Langmuir 2007, 23, 11259-11265
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Hydration Changes of Poly(2-(2-methoxyethoxy)ethyl Methacrylate) during Thermosensitive Phase Separation in Water Yasushi Maeda,* Tomoyuki Kubota, and Hideo Yamauchi Department of Applied Chemistry and Biotechnology, UniVersity of Fukui, Fukui 910-8507, Japan
Tadashi Nakaji and Hiromi Kitano Department of Applied Chemistry, Graduate School of Science and Engineering, UniVersity of Toyama, 3190 Gofuku, Toyama 930-8555, Japan ReceiVed May 31, 2007. In Final Form: August 6, 2007
Hydration changes of poly(2-(2-methoxyethoxy)ethyl methacrylate) (PMoEoEMa) during thermosensitive phase separation in water have been investigated by infrared spectroscopy. The CdO stretching band can be separated into three components assigned to non-hydrated carbonyl groups and singly and doubly hydrogen-bonded carbonyl groups (1728, 1709, and 1685 cm-1, respectively). Relatively large parts of the carbonyl groups (50% in 30 wt % solution) do not form hydrogen bonds even below the transition temperature (Tp) probably because they possess crowded positions near the backbone. The fraction of hydrogen-bonding carbonyl groups decreased during phase separation by ∼0.2. Among five ν(C-H) bands, the highest- and the lowest-frequency bands (ν(C-H)A and ν(C-H)E) exhibited relatively large red shifts of 8 and 11 cm-1, respectively. DFT calculations indicate that the formation of a H-bond between the ether oxygen and water leads to blue shifts of ν(C-H) of adjacent alkyl groups and has a larger effect than a direct H-bond to the alkyl groups, namely, C-H‚‚‚O H-bonds. The fraction of hydrogen-bonding methoxy oxygens estimated from the position of the ν(C-H)A is 1 at Tp. This result indicates that the methoxy oxygens and the carbonyl are more favorably hydrated than the other at Tp, respectively.
Introduction Temperature-sensitive polymers, whose aqueous solutions exhibit phase separation above lower critical solution temperatures (LCSTs), have attracted considerable attention from both fundamental and applicational points of view. A variety of applications have been reported, ranging from drug delivery systems to the creation of smart surfaces. A typical example of such polymers is poly(N-alkyl acrylamide)s including poly(Nisopropylacrylamide) (PNiPAm),1 which has been the most intensively studied. It has been shown recently that the introduction of an oligo(ethylene glycol) chain of the appropriate length to the polymer backbone as a pendant group yields temperaturesensitive polymers. Poly(vinyl ether)s,2 poly(meth)acrylates,3 and polystyrenes4 with pendant oligo(ethylene glycol) groups have been prepared, and their LCSTs were tuned by the side-chain length and end group. However, hydration changes in these polymers during phase transition have scarcely been studied until now. During such a temperature-dependent phase separation, the transition from the hydrophobic hydration of apolar sites of the polymer in soluble states to the hydrophobic interaction between them in insoluble states is considered to play an important role.5 * To whom correspondence should be addressed. Fax: +81-776-278747. E-mail:
[email protected]. (1) Fujishige, S.; Kubota, K.; Ando, I. J. Phys. Chem. 1989, 93, 3311. (2) (a) Aoshima, S.; Oda, H.; Kobayashi, E. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 2407. (b) Sugihara, S.; Kanaoka, S.; Aoshima, S. Macromolecules 2004, 37, 1711. (3) (a) Han, S.; Hagiwara, M.; Ishizone, T. Macromolecules 2003, 36, 8312. (b) Kitano, H.; Hirabayashi, T.; Gemmei-Ide, M.; Kyogoku, M. Macromol. Chem. Phys. 2004, 205, 1651. (4) Zhao, B.; Li, D.; Hua, F.; Green, D. R. Macromolecules 2005, 38, 9509. (5) (a) Otake, K.; Inomata, H.; Konno, M.; Saito, S. Macromolecules 1990, 23, 283. (b) Grinberg, V. Y.; Dubovik, A. S.; Kuznetsov, D. V.; Grinberg, N. V.; Grosberg, A. Y.; Tanaka, T. Macromolecules 2000, 33, 8685. (c) Cho, E. C.; Lee, J.; Cho, K. Macromolecules 2003, 36, 9929.
