Article pubs.acs.org/JPCA
Hydration Structure and Mobility of the Water Contained in Poly(ethylene terephthalate) Reikichi Iwamoto* and Hiroshi Kusanagi NIRS Institute of Water, Yuyamadai 2-7-10, Kawanishi, Hyogo Pref. 666-0137, Japan ABSTRACT: We have studied the state of the water contained in poly(ethylene terephthalate) (PET), which consists of repeat units of OCC6H4COOCH2CH2O, in variously hydrated states. We first determined the hydration structure of the water therein not only from its OH stretching spectrum in a thinner sample but also from the hydration energy, the effect of the hydrogen bonding on the lengths of the donor and acceptor bonds, and the OH stretching frequencies of the water for the optimized 1:1 hydrate structures (quantum-chemically calculated). It has been found that the water bridges two ester CO’s in the manner of CO···HOH···OC therein and that about a 0.05 mole fraction of the CO groups is bridged by the water in a PET sample hydrated in open air. We then carefully analyzed the state of the hydrating water in a thicker sample from its combination band around 5240 cm−1, which significantly changes in frequency and bandwidth depending on the quantity of the contained water. It has been shown that the hydrating water molecules are so mobile as to begin to intermolecularly interact among themselves at a low hydration density of 10−15 water molecules per 1000 repeat units of OCC6H4COOCH2CH2O in the solid matrix.
1. INTRODUCTION Water, which is a small actively interactive molecule, ubiquitously exists and penetrates most polymeric materials by weak or strong hydrogen-bonding interactions.1 Not only the quantity of the contained water but also the interaction pattern vary from one polymer to another.1−3 Synthetic polymer materials in use in the atmosphere should absorb an amount of water. The water may have an influence on some “intrinsic” physical or chemical properties of a polymer.4 To have correct knowledge of the possible effect, it is important to analyze the state of the contained water. Fourier transform infrared (FT-IR) spectroscopy is one of the most powerful methods used to study the water contained in a polymer.2,3,5−7 A water molecule shows its OH stretching bands in the 3500−3700 cm−1 region.8 The frequencies and shapes of the OH stretching bands sensitively change in response to how the contained water interacts in a polymer.3,7 Recently, an increasing number of studies have been made to investigate the water dissolved in organic and polymeric materials.2,3,7,9−25 Interactions of the hydrating water therein have been considerably well understood with respect to the early stage of the interactions by the applications of infrared spectroscopy, and it has been established that the water interacts in most cases with the functional groups possessed therein.2,3,7,9,16 In the present paper we study the water contained in poly(ethylene terephthalate) (PET) or (OCC6H4COOCH2CH2O)n, which is a representative hydrophobic polymer of excellent quality.26 PET is one of the most important polymers and is widely utilized for textiles, films, recording tapes, etc.26 Few studies have been made with respect to the state of the water contained therein,13 although its transportation property was reported in a few papers.13,27 PET possesses the common functional groups of phenyl and ester moieties, both of which can interact with water, and it is an © 2015 American Chemical Society
appropriate polymer for which we can investigate how the hydrating water interacts with the two types of functional groups contained. Knowledge about the interactions of water in PET should give useful information that contributes to generally understanding the hydration property of the synthetic or natural polymeric materials that have two or more different types of functional groups. We apply Fourier transform infrared and near-infrared (FTIR/NIR) spectroscopy and the quantum-chemical method to study interactions of the water in PET. First, we study a basic interaction pattern of the water in PET, and then we investigate the intermolecular interactions among the hydrating water molecules in the matrix using the combination band of the water in the thick PET samples, the hydration extents of which change from nearly perfect dryness to fully saturated hydration.
2. EXPERIMENTAL SECTION 2.1. Materials. Pellet samples of PET were commercially obtained from General Science Corporation, Ltd. Thinner film samples (0.1−0.5 mm thickness) were prepared from the pellets by hot pressing at temperatures of 180−220 °C. The PET plate samples of 2 mm thickness, which were used to measure both the near-infrared spectra and the weights of the contained water in the variously hydrated states, were obtained from Kuraray Co. Ltd. 2.2. Spectroscopic Measurements. We measured the infrared and near-infrared spectra of a sample in the 400− 11000 cm−1 range by transmission at room temperature using a Nicolet Magna 760 Fourier transform infrared/near-infrared spectrometer (FT-IR/NIR) with a detector of DTGS, a beam Received: January 14, 2015 Revised: February 26, 2015 Published: March 11, 2015 2885
DOI: 10.1021/acs.jpca.5b00385 J. Phys. Chem. A 2015, 119, 2885−2894
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The Journal of Physical Chemistry A
Figure 1. Infrared and near-infrared spectra of the 0.5 mm thick PET sample, which had been kept hydrated in open air before the measurement, during dehydration (from c to a) under the flow of dry air.
