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Effects of Ionization of Incorporated Imidazole Groups on the Phase Transitions of Poly(N-isopropylacrylamide), Poly(N,N-diethylacrylamide), and Poly(N-vinylcaprolactam) in Water Yasushi Maeda,* Hiroki Yamamoto, and Isao Ikeda Department of Applied Chemistry and Biotechnology, Fukui University, Fukui 910-8507, Japan Received April 30, 2001. In Final Form: July 30, 2001
The effects of ionization of N-vinylimidazole (VIm) units that are incorporated into poly(N-isopropylacrylamide) (PiPA), poly(N,N-diethylacrylamide) (PdEA), and poly(N-vinylcaprolactam) (PVCL) on their phase behaviors in water have been investigated by means of turbidimetry, Fourier transform infrared spectroscopy, and differential scanning calorimetry (DSC). The phase transition temperatures (Tp) of these copolymers increase with the degree of ionization of VIm units, which in turn is dependent on the pH of the solutions. Apparent values of pKa for the VIm units that are determined from the pH dependencies of Tp are 5.2, 4.5, and 6.7 for PiPA-VIm, PdEA-VIm, and PVCL-VIm, respectively. Changes in Tp arising from the incorporation of an ionized form of VIm unit into these polymers (∆Tp ) (Tp,copolymer - Tp,homopolymer)/x, where x is the comonomer content in mol %) are calculated to be 1.7, 2.2, and 1.5 °C/mol % for PiPA-VIm, PdEA-VIm, and PVCL-VIm, respectively. DSC thermograms for each copolymer measured at a different pH show a linear relationship between the enthalpy of the transition (∆H) and the Tp probably because the hydrogen-bonding structure of water that surrounds the hydrophobic moieties of the copolymer is gradually broken with increasing temperature. IR measurements show that profiles of IR bands due to the amide and alkyl groups of these polymers exhibit critical changes at Tp, indicating that the hydration states of these groups change upon the phase separation.
Introduction Some water-soluble polymers undergo phase transitions at critical temperatures (lower critical solution temperature, LCST) in water, above which the solutions separate into two phases and polymer chains experience large structural changes. Until now, studies concerning the LCST type phase transition phenomena of aqueous polymer solutions have been concentrated on one of these polymers. In other words, poly(N-isopropylacrylamide) (PiPA) has been extensively studied by using various methods including calorimetry,1-3 fluorescence spectroscopy,4-6 light scattering,7,8 and many other techniques.9-11 In addition, many researchers have also investigated the effects of incorporation of various kinds of comonomer units into PiPA12-14 and the effects of addition of salts,15 * Corresponding author fax:
[email protected].
+81-776-27-8747;
e-mail:
(1) Grinberg, V. Y.; Dubovik, A. S.; Kuznetsov, D. V.; Grinberg, N. V.; Grosberg, A. Y.; Tanaka, T. Macromolecules 2000, 33, 8685. (2) Tiktopulo, E. I.; Bychkova, V. E.; Ricˇka, J.; Ptitsyn, O. B. Macromolecules 1994, 27, 2879. (3) Schild, H. G.; Muthkumar, M.; Tirrell, D. A. Macromolecules 1991, 24, 948. (4) Winnik, F. M. Macromolecules 1990, 23, 233. (5) Mylonas, Y.; Staikos, G.; Lianos, P. Langmuir 1999, 15, 7172. (6) Chee, C. K.; Rimmer, S.; Soutar, I.; Swanson, L. Polymer 2001, 42, 5079. (7) Fujishige, S.; Kubota, K.; Ando, I. J. Phys. Chem. 1989, 93, 3311. (8) Wang, X.; Qiu, X.; Wu, C. Macromolecules 1998, 31, 2972. (9) Tokuhiro, T.; Amiya, T.; Mamada, A.; Tanaka, T. Macromolecules 1991, 24, 2936. (10) Richardson, R. M.; Pelton, R.; Cosgrove, T.; Zhang, J. Macromolecules 2000, 33, 6269. (11) Vesterinen, E.; Dobrodumov, A.; Tenhu, H. Macromolecules 1997, 30, 1311. (12) Kuckling, D.; Adler, H.-J. P.; Arndt, K.-F.; Ling, L.; Habicher, W. D. Macromol. Chem. Phys. 2000, 201, 273. (13) Bokias, G.; Vasilevskaya, V. V.; Iliopoulos, I.; Hourdet, D.; Khokholov, A. R. Macromolucules 2000, 33, 9757.