It has been believed that water clathrates or iceberg structures exist around the hydrophobic moieties, where water molecules have a well-ordered structure and form a larger number of waterwater hydrogen bonds (H-bonds) than in bulk water. Recently, a new insight has been introduced into the hydrophobic hydration since H-bonds between alkyl groups and water, C-H‚‚‚O H-bonds, became known.6 One of critical features of C-H‚‚‚O H-bonds is blue shifts of the C-H stretching (ν(C-H)) frequency, which is opposite to normal H-bonds such as O-H‚‚‚O and N-H‚‚‚O where the X-H stretching bands undergo red shifts. Many water-soluble molecules, including ethanol,7 acetone,8 and 1,4-dioxane,9 have been reported to exhibit a blue shift of ν(C-H) on hydration. However, we have to take care to interpret these observations as direct evidence of blue-shifting C-H‚‚‚O H-bonding. For example, H-bonding of water to the ether oxygen of dimethyl ether in argon matrices induces significant blue shifts of the C-H stretching frequencies.10 This phenomenon was explained by a decrease in the oxygen lone pair electron density leading to a decrease in the extent of negative hyperconjugation and thus to a strengthening and shortening of the C-H bonds. We have previously investigated the temperature and concentration dependencies of the hydration of several thermosensitive polymers including poly(N-alkyl (meth)acrylamide)s11 and poly(vinyl ether)s12 by using IR spectroscopy. We found that the hydration of these polymers leads to an increase in the ν(C-H) frequencies and that the phase separation leads to a decrease in (6) (a) Barnes, A. J. J. Mol. Struct. 2004, 704, 3. (b) Hobza, P.; Havlas, Z. Chem. ReV. 2000, 100, 4253. (7) Mizuno, K.; Miyashita, Y.; Shindo, Y.; Ogawa, H. J. Phys. Chem. 1995, 99, 3225. (8) Chang, H.-C.; Jiang, J.-C.; Lin, S. H.; Weng, N.-H.; Chao, M.-C. J. Chem. Phys. 2001, 115, 3215. (9) Mizuno, K.; Imafuji, S.; Fujiwara, T.; Ohta, T.; Tamiya, Y. J. Phys. Chem. B 2003, 107, 3972. (10) Barnes, A. J.; Beech, T. R. Chem. Phys. Lett. 1983, 94, 568.
10.1021/la7016006 CCC: $37.00 © 2007 American Chemical Society Published on Web 09/22/2007
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them. In particular, poly(vinyl methyl ether) (PVME), a polymer possessing pendant ether groups, exhibits remarkably large red shifts (∼20 cm-1) in the symmetric and the antisymmetric C-H stretching of the methyl group on its phase separation. Ab initio quantum chemical calculations by Zeng and Yang using dimethyl ether as a model compound showed that the blue shift is mainly due to the hydrogen bond between the ether oxygen and water at a low water/DME ratio and is partially due to C-H‚‚‚O H-bonds at a high water/DME ratio.13 Vibrational spectroscopic observations supported by molecular simulation based on quantum chemical calculations are quite useful in revealing hydration changes of thermosensitive polymers during phase transitions. In the present study, we investigate the phase separation of aqueous solutions of PMoEoEMa. The polymer possesses both ether and ester groups as polar sites. Another structural feature of PMoEoEMa is a relatively long side chain and the presence of four different kinds of oxygen atoms, which are able to accept H-bonds from water molecules. The purpose of this study is to elucidate (1) H-bonding between the ether oxygen and water and its effects on the C-O stretching vibration and the C-H stretching vibrations of the alkyl groups located nearby and (2) H-bonding between the carbonyl group and water as compared with that of the amide CdO groups of previously studied poly(N-alkyl(meth)acrylamide)s. We also discuss the effects of the positions of these oxygen atoms in the polymer chain on their hydration.
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Figure 1. (a) Structure of PMoEoEMa. (b) Structure and serial numbers of atoms of MoEoEMPa. (c) Structure of isolated MoEoEMPa optimized by DFT calculations at the B3LYP/6-31G(d) level.
