Structure and Conformation in Mixtures of Methyl-Terminated Poly

Oct 2, 2009 - Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, ... the water O−H stretching region suggest that the degree of wat...
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J. Phys. Chem. B 2009, 113, 14229–14238

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Structure and Conformation in Mixtures of Methyl-Terminated Poly(ethylene oxide) and Water. Principal Component Analysis and Band Fitting of Infrared Absorptions Jacob J. Shephard, Phil J. Bremer, and A. James McQuillan* Department of Chemistry, UniVersity of Otago, P.O. Box 56, Dunedin, New Zealand ReceiVed: June 2, 2009; ReVised Manuscript ReceiVed: September 2, 2009

Infrared absorption spectra of aqueous mixtures of poly(ethylene oxide) dimethyl ether (PEO), with a number average molecular weight of 500 and a water volume fraction between 0 and 0.96 have been recorded at 18, 26, and 40 °C. Composition and temperature were found to influence the intensity and wavenumber of PEOrelated absorptions. Spectral band fitting of the CH2 wagging/twisting region of the spectra showed that the proportion of gauche and trans conformers of the C-C and C-O bonds varied with composition. Principal component analysis (PCA) of the PEO-related absorptions showed that the configuration with a gauche conformation about the C-C bond and trans conformation about the C-O bond was favored at all the compositions and temperatures tested but was decreasingly favored as the temperature was increased. PCA also enabled the assignment of spectral features associated with a specific conformation of either the C-C or C-O bond. Water-related O-H stretching absorptions indicated that hydrogen bonds were formed between water and PEO ether oxygen atoms. Variation in the wavenumber and integrated absorbance of band-fitted contributions to the water O-H stretching region suggest that the degree of water-water hydrogen bonding changes with composition. At water volume fractions between 0.2 and 0.6, almost all of the water appeared to be strongly hydrogen bonded. The smallest infrared absorption intensity from weakly hydrogen bonded water was recorded for mixtures with a water volume fraction of around 0.4. Such PEO-water mixtures had the greatest proportion of trans conformers about the C-O bond, and there were unexpectedly large PEO and water absorption intensities. Correlations between the average PEO configuration and the water O-H stretching absorptions suggest that, while the conformation about the C-C bond was mainly determined by the polarity of the mixture, the conformation about the C-O bond was influenced by the formation of a strongly hydrogen bonded PEO-water network. Introduction Poly(ethylene oxide) (PEO), also known as poly(ethylene glycol) (PEG), is an important polymeric material which contains both polar ether and nonpolar alkyl groups. The commonly available forms of PEO are hydroxyl or methyl ether terminated. The polymer is flexible and is able to change the relative position of its constituent groups and alter its local dipole moment. Rotation about the C-C and C-O bonds modifies the polymer configuration and interaction with its environment.1 PEO is miscible with water at room temperature but, for PEO with a sufficient chain length, the phase diagram includes a closed loop immiscibility gap.2 It has been suggested that the instability of the single phase above a critical temperature may be partly due to hydrophobic hydration of the alkyl groups and this has been successfully modeled using statistical associating fluid theory.3,4 However, reports of PEO aggregates and water clusters in regions of the phase diagram where PEO and water are expected to be miscible suggest that other factors such as the PEO configuration may play a role.5-12 The configuration with a gauche conformation about C-C bonds and a trans conformation about C-O bonds (TGT) adopted by solid, high molecular weight (Mw) PEO has been predicted to be the most favored PEO configuration in PEO-water mixtures due to the formation of PEO-hydration structures.3,13-15 Further understanding of how PEO miscibility is influenced by the impact of PEO configuration on PEO hydration is needed to better explain * Corresponding author.

the properties of these mixtures and advance the application of PEO in the fields of biological and materials science. The infrared spectrum of PEO is sensitive to the average PEO configuration, and the intensity of bands in the CH2 wagging/ twisting and CH2 rocking regions of the spectra have been used to analyze changes in the average PEO configuration with temperature, the degree of PEO polymerization, and the solvent composition.13-18 In an infrared spectroscopic study by Matsuura and Wahab16 on PEO-water mixtures with a Mw of around 200, the conformation of the C-C bond at room temperature was found to be predominantly gauche and was increasingly so as the proportion of water increased. The number of gauche and trans conformers about the C-O bond was found to be similar at all compositions. The variation in the polarity of the mixture was used to explain the variation in C-C conformation with composition as the gauche conformer is expected to interact more strongly with water due to its larger dipole moment. In the Matsuura and Wahab study16 the preference for the trans conformation about the C-O bond as predicted by Kjellander and Florin3 was not observed. Measured parameters of enthalpy of mixing, PEO and water dynamics, dielectric relaxation time, and adiabatic compressibility for the PEO-water system vary significantly with composition.11,19-21 The minimum PEO mobility and maximum dielectric relaxation time occur at compositions between about 10 and 35% water, and the greatest negative enthalpy of mixing and lowest adiabatic compressibility occur at a composition of around 45% water (around two water molecules per oxygen atom). The variation in the enthalpy of

10.1021/jp905149z CCC: $40.75  2009 American Chemical Society Published on Web 10/02/2009

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Figure 1. Three possible poly(ethylene oxide) configurations, TTT, TGT, TTG.

