurements being ∼10 min at first and ∼3 h at last. The spectrum of the sample of 1-mm thickness, which had been hydrated in open air, was intermittently measured under the flow of dry air, while it was being dehydrated in a small bottle half-filled with molecular sieve, until even the OH stretching band of water at 3456 Analytical Chemistry, Vol. 79, No. 9, May 1, 2007
∼3630 cm-1 completely disappeared. On the other hand, the spectrum of the thick sample, which was being hydrated by immersion in liquid water, was intermittently measured in open air, until the combination band of water did not further increase in intensity. The weight of the thick sample was measured each time after the spectrum was measured, during the process of dehydration or hydration, by a balance with the weighing resolution of 0.0001 g. The peak height intensities of a combination band at 5240 cm-1 (P5240) and an OH stretching band at 3630 cm-1 (P3630) of water and a combination around 4435 cm-1 (P4435) of PMMA as a reference were used, rather than their area intensities, for quantitative analysis, because the errors caused by baseline drawing are significantly smaller for the former than the latter. If needed, the bandwidth of a band is obtained by dividing the area by the peak height. We obtained the peak height and the area intensity of a band by the built-in program in the spectrometer. RESULTS AND DISCUSSION Quantitative Analysis of Water. To develop the quantitative relationship between the weight of contained water and the absorption intensity of a near-infrared or infrared band of water in PMMA up to a highly hydrated state, the following three conditions should be satisfied. First, the sample should contain a sufficient quantity of water that can be gravimetrically measurable. Second, the near-infrared or infrared band of interest should be sufficiently below the photometric saturation of a detector even in the most hydrated state. Third, the weight loss of water by evaporation, which occurs during a spectral measurement, must be small enough to be neglected. We checked as to how the weight loss of water during a spectral measurement depended on its thickness. It was found that the thickness of a sample should be ∼0.15 mm or less, in order that OH stretching bands of water are sufficiently below the photometric measurable limit even in the most hydrated state. However, the quantity of contained water was not only too small to be gravimetrically measured for that thickness, but also, water significantly escaped by evaporation during the time of a spectral
Table 1. Assignments of Main Bands of Interest Mainly in the Spectra in Figure 1 bands (cm-1)
assignmentsa
3416b 3438c 3546 3552 3629 4500-3800 4436 4681 5235 5238 6000-5500 5951
2ν(CdO) of hydrated CdO (PMMA) 2ν(CdO) (PMMA) (PMMA)d νs(OH) (H2O) νa(OH) (H2O) ν(CH) + δ(CH) of CH, CH2, and CH3 of PMMA ν(CH) + δ(CH) (CH3 of PMMA) 2ν(CdO) + ν(C-O) (COO of PMMA) (PMMA)d νa(OH) + δ(OH) (H2O) 2ν(CH) of CH, CH2, and CH3 of PMMA 2ν(CH) (CH3 of PMMA)
a 2ν(CdΟ), the overtone band of CdO stretching; ν (OH) and s νa(OH), symmetric and antisymmetric OH stretching, respectively, of H2O; ν(CH) + δ(CH), the first combination of CH stretching and CH deformation; 2ν(CdO) + ν(C-O), the second combination of CdO stretching and C-O stretching of COO; νa(OH) + δ(OH), the first combination of the antisymmetric OH stretching and the OH deformation; 2ν(CH), the first overtone of CH stretching. See Refs 27 and 30. b This band is from the spectrum in Figure 5. c This band is from the spectrum in Figure 4. d The bands are certainly assigned to PMMA, although it is not possible to assign any specific combination modes at present.