Experimental Section Materials. 2-(2-Methoxyethoxy)ethyl methacrylate was purchased from Aldrich. PMoEoEMa was synthesized by radical polymerization in methanol at 70 °C for 7 h using 2,2′-azobis(isobutyronitrile) as an initiator. The polymers obtained were purified by dialysis against water and then freeze dried. The number-average molecular weight (Mn) is 1.0 × 104 (poly(ethyleneglycol) standard), and the polydispersity, Mw/Mn, is 1.9 as determined by size exclusion chromatography in chloroform at 40 °C. D2O was purchased from Aldrich. Measurements. IR, differential scanning calorimetry (DSC), and cloud-point measurements were essentially the same as those described previously.11,12 IR spectra were measured at a resolution of 1 cm-1 by using a Fourier transform infrared spectrometer (FTS3000, Varian) equipped with a deuterated triglycine sulfate detector. Each polymer solution was placed between two CaF2 windows directly (>50 wt %) or with a 10-µm-thick spacer (e50 wt %), and the temperature was controlled by using a circulating water bath. The IR spectra of each solution at different temperatures were continuously collected on heating at a rate of ∼1 °C/min. Analyses of the spectra were performed by using a software package from Varian (Win-IR Pro). DSC measurements were performed using a microcalorimetry system (MicroCal Inc.) at a scanning rate of 0.75 °C/min. (11) (a) Maeda, Y.; Higuchi, T.; Ikeda, I. Langmuir 2000, 16, 7503. (b) Maeda, Y.; Nakamura, T.; Ikeda, I. Macromolecules 2001, 34, 1391. (c) Maeda, Y.; Nakamura, T.; Ikeda, I. Macromolecules 2002, 35, 10172. (12) (a) Maeda, Y. Langmuir 2001, 17, 1737. (b) Maeda, Y.; Yamauchi, H.; Fujisawa, M.; Sugihara, S.; Ikeda, I.; Aoshima, S. Langmuir 2007, 23, 6561. (13) Zeng, X. G.; Yang, X. Z. J. Phys. Chem. B 2004, 108, 17384. (14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998.
Figure 2. (a) Temperature dependence of the turbidity (500 nm) of an aqueous PMoEoEMa solution (Wp ) 0.005) upon heating (∼1 °C/min). (b) DSC thermogram of the solution upon heating. DFT Calculations. DFT calculations were performed using Gaussian 9814 running on a Linux PC at the B3LYP level with the 6-31G(d) basis set. A model compound, 2-(2-methoxyethoxy)ethyl 3-methyl-3-pentanate (MoEoEMPa, Figure 1b), was used for the calculation. Two dummy methyl groups (C13D3), which are not included in the repeat unit of PMoEoEMa, were deuterated to remove unnecessary vibrational coupling with the remaining parts of the molecule. The serial numbers of atoms used in this report are also shown. As a matter of convenience, the same numbering is also used to designate the atoms of PMoEoEMa.
Results Turbidimetry and DSC. First, we measured the phasetransition temperature of the aqueous PMoEoEMa solution by using turbidimetry and DSC (Figure 2). The solution was clear at low temperatures and became turbid above a cloud point of Tc ) 26 °C, which was determined as the onset temperature of its turbidity at heating. The value is consistent with that reported in the literature3a and is due to the phase separation of the solution as observed by optical microscopy. The phase separation is accompanied by endothermic heat as shown in the DSC thermogram of the solution (Figure 2b). The enthalpy of the transition, ∆H, as the area of the peak is 36 J/g-polymer (6.7 kJ/mol of monomer unit). The value is smaller and larger than those of both PNiPAm (51 J/g, 5.8 kJ/mol) and PVME (85 J/g, 4.8 kJ/mol) when they are compared on a weight basis and a number basis, respectively. The largest ∆H per monomer unit of PMoEoEMa may reflect the largest hydration number of its side chain. IR Spectrum of Solid PMoEoEMa. The IR absorption spectrum of bulk solid PMoEoEMa is shown in Figure 3. Fourier
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The values of ∆∆A(T), which are given by the following equation, are plotted against temperature to demonstrate the progress of the phase separation (Figure 5),
∆∆A(T) ) ∆A(T, ν1) - ∆A(T, ν2)
Figure 3. IR absorption (red), Fourier self-deconvoluted (blue), second-derivative (green), and simulated (violet) spectra of PMoEoEMa in the bulk solid state.