again through the rehydration process of the sample in open air and then by immersion in liquid water at room temperature until the sample was fully hydrated to saturation after prolonged immersion (about three months in water). The saturation was confirmed by observing that the combination band around 5240 cm−1 of the water did not increase further in intensity. Two PET samples of 3.0782 (no. 1) and 3.0913 g (no. 2) in dry weight were used for the measurements. To estimate the area intensity of either a combination band at 5240 cm−1 (denoted as A5240) due to the contained water or a combination band at 4632 cm−1 (A4632) due to the PET (used as the intensity reference for the band of the water), we appropriately drew a linear baseline tangent to the spectrum at the wings of the band in question. 2.3. Quantum-Chemical Calculation. Methyl phenylcarboxyethylene 4-terephthalate (MPT) with the chemical structure of CH3OOCC6H4COOCH2CH2OOCC6H5, which contains one repeat unit OCC6H4COOCH2CH2O of PET, was chosen for a model compound of PET. Quantum-chemical calculations were carried out using a personal computer (Hewlett-Packard s5330jp) on which the Gaussian 03 program was installed. The purpose of the calculation is to quantumchemically predict, under the limited power of the computer used, whether the water is hydrogen-bonded either to only the ester CO or to both of the CO’s and the phenyl group in PET. Therefore, we treated the 1:1 hydrate complex of MPT, in which one OH of the water is hydrogen-bonded to the ester CO or the phenyl group of the phthalate part, instead of the 1:2 complex. The total quantum-chemical attractive energy, including the hydrogen bond and van der Waals energies, is sometimes denoted as the hydration energy hereafter and was calculated for the hydrate complexes. The starting structure was first selected by trial and error from some plausible hydrate structures of MPT, in which one water is hydrogen-bonded to the ester CO or the phenyl group. The selected structures were then refined by the density functional method (B3LYP/631G(d)), and the optimized structure was obtained for each of the two types of the hydrate complexes of MPT. Normal vibrations were calculated for the water in the optimized structures, and the frequencies obtained here were multiplied by a scale factor of 0.9613.28
splitter of Ge/KBr or CaF2, an infrared light or a white light source, a resolution of 4 cm−1, and 100 scans. PET samples of three different thicknesses (0.1, 0.5, and 2 mm) were used in the present study. The 0.1 mm thick sample was used to obtain the infrared spectrum in the 400−4000 cm−1 region of the water contained therein. The 0.5 mm thick one was used to measure the OH stretching and combination spectra of the contained water. The 2 mm thick ones were used especially to measure both the near-infrared spectra and the quantities of the contained water in the variously hydrated states, as explained below in detail. The spectrum of the 0.1 or 0.5 mm sample, which had been kept exposed to open air, was consecutively measured during its continuous dehydration under the flow of dry air for about 10 h in the spectrometer. The spectra of the contained water were separated by subtracting the spectrum of the PET sample in the most dehydrated state from those in the hydrated states during the process of dehydration, with a subtraction factor of 1.0. The PET sample of 2 mm thickness was used for the following two reasons. First, a sample weight of several grams was needed to weigh the quantity of contained water with sufficiently high accuracy because the weighing resolution of the balance in use was 0.0001 g. Second, the spectrum and the weight of a sample were measured at different places, and the contained water had to be kept without any significant evaporation loss during the transfer. A series of spectral and weight measurements of a 2 mm thick sample was carried out through the whole process from its complete dryness to its fully saturated hydration in liquid water as follows. To catch the exact hydration extent of a PET sample in open air, we first measured the spectrum of a sample, which had been kept hydrated in open air at room temperature, without any flow of dry air in the spectrometer, and we also weighed the sample. Subsequently, the spectrum and weight of the sample were continuously measured in the sample’s dehydration process under the flow of dry air in the spectrometer for as long as the drying procedure was effective. When this became ineffective, the sample was further dehydrated in a small bottle half-filled with molecular sieves. The spectral and weight measurements were similarly continued until the sample was perfectly dried, with the dehydration being detected by the disappearance of the OH stretching band of the water around 3630 cm−1. After this, the spectral and weight measurements were continued 2886
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The Journal of Physical Chemistry A Table 1. Assignments of the Infrared and Near-Infrared Absorptions of PET frequency region of absorption bands (cm−1)
intensity
3100−3200 3200−3800 3432 3553 3500−4500 4085 4500−4700 5122 5600−5900 5900−6200
strong to weak weak very strong strong medium strong, sharp weak very weak weak, broader weak, sharper
combination or overtone modes of aliphatic CH
combination or overtone modes of aromatic CH
overtone modes of ester CO
the tail of ν1(CH) 3δ1(CH) 2ν1(CO) 2ν1(CO) ν1(CH) + δ1(CH)
ν1(CH) + δ1(CH) ν1(CH) + ν1(CC) ν1(CH) + ν1(CC) 3ν1(CO)
1
2ν (CH) 2ν1(CH)
Figure 2. Spectrum of the contained water in sample S, obtained by subtracting spectrum a from spectrum c in Figure 1, with the ordinate scale of the upper spectrum in the 4500−6500 cm−1 region being expanded to 15 times that of the shown ordinate.