measurement. Thus, it was not possible to directly develop a quantitative analytical method that uses the OH stretching band, but this method was developed via utilization of the combination band, as will be discussed later. By contrast, for the sample of 1-mm thickness, the combination band of water was sufficiently below the measurable limit in intensity even in the most hydrated state, and the band did not actually change in intensity during a spectral measurement. The quantity of contained water was obtained from the change in weight during dehydration of a sample. Thus, we developed a near-infrared spectrometric method to quantitatively determine the content of water, as will be discussed below. Figure 1 shows representative near-infrared spectra of a hydrated sample of 1-mm thickness during dehydration. The strong absorptions in the 4500-3800-cm-1 region, which little change in intensity by dehydration, are assigned to the first combination bands of CH stretching and CH deformation of CH, CH2, and CH3 groups in PMMA.27,30 The absorptions in the 6000-5500-cm-1 region, which little change during dehydration, are assigned to the overtone bands of the CH stretching of the groups.27,30 The saturated band at ∼3400 cm-1, which does not change much in intensity during dehydration, as will be discussed later for Figure 4, is assigned to the overtone band of the CdO group.27 The weak bands at ∼4700 cm-1, which change little during dehydration, are assigned to the combination bands of the COO group.27 The absorption band at ∼3630 cm-1, which is saturated for the spectra of (a-d) during dehydration, clearly appears for (e), and completely disappears for (f), is assigned to the antisymmetric OH stretching band [which is denoted as νa(OH)] of water in the sample.27,28 The band at 3552 cm-1 is assigned to the symmetric OH stretching band [νs(OH)]. The weak band at 3546 cm-1, which appears even after complete dehydration, is assigned to PMMA. The band at ∼5240 cm-1, which is assigned to the first combination [νa(OH) + δ(OH)] of the OH stretching and OH deformation (30) Iwamoto, R.; Nara, A.; Matsuda, T. Appl. Spectrosc. 2006, 60, 450-458.
Figure 2. A series of the separated combination band of water during dehydration, in which the spectrum f at the complete dehydration was subtracted from the other spectra in Figure 1. The ordinate is common for all the spectra.
[δ(OH)] of the contained water,28 has an intensity of less than 0.5 in absorbance even in the most hydrated state and decreases in intensity with the progress of dehydration. The very weak peak, which appears at 5235 cm-1 for the spectrum of (f) at the complete dehydration, is assigned to PMMA. The assignments of the bands of interest are summarized in Table 1. Figure 2 shows a series of the combination band of water during dehydration, which we separated by subtracting the spectrum of (f) from the others in Figure 1 with the subtraction factor of 1. The intensity of the band was expressed as the ratio against the band of PMMA at ∼4435 cm-1, so that we do not need the thickness of a sample to determine the content of water. The quantity of water that was contained to its saturation in liquid water at room temperature was 2.2% in weight per dry PMMA at the relative intensity of P5240/P4435 of 0.183. The saturated content of water in open air was similarly ∼0.8% at the P5240/P4435 of 0.085. All the measured contents of water in weight percent per dry PMMA were well plotted against P5240/P4435 on a slightly curved line as in Figure 3. The relation of the content in weight percent or y to the relative intensity of P5240/P4435 or x is given by the equation,
y ) 7.75x + 23.16x2
(1)
with the correlation factor of 0.9998. The standard deviation was 0.012 wt % for the equation for 14 measured values from 0.082 to 2.19 wt %. As is seen from eq 1 or the curve in Figure 3, y is not exactly linear to x but contains some quadratic term. This is because the state of water somewhat depends on its content, as will be discussed later. The weakest observed intensity or P5240 for the sample of 1-mm thickness in the almost dehydrated state was 0.000 98 for the combination band at 5240 cm-1, although Analytical Chemistry, Vol. 79, No. 9, May 1, 2007
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measurement, because the evaporating portion cannot be negligible especially at the early stage of successive measurements. The quantity of water contained in the sample was actually determined for each spectrum in Figure 4, from the relative intensity (P5240/P4335 ) x) of the separated combination band, for example, as in Figure 5, according to eq 1. The quantities of water in weight percent (or y) were plotted against the relative intensity of the OH stretching band at 3630 cm-1 or P3630/P4435 ()xi) on a slightly curved line with the correlation factor of 0.9999, which is given by the following equation,
y ) 0.234xi + 0.0119xi2
Figure 3. Relation of the contained water in wt % per dry PMMA against the relative intensity of the combination of water at ∼5240 cm-1 or P5240/P4435 in the sample of 1-mm thickness.