mixing and adiabatic compressibility with composition supports the theory that there is significant hydrophobic hydration of alkyl groups.3 Vibrational spectroscopic studies of the water-related absorptions suggest that the variation in the PEO mobility and dielectric relaxation time with composition may be partly due to the formation of hydrogen bond bridges between a water molecule and two PEO molecules in concentrated PEO-water mixtures.22-25 Three of the twenty seven possible configurations3 for the O-C-C-O segment of PEO are shown in Figure 1. The most noticeable structural difference with conformation change is a reduction in distance between PEO ether oxygen atoms when the C-C bond (bond 2) rotates to be in the gauche conformation. If the drawn structure with a gauche conformation about the C-C bond and a trans conformation about the C-O bond (TGT) is expanded to include several PEO segments, then the resulting three-dimensional (3D) structure is helical. For PEO in the TGT configuration, the hydrogen atoms point outward from the center of the helix and the oxygen atoms point inward. The introduction of C-C bonds with a trans conformation and C-O bonds with a gauche conformation causes the drawn 3D structure of the polymer to deviate from the helix into a more random coil. Configurations are theoretically possible where PEO ether oxygen atoms and alkyl groups are pushed to opposite sides of the molecule, creating hydrophobic and hydrophilic faces. Such configurations are expected be less water miscible and may have a tendency to aggregate. The stability of the TGT configuration may have a strong dependence on temperature due to the significant amount of ordering of the chain configuration and hydrating water. The average configuration of the polymer may also be influenced by composition and PEO-water stoichiometry, as the hydration structure proposed by Kjellander and Florin to complement the TGT configuration requires approximately two water molecules per oxygen atom.3 The present work examines the infrared (IR) spectra of PEO dimethyl ether with a number average molecular weight (Mn) of 500 and its mixtures with water at water volume fractions between 0 and 0.96 at temperatures of 18, 26, and 40 °C. A number average molecular weight of 500 corresponds to 11 ethylene oxide monomer units. This work has been completed partly because significant band shape variation with surface coverage and temperature was observed in a study of the IR absorption spectra of a PEO self-assembled monolayer (SAM), to appear in a future publication. Interestingly, this surface also showed distinct variation with coverage in the dynamic water contact angle and rate of bacterial adhesion. The unavailability of literature assignments for the absorptions of specific conformers, other than in the CH2 wagging/twisting region, limited interpretation of IR spectral variations in this surface study. The present work looks at variation in composition of a PEO-water mixture analogously to the variation in surface coverage of a PEO SAM and aims to improve understanding of the variation in the IR spectrum, PEO configuration, and water structuring ability of PEO with composition/surface coverage. Due to spectral bands arising from the modes of vibration of the many possible configurations of PEO, the spectra of PEO-water

mixtures are complex with overlapping bands, some of which scale together as the composition and temperature are changed. Spectral band fitting is a technique frequently used to identify contributions to overlapping spectral bands and has been used in this work to separate IR absorption contributions from specific conformers in the CH2 wagging/twisting spectral region.26 Principal component analysis (PCA) is a method often used to identify spectral features which scale together in mixtures and has been used in this work to isolate spectral features corresponding to conformational change about C-C and C-O bonds.27 Analysis of overlapping bands in the water O-H stretching region has been employed to correlate variation in the average PEO configuration with the degree of hydrogen bonding present in the mixture. These correlations have given useful insights into the factors influencing the average conformation about the C-C and C-O bonds in PEO. Materials and Methods PEO dimethyl ether with a number average molecular weight of 500 (Sigma Aldrich) was used without further purification. Deionized water (Milli-Q, Millipore) was used throughout. A series of PEO-water mixtures were prepared by volume dilution of PEO. A Digilab FTS 4000 spectrometer was used to record IR spectra from 64 scans at a resolution of 4 cm-1 (data point interval of 2 cm-1) using Win IR Pro 3.4 (Digilab) software. Attenuated total reflection infrared (ATR-IR) spectroscopy was employed in this work because, in comparison with conventional IR transmission sampling, it conveniently provides a short path length which is necessary for aqueous media. ATR-IR spectra of 1 mL samples were recorded using a 45° ZnSe internal reflection element with approximately 13 reflections (Harrick, Horizon). The ATR-IR spectra were corrected for wavelengthdependent variation in path length and are referred to as IR spectra in the following text. The ATR-IR cell was equipped with a heating/cooling block for temperature control through which was pumped water from an external water bath. The temperature of the PEO-water mixture in the IR cell was measured using a thermocouple. Spectral bands were fitted to Gaussian peaks after baseline correction using a least-squares fitting method as part of the analysis and graphing program OriginPro 8.0 (OriginLab).28 The number of peaks was chosen to optimize the consistency of parameters for fitted peaks, and the initial wavenumber was chosen using variation in the second derivative.29 Principal component analysis (PCA) of the PEO-related regions of the spectra was carried out using the program Unscrambler 9.8 (CAMO). Absorbance spectra were baseline corrected, normalized to PEO-related integrated absorbance (1500-900 cm-1), and mean centered prior to PCA. Random cross validation with 5 samples per segment was used.29 PEO-related integrated absorbance was calculated by removal of water-related absorptions using the baseline correction function and summing absorbance measurements between 1500 and 900 cm-1, which are at 2 cm-1 intervals, and multiplying by two. Water-related integrated absorbance was calculated by summing twice the

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Figure 2. ATR-IR spectra PEO-water mixtures at 18 °C, for water volume fractions of 0.04 (thick solid line), 0.52 (solid line), and 0.96 (dashed line).