Figure 3. Infrared spectrum, separated by subtraction, of the contained water in the 0.1 mm thick PET sample.
3. RESULTS AND DISCUSSION 3.1. Infrared and Near-Infrared Spectrum of the Water Contained in PET. Figure 1 representatively shows three spectra of a 0.5 mm thick film sample (denoted as sample S) in the dehydration process. Of the many absorptions in the spectra, we found that the bands around 3550 and 3630 cm−1 and the much weaker one around 5240 cm−1, all of which decrease in intensity with the progress of dehydration, are totally or partly assigned to the contained water. All the other absorptions, which remain unchanged in intensity during dehydration, are assigned to the combinations or overtones of the CH and CC vibrations of aliphatic or aromatic groups
and of the CO stretching vibration of the ester CO in the repeat unit OCC6H4COOCH2CH2O of PET. The absorptions are classified into several different combination or overtone modes, as shown in Table 1. In the table, the notations ν1 and δ1 are used to denote the stretching and deformation vibrations, respectively, of anharmonic character instead of notations ν and δ, which are conventionally used to denote harmonic vibrations. The very strong band at 3432 cm−1 is assigned to the CO stretching overtone, or [2ν1(CO)], of the ester CO. The characteristic weak triplet bands in the 4500−4700 cm−1 region are assigned to the combinations with the [ν1(CH) + ν1(C C)] mode of the aromatic ring.29 2887
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The Journal of Physical Chemistry A Figure 2 shows the spectrum of the water contained in sample S. The distinct OH stretching bands at 3634 and 3554 cm−1 are respectively assigned to the asymmetric and symmetric OH stretching vibrations (denoted as ν1a(OH) and ν1s(OH), respectively).16,30 The spectrum additionally shows a weak clear band at 5243 cm−1 that is assigned to the combination between the asymmetric OH stretching and the OH deformation (denoted as δ1(OH)) of the [ν1a(OH) + δ1(OH)] mode.30,31 To observe the δ1(OH) band of the water, we measured the spectra of a 0.1 mm thick PET film sample during dehydration. The infrared spectrum (Figure 3) of the water contained therein shows the OH deformation band at 1627 cm−1 as well as the OH stretching bands around 3600 cm−1, although the spectrum is somewhat disturbed by interference fringes and the noise caused by the subtraction of the strong absorptions of the PET substrate itself. The frequencies and the spectral feature of the OH stretching bands of the water perfectly agree with those in the spectrum in Figure 2. Note that the observed frequency (5243 cm−1) of the [ν1a(OH) + δ1(OH)] combination reasonably agrees with the sum frequency of the components as 5261 = 3634 + 1627, with the discrepancy of 18 cm−1 being caused by the anharmonic nature of the vibrations.31,32 3.2. Hydration Structure of the Water in PET. 3.2.1. Spectroscopic Analysis. According to previous reports,16,30 the hydrating water molecule does not usually aggregate itself in hydrophobic organic materials if these do not possess electrolytic, ionic, or self-clustering functional groups,3,17,22,33 but it becomes individually hydrogen-bonded to one or two functional groups possessed in the manner shown in I and II,
A ··· HbOH f A ··· HbOHb ··· A
Table 2. OH Stretching Bands of the Contained Water in PET in Comparison with Those of the One-Bonded and Two-Bonded Watersa functional groups PET ester CO of MD onebonded twobonded ester CO of PVAc and EVAc onebonded twobonded ester CO of PMMA amide CO of DEDA onebonded twobonded
ν1s(OHb)
ν1(OHb)
3554
ν1a(OHb)
ν1f(OHf)
3634
3564 3558
ref this work 30
3698 3642 16
3550
3688
3550
3629
3552
3629
18 30
3487 3468
3692 3536
a
One- and two-bonded waters are hydrogen-bonded to the ester C O of methyl decanoate (MD) in heptane solution, poly(vinyl acetate) (PVAc) and poly(ethylene-co-vinyl acetate) (EVAc), and poly(methyl methacrylate) (PMMA), and to the amide CO of N,Ndiethyldodecylamide (DEDA) in heptane solution.
