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Confocal Micro-Raman and Infrared Spectroscopic Study on the Phase Separation of Aqueous Poly(2-(2-methoxyethoxy)ethyl (meth)acrylate) Solutions Yasushi Maeda,* Hideo Yamauchi, and Tomoyuki Kubota Department of Applied Chemistry and Biotechnology, UniVersity of Fukui, Fukui 910-8507, Japan ReceiVed September 2, 2008. ReVised Manuscript ReceiVed October 6, 2008 Thermosensitive phase separation of aqueous solutions of acrylate polymers with oligooxyethylene side groups, poly(2-(2-methoxyethoxy)ethyl acrylate) (PM2A) and poly(2-(2-methoxyethoxy)ethyl methacrylate) (PM2Ma), has been investigated by using infrared and confocal micro-Raman spectroscopy. PM2A and PM2Ma exhibited phase separation at 45 and 26 °C with a transition enthalpy of 21 and 36 J/g, respectively. The Raman and infrared bands for the C-H and CdO stretching modes (ν(C-H) and ν(CdO)) of the polymers exhibited red and blue shifts on phase separation, respectively, whereas the C-O stretching bands of the ether groups hardly shifted. The ν(C-H) and ν(CdO) bands also shifted with an increase in polymer concentration. Though free carbonyl groups exist in both of the polymers even at low polymer concentrations and below the phase separation temperatures, the average number of H-bonds is lower in PM2Ma than in PM2A probably because of the steric effects of the R-methyl groups. Raman microscopic observation showed that polymer-rich domains appeared above the phase separation temperature and the polymer concentration reached 52%.
Introduction Aqueous solutions of some kinds of water-soluble polymers exhibit phase separation above critical temperatures (Tp). Typical examples are poly(N-substituted acrylamide)s such as poly(Nisopropylacrylamide)1 (PiPA) and poly(N,N-diethylacrylamide)2 and poly(vinyl ether)s3 such as poly(vinyl methyl ether)4 and poly(methoxyethyl vinyl ether).5 It is generally believed that breaking of some parts of hydrogen bonds (H-bonds) between polar sites of these polymers and water and the change in the interaction of apolar sites from unfavorable hydrophobic hydration to hydrophobic interaction between them at heating drive the phase separation. Infrared and Raman spectroscopies are suitable to investigate these interactions because they are based on molecular vibration, which is sensitive to the conformation and interaction of the molecule. Recently, poly(meth)acrylates with an oligo(oxyethylene) of appropriate length as the pendant group have been shown to exhibit phase separation, and their Tp values were tuned by the side chain length and end group.6 In the previous study we have investigated phase separation of poly(2-(2-methoxyethoxy)ethyl methacrylate) (PM2Ma) by using IR spectroscopy.7 Hydration of the ester groups was examined through the analyses of the carbonyl stretching bands, which exhibit red shifts of ∼30 cm-1 * To whom correspondence should be addressed. Fax: 0776-27-8747. E-mail:
[email protected]. (1) (a) Fujishige, S.; Kubota, K.; Ando, I. J. Phys. Chem. 1989, 93, 3311. (b) Winnik, F. M. Polymer 1990, 31, 2125. (2) Maeda, Y.; Nakamura, T.; Ikeda, I. Macromolecules 2002, 35, 10172. (3) (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. (4) (a) Scha¨fer-Soenen, H.; Moerkerke, R.; Berghmans, H.; Koningsveld, R.; Dusˇek, K.; Sˇolc, K. Macromolecules 1997, 30, 410. (b) Maeda, Y. Langmuir 2001, 17, 1737. (5) Maeda, Y.; Yamauchi, H.; Fujisawa, M.; Sugihara, S.; Ikeda, I.; Aoshima, S. Langmuir 2007, 23, 6561. (6) (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. (7) Maeda, Y.; Kubota, T.; Yamauchi, H.; Nakaji, T.; Kitano, H. Langmuir 2007, 23, 11259.
on H-bonding. The result showed that only 50% of the carbonyl groups form H-bonds with water even at T < Tp (homogeneous solution), which is much lower than that of the amide CdO groups of poly(N-alkyl(meth)acrylamide)s. The ν(C-H) bands of the alkyl groups also exhibited relatively large blue shifts (∼20 cm-1) upon hydration of the polymer, though the ν(C-O) bands of the ether groups were not sensitive to the formation of H-bonds. In the present study, we have investigated the phase separation of poly(2-(2-methoxyethoxy) ethylacrylate) (PM2A), which lacks the R-methyl groups as compared with PM2Ma, by using IR spectroscopy and compared its hydration and phase behavior with those of PM2Ma to know the effects of the R-methyl group. In addition, we have observed the phase separation behaviors of both PM2A and PM2Ma by using confocal microRaman spectroscopy. The advantage of the method is a relatively high spatial resolution (∼1 µm), which allowed us to examine each individual polymer-rich droplet appearing in the system above Tp.