self-deconvolution is applied to the spectrum to reduce the width of overlapping peaks and to make their positions clear. The second derivative spectrum is also shown, in which the sharpened minima correspond to the maxima in the original absorption spectrum. Major IR bands are the ν(C-H) bands (2800-3000 cm-1), the CdO stretching bands of the ester group (ν(CdO)) (1728 cm-1), the C-H deformation bands (1350-1500 cm-1), and the C-O stretching (ν(C-O)) bands of the ether groups (1000-1200 cm-1). We can recognize five peaks in the deconvoluted and second derivative spectra in the ν(C-H) region. To assign the observed IR bands precisely, we applied vibrational analysis based on DFT to a model compound, MoEoEMPa. The optimal structure of isolated MoEoEMPa is shown in Figure 1c. The calculated frequencies are scaled down by multiplying by a factor of 0.95 to obtain a best fit to the observed spectrum of PMoEoEMa, and the positions are shown in the bottom of Figure 3. The observed frequencies and assignments are also compiled in Table 1. We mainly assign the IR bands in the ν(C-H) region. Because each IR peak still consists of several vibrational modes, we hereafter use abbreviations shown in the last column in Table 1 to designate them. We can observe three major bands at 1158, 1143, and 1110 cm-1 (solid state) in the ν(C-O) region of the IR spectra. The vibrational analysis shows that the antisymmetric C1-O2-C3 and C4-O5-C6 stretching vibrations possess major contributions in the 1158 and 1110 cm-1 bands, and we abbreviate these modes as ν(C-O)A and ν(C-O)C, respectively. The antisymmetric and C4-O5-C6 and C7-O8-C9 stretching vibrations possess major contributions to the ν(C-O)B band (1143 cm-1). Temperature Dependence of the IR Spectra of PMoEoEMa in Water. The IR absorption spectra of PMoEoEMa (weight fraction, Wp ) 0.1) measured in D2O or H2O at different temperatures are shown in Figure 4. The ν(C-H) and ν(CdO) regions of the D2O solution and the ν(C-O) region of the H2O solution are shown to prevent hindrance by the IR absorption of the solvents. The deconvoluted, second derivative, and difference (∆A(T, ν)) spectra are also shown to enhance small changes in the IR spectra induced by phase separation. Each difference spectrum is obtained by subtracting the IR absorption spectrum measured at the initial temperature (T0) from that measured at an elevated temperature (T). These spectra show that the ν(CH) bands shift downward and the ν(CdO) band shifts upward during phase separation. The ν(C-O)A and ν(C-O)C bands shift downward and upward during phase separation, respectively, while the ν(C-O)B frequency is constant.
where ν1 and ν2 are the wavenumbers at the maximum or minimum in the difference spectrum (shown with violet triangles in Figure 4d). For instance, the values of ∆∆A(T) ) ∆A(T, 1736 cm-1) - ∆A(T, 1711 cm-1) are plotted to follow the changes in the ν(CdO) band (closed triangles). The onset temperatures (Tp) of the ∆∆A(T) curves for the ν(C-H)A, ν(CdO), and ν(C-O)C bands are ∼24 °C and agree with the onset temperature (cloud point) of the simultaneously measured transmittance of visible light from a light-emitting diode through the solution. The values of ∆∆A(T) change rapidly over the range of 24-40 °C, which is comparable to the width of the endothermic peak in the DSC thermogram. Gradual changes in ∆∆A(T) of the ν(CdO) and ν(C-O)C bands observed outside this range (T < Tp or T > ∼40 °C) may be due to a steady change in the H-bonding of the ester and ether groups of the polymer (the strength and/or the number of H-bonds). Concentration Dependence of the IR Spectra. Next, we underwent IR measurements of PMoEoEMa/D2O solutions at different concentrations. As expected in an LCST phenomenon, the values of Tp have a minimum at Wp ) 0.1-0.2 (Figure S1). The wavenumbers of ν(C-H)A and ν(C-H)E are plotted against Wp (lower axis) and the number of water molecules per monomer unit (Nw, upper axis) in Figure 6. The open circles in the Figures show the wavenumbers at T < Tp, which shift downward with increasing Wp and decreasing Nw. The closed circles in Figure 6 indicate the wavenumbers of these bands at T > Tp. Though two phases coexisted at this temperature, the contribution