absorbance measurements between 4000 and 2600 cm-1 and subtracting the PEO-related integrated absorbance in this spectral region. Results and Discussion (a) IR Spectra of PEO-H2O Mixtures. Figure 2 shows the IR spectra of PEO-water mixtures at water volume fractions of 0.04, 0.52, and 0.96. These compositions represent the dilute solution limits for PEO and water as well as an intermediate composition. With increasing water volume fraction, absorptions due to water O-H stretching (3800-2900 cm-1), bending (1680-1580 cm-1), and libration modes (1100-0 cm-1) increase in intensity, and there are decreases in intensity of PEOrelated absorptions. Principal PEO absorptions arise from CH2 and CH3 stretching modes (3100-2800 cm-1), CH2 and CH3 scissor/bending modes (1500-1400 cm-1), CH2 and CH3 wagging and twisting modes (1400-1180 cm-1), polyether backbone modes (1180-1000 cm-1), and CH2, CH3 rocking modes (800-1000 cm-1). Literature assignments for absorptions from the polarized IR spectra of PEO in the TGT configuration are given in the work of Miyazawa et al.30 and Yoshihara et al.,31 and assignments from normal coordinate analysis and density functional theory studies of the various conformers have been reported by Matsuura and co-workers.32,33 An analysis of the variation of integrated absorbance for PEO (1500-900 cm-1) and for water (4000- 2600 cm-1) with composition is made in Figures 3A and 3B, respectively. The integrated absorbance data (9) as shown in Figure 3 varies with composition almost as expected. However, Figure 3 also contains integrated absorbance data which is normalized for composition (0) to 100% PEO in 3A and to 100% water in 3B. For an ‘ideal mixture’, such intensity data might be expected to be constant, but this data shows distinct variation with composition. The composition-normalized data for the PEO absorptions (Figure 3A) shows increased intensities up to a water volume fraction of 0.6 and decreased intensities for compositions with a greater water volume fraction. The corresponding data for the water-related absorptions (Figure 3B) shows increased intensities for increasing PEO volume fractions up to 0.8 (water volume fraction of 0.2) and pronounced intensity reductions for compositions with a greater PEO volume fraction. This data

Figure 3. Integrated absorbance (9) and that corrected for composition (0) for (A) the C-O-C stretching and C-H bending regions (1500-900 cm-1) and (B) the O-H stretching region (4000-2600 cm-1).

may partly reflect that parameters related to ATR-IR absorption intensity (sample penetration depth),34 such as the refractive index and density of the mixture, change with composition.35-37 The similar variation of this data with composition to that reported for enthalpy of mixing measurements in similar systems suggests that the increased absorption intensities reported in this work are associated with attractive interactions in the mixture. In addition to irregular intensity variations there are also spectral band changes with composition which are more clearly shown in Figure 4 which contains the PEO-related sections of the Figure 2 spectra. Most obvious are changes occurring in the 3000-2700 cm-1 and 1200-1000 cm-1 regions. There are also changes in the band shape of absorptions due to water that will be considered in later sections. Changes in the band shape of PEO-related absorptions with composition are recognized from previous vibrational spectroscopic studies to be associated with changing polymer configuration and variation in the extent of interaction between PEO and water.17,18 Less obvious changes in the spectra between 1400 and 1200 cm-1 are also evident, which have been assigned by Matsuura and Wahab16 to the CH2 wagging/twisting vibrations of PEO in various conformations. In the following section, the relative integrated absorbance of fitted peaks in this spectral region has been used to monitor the average PEO configuration. (b) Band Fitting of PEO-Related Absorptions in the 1400 to 1230 cm-1 Spectral Region. In the IR spectra of PEO-water mixtures, peaks centered at 1350 and 1325 cm-1 have been assigned by Matsuura and co-workers16,32,33 to gauche and trans conformations, respectively, about the C-C bond while peaks centered at 1305 and 1288 cm-1 have been assigned to gauche and trans conformations, respectively, about the C-O bond. It

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Figure 4. IR spectra of PEO-water mixtures in the 3100-2650 cm-1 and 1500-900 cm-1 spectral regions for compositions with water volume fractions of (A) 0.04, (B) 0.52, and (C) 0.96.

Figure 5. IR spectra of 100% PEO at 18 °C in the CH2 wagging/ twisting region with Gaussian peaks from spectral band fitting.

is notable that the 1325 and 1305 cm-1 peaks are missing in the spectra of solid, high Mw PEO which exists in the TGT configuration (Figure 1).30 Figure 5 shows the spectrum of 100% PEO and absorption contributions from fitted peaks in the CH2 wagging/twisting region. The lower wavenumber limit was chosen to provide a good fit to overlapping absorptions at 1288 and 1305 cm-1. The Figure 5 data shows the existence of six overlapping absorptions at 1358, 1349, 1325, 1305 1289, and 1248 cm-1. Spectral band fitting in this region of the spectra was also carried out for all PEO-water compositions. These PEO-water mixtures showed recognizably similar absorptions but with small wavenumber shifts and significant variation in intensity with composition. In subsequent discussion the wave-

number of these peaks in the spectrum of 100% PEO is used to identify peaks in the spectra of PEO-water mixtures in which the peak wavenumbers are slightly shifted. Due to the peak wavenumber shifts with composition, integrated absorbance rather than absorbance at a specific wavenumber was used for the intensity of absorptions. If it is assumed that the intensity of absorptions (transition dipoles) of the CH2 wagging and twisting vibrations of PEO did not vary with PEO configuration or the degree of hydrogen bonding in the mixture, the variation of average polymer conformation can be derived from the integrated absorbance data obtained from the peak fitting procedure. Further computational studies of PEO configurations may be able to address the validity of this assumption. Figure 6A shows the variation in integrated absorbance with composition of the 1350 to 1325 cm-1 peaks giving the gauche/trans conformation ratio about the C-C bond. For the spectrum of 100% PEO, the integrated absorbance of the 1350 cm-1 peak, corresponding to a gauche conformation about the C-C bond, has approximately twice the integrated absorbance of that centered at 1325 cm-1 corresponding to a trans conformation. As the water volume fraction increases, the gauche/trans conformation ratio increases markedly from about 2 to 15. While this finding is in general agreement with those of Matsuura and Wahab16 who used a shorter chain PEO, the gauche/trans conformation ratio of 15/1 found in this work is greater than that previously reported. The difference between the gauche/trans conformation ratio about