bands are similar in frequency to those of the two-bonded water that is hydrogen-bonded to the ester CO of various compounds, as shown in Table 2. From the frequency similarity it is reasonably inferred that the contained water is totally hydrogen-bonded to the two ester CO’s in the PET in the manner shown in III:
(I) (II)
where A is the functional group of an acceptor type. Here, O Hf and OHb denote free and bonded OH, respectively, of the hydrating water. The HbOHf and HbOHb in I and II are denoted as “one-bonded” and “two-bonded” water, respectively. The one-bonded water shows a very sharp band of the OH stretching around 3700 cm−1 due to the OHf (denoted as ν1(OHf)) and a broader one due to the OHb (ν1(OHb)), the frequency of which changes in the 3600−3400 cm−1 range depending on the hydrogen bond strength with A.2,16,30 In contrast, the two-bonded water, which is symmetric, shows symmetric and asymmetric OH stretching bands (ν1s(OHb) and ν1a(OHb), respectively).34 Both bands change in frequency depending on the hydrogen bond strength with A. At a very low concentration of A the dissolved water exists only as the one-bonded form shown in I, and as the concentration of A increases, the two-bonded water form shown in II begins to coexist with the one-bonded form and increases in its relative population.2,16,30 The one-bonded water is certainly identified without fail from the characteristic sharp ν1(OHf) band around 3700 cm−1.2,16,30 The observed frequencies of the water dissolved in the PET are compared with those of the ν1s(OHb), ν1(OHb), ν1a(OHb), and ν1(OHf) observed for the water hydrogen-bonded to ester CO and also to amide C O, as shown in Table 2. The spectrum of the contained water in PET, shown in Figures 2 and 3, exhibits clearly two separate bands with peaks at about 3635 and 3555 cm−1 and does not show any high frequency absorption around 3700 cm−1. In addition, the two
CO ··· HbOHb ··· OC
(III)
The spectrum in Figure 2 shows a weak sideband at 3420 cm−1, which is lower by 12 cm−1 than that of the overtone band at 3432 cm−1 of the CO stretching vibration (see the spectra in Figure 1). Appearance of the sideband is explained as follows. The hydration should cause some intensity decrease of the overtone band at 3432 cm−1, which is due to the “nonhydrated” CO, and give rise to the appearance of a new band due to the “hydrated” CO at a lower frequency. Subtraction of the spectrum of the less-hydrated sample (spectrum a in Figure 1) from that of the more-hydrated one (spectrum c) at the subtraction constant of 1.0 should bring about a “minus” peak at the frequency of the overtone band of the nonhydrated CO and a “plus” peak at that of the hydrated one. The former and the latter correspond to the hollow at 3432 cm−1 and the weak sideband at 3420 cm−1, respectively, in Figure 2. On the one hand, the above observation directly verifies that the water is hydrogen-bonded to the ester CO in PET, consistent with the previous reports.16,18,30 On the other hand, PET also has phenyl groups, which can interact with water.7 In fact, polystyrene (PS) (consisting of the repeat unit of CH2CHC6H5), in which the phenyl group is the only functional group that can interact with water, absorbs water, showing clearly two separated bands at 3586 and 3672 cm−1.7,35 The frequencies are higher by 30−40 cm−1 than those of the water in PET, indicating that the hydrogen bond strength that exerts 2888
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donor OH of the water, which is placed out of the terephthalate plane, is hydrogen-bonded almost to the ortho carbon atom of the phenyl group, leaving the other OH free. Table 3 gives the structure parameters of the optimized structures of A and B, together with the starting molecular parameters used. In structure A, both OHb and OC of the HfOHb···O C linkage are considerably lengthened by the hydrogen bonding, indicating that the hydrogen bond between the OH of the water and the ester CO is so strong as to cause the lengthening. The distance d(H···O(C)) (1.956 Å) between the H atom of the donor OH of the water and the