from the polymer existing in the concentrated phase to the IR spectra is dominant. Above Tp, the wavenumber of ν(C-H)A is nearly independent of PMoEoEMa concentration and close to that of the dried polymer film, suggesting that the corresponding groups of the polymer are fully dehydrated. The wavenumber of ν(C-H)E is also independent of polymer concentration at T > Tp and Wp e 0.8 but is higher than that of the dried film by 2 to 3 cm-1, suggesting that the corresponding groups are partially hydrated. Figure 7 shows the profiles of the ν(CdO) band of PMoEoEMa at different concentrations. The center of the band of dried PMoEoEMa is found at 1728 cm-1 and is comparable to that of bulk poly(methyl methacrylate) (PMMA).15 The band can be fitted with a single Gaussian curve and is assigned to the carbonyl group without H-bonding. The ν(CdO) band of PMoEoEMa measured in D2O has a considerable contribution at lower wavenumbers. The band can be fitted with one or two more Gaussian components at 1709 and 1685 cm-1 in addition to the 1728 cm-1 component (free carbonyl). The 1685 cm-1 component makes only a small contribution and is absent at Wp g 0.3. It is well known that the ν(CdO) band shifts toward lower wavenumbers when the carbonyl group is hydrogen bonded.15 The 1709 and 1685 cm-1 components can be assigned to singly and doubly H-bonding carbonyl groups with water molecules as simulated by DFT calculations (see below). (15) (a) Gonza´lez-Benito, J.; Koenig, J. L. Macromolecules 2002, 35, 7361. (b) Berquier, J.-M.; Arribert, H. Langmuir 1998, 14, 3716.
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Table 1. Observed IR Frequencies and Assignments of PMoEoEMa in Water and the Bulk State wavenumber (cm-1) neat
assignmenta,b
abbreviationb
νas(C11H3), νas(C7H2), νas(C1H3) νas(C12H2), νs(C7H2) νs(C11H3) νs(C12H2), [νas(C3H2) + νas(C4H2)], νas(C1H3)′, νas(C6H2) [νs(C4H2) + νs(C6H2)], [νs(C1H3) + νs(C3H2)] ν(CdO) δ(C-H)
ν(C-H)A ν(C-H)B ν(C-H)C ν(C-H)D ν(C-H)E ν(CdO) δ(C-H)A δ(C-H)B w(C-H)A w(C-H)B w(C-H)C t(C-H)A t(C-H)B ν(C-C) ν(C-O)A ν(C-O)B ν(C-O)C
aqueous solution
2989 2947 2928 2876 2817 1729 1484 1452 1402 1388 1353 1269 1246 1199 1158 1143 1110
T < Tp 2997 2952 2928 2881 2830 1718 1485 1455 1398 1388 1356 1280 1252 1199 1178 1136 1100
T > Tp 2989 2945 2928 2874 2819 1727 1484 1453 1398 1388 1356 1270 1244 1199 1163 1137 1107
w(C-H) t(C-H) νas(O8C9C10) [νas(C1O2C3) + νas(C4O5C6)] [νas(C4O5C6) + νas(C7O8C9)] [νas(C1O2C3) + νas(C4O5C6)]′
a Assignment based on the vibrations of 0 calculated at the B3LYP/6-31G(d) level. b ν, δ, w, and t indicate stretching, bending, wagging, and twisting modes, respectively.
Figure 4. (a) IR absorption, (b) Fourier self-deconvoluted, (c) second-derivative, and (d) difference spectra of PMoEoEMa (Wp ) 0.1) measured in D2O (ν(C-H) and ν(CdO) regions) and in H2O (ν(C-O) region) at T ) 12 (blue line) and 54 (red line) oC). The green arrows in b indicate the direction of the peak shifts upon heating. The violet triangles in d indicate the peaks used to calculate ∆∆A(T) in Figure 5.
Area intensities of the 1728 cm-1 and 1709 cm-1 components (A1728 and A1709) are related to the molar fractions of the free and singly H-bonded carbonyl groups (ffree and fH-bond) as
(because of the absence of the doubly H-bonded species) and obtain the relationship between A1728 and A1709 as
A1728 ) cl1728ffree and A1709 ) cl1709fH-bond
1709 A1728 ) cl1728 - A1709 1728
(1)
where c, l, and represent the total concentration of the carbonyl groups, the cell length, and the area absorption coefficient. We can approximate the sum of ffree and fH-bond to be 1 at Wp g 0.3
(2)
We plotted A1728 against A1709 in Figure S2 and estimated the value of 1709/1728 to be 1.1 by linear regression analysis. This value is slightly smaller than the value calculated by DFT (1.27,
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Figure 8. Molar fraction of the H-bonding carbonyl groups (fH-bond) plotted against (a) temperature (Wp ) 0.3) and (b) Wp below (O) and above (b) Tp. Figure 5. Values of ∆∆A(T) for the selected IR bands and transmittance at 520 nm (green lines) of PMoEoEMa (Wp ) 0.1) in D2O plotted against temperature. The symbols represent 2, ν(C-H)A; b, ν(C-O)C; and 1, ν(CdO).