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Figure 6. Variation with composition of the ratio of integrated peak area corresponding to gauche/trans conformers of (A) the C-C bond, 1351/1325 cm-1, and (B) the C-O bond, 1305/1288 cm-1.

the C-C bond reported in this study and that reported by others may be due to different PEO chain lengths or due to the employment of spectral band fitting. The integrated absorbance data obtained by spectral band fitting carried out in this study provides a more quantitative measure of average polymer configuration than has hitherto been available. Figure 6B shows the variation of integrated absorbance with composition of the 1305 to 1288 cm-1 peaks giving the gauche/ trans conformation ratio about the C-O bond. Although the absorbance at 1305 and 1288 cm-1 in the measured spectrum of 100% PEO appear to be similar, spectral band fitting indicates that the integrated absorbance of the peak at 1305 cm-1 corresponding to the gauche conformer is about half of that at 1288 cm-1 corresponding to the trans conformer. As the water volume fraction in the mixture increases, the gauche/trans conformation ratio decreases from about 0.6 to reach a minimum of about 0.35 at a water volume fraction of around 0.4, and then increases gradually, reverting to a ratio similar to that observed for 100% PEO in the most dilute PEO-water mixtures (large water volume fraction). Previous IR studies of PEO-water mixtures have concluded that there is little variation with composition in the average conformation about the C-O bond.16 The present study has revealed that there are relatively small but significant changes in the average C-O bond conformation with composition for PEO dimethyl ether (Mn of 500). Band fitting in other spectral regions gave less consistent outcomes for the number of contributing peaks and did not isolate other spectral features associated with specific C-C and C-O bond conformations. Principal component analysis (PCA) has been used as an alternative means of identifying further spectral features, which scale together as the composition is changed, and include those due to conformational change about the C-C and C-O bonds. The average configuration of the polymer in PEOswater mixtures is known to be affected by temperature in addition to composition.14,17 PCA has also enabled this extra variable to be considered.

J. Phys. Chem. B, Vol. 113, No. 43, 2009 14233 (c) Principal Component Analysis (PCA) of PEO Absorptions. PCA was applied to the 1500-900 cm-1 and the 3100-2650 cm-1 PEO-related spectral regions separately for spectra of PEO-water mixtures with a water volume fraction between 0 and 0.88 recorded at 18, 26, and 40 °C. The separation of these two spectral regions provided more precise PCA data, partly because spectra in the CH stretching region (3100-2650 cm-1) are primarily influenced by rotation about the C-C bond while those in the 900-1500 cm-1 spectral region are influenced by rotation about the C-O bond (see Figure 1). The spectra of the most dilute PEOswater mixtures were omitted from the PCA because of low signal-to-noise ratios. Figure 7A-C show PCA data from the 1500-900 cm-1 spectral region and Figure 7D-F show PCA data from the CH2 stretching spectral region. The first and second principal components (PCs) describe 90 and 8%, respectively, of the total spectral variance in the 1500-900 cm-1 spectral region and 97 and 2%, respectively, of the total spectral variance in the CH2 stretching region. The spectral variance described by each PC was in close agreement with that predicted by cross validation, and further PCs with low variance have not been considered. Figure 7A and 7B show the first and second PC scores, respectively, for the 1500-900 cm-1 spectral region, and Figure 7C shows loading weights. The score for a sample indicates the variation in absorbance between the sample spectrum and the mean spectrum of the mean-centered data. The loading weights (LW) show the corresponding variance in absorbance at each point in the spectrum.33 The main features in the LW plot for the first PC shown in Figure 7C (solid line) include positive LW peaks at 1140 and 1325 cm-1 and negative LW peaks at 1080 and 1350 cm-1. These correspond to PEO-related backbone vibrational modes (1200-1000 cm-1) and CH2 wagging/twisting modes (1200-1400 cm-1). There are also variations in LW for the C-H bending (1500-1400 cm-1) and CH2 rocking (1000-800 cm-1) regions which are less pronounced. The first PC score shown in Figure 7A decreases almost linearly with increasing water volume fraction. Therefore concentrated PEO-water mixtures (small water volume fraction) have strong absorptions in regions of the spectrum which show positive LWs (1140 and 1325 cm-1), and the most dilute PEO-water mixtures (large water volume fraction) have strong absorptions in regions of the spectra which show negative LWs (1080 and 1350 cm-1). For spectra of PEO-water mixtures recorded at 40 °C, PCA of the spectral data gives a mildly increased score at compositions above a water volume fraction of 0.2 when compared to spectra recorded at 18 °C. The second PC score shown in Figure 7B shows a nonlinear variation with composition, which is positive in concentrated and dilute PEO-water mixtures and negative at compositions with a water volume fraction between 0.1 and 0.6. The range of values for the score of the second PC is smaller than for the first PC which reflects the smaller proportion of the total spectral variance accounted for by this PC. The effect of increasing the temperature from 18 to 40 °C is more significant than for the first PC with an increase in score for spectra recorded at 40 °C in PEO-water mixtures with a water volume fraction up to 0.7. The LW plot for the second PC shown in Figure 7C (dashed line) includes a positive LW peak at 1100 cm-1 (backbone vibrational modes) and negative LW peaks at 950 and 1020 cm-1 (CH2 rocking modes). In the CH2 wagging and bending regions there is also some variation in LW which is less pronounced. Figures 7D-F show PC analysis of the CH2 stretching region of the spectra. Figure 7D and 7E show the score for the first