Experimental Section Materials. 2-(2-Methoxyethoxy) ethylacrylate was synthesized via coupling of acryloyl chloride (Tokyo Kasei, 8.12 mL) with 2-(2methoxyethoxy)ethanol (Wako Pure Chemicals, 13.1 mL) in benzene (120 mL) in the presence of triethylamine (16.7 mL) and purified by vacuum distillation. PM2A (Figure 1a) was synthesized with radical polymerization in methanol at 70 °C for 7 h using 2,2′azobis(isobutyronitrile) as an initiator. After evaporation the polymer was purified by dialysis in water (seamless cellulose tube, exclusion limit 12 000) and freeze-dried. The roughly estimated weight-average molar mass (Mw) and polydispersity (Mw/Mn) of the polymers determined by gel permeation chromatography (column, Toso TSK -1 gel GMHHR-M; mobile phase, chloroform (0.6 mL · min ); standard, poly(ethylene glycol)s) were 51 000 g/mol and 3.4. PM2Ma used here was the same as that used in the previous study.7 Measurements. The polymers were dissolved in H2O or D2O (Aldrich). The weight fraction of the polymer (Wp) is used to represent the polymer concentration. Sample solutions were placed between two CaF2 windows with a spacer 10 µm thick. IR spectra were continuously collected at a resolution of 1 cm-1 using an FTS-3000 IR spectrometer (Varian) equipped with a deuterated triglycine sulfate
10.1021/la802869j CCC: $40.75 2009 American Chemical Society Published on Web 12/03/2008
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Figure 1. Structures of (a) PM2A, (b) MoEoEPa as a model for the repeating unit of PM2A (serial numbers of the atoms are also shown), and (c) MoEoEPa optimized by DFT calculation at the B3LYP/6-31G(d,p) level.
Figure 2. (a) Turbidimetric and (b) DSC thermograms of PM2A (solid line)/H2O and PM2Ma (broken line)/H2O solutions (Wp ) 0.005) at heating.
detector during heating or cooling of the sample solutions at a rate of ca. 1 °C/min controlled by a circulating water bath. Analyses of the spectra were performed by using a software package from Varian (Win-IR Pro). Raman spectra were measured at a resolution of ca. 1 cm-1 using a confocal micro-Raman spectrometer (NRS-1000, JASCO) equipped with an Ar laser (GLG2169, Showa Optronics) operated at 514.5 nm and an electronically cooled (-70 °C) CCD detector (DU401FI, Andor). A pinhole aperture (50 µm) and a 100× objective lens give a spatial resolution of ca. 1 and 2 µm to the lateral and vertical direction, respectively. Sample solutions were placed between a holed slide glass and a coverglass and put on a metal plate thermostated by a circulating water bath. Point-by-point mapping was carried out using a motorized scanning stage controlled by a computer. DSC measurements were performed using a microcalorimetry system (MicroCal Inc.) at a scanning rate of 1 °C/min. DFT Calculations. DFT calculations were performed using Gaussian 98 running on a Linux PC at the B3LYP level with the 6-31G(d,p) basis set. A model compound, 2-(2-methoxyethoxy)ethyl 3-pentanate (MoEoEPa; Figure 1b) was used for the calculation. Two dummy methyl groups (C12D3), which are not included in the repeating unit of PMoEoEMa, were deuterated to remove unnecessary vibrational coupling with the remaining parts of the molecule. The serial numbers of the atoms used in this paper are also shown. As a matter of convenience, the same numbering is also used to designate the atoms of PM2A.