Figure 9. Structures of MoEoEMPa + H2O optimized by DFT calculations at the B3LYP/6-31G(d) level.
Figure 6. Positions of (a) ν(CH)A and (b) ν(CH)E of PMoEoEMa measured below (O) and above (b) Tp plotted against Wp (lower axis) and Nw (upper axis, uneven interval).
Figure 7. Baseline-subtracted and normalized ν(CdO) bands of PMoEoEMa measured below (left) and above (right) Tp and at different concentrations. The broken lines indicate the Gaussian fit to separate the free and H-bonding contributions.
see below). Finally, we can calculate fH-bond by the following equation:
fH-bond )
1 1709A1728 1+ 1728A1709
(3)
The values of fH-bond at different temperatures and concentrations are shown in Figure 8. Figure 8 shows that even at low polymer concentrations a considerable number of carbonyl groups
remained free. For example, ∼50% of the carbonyl groups are not H-bonding at Wp ) 0.3 even at T < Tp (homogeneous solution). We can compare the present result with H-bonding of the amide CdO groups of poly(N-alkyl(meth)acrylamide)s. The CdO groups of poly(N-isopropylacrylamide), poly(N-n-propylacrylamide), and poly(cyclopropylacrylamide) are fully H-bonded (fH-bond ) 1) at T < Tp.11a,b However, the CdO groups of poly(N-isopropylmethacrylamide) and poly(N-n-propylmethacrylamide) are partially dehydrated (fH-bond ≈ 0.9) even at T < Tp.11c This phenomenon is explained by the steric hindrance of the R-methyl groups of the latter polymers, which restrict water molecule access to CdO groups. Furthermore, the large degree of dehydration of the CdO groups of PMoEoEMa may be related to the steric hindrance of its long side chains. Because the carbonyl groups are located near the backbone of the polymer, they are easily surrounded by the R-methyl, the other end of the side groups, and the backbone (Figure S3). Moreover, a remarkable difference is evident in the dependences of the ν(C-H) frequencies and fH-bond on Wp at T > Tp (phase-separated state). Though the ν(C-H) frequencies are independent of Wp, fH-bond linearly decreases with Wp at T > Tp. DFT Calculations. In this section, we show the results of the DFT calculations using MoEoEMPa as a model (Figures 1 and 9) and discuss the effect of H-bonding on the vibrational frequencies. First, we deal with four monohydrated species (1a1d) to determine the effects of H-bonding on each oxygen atom. The vibrational frequencies are compiled in Table 2. The calculated wavenumbers (ν) of 0 (anhydrous MoEoEMPa, Figure 1c) are shown in the second column of the Table, and the shifts in wavenumber (∆ν) of the hydrated species (1a-1d) as compared with ν of 0 are shown in the following columns. We recognize that H-bonding to these oxygen atoms affects the vibrational frequencies of the adjacent alkyl groups to a considerable extent. In general, there are two mechanisms for the shifts of ν(C-H) frequencies during hydration. The first one is related to a direct interaction between the C-H group and water molecules including the C-H‚‚‚O H-bond. The second one is related to H-bonding to a polar group located near the C-H group under consideration. H-bonding to the ether oxygen, in particular, has a large effect
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Table 2. Calculated Vibrational Frequencies of the Complexes of MoEoEMPa and Watera ∆ν (cm-1)
ν vibrational mode 1
νas(C H3) νas(C1H3)' [νs(C1H3) + νs(C3H2)] [νas(C3H2) + νas(C4H2)] [νs(C4H2) + νs(C6H2)] νas(C6H2) νas(C7H2) νs(C7H2) νas(C11H3) νs(C11H3) νas(C12H2) νs(C12H2) ν(C9dO9)
0
1a
1b
1c
1d
2977 2880 2843 2887 2848 2869 2977 2929 2970 2904 2941 2890 1728
10 23 15 20 4 6 -1 -1 0 -0 0 -0 1
5 2 -1 22 20 30 3 -3 1 1 -2 -1 3
0 -0 -2 3 4 16 11 3 -0 2 5 4 7
-1 -0 0 0 -1 -0 18 8 1 1 -2 1 -36
a
Wavenumbers calculated at the B3LYP/6-31G(d) level are corrected by a factor of 0.95.