O atom of the CO group, which is significantly smaller than the sum of the respective van der Waals radii (O, 1.52 Å; H, 1.20 Å),36 is reasonable for the hydrogen-bonded linkage when compared to reported values.37 In structure B, the OHb bond of the water, which is hydrogen-bonded to the phenyl moiety, is only slightly lengthened, indicating that the hydrogen bond is weak. The interatomic distance d(H···C(ortho)) (2.614 Å) is shorter than either the reported distance (2.66 Å) for OH···Ph (Ph: phenyl plane)38 or the sum of the van der Waals radii (C, 1.70 Å; H, 1.20 Å).36 For consideration, Table 3 additionally gives some interatomic distances and angles related with the hydrogen bonding in the hydrate structures. Table 4 gives the hydration energies obtained for optimized structures A and B, together with the OH stretching
the water is considerably weaker in PS or in benzene than in PET. The observations strongly and consistently suggest that the water is hydrogen-bonded to only the ester CO’s. However, the relatively broader bands in question in Figures 2 and 3 do not completely deny the possibility that they are overlapped, to some extent, by the absorptions due to the water hydrogen-bonded to the phenyl group. 3.2.2. Theoretical Investigation of the Hydration Structure. In what follows, we investigate the possibility that the water can be hydrogen-bonded to the phenyl group in addition to the ester CO in PET with the results of quantum-chemical calculations. Figure 4 shows the obtained optimized structures, in which a water molecule is hydrogen-bonded to the ester CO (A) or
Table 4. Calculated OH Stretching Frequencies of the Water, Hydrogen-Bonded to the Ester CO or the Phenyl Group, and the Calculated Hydration Energies for Optimized Hydrate Complexes A and B of MPT A, ester CO···HbOHf B, Ph···HbOHf hydration energy (kcal/mol) ν(OHf) (cm−1) ν(OHb) (cm−1)
Figure 4. Optimized structures of the hydrate complexes, in which one OH of the water is hydrogen-bonded to (A) the ester CO and (B) the phenyl group of MPT. Shown are (b1) the view projected onto the terephathalate plane and (b2) the side view to the plane.
8.37
4.71
3659 3526
3689 3577
frequencies of the hydrating water. The calculated hydration energy is much larger for structure A than for structure B, which is consistent with the markedly different effect of the hydrogen bond on the lengths of the donor and acceptor bonds for the two structures given in Table 3. In harmony with this, the calculated frequency (3526 cm−1) for the bonded OH or ν(OHb) of the one-bonded water in hydrate A is lower by as much as 51 cm−1 than the calculated frequency (3577 cm−1) in hydrate B, as shown in Table 2. The above-discussed effect of the hydrogen bonding on the bond lengths of the donor OH and the acceptor CO, the
the phenyl group (B) of the terephthalate part of MPT. In structure A, one OH of the water is hydrogen-bonded as a donor to the CO of the ester group that bridges the terephthalate unit and the ethylene oxide part, leaving the other OH free. The donor OH of the water lies in the plane of the terephthalate group with the free OH directing outward from the plane. In structure B, the b1 and b2 pictures are shown to display the spatial arrangement in the hydration structure. The
Table 3. Structure Parameters Obtained for Optimized Hydrate Structures A and B, and the Starting Parameters of the Water and the CO bonds, interatomic distances, or angles
starting parameters
A, HfOHb···OC
B, HfOHb···Ph
r(CO) (Å) r(OHb) (Å) r(OHf) (Å) ∠HOH (deg) d(H···O(C)) (Å) d(H···C(ortho)) (Å) ∠OH···O(C) (deg) ∠OH···C(ortho) (deg) ∠CO···H(O) (deg) ∠(O)H···C(ortho)Ph(plane) (deg)
1.215 0.969 0.969 103.6
1.222 0.975 0.969 103.1 1.956
0.970 0.969 107.3 2.614
135.3 151.4 163.3 100.9 2889
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Figure 5. (A) Series of the representative near-infrared spectra of a 2 mm thick PET sample (no. 1) at widely hydrated states from the perfect dehydration (spectrum a) through the intermediate (b to m) to the saturated hydration (n), and (B) the infrared region of spectra a, b, and c.