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Figure 7. PCA of the spectral data for PEO-water mixtures with a water volume fraction between 0 and 0.88 at 18, 26, and 40 °C. Parts A, B, and C show first PC score, second PC score, and corresponding loading weight plots, respectively, for the 1500-900 cm-1 spectral region. Parts D, E, and F show first PC scores, second PC scores, and corresponding loading weight plots, respectively, for the CH2 stretching region (2960-2650 cm-1). Loading weights are given for the 1st PC (solid line) and the 2nd PC (dashed line). Scores are given for spectra recorded at 18 °C (∆) 26 °C (O) and 40 °C (0).

and second PCs, respectively. The variation in score with composition for PCA of the CH2 stretching spectral region is similar to that for the 1500-900 cm-1 region; however, the range of values for the second PC score for the CH2 spectral region is smaller than for the second PC score for the 1500-900 cm-1 spectral region which is due to the smaller spectral variance described by this PC. In Figure 7F is a LW plot for the first PC (solid line) which shows a positive LW peak centered at 2860 cm-1. The second PC (dashed line) shows negative LW peaks at 2900 and 2870 cm-1 and a positive LW peak at 2850 cm-1. Absorptions in this spectral region are likely to include combination bands which make assignments more difficult; however, it is noteworthy that spectral bands at 2890

and 2865 cm-1 have been assigned to CH2 stretching of solid, high Mw PEO in the TGT conformation.30 The variation of score and corresponding LW plots with composition suggests that PC analysis has successfully separated spectral variance due to rotation about the C-C bond (first PC) and the C-O bond (second PC). Assigned spectral features corresponding to CH2 wagging vibrations of gauche (1350 cm-1) and trans (1325 cm-1) conformers about the C-C bond are well resolved in the LW plot of the first PC shown in Figure 7C. In the current study, the intensities of these peaks have been shown to vary with composition in PEO-water mixtures using PCA and spectral band fitting methods and by the spectral analysis of Matsuura et al.16,32,33 Further absorptions at 1140 and 2860

Methyl-Terminated Poly(ethylene oxide) and Water cm-1 which exist in the spectra of PEO-water mixtures with a large PEO content contain contributions from vibrations of PEO with a trans conformation about the C-C bond. An absorption centered at 1080 cm-1 which exists in the spectra of PEO-water mixtures with a large water content may include a contribution from vibrations of the gauche C-C conformation. The LW plot for the second PC (dashed line) shown in Figure 7C has unresolved features in the CH2 wagging region, and it is more difficult to determine the structural cause for this spectral variance. As rotation about the C-O bond changes the relative positions of the C-O bond and adjacent hydrogen atoms, absorptions due to C-O-C stretching vibrations coupled to CH2 rocking vibrations, which absorb between 1200 and 800 cm-1, are expected to be effected by conformational change of the C-O bond (see Figure 1). The main LW features for this PC occur at 950, 1020, and 1100 cm-1, suggesting that spectral variance of the second PC is mainly associated with rotation about the C-O bond. The negative LW peaks at 2900 and 2870 cm-1, which are likely due to CH2 stretching absorptions of the TGT conformer, support this conclusion. The variation with composition in the average configuration of PEO in PEO-water mixtures, as found from PCA of the spectra, is in agreement with that found in this work from spectral band fitting of absorptions in the CH2 wagging/twisting region. Increasing the temperature from 18 to 40 °C appears to reduce the stability of the TGT conformation in PEO-water mixtures at compositions with a water volume fraction between 0.1 and 0.6. (d) Analysis of Water-Related Spectral Regions Including Curve Fitting of the O-H Stretching Absorptions. Hydrogen bonding in liquid water is extensive with a reasonably wellaccepted model including the existence of two main components: strongly hydrogen bonded water which has a symmetric O-H stretching absorption at around 3200 cm-1 and less strongly hydrogen bonded water which has a symmetric O-H stretching absorption at around 3400 cm-1.38,39 The O-H bending region (1700-1550 cm-1) also contains absorptions from both components, but these are not resolved. Increasing the temperature of liquid water is known to disrupt the hydrogen bonding structure, giving rise to an increased absorbance at ∼3400 cm-1 and a decreased absorbance at ∼3200 cm-1. The addition of solutes is also known to affect the degree of hydrogen bonding in liquid water, giving similar spectral variation. It was of interest to ascertain the effect of PEO composition on the water-related absorptions and hydrogen bonding structure, as the variation in PEO conformation with composition and temperature may be partly due to the formation of hydration structures, with extensive hydrogen bonding. Vibrational spectroscopic studies have also determined that hydrogen bonds are formed between water and the PEO ether oxygen atoms in PEO-water mixtures. Water molecules hydrogen bonded to PEO ether oxygens are expected to have O-H stretching absorptions in the range of 3700-3400 cm-1 with a wavenumber dependent on the proportion of water molecules which form hydrogen bond bridges between two PEO ether oxygen atoms.23-25 Figure 8 shows the water-related sections (O-H stretch, 3700-2900 cm-1 and O-H bend 1700-1550 cm-1) of the spectra for PEO mixtures with water volume fractions of 0.04, 0.52, and 0.96. Absorptions corresponding to O-H bending vibrations increase in intensity with the increasing water volume fraction and have a band shape fairly well described by a Gaussian distribution at all compositions. The most notable change in the O-H bending spectral band with increasing water volume fraction is an increase in its bandwidth. Absorptions