on ν(C-H) frequencies. The calculations show that H-bonding to O2 induces blue shifts in the ν(C1-H) and ν(C3-H) frequencies to a large extent. H-bonding to O5 also induces blue shifts of the ν(C4-H) and ν(C6-H) frequencies. The observation is related to the hyperconjugation between the ether oxygen and the adjacent alkyl groups of the molecule in which electron density from the lone pairs on the oxygen are donated to π* orbitals on the alkyl groups,13 therefore causing the C-H bonds to be weakened. Recently, Katsumoto et al. revealed that an intramolecular n f π* hyperconjugative interaction with a stabilization energy of 24.5 kJ mol-1 occurs between the ether oxygen and the R-CH group of 2-butoxyethanol by using natural bond orbital (NBO) analysis.16 When a water molecule binds to the oxygen atom, its lone pair electron density on the oxygen will decrease, which leads to a decrease in the extent of hyperconjugation. Thus, the C-H bonds are shortened, and finally the ν(C-H) mode undergoes a blue shift. The tendency is enhanced for the formation of the second H-bond to the same oxygen atom. Engdahl et al. have experimentally obtained a similar result by using a matrix isolation method in which the νs(CH3) band of dimethyl ether shifts upward when an H-bond is formed between the ether oxygen and water.17 Similarly, when O8 (esteric ether) forms a H-bond with water (1c), the C6-H6 and C7-H7 bonds are contracted, and finally ν(C6-H) and ν(C7-H) frequencies undergo blue shifts. When O9 (carbonyl) forms H-bonds (1d), C7 and H7 acquire a more negative and a more positive charge, respectively, which leads to a contraction of C7-H bonds and an increase in νs(C7H2) and νas(C7H2) frequencies. The DFT calculations show that the ν(CdO) frequency of the free carbonyl group, C9dO9, is 1728 cm-1 and single and double H-bonds lead to red shifts of 36 and 55 cm-1, respectively. Therefore, the assignments of the three ν(CdO) components of PMoEoEMa (1728, 1709, and 1685 cm-1, respectively) to nonhydrated carbonyl groups and singly and doubly H-bonded carbonyl groups are reasonable. The H-bonds also enhance the intensity of the ν(CdO) band as the relative intensity of the ν(CdO) band for non-hydrated carbonyl groups and singly and doublely H-bonded carbonyl groups is calculated to be 1:1.27:1.66. H-bonding to O8 also leads to a small blue shift of the ν(CdO) frequency by 7 cm-1. However, because the width of the ν(CdO) components (20-30 cm-1) is larger than the shift (16) Katsumoto, Y.; Komatsu, H.; Ohno, K. J. Am. Chem. Soc. 2006, 128, 9278. (17) Engdahl, A.; Nelander, B. J. Chem. Soc., Faraday Trans. 1992, 88, 177.
induced by the H-bonding to O8, it is difficult to evaluate its effect experimentally. However, the observed changes in the ν(C-O) region were not successfully explained by simulation. It shows that H-bonding to O2 induces increases in the net negative charges on both O2 and adjacent C1 and C3 and the extension of the C1-O2 and C3-O2 bonds. H-bonding to O5 also has a similar influence. However, we failed to find a reliable trend in the shifts of the ν(C-O) frequencies. The reasons are that two C-O-C stretching vibrations are coupled in a complex manner with each other and also with the C-H deformations and that conformational changes in the side chain also affect the ν(C-O) frequencies. With an increase in the number of water molecules, we found another type of interaction in the models. For example, C-H‚‚‚O H-bonds (C1-H in 3a and C4-H in 3b, Figure S4) are recognized in 1:3 complexes of MoEoEMPa/water. The C-H‚‚‚O H-bonds are similar to a normal H-bond in their bond lengths and angles and stabilize the complexes to some extent. The water molecules binding to the C-H groups also form H-bonds with the water molecules binding directly to oxygen atoms (O2). Therefore, the water molecules affect the ν(C-H) frequencies in two different ways: (1) the C-H‚‚‚O H-bond may lead to a small blue shift and (2) the water-water H-bonds may have an influence on the hyperconjugation between O2 and C1-H and C4-H. However, overall effects of the third water molecule on ν(C-H) is not very significant as compared with that of the first water molecule binding directly to O2.