weak band at 5258 cm−1 is not of the water but of the PET. Spectrum b, which has a slightly stronger absorption around 5240 cm−1 than that in spectrum a, displays an enhanced shoulder around 3630 cm−1 with an intensity of about 1.5 in absorbance, as shown in Figure 5B. The combination band in spectrum c is stronger than that in spectrum b, and the corresponding fundamental band shows a clear separate peak at 3629 cm−1 with an intensity of about 2.4 in absorbance, as shown in Figure 5B. The spectra in Figure 5A,B typically exhibit the spectral characteristics of the OH stretching fundamental and the combination band, respectively, of the water. The combination band, which is well separated and has a suitable intensity range below 0.7 in absorbance for the water contained in the 2 mm thick sample up to its full hydration, should be useful for determining the quantity of the contained water. In contrast, the OH stretching band is useful for detecting a very low content but is too strong for using to quantitatively analyze the water in the sample. We examined the relationship between the intensities of the combination band in the spectra of the two PET samples that were 2 mm thick and the quantities of the water contained therein. Here, we divided the area intensity (A5240) of the combination band of the water by that (A4632) of the 4632 cm−1 band due to PET itself, changing it to the “referenced” intensity or A5240/A4632. The quantity of the contained water is given as percent to the dry weight of the PET sample, or {100[wt(H2O)/wt(PETdry)]}. The quantities of the water in weight percent are plotted against the referenced intensities of the combination bands as the abscissa in Figure 6. All the observed intensities of the combination band are excellently related to
hydration energies, and the OH stretching frequencies of the water in the optimized structures show altogether that hydrate A (Figure 4), in which the water is hydrogen-bonded to the ester CO, is much more stable than hydrate B, in which the water is hydrogen-bonded to the phenyl group. Considering this and the above inference deduced from the infrared observations, we have finally concluded that the water is predominantly hydrogen-bonded to the ester CO as in III in the PET matrix. 3.3. State of the Hydrating Water in the PET Matrix. Here we consider the state of the water molecules contained in the PET matrix in light of its combination band. First, we discuss how the intensity of the combination band changes with the increasing quantity of the contained water. Then, we consider the state of the water from the combination band, the frequency and bandwidth of which notably depend on the quantities of contained water. 3.3.1. Combination Band and the Quantities of Contained Water. Figure 5A representatively shows a series of the nearinfrared spectra of the 2 mm thick PET sample (no. 1) in the transition from perfect dryness (spectrum a) to fully saturated hydration (spectrum n). The combination band around 5240 cm−1 of the water increases in intensity with increasing water content, being maximized to about 0.65 in absorbance. In contrast, the three distinct bands in the 4500−4700 cm−1 region, which are assigned to the phenyl group (see Table 1), do not change in intensity. Figure 5B shows the OH stretching fundamental region of spectra a, b, and c at low hydrations for comparison. Spectrum a shows a smooth valley curvature around 3630 cm−1, verifying that the PET sample is completely dehydrated at the measurement and also indicating that a very 2890
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A difference spectrum between a neighboring pair among the spectra in Figure 5 should represent the state of the water at the “average” hydration between the relevant hydrated states as well as the difference in water content between them. Here we look at the state of the water at (i) the initial (denoted as stage 1), (ii) the middle (stage 2), and (iii) the last stage (stage 3) in the hydration course. These may be represented by the differences between spectra b and a, i and h, and n and m (denoted as b − a, i − h, and n − m, respectively), judging from the water contents given in Table 5. Figure 7 shows the difference spectra of b − a, i − h, and n − m, which are respectively denoted as p, q, and r. The peak
Figure 6. Plot of water contents in wt % {100 × [wt(H2O)/ wt(PETdry)]} against the referenced intensities (A5240/A4632) of the combination band for the two 2 mm thick samples used.
the quantities of the water by the linear relationship of eq 1, as shown in Figure 6, y = 0.272x − 0.010 (1) with the correlation factor R2 = 0.999. This indicates that the quantity in wt % of the water contained in a PET sample can be usefully estimated from the referenced intensity of the combination band with eq 1. The water quantities contained (which correspond to the spectra a, b, and c shown in Figure 1), for example, in the 0.5 mm thick sample during dehydration from the hydrated state in open air, are estimated as 0.05, 0.10, and 0.18% in weight, respectively, from the A5240/A4632 values by eq 1. 3.3.2. Mobility of the Hydrating Water. Here we investigate the intermolecular interactions among water molecules contained in the PET matrix, represented by the combination band in the spectra of sample no. 1, shown in Figure 5A. Table 5 gives the water contents in wt % or {100[wt(H2O)/ wt(PETdry)]} and the number of water molecules per repeat unit (or mole ratio of H2O/repeat unit) in the variously hydrated states, which correspond to the spectra from a to n in Figure 5A. Table 5. Water Contents Relative to Dry PET Weight and Mole Ratios of the Water per Repeat Unit in the Hydrated States Which Correspond to the Spectra from a to n in Figure 5A spectrum notationa
water content (wt %)
mole ratio (H2O/repeat unit)
a b c d e f g h i j kb l m n
0 0.042 0.084 0.12 0.21 0.25 0.30 0.36 0.41 0.50 0.56 0.69 0.79 0.86
0 0.005 0.009 0.013 0.022 0.026 0.032 0.038 0.044 0.053 0.059 0.074 0.085 0.092
Figure 7. Differences b − a, i − h, and n − m (denoted as p, q, and r, respectively) among the spectra in Figure 5A, with the shown ordinate being that of spectrum r and the ordinate magnitude of the frame being 0.065, 0.055, and 0.064 in absorbance for spectra p, q, and r, respectively.