the water content could not be measured by weight, and 0.0364 for the fundamental band at 3630 cm-1. This corresponds to the water content of 34 ppm in weight. Their S/N ratios were 9.5 and 117 for the former and latter bands, respectively, on the basis of peak-to-peak noises. The observation demonstrates that we can analyze water to a content of less than 10 ppm in weight with the combination band for water in a PMMA sample of 1-mm thickness. Water interacts only with CdO groups in PMMA, as was reported in a previous paper.27 That is, the number of water molecules per CdO, which is denoted as the H2O/CO ratio, is a key factor that defines the hydrated state in PMMA. To obtain the H2O/CO ratio, eq 1 has to be changed to,
yr ) 0.411x + 1.395x2
(2)
where yr is the mole ratio of H2O to CO (or COO) in PMMA and x is the same as in eq 1. The standard deviation is 0.0012 in the mole ratio for the equation with 14 measured values from 0.0046 to 0.122. Figure 4 shows the spectra (from a to d) of the PMMA sample of 0.15-mm thickness, which was hydrated in liquid water, during dehydration in open air and those (from e to h) of the same sample during dehydration after (d) under the flow of dry air to the most dehydrated. We assigned the bands in the 4500-4000 and 60005500-cm-1 regions in the above. The band at 3438 cm-1, which was assigned to the overtone of the CdO group (Table 1), is clear but not saturated for the thickness. All the spectra in Figure 4 show the very weak combination bands of contained water at ∼5240 cm-1 in addition to the strong OH stretching bands in the 3700-3500 cm-1 region. The combination band is clearly separated even for the weakest one in the g spectrum (Figure 4), as in Figure 5. The weak band at 3416 cm-1 in Figure 5 is assigned to the overtone of the hydrated CdO.27 The amount of contained water in the 0.15-mm-thick sample was determined in weight percent or H2O/CO ratio from the relative intensity of the combination band separated, according to eq 1 or 2, which is a function of the relative intensity of the combination band to the band at 4435 cm-1 of PMMA but not of the thickness of a sample. However, it should be kept in mind that the determined quantity is the average over the time period (130 s) of a spectral 3458
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(3)
The standard deviation is 0.006 wt % for the equation with eight obtained quantities from 0.015 to 1.582 wt %. The weakest observed intensities in peak height in absorbance were 0.0261 and 0.00064 for the bands at 3630 and 5240 cm-1, respectively, for a PMMA sample of 0.15-mm thickness. The intensities correspond to a water content of 170 ppm in weight. The S/N ratios (based on a peak-to-peak noise) were 139 and 4.0 for the former and latter, respectively. The combination band, the S/N ratio of which is as small as 4.0, shows a fairly clear signal, and this indicates that we can detect a band to the level of an S/N ratio of a little more than 1. This means that we can analyze the quantity of water to the content of a few ppm in weight by the band at 3630 cm-1 for the sample of 0.15-mm thickness. State of Water in the Matrix of PMMA. We obtained the infrared spectra of the water in a PMMA sample, the content of which was much changed from a nearly dehydrated state to that actually saturated in liquid water. We have established nearinfrared and infrared spectrometric methods to quantitatively analyze the quantity of water therein. Now we can investigate how the state of water, about which the OH stretching bands should give key information, depends on its quantity in PMMA or the H2O/CO ratio. Among a series of the spectra in Figure 4 of the water in PMMA during dehydration, a difference spectrum between a pair of successive spectra (a-b, for example) is of the water, which was contained at the time when the first spectrum (a, for example) was measured and evaporated before the time when the succeeding one (b, for example) was measured. Consequently, a series of the difference spectra of a - b, b - c, etc., should show how the spectra, which symbolize the state of the water, depend on the holding time in the sample before evaporation. Evaporation should occur from loosely bound water in the outer shell to more tightly bound water in the inner one in the hydration sphere. This means that a series of difference spectra of (p), (q), etc., in Figure 6, which correspond to the difference spectra of a - b, b - c, etc., respectively, among the spectra in Figure 4, should show how the OH stretching bands (or the state) of the water changes from the outer shell to the inner. The following changes are observed for the spectra in Figure 6. The spectra p and q, which are of the water in the outermost shells, have one peak only at ∼3605 cm-1. Spectrum r shows the main band at 3616 cm-1 with a shoulder at ∼3554 cm-1. Spectrum s shows two separate peaks at about 3630 and 3550 cm-1, and the spectral feature does not change after this. We will discuss the OH stretching bands in relation to the H2O/CO ratio, which is estimated by eq 2. Table
Figure 4. A series of near-infrared and infrared spectra a-h for a hydrated PMMA sample of 0.15-mm thickness, which was fully hydrated in liquid water, during dehydration. The ordinate is common for all the spectra.