Discussion It is of great value to discuss the effect of the position of each polar group in the polymer chain on its hydration. The binding energy of the H-bond between each oxygen atom of the monomer model, MoEoEMPa, and water decreases in the order O9 > O5 > O2 > O8. The order is consistent with the result obtained for some small organic molecules using quantum chemical calculations with a higher-level basis set.18 This indicates that the carbonyl group is the most favorably hydrated. However, the hydration of PMoEoEMa is not consistent with the simulation as described below. The effect of the positions of polar groups can be examined experimentally, for example, by comparing the hydration of the carbonyl group located near the backbone and that of O2 located near the free end of the side chain. The ν(C-H)A band can be used to monitor the H-bonding at O2. The DFT calculation shows that the band contains three νas(C-H) modes and that νas(C1H3) and νas(C7H2) exhibit large and medium shifts upon H-bond formation at O2 and O8, respectively. In contrast, H-bonding to any oxygens has a negligible effect on the νas(C11H3) frequency. Therefore, the red shift of the ν(C-H)A band is largely attributed to the shift of νas(C1H3) during the dehydration of O2. The wavenumbers at the maximum (2982 cm-1) and minimum (3002 cm-1) in the difference ν(C-H)A spectra (Figure 4d) indicate that the νas(C1H3) band shifts at least 20 cm-1 during phase separation because the νas(C11H3) and νas(C7H2) bands might make minor contributions. The shift is as large as the calculated value (∆n of 1a, 23 cm-1) and that of the νas(CH3) band of PVME (20 cm-1).12a In addition, the ν(C-H)A band shifts upward with an increasing number of water molecules at T < Tp. The magnitude of the shift is significant at low Nw and becomes gentle at high Nw, suggesting that the first H-bond to O2 leads a relatively large shift and the second H-bond to O2 and the CH‚‚‚O H-bond occurring at high Nw lead to a smaller shift. It (18) Rablen, P. R; Lockman, J. W.; Jorgensen W. L. J. Phys. Chem. A 1998, 102, 3782.
PMoEoEMa Hydration Changes during Phase Separation
is reasonable to believe that O2 accept at least one H-bond at T < Tp and high Nw. In contrast, because the position of the band at T > Tp is close to that of the dried polymer film, O2 is considered to be fully dehydrated. This result means that O2 experiences a large change in its degree of hydration, from 1 to zero, during phase separation. Though the bond energy indicates that H-bonding to the carbonyl O9 is the most stable, the fraction of H-bonding at the carbonyl groups of PMoEoEMa is not high. For example, half of them do not form H-bonds at all at Wp ) 0.3 and T < Tp. The carbonyl group hardly accepts the second H-bond even at low Wp. However, the decrease in fH-bond during phase separation is also small (∼0.2). Therefore, at the end of the phase separation the degree of hydration of the carbonyl group becomes higher than that of the ether O2. The different hydration behaviors of the carbonyl and the ether groups may be explained by a steric restriction. Because the polymer chains have an extended conformation and are isolated from one another at T < Tp, O2 located near the end of the side chain can easily accept H-bonds from water molecules. However, the carbonyl groups possess such crowded positions
Langmuir, Vol. 23, No. 22, 2007 11265
near the backbone that water molecules are prevented from binding to them. In the phase-separated state at T > Tp, the polymer chains shrink to a compact conformation, and intermolecular association causes the surroundings of O2 to be crowded and hydrophobic, which induces dehydration of O2. Because the H-bond between the carbonyl O9 and water is thermodynamically more stable than that of O2, large parts of them will be retained even at T > Tp. The effects of the position of the polar groups in the polymer chain on their hydration can be simulated by molecular dynamics. Such a study is desired to explain the results obtained here in more detail. Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research (17550115) from the Japan Society for the Promotion of Science. We thank anonymous reviewers for helping to improve a previous version of this article. Supporting Information Available: Atomic charges and bond lengths of MoEoEMPa calculated at the B3LYP/6-31G(d) level. This material is available free of charge via the Internet at http://pubs.acs.org. LA7016006