frequency of the combination band shifts notably downward from 5243 cm−1 in spectrum p at stage 1 to 5228 cm−1 in spectrum r at stage 3. Note here that the ν1a(OH) component of the [ν1a(OH) + δ1(OH)] mode of the combination shifts downward with increasing strength of the hydrogen bond involved but the other component δ1(OH) shifts upward, with the shift of the latter being considerably smaller than that of the former.31 This means that the increased hydration should have a larger effect on the state of the water than expected from the observed frequency shift of the combination band. On the one hand, the above-mentioned spectral change of the combination band, which follows the increasing hydration, should be most probably caused by intermolecular interactions among the hydrating water molecules in the PET matrix. On the other hand, there is the possibility that a pair of the CO
a The notations correspond to those of the spectra in Figure 5A. bThe spectrum of the sample that was kept hydrated in open air at room temperature (see the text).
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The Journal of Physical Chemistry A Table 6. Spectral Characteristics of the Water and Hydration Indexes at Several Hydration Stages
a
hydration stage
spectrum notation
hydration rangea
hydration index (H2O/1000 repeat units)
peak freq (cm−1)
bandwidth (cm−1)
1 1-1 1-2 2 3
p
0.005−0.000 0.013−0.009 0.026−0.022 0.044−0.038 0.092−0.085
5 11 24 41 89
5243 5243 5241 5240 5228
76.6 77.8 89.6 92.6 110.2
q r
The hydration range in mole ratio (H2O/repeat unit) at the hydrations in Table 5.
groups, which are favorably arranged for the water bridging as in III, should be successively occupied with increasing hydration, decreasing in the capacity to form the water bridge, giving rise to the formation of the one-bonded water as in C O···HbOHf, and bringing about the “observed” spectral change in the combination band. However, this possibility is rejected because an absorption component, which the [ν1(OHf) + δ1(HfOHb)] combination of the one-bonded water should show,30,31 does not appear at the high frequency side of the combination band even at high hydrations. On the contrary, the combination band increasingly shifts downward as the hydration proceeds, as observed in Figure 7. To examine the physical meaning of the hydration effect on the combination band in more detail, we consider the frequency and the bandwidth of the band in connection with the hydration index, which is defined as the number of hydrating water molecules per 1000 repeat units, at the 1, 2, and 3 stages, as shown in Table 6. These stages correspond to spectra p, q, and r, respectively, in Figure 7. The frequency shift between stages 1 and 2 is only 3 cm−1, but correspondingly the bandwidth largely increases from 76.6 to 92.6 cm−1. To look at the changes between the two stages in more detail, we added two more stages (1-1 and 1-2) between them. The peak frequency is the same at stages 1 and 1-1, and similarly the bandwidth at both stages is almost the same. In contrast, the peak frequency shifts downward by 2 cm−1 from 5243 cm−1 at stage 1-1 to 5241 cm−1 at stage 1-2, and the bandwidth correspondingly increases from 77.8 to 89.6 cm−1. The hydration index is only 5 at stage 1, and it increases from 11 at stage 1-1 to 24 at stage 1-2. It was confirmed that the above hydration dependence of the combination band observed for the spectra of the sample (no. 1) was also detected to a similar extent for those of the other sample (no. 2) used in the present study. Judging from the spectral features of the combination band in connection with the hydration index, it should be true that the hydrating water molecule at stage 1, being hydrogenbonded to the neighboring two ester CO’s as in III, is so separate from the other surrounding ones that it is perfectly isolated without any intermolecular interaction. Here we discuss the “absorption component” that is caused by the intermolecular interactions among hydrating water molecules. From the above discussion, an example of this may be isolated by subtracting spectrum p, which is assigned to the isolated water at stage 1, from spectrum q or r in Figure 7. Using the subtraction, we obtained the spectra shown in Figure 8, where t and u indicate differences q − p and r − p, respectively. Here, the subtraction factor was carefully determined so that the position of the peak at 5243 cm−1 became as smooth as possible on the slope of the newly appearing peak as in the difference spectra shown. Note that the separated new peaks happen to have almost the same frequency around 5215 cm−1, as shown in the spectra of t and u in Figure 8. The peak is lower by about 30 cm−1 than that
Figure 8. Differences q − p and r − p (denoted as t and u, respectively) among the spectra in Figure 7, the shown ordinate being that of spectrum u and the ordinate magnitude of the frame being 0.013 and 0.03 in absorbance for spectra t and u, respectively.