Results and Discussion Turbidity and DSC Measurements. An aqueous solution of PM2A exhibited phase separation above a Tp of 45 °C as recognized with a change in its turbidity (Figure 2a). The phase separation was accompanied by endothermic heat as shown in the DSC thermogram of the solution (Figure 2b). Turbidity and DSC thermograms of PM2Ma are also shown for comparison. PM2A had a higher Tp and a lower enthalpy of transition, ∆H
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(21 J/g of polymer, 3.7 kJ/mol of monomer), as the area of the peak, than PM2Ma (36 J/g of polymer, 6.7 kJ/mol of monomer). It is reasonable that PM2Ma has a lower Tp than PM2A because the additional R-methyl groups of PM2Ma are hydrophobic. However, the opposite is true in the cases of poly(N-alkyl(meth)acrylamide)s; that is, PiPMA and PnPMA8 have higher Tp values than PiPA9 and PnPA,10 respectively. Because the latter polymers possess primary amide groups that can form H-bonds, their intra- and/or intermolecular interaction may be responsible for the difference. The lower value of ∆H for PM2A than PM2Ma is rationalized by the fact that a polymer with a higher Tp has a lower ∆H in general.11 The enthalpy is due mainly to breaking of H-bonding between water molecules surrounding hydrophobic moieties of the polymer chains during phase separation.8,12 Because the number of H-bonds between water molecules is reduced at high temperatures, the value of ∆H decreases. IR Spectra of PM2A. IR absorption spectra of a dry polymer film are shown in Figure 3a. Fourier self-deconvolution was applied to the spectra to reduce the width of overlapping peaks and to make their positions clear (the blue lines). Major IR bands of PM2A were 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). To assign the observed IR bands, we applied vibrational analysis based on DFT to the model compounds, whose optimal structures are shown in Figure 1c. The observed frequencies and assignments are compiled in Table 1. We hereafter use abbreviations shown in the first column in Table 1 to designate the IR bands. The IR spectra of PM2A were also measured in H2O and D2O at different temperatures (Figure 3b). Fourier self-deconvolution was applied except for the ν(CdO) band to enhance spectral resolution. Because the polymers do not have any exchangeable protons, the positions of the IR bands measured in D2O were essentially the same as those measured in H2O. The ν(C-H) bands and ν(CdO) bands were measured in D2O, and the ν(C-H) and ν(C-O) bands were measured in H2O, because each solvent does not have IR bands in the used region. Five peaks were recognized in the ν(C-H) region of the deconvoluted spectra of these polymers. The intensity of band a of PM2A was lower than that of PM2Ma because the R-CH3 groups absorb in this region. Remarkable changes were observed at bands a and e during phase separation. The wavenumbers of bands a and e are plotted against temperature in Figure 4. The peaks started to shift downward at Tp. The red shift of these C-H stretching bands can partly be explained with the dehydration of the alkyl groups. The interaction between alkyl groups and water, C-H · · · O interaction, is known to induce a blue shift of the C-H stretching vibration.13 The red shift is also related to the hyperconjugation between the alkyl groups and adjacent ether oxygen. Donation of electron density from the lone pairs of the oxygen to π* orbitals of the alkyl groups causes the C-H bonds to be weakened.14 When a water molecule binds to the oxygen, its lone pair electron density 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 (8) Maeda, Y.; Nakamura, T.; Ikeda, I. Macromolecules 2001, 35, 1391. (9) Maeda, Y.; Higuchi, T.; Ikeda, I. Langmuir 2000, 16, 7503. (10) Maeda, Y.; Nakamura, T.; Ikeda, I. Macromolecules 2001, 35, 8246. (11) Feil, H.; Bae, Y. H.; Feijen, J.; Kim, S. W. Macromolecules 1993, 26, 2496. (12) Otake, K.; Inomata, H.; Konno, M.; Saito, S. Macromolucules 1990, 23, 283. (13) Hobza, P; Havlas, Z. Chem. ReV. 2000, 100, 4253. (14) Zeng, X. G.; Yang, X. Z. J. Phys. Chem. B 2004, 108, 17384.