J. Phys. Chem. B, Vol. 113, No. 43, 2009 14235 corresponding to O-H stretching vibrations also increase in intensity with an increasing water volume fraction; however, the band shape cannot be described by a single Gaussian distribution and shows more considerable changes in shape with composition. Three Gaussian peaks were found to provide a satisfactory fit for the O-H stretching absorptions of PEO-water mixtures at all compositions and temperatures tested and are shown in the Figure 8 spectra. The inclusion of a fourth peak was found to improve the curve fit but gave less consistent data with respect to peak width and integrated absorbance. The peak at the lowest wavenumber (3350-3260 cm-1) has its main contribution from the symmetric stretching vibration of O-H bonds strongly hydrogen bonded to adjacent water molecules. The peak at the second lowest wavenumber (3400- 3550 cm-1) has contributions from the symmetric stretching vibration of O-H bonds with less strong hydrogen bonding. The third peak at the highest wavenumber (3500-3600 cm-1) has its main contribution from the vibrations of water molecules hydrogen bonded to PEO via the ether oxygen atoms of the polymer. In the following sections, integrated absorbance and peak wavenumber for the three peaks will be used to better describe hydrogen bonding in the PEO-water mixtures and its variation with composition and temperature. Figure 9 shows the variation of integrated absorbance with composition for the three peaks obtained from spectral band fitting of the O-H stretching spectral region as shown in Figure 8. The integrated absorbance of peak 1, which has its main contribution from strongly hydrogen bonded water, increases as expected with the increasing water volume fraction. However, the variation of integrated absorbance with composition is nonlinear. With increasing water volume fraction, the integrated absorbance is relatively low up to a water volume fraction of 0.12, increases more steeply to a water volume fraction of 0.6, and subsequently increases more gradually. The integrated absorbance of peak 2 (note the scale change), which includes a contribution from weakly hydrogen bonded water, initially increases with water volume fraction but decreases at water volume fractions between 0.12 and 0.28. For compositions with water volume fractions between 0.2 and 0.6, the dramatic increase in the integrated absorbance due to strongly hydrogen bonded water (peak 1) with water volume fraction and the relatively small absorption from weakly hydrogen bonded water (peak 2) suggest that PEO-water mixtures in this composition range have considerable hydrogen bonding in their structure. The small absorption due to weakly hydrogen bonded water suggests that there is little water in the second hydration layer of PEO for mixtures in this composition range, which is in agreement with the hydration structures predicted by Kjellander and Florin.3 The integrated absorbance of peak 3, which has its main contribution from water molecules that are hydrogen bonded to PEO, has a nonlinear relationship with composition as expected. The integrated absorbance is small relative to peaks 1 and 2 for spectra of PEO-water mixtures with small and large water volume fractions because at these compositions the proportion of water and PEO, respectively, are small and limit the absorption intensity and integrated absorbance. In addition to the variation with composition of the integrated absorbance of these three peaks, there are also variations in peak wavenumber. The wavenumber of peaks 1, 2, and 3 determined from spectral band fitting of the IR spectra of PEO-water mixtures recorded at compositions with water volume fractions between 0.04 and 0.96 at 18, 26, and 40 °C are given in Figure 10. The wavenumber of absorptions in the O-H stretching region is determined by the O-H bond strength and is therefore an

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Figure 8. IR spectra of PEO-water mixtures in the 3100-2650 cm-1 spectral region with Gaussian peaks from curve fitting and in the 1500-900 cm-1 spectral region without peaks from curve fitting for compositions with a water volume fraction of (A) 0.04, (B) 0.52, and (C) 0.96.

Figure 9. Variation of integrated absorbance with composition from curve fitting of the O-H stretching spectral region (4000-3050 cm-1) into 3 Gaussian peaks, integrated absorbance of peak 1(3) centered at ∼3300 cm-1, for peak 2 (star) centered at ∼3400 cm-1 and peak 3 (]) centered at ∼3550 cm-1 for IR spectra of PEO-water mixtures recorded at 18 °C. The peak wavenumber varies with composition.

indication of the average degree of hydrogen bonding in such an environment. Peak 1 with contributions from strongly hydrogen bonded water has a peak wavenumber that varies between 3260 and 3340 cm-1. Variation in the wavenumber of peak one at water volume fractions less than 0.2 is mainly due to the small area of this peak at these compositions and the decreased precision of fit for such a small peak. The wavenum-

ber of this peak appears to be greatest at a water volume fraction of around 0.2 and decreases linearly with increasing water volume fraction. For spectra recorded at 40 °C the peak wavenumber is around 15 cm-1 greater than for spectra recorded at 18 °C. This data indicates that the degree of hydrogen bonding in the PEO-water mixtures became more extensive as the water volume fraction was increased and was less extensive at 40 °C than at 18 °C. This could be due to the increased polarity of the mixture as the proportion of water was increased and due to the increased thermal motion at elevated temperatures. The wavenumber of peak 2 with contributions from more weakly hydrogen bonded water also showed variation with composition and temperature. The wavenumber of this peak was found to be greatest in the spectra of concentrated PEO-water mixtures at 3520 cm-1 and lowest at around 3400 cm-1 in the spectra of PEO-water mixtures with a water volume fraction of around 0.4. The wavenumber of peak 3 was found to vary in a similar way to that of peak 2, being 3600 cm-1 in the most concentrated PEO-water mixtures and around 3520 cm-1 at a water volume fraction of around 0.4. The wavenumber of these peaks was found to be approximately 20 cm-1 greater when spectra were recorded at 40 °C than at 18 °C. The large wavenumber variation with composition may indicate that these peaks include absorbance contributions from more than one absorption. Three absorptions have been isolated by Matsuura and co-workers in the spectra of PEO-water-CCl4 mixtures corresponding to the