(5243 cm−1) of the isolated water in spectrum p. The 5215 cm−1 peak accounts for the 0.23 and 0.59 fractions of the area intensities of the 5240 and 5228 cm−1 bands in spectra q and r (Figure 7), respectively. A new peak shown in Figure 8 was similarly separated for the spectra of sample no. 2 at the corresponding hydration indexes, the peak frequency being 5220 cm−1 in this case. From the observations, it is certain that the new absorptions in Figure 8 are brought about by the intermolecular interactions among the contained water molecules in the PET matrix. Let us consider the nature of the intermolecular interactions that give rise to the band around 5215 cm−1. At stage 1, where the hydration index is only 5, the hydrating molecule is isolated and not interacting with the other water molecules. In contrast, at stage 1-2 where the hydration index is 24, the water molecules begin to detectably interact with each other, giving rise to a small frequency shift of 2 cm−1 and widening the bandwidth by more than 10 cm−1 (see Table 6). This indicates that the water, which is basically hydrogen-bonded to the surrounding CO’s as in III, performs its molecular motion around the hydrogen-bonded site so that it can interact with the moderately distant neighboring molecules through the interstice of the bulky repeat units in “the hydrophobic solid matrix” of the PET. When the hydration index increases to 89 2892
DOI: 10.1021/acs.jpca.5b00385 J. Phys. Chem. A 2015, 119, 2885−2894
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the hydrating water may possibly have a significant influence on the physical properties of PET in the atmosphere. We have investigated the dependence of intermolecular interactions among the hydrating water molecules on the distribution density of the water in the PET matrix from the combination spectrum. It has been found that the hydrating water is completely isolated or almost so in the matrix at the low hydration density of 5−11 molecules per 1000 repeat units, and also that as the hydration density increases above this, the hydrating water molecules begin to interact with the surrounding water molecules. The onset distribution density of the intermolecular interactions indicates that the water molecule, hydrogen-bonded to ester CO’s, vigorously moves around the hydrogen-bonded site in the PET matrix.
at stage 3 (see Table 6) at its full hydration (the index being 91 for sample no. 2), the interactions become more conspicuous so as to cause a frequency shift of 15 cm−1 and to widen the bandwidth by about 35 cm−1 (Table 6). Correspondingly, the absorption component caused by the intermolecular interactions accounts for a fraction of 0.59 of the total intensity of the 5228 cm−1 band in spectrum r (Figure 7). The intermolecular interactions may be characterized as “the initial stage of the molecular clustering” or “the instantaneous molecular aggregation” caused by the “mobile” hydrating water. This is consistent with the previous report that the hydrating water behaves similarly in poly(methyl methacrylate) (PMMA).18
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4. CONCLUSION In previous papers,2,7,16,18,30 we studied the water contained in various hydrophobic organic or polymeric materials which possess only one type of an acceptor type functional group and aimed at establishing the basic interaction pattern between the water and the attached functional group. In the present paper, we have studied the water dissolved in PET, which possesses two types of the functional groups that can both interact with water. From the combined approach of the infrared analysis and the quantum-chemical calculation, we have concluded that the water is hydrogen-bonded to the ester CO in the PET matrix. However, it is true that water is hydrogen-bonded to phenyl groups in polystyrene or benzene, in which only the phenyl group can interact with water.7,35 The present results have given an important implication with respect to the factor that controls the hydration properties of synthetic or natural polymeric materials, which contain two or more different types of functional groups. Let us note the following interesting observation. In homogeneous polymers such as PET as well as PMMA and PVAc,16,18 the hydrating water is totally hydrogen-bonded to two CO groups and the one-bonded water was not spectrally identified. In contrast, in a microscopically heterogeneous material such as poly(ethylene-co-vinyl acetate),16 which consists of vinyl acetate and hydrophobic ethylene parts, as well as in a heptane solution of an organic compound with a functional group,30 the two-bonded water and the one-bonded water coexist and the proportion of the latter relatively increases with the increasing composition of the hydrophobic component. This seems to imply that a CO group, which is separated from the other functional group by more than the water-bridgeable distance, does not stably form a hydrate such as CO···HbOHf in a homogeneous polymer matrix such as PET, whereas the hydrate is stable enough in the heterogeneous matrix that contains the hydrophobic hydrocarbon portions. The water, which should penetrate the amorphous part of the semicrystalline polymer of PET,39,40 bridges two CO groups as in III in the matrix. Thus, the water should have the effect of significantly strengthening the bonding between the bridged CO groups, which mainly belong to different chains located in an amorphous portion in the PET matrix. In the atmosphere at room temperature, the water-bridged CO part accounts for about 5% of the whole of the CO groups of a PET sample, according to the hydration mole ratio in Table 5 and with the assumption that the sample is totally amorphous. In reality, PET is semicrystalline,39,40 and the fraction of the water-bridged CO parts should be larger. This suggests that
AUTHOR INFORMATION
Corresponding Author
* R.I. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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