Figure 5. Difference spectrum, in which the spectrum h was subtracted from that of g in Figure 4 with the subtraction factor of 1. The upper spectrum in the region above 4800 cm-1 is expanded by 28.4 times against the given ordinate.
2 gives the obtained H2O/CO ratios in the second column, of the water that shows spectra a, b, etc. (Figure 4) in the first column. The spectra s, t, u, and v, each of which shows two separate OH stretching bands as is seen from Figure 6, correspond to the differences of d - e, e - f, f - g, and g - h, respectively, among the spectra in Figure 4. This means that if the H2O/CO ratio is 0.032 or less (Table 1), the water shows clearly separated two bands. By contrast, spectrum r in Figure 6, which is of the water in the hydration shell slice from the ratio of 0.032 (d) to that of 0.047 (c), is considerably distorted from the above, and those of (q) and (p), which are of the water in the shell slices from the ratio of 0.047 (c) to that of 0.065 (b) and from the ratio of 0.065 (b) to that of 0.087 (a), respectively, show one broader band only, where (a), (b), etc., in parentheses indicate the notations of the spectra in Figure 4. This observation indicates that the H2O/CO ratio of 0.032 is a turning point that defines the hydrating structure of water in PMMA.
The water at the H2O/CO ratio of 0.032 or less, which shows the separate two OH stretching bands, should be hydrogenbonded to two CdO groups in the manner of
CdO::H-O-H::OdC
(I)
according to a previous paper,27 the structure being denoted as “the core hydration”. On the other hand, the rather broad OH stretching band with one peak only, which occurs above the H2O/ CO ratio of 0.032, suggests that the water is not hydrogen-bonded with order as in I but is somewhat disordered. From the observation we consider that the water aggregates around hydrated sites to form the outer hydration shell outside the core hydration. The dividend (denoted as n) of the H2O/CO ratio by 0.032, in the third column in Table 1, should give the average number of water molecules at one hydrated site above the H2O/ CO ratio of 0.032, although the number is 1 below the ratio. The water in the outer hydration shell, consisting of (n - 1) molecules, Analytical Chemistry, Vol. 79, No. 9, May 1, 2007
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obtained for the four points;
y ) 8.85x
(1′)
yr ) 0.493x
(2′)
instead of (1), and
Figure 6. A series of difference spectra in the region of OH stretching bands between a pair of successive spectra from a to h in Figure 4. Each spectrum from p to v gives the difference spectrum of (p) a - b, (q) b - c, (r) c - d, (s) d - e, (t) e - f, (u) f - g, and (v) g - h, the subtraction factor being 1 for all the cases. The given ordinate is of the top spectrum and is arbitrary for the other spectra. Table 2. Mole Ratios of H2O/CO, Estimated According to Eq 2 from the P5240/P4435 Valuesa notation of spectrum
mole ratio of H2O/COb
a b c d e f g h
0.087 0.065 0.047 0.032 (0.032) 0.025 (0.025) 0.015 (0.016) 0.0063 (0.0072) 0.0
n 2.7 2.0 1.5 1 1 1 1
a The P 5240 values were actually read from the separated spectra of water, in which spectrum h is subtracted from the others in Figure 4. The number n is the number of water molecules per hydration site (see the text). b The values in parentheses were calculated according to eq 2′.