Methyl-Terminated Poly(ethylene oxide) and Water

Figure 10. Variation of peak wavenumber with composition from curve fitting of the O-H stretching region (4000-3050 cm-1) into three Gaussian peaks, for spectra recorded at 18 (∆), 26 (O), and 40 °C (0).

symmetric (3525 cm-1) and asymmetric (3590 cm-1) stretching vibration of bidentate hydrogen bonded water and the stretching vibration of the monodentate hydrogen bonded water (3490 cm-1).25 Based on these assignments, the wavenumber downshift of peak 3 for water volume fractions between 0.2 and 0.6 indicates an increased proportion of water molecules which have formed hydrogen bonds with a single PEO ether oxygen atom. Conclusions The number of peaks and their wavenumber in the PEOrelated sections of the IR spectra of PEO-water mixtures suggest that PEO dimethyl ether (Mn of 500) exists in a range of configurations and that the average configuration varies with composition. In the present IR spectroscopic work, based on and extending the assignments of Matsuura and co-workers,16 spectral band fitting of absorption bands in the CH2 wagging/ twisting region of the spectra revealed that a gauche conformation about the C-C bond and a trans conformation about the C-O bond were favored in 100% PEO and in all the PEOswater mixtures tested. The gauche conformation about the C-C bond became increasingly favored as the water volume fraction was increased while the greatest proportion of trans C-O conformers existed in PEOswater mixtures with a water volume fraction of around 0.4. PCA of the PEO-related regions of the spectra suggested a similar variation of average conformation with composition and identified spectral features associated with specific conformers in addition to peaks in the CH2 wagging/twisting spectral region. The spectra of PEO with a predominantly gauche conformation about the C-C bond had

J. Phys. Chem. B, Vol. 113, No. 43, 2009 14237 a strong absorption at ∼1080 cm-1, and the spectra of PEO with a trans conformation about the C-C bond had a strong absorption at ∼1140 cm-1. The spectra of PEO with a gauche conformation about the C-O bond contained an absorption centered at ∼1100 cm-1 while the spectra of PEO with the more favored trans conformation about the C-O bond had absorptions at ∼950 and ∼1020 cm-1. PCA of spectra recorded at 18, 26, and 40 °C revealed that increasing the temperature from 18 to 40 °C at compositions with water volume fractions between 0.2 and 0.6 decreased the relative proportion of gauche conformers about the C-C bond and trans conformers about the C-O bond (TGT). The increased proportion of gauche conformers about the C-C bond with water content is understood to be due to the increasing polarity of the mixture and greater dipole character of this conformer. The variation in the average conformation about the C-O bond with composition and reduced stability of the TGT configuration as the temperature was increased is not easily explained but may be due to the formation of PEO-hydration structures which require a trans conformation about the C-O bond. The absorptions due to water in the spectra of PEO-water mixtures also showed variation with composition and temperature which is not surprising, as the variation in dielectric constant, nonideality of the mixture, and the effect of temperature on water hydrogen bonding are well-known. However, analysis of the wavenumber distribution of absorptions in the water O-H stretching region highlighted variations in the proportion of water which was strongly hydrogen bonded, weakly hydrogen bonded, and hydrogen bonded to PEO which were unexpected. At compositions with water volume fractions between 0.2 and 0.6, there was almost no IR absorption from weakly hydrogen bonded water, with the smallest absorption occurring at a water volume fraction of around 0.4. Significant variation with composition in the wavenumber of absorptions corresponding to PEO solvating water suggested that there were several possible modes by which water could form hydrogen bonds to the PEO ether oxygen atoms. A monodentate interaction was favored at compositions with a water volume fraction between 0.2 and 0.6. The increased proportion of weakly hydrogen bonded water at compositions with a water volume fraction greater than 0.4 supports the conclusions of other researchers that approximately two water molecules per ethylene oxide segment are required for complete hydration of PEO.3 Interestingly PEO-water mixtures with a water volume fraction of around 0.4 also gave the greatest enhancement in water and PEO-related IR absorption intensity and had the greatest proportion of trans conformers about the C-O bond. The correlation between the proportion of trans C-O conformers and the wavenumber distribution of water O-H stretching absorptions suggests that the conformation about the C-O bond is influenced by the local hydrogen bonding structure which is further influenced by factors such as composition and temperature. A conclusion which may be derived from these findings, and which is relevant to many PEO applications, is that PEO-water mixtures with a nonideal stoichiometry or at elevated temperatures are likely to have some entropically driven deviation from the TGT configuration which may result in a reduction in polymer hydration and water miscibility. These finding may help to rationalize the unusual behavior of PEO aggregation, allow for improved modeling of PEO phase behavior, and lead to an improved understanding of aqueous PEO systems. Acknowledgment. The work was supported by the New Zealand Foundation for Research Science and Technology,