Phase Separation of PM2A and PM2Ma Solutions
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Figure 3. (a) IR absorption (green) and Fourier self-deconvoluted (pink) spectra of dry PM2A. Broken lines indicate the position of each band. (b) IR absorption spectra of PM2A (Wp ) 0.1) measured in D2O (3030-2780 and 1780-1680 cm-1) and H2O (1500-1000 cm-1) at different temperatures (30 (blue) and -58 (red) °C). The deconvolution was applied to the 3030-2780 and the 1500-1000 cm-1 ranges. Raman spectra of (c) PM2A/H2O and (d) PM2Ma/H2O solutions at different concentrations (Wp ) 0.1 (orange) and -1 (red)) at 25.0 °C. Table 1. Observed Infrared and Raman Frequencies (cm-1) and Assignmentsa of PM2A in Water and the Bulk Statea,b infraredb neat a b c d
2979 2950 2924 2879
e
2823
f g h i j k l m n o p
1733 1451 1396 1354 1251 1198 1167 1140 1110 1026
0.5. The PM2A and PM2Ma solutions of Wp ) 0.5 correspond to 9.7 and 10.4 water molecules per monomer unit (W/M), respectively. The result indicates that water molecules have a small effect on the ν(C-H) and ν(CdO) frequencies at W/M > 10. At high polymer concentrations, the hydration of hydrophilic ester and ether groups is preferred to that of hydrophobic alkyl groups. The bending point of band f suggests that H-bonding to the carbonyl groups is saturated around W/M ) 10. Even in the saturated situation, that is, when the carbonyl groups are surrounded by a satisfactorily large number of water molecules, the formation of the carbonyl-water H-bonds is relatively low as shown in Figure 5. The bending point of band e suggests that H-bonding to the ether oxygen, which induces relatively large blue shifts of the ν(C-H) frequencies including band e according to the second mechanism mentioned above, is saturated around W/M ) 10. A further increase of water molecules initiates hydration of the alkyl groups, which induces a slight shift of band e according to the first mechanism mentioned above. By using an optical microscope, we observed that small particles appeared just above Tp and gradually increased their sizes by collision. The optical microscopic image of the phaseseparated system (Wp ) 0.2) at 36 °C is shown in Figure 7 as a background gray image. A point-by-point Raman mapping was carried out using a motorized scanning stage, and the value of AC-H/AO-H (area ratio of the ν(C-H) band of PM2A and the
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Figure 8. (a) Calculated weight fraction (Wp,cal) of PM2A (red) and PM2Ma (blue) in the domain and matrix phases at cooling plotted against temperature. The overall weight fraction (Wp) was 0.2. (b) Temperature dependences of Raman shifts of band e of PM2A and PM2Ma in the domain phase (T > Tp) and homogeneous solution (T < Tp).
ν(O-H) band of water) is shown as a colored contour map. The map indicates that the domain and the matrix are a polymer-rich phase and a solvent-rich phase, respectively. Next we measured Raman spectra at a point in the domain phase and a point in the matrix phase to reveal a temperatureinduced change in the polymer concentration at a cooling process. The estimated weight fraction of PM2A and PM2Ma (Wp,cal) is plotted against temperature in Figure 8a. To obtain the values, we used the relationship between AC-H/AO-H and Wp observed at 25 °C (PM2A) and 15 °C (PM2Ma) (homogeneous solution). The value of Wp,cal for both of the polymers reached 0.52 at the highest temperatures of the measurements. Measurement above 60 °C was hard because of the limit of our experimental setup. The polymer concentration decreases with decreasing temperature at 58-45 °C (PM2A) and 30-20 °C (PM2Ma). Optical microscopic images showed that the polymer-rich droplets swelled in the temperature ranges and disappeared at around 45 °C (PM2A) and 20 °C (PM2Ma). Below Tp the value of Wp,cal becomes independent of the position in the mixtures and also temperature. Though the values of Tp for the polymers were different, the concentration changes are quite similar to each other. Figure 8b shows the temperature dependence of the wavenumber of band e measured in the domain phase. The wavenumber of the band decreases with increasing temperature in the transition temperature region, which is consistent with the result obtained by IR measurements.
Conclusions Thermosensitive phase separations of aqueous solutions of PM2A and PM2Ma have been investigated by using infrared and confocal micro-Raman spectroscopy. PM2A and PM2Ma exhibited phase separation at 45 and 26 °C with a transition enthalpy of 21 and 36 J/g, respectively. The behaviors of the polymers during phase separation were quite similar to each other with the exception of H-bonding to the carbonyl groups. Though free carbonyl groups existed in both of the polymers even at temperatures below Tp and low polymer concentrations, the average number of H-bonds was higher in PM2A than in PM2Ma. The additional R-methyl groups of PM2Ma may prevent water from forming H-bonds because of steric effects. After all, the hydrophobicity and the steric effects of the R-methyl groups make aqueous PM2Ma solutions unstable as compared with PM2A solutions. Acknowledgment. This work was supported by a Grant-inAid for Scientific Research (20550109) from the Japan Society for the Promotion of Science. LA802869J