may be hydrogen-bonded through the H to the O of other water molecules in the hydration sphere or to a surrounding CdO and through the O to the H of the other water molecule therein. The diverse hydrogen-bonding modes may give the broader band with one peak only. The above finding that one water molecule is hydrogen bonded to two CdO groups in the manner of I suggests that the water content in PMMA should be linear to the relative intensity of P5240/ P4435 below the H2O/CO ratio of 0.032 instead of the quadratic eq 1 or 2. The value of the ratio corresponds to the P5240/P4435 of 0.0639. The four observed points below this ratio can be linear to the relative intensity of P5240/P4435, as is seen from Figure 3. The following linear equations below x ) 0.0639 were tentatively 3460
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instead of (2), although standard deviations are not given because the observed points are too small. Actually, the H2O/CO ratios, which were calculated according to eq 2′ for the spectra d, e, f, and g in Figure 4, were not significantly different, as is seen from the values given in parentheses in Table 2. We also investigated how the combination band depended on the H2O/CO ratio. The frequency of the combination is ∼5245 cm-1, if the ratio is ∼0.03 or less, but as the ratio increases above this, it gradually shifts down, becoming 5236 cm-1 in the most hydrated state or at the H2O/CO ratio of 0.122. In addition, the bandwidth of the combination band, which is ∼90 cm-1 below the ratio of ∼0.03, increases with the increase of the H2O/CO ratio above this, to ∼120 cm-1 at the ratio of 0.122. Thus, what is observed for the combination is totally consistent with the interpretation that above the H2O/CO ratio of 0.032 water aggregates around the core hydration. However, it should be added for comparison that the water contained in poly(ethylene terephthalate), which is hydrogen-bonded to two CdO groups just similar in PMMA below the H2O/CO ratio,31 has the hydrogenbonding structure up to the almost saturated content of water, which is 0.087 as the H2O/CO ratio.31 This indicates that the aggregating behavior of water in a polymer changes, depending on the property of a polymer matrix. The OH stretching band at 3605 cm-1 in the spectra p and q in Figure 6 is somewhat similar in band shape to that of liquid water but significantly higher in frequency than that of liquid water at ∼3420 cm-1.29 The frequency of the combination band at 5226 cm-1 for the sliced outermost hydration shell32 is also significantly higher than that of liquid water at 5176 cm-1.29 This may be explained as follows. If an outer hydration shell consists of a small number of molecules as in the present case, the interactions of the water with surrounding CdO groups in the surrounding matrix should have a larger contribution to the observed frequency. The spectrum of the water in the outer hydration shell is similar in frequency and shape to that of the water that is dissolved in propylene carbonate at a very low concentration, which shows some peculiar properties.33 As the outer hydration shell consists of more water molecules, the contribution of the interactions with the surrounding matrix should become smaller and, instead water-water interactions, should have a larger contribution, giving rise to downshifts of the OH stretching and combination bands. CONCLUSION The present study has demonstrated that the combination band of water is useful and sensitive to analyze the quantity of water in (31) Iwamoto, R. Unpublished. (32) The combination band in the difference spectrum, in which we subtracted the spectrum at the H2O/CO ratio of 0.10 from that at the ratio of 0.12, shows the frequency of 5226 cm-1. The band is of the outermost hydration shell in the present study. The largest content of water had the n number of 3.8 for a sample of 1-mm thickness; that is, the outer hydration layer consists of about three molecules. (33) Dei, L.; Grassi, S. J. Phys. Chem. B 2006, 110, 12191-12197.
PMMA. It is important to note that the combination band appears in the frequency region where most polymers do not have any seriously disturbing absorptions, even if they have an alcoholic OH.29 The OH stretching bands can be used to more sensitively analyze water in thinner polymer films than the combination band and to follow the rapidly changing content of water therein. On the basis of spectral analysis of contained water in relation to the H2O/CO ratio in PMMA, we have found that the hydration structure of the water therein changes at the H2O/CO ratio of (34) It should be noted that the value of 0.032, which was roughly determined from the feature of the spectra in Figure 6 during dehydration in the present study, is not exact but approximate.
0.032.34 At this ratio only ∼6 out of 100 CdO groups are hydrated as in I, but the remaining ones (or 96 or so) are free from hydration. This means that if the H2O/CO ratio is higher than that value, the newly coming water is hydrogen-bonded to a hydrated CdO site rather than to a nonhydrated one, although the latter is much larger in population. This may imply that above the ratio the water becomes mobile under the interactive influence of hydrated sites in the neighborhood in the partly hydrated matrix in PMMA. Received for review November 21, 2006. Accepted March 2, 2007. AC0621928
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