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Contract number CO8X0409. The authors acknowledge Cushla McGoverin for her assistance with principal component analysis. References and Notes (1) Karlstrom, G.; Engkvist, O. In Poly(ethylene glycol): Chemistry and Biological Applications; Harris J. M., Ed.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997; pp 16-30. (2) Saeki, S.; Kuwahara, N.; Nakata, M.; Kaneko, M. Polymer 1976, 17, 685. (3) Kjellander, R.; Florin, E. J. Chem. Soc., Faraday Trans. 1981, 77, 2053–2077. (4) Clark, G.; Galindo, A.; Jackson, G.; Rogers, S.; Burgess, A. N. Macromolecules 2008, 41, 6582–6595. (5) Borodin, O.; Bedrov, D.; Smith, G. D. J. Phys. Chem. B 2002, 106, 5194–5199. (6) Polverari, M.; van de Ven, T. G. M. J. Phys. Chem. 1996, 100, 13687–13695. (7) Devanand, K.; Selser, J. C. Nature 1990, 343, 739–741. (8) Trouw, F. R.; Boridin, O.; Jeremy, C. C.; Copley, J. R. D.; Smith, G. D. J. Phys. Chem. B 2003, 107, 10446–10452. (9) Nickolov, Z. S.; Goutev, N.; Matsuura, H. J. Phys. Chem. A 2001, 105, 10884–10889. (10) Fenn, E. E.; Moilanan, D. E.; Levinger, N. E.; Fayer, M. D. J. Am. Chem. Soc. 2009, 131, 5530–5539. (11) Schmelzer, C. E. H.; Zwirbla, W.; Rosenfeld, E.; Linde, B. B. J. J. Mol. Struct. 2004, 699, 47–51. (12) Sato, T.; Niwa, H.; Chiba, A. J. Chem. Phys. 1998, 108, 4138– 4147. (13) Begum, R.; Matsuura, H. J. Chem. Soc., Faraday Trans. 1997, 93, 3839–3848. (14) Wahab, S. A.; Matsuura, H. J. Mol. Struct. 2001, 606, 35–43. (15) Kuroda, Y.; Kubo, M. J. Polym. Sci. 1959, 36, 453–459. (16) Wahab, S. A.; Matsuura, H. Phys. Chem. Chem. Phys. 2001, 3, 4689–4695. (17) Koenig, J. L.; Angood, A. C. J. Polym. Sci. 1970, 8, 1787–1796. (18) Liu, K. J.; Parsons, J. L. Macromolecules 1969, 2, 529–533. (19) Hanke, E.; Schulz, U.; Kaatze, U. Chem. Phys. Chem. 2007, 8, 553–560.

Shephard et al. (20) Hanke, E.; von Roden, K.; Kaatze, U. J. Chem. Phys. 2006, 125, 084507. (21) Branca, C.; Magazu, S.; Maisano, G.; Migliardo, F.; Romeo, G. J. Phys. Chem. B 2002, 106, 10272–10276. (22) Goutev, N.; Nickolov, Z. S.; Georgiev, G.; Matsuura, H. J. Chem. Soc., Faraday Trans. 1997, 93, 3167–3171. (23) Kitano, H.; Ichikawa, K.; Ide, M.; Fukuda, M.; Mizuno, W. Langmuir 2001, 17, 1889–1895. (24) Ide, M.; Motonaga, T.; Kitano, H. Langmuir 2006, 22, 2422–2425. (25) Matin, R.; Katsumoto, Y.; Matsuura, H.; Ohno, K. J. Phys. Chem. B 2005, 109, 19704–19710. (26) Gold, H. S.; Rechsteiner, C. E.; Buck, R. P. Anal. Chem. 1976, 1540–1546. (27) Kwak, C. W.; Choung, D.; Min, S. R.; Kim, S. W.; Liu, J. R.; Chung, H. Anal. Sci. 2007, 23, 895–889. (28) Pierce, J. A.; Jackson, R. S; Van Every, K. W.; Griffiths, P. R. Anal. Chem. 1990, 62, 477–484. (29) De Haseth, James A.; Griffiths, Peter R. Fourier Transform Infrared Spectrometry, 2nd ed.; Wiley: NY, 2007; pp 213-215, 235-245. (30) Miyazawa, T.; Fukushima, K.; Ideguchi, Y. J. Chem. Phys. 1962, 37, 2764–2776. (31) Yoshihara, T.; Hiroyuki, T.; Murahashi, S. J. Chem. Phys. 1964, 41, 2902. (32) Matsuura, H.; Fukuhara, K. J. Pol. Sci. B 1986, 24, 1383–1400. (33) Yoshida, H.; Matsuura, H. J. Phys. Chem. A 1998, 102, 2691– 2699. (34) Harrick, N. J. Internal Reflection Spectroscopy; Wiley: New York, 1967; p 30. (35) Douheret, G.; Reis, J. C. R.; Davis, M. I.; Fjellanger, I. J.; Hoiland, H. Phys. Chem. Chem. Phys. 2004, 6, 784–792. (36) McGee, R. L.; Wallace, W. J.; Rataiczak, R. D. J. Chem. Eng. Data 1983, 28, 305–307. (37) Donato, D. I.; Jannelli, M. P.; Magazu, S.; Majolino, D.; Maisano, G.; Migliardo, P.; Ponterio, R. J. Mol. Struct. 1996, 381, 213–217. (38) Shen, Y. R.; Ostroverkhov, V. Chem. ReV. 2006, 106, 1140–1154. (39) Bain, C. D. J. Chem. Soc., Faraday Trans. 1995, 91, 1281–1296.

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