cosolvents,16,17 and surfactants18,19 to the solutions of PiPA. Poly(N,N-diethylacrylamide) (PdEA)20-22 and poly(Nvinylcaprolactam) (PVCL)23-26 also exhibit phase transitions at temperatures close to room temperature. It is known that small differences in the structures of polymers make their phase behaviors quite different. For example, the phase transitions of PiPA and poly(N-n-propylacrylamide) are much sharper than those of poly(N-cyclopropylacrylamide) and PdEA. Poly(N-n-propylacrylamide) and poly(N-isopropylmethacrylamide) show a much larger thermal hysteresis (characterized by differences in the transition temperatures for heating and cooling processes) than PiPA.27,28 The addition of methanol into an aqueous solution of PiPA lowers the transition temperature toward (14) Feil, H.; Bae, Y. H.; Feijen, J.; Kim, S. W. Macromolecules 1993, 26, 2496. (15) Inomata, H.; Goto, S.; Otake, K.; Saito, S. Langmuir 1992, 8, 687. (16) Schild, H. G.; Muthkumar, M.; Tirrell, D. A. Macromolecules 1991, 24, 948. (17) Winnik, F. M.; Ottaviani, M. F.; Bossmann, S. H.; Pan, W.; GarciaGaribay, M.; Turro, N. J. Macromolecules 1990, 23, 233. (18) Zhu, P. W.; Napper, D. H. Langmuir 1996, 12, 5992. (19) Walter, R.; Ricˇka, J.; Quellet, C.; Nyffenegger, R.; Binkert, T. Macromolecules 1996, 29, 4019. (20) Kobayashi, M.; Ishizone, T.; Nakahama, S. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4677. (21) Idziak, I.; Avoce, D.; Lessard, D.; Gravel, D.; Zhu, X. X. Macromolecules 1999, 32, 1260. (22) Speˇa´cˇek, J.; Geschke, D.; Ilavsky´, M. Polymer 2001, 42, 463. (23) Tager, A. A.; Safronov, A. P.; Sharina, S. V.; Galaev, I. Yu. Colloid Polym. Sci. 1993, 271, 868. (24) Kirsh, Y. E.; Soos, T. A.; Karaputadze, T. M. Eur. Polym. J. 1983, 19, 639. (25) Lau, A. C. W.; Wu, C. Macromolecules 1999, 32, 581. (26) Gao, Y.; Au-Yeung, S. C. F.; Wu, C. Macromolecules 1999, 32, 3674. (27) Maeda, Y.; Higuchi, T.; Ikeda, I. Langmuir 2000, 16, 7503. (28) Maeda, Y.; Nakamura, T.; Ikeda, I. Macromolecules 2001, 34, 1391.
10.1021/la0106438 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/29/2001
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Chart 1
minimum and then induces a steep increase.29 In contrast, the addition of methanol induces a monotonic increase in the transition temperature of the aqueous PdEA solution. To understand the phase transition phenomena of aqueous polymer solutions in molecular terms, it is necessary to reveal the relationship among (i) the structures of repeating units of polymers, (ii) the interaction between water and individual chemical groups in the polymers, and (iii) their phase behaviors. In previous studies, we have analyzed the phase behaviors of several polymers including poly(N-substituted acrylamide)s,28 poly(N-substituted methacrylamide)s,30 poly(N-vinyllactam)s, and polyethers31 by using turbidimetry, IR spectroscopy, and calorimetry. In particular, IR spectra have provided important information concerning changes in the hydration states of these polymers upon their phase transitions. The objective of the present study is to reveal the effects of charges of ionizable units that are incorporated into thermosensitive polymers on their phase behaviors. For this purpose, we have incorporated vinylimidazole (VIm) as an ionizable unit into PiPA, PdEA, and PVCL (Chart 1), whose aqueous solutions undergo phase transitions at 31-34 °C. The phase behaviors (transition temperatures, heats of transition) and hydration of these copolymers have been investigated at various pH values by means of turbidimetry, IR spectroscopy, and differential scanning calorimetry (DSC).
Figure 1. Temperature-induced turbidity changes in the heating processes of (a) PiPA-VIm10 and (b) PdEA-VIm10 measured in H2O at different pH values. DSC thermograms of (c) PiPA-VIm10 and (d) PdEA-VIm10 solutions in the heating processes. D2O), which has an IR absorption at 1465 cm-1 (bending vibration of HOD). The polymer was then dissolved in D2O (20 wt %) and was placed between two CaF2 windows with a spacer (10 µm thick). A background spectrum for one cycle of a measurement was obtained with the polymer solution in the IR cell equilibrated at a starting temperature (To). Then, time-resolved IR spectra were continuously collected at a resolution of 2 cm-1. The collected spectra were differential spectra between the absorption spectra at various temperatures (T) and the absorption spectrum at To, and they are designated as ∆AT-To. The difference between absorbance at the positive peak and absorbance at the negative peak of a selected vibration mode in each difference spectrum (∆AT-To) is defined as
∆∆AT-To(ν1, ν2) ) ∆AT-To(ν1) - ∆AT-To(ν2) where ν1 and ν2 denote wavenumbers at the positive or negative peaks in the IR difference spectrum and ν1 is defined to be lower than ν2. DSC measurements were performed using a Micro Calorimetry System (MicroCal, Inc.) with a scanning rate of 0.75 °C/min. Turbidity and DSC measurements were performed by using 0.5 wt % polymer solutions. Because the copolymers themselves act as buffers, the pH of the solutions was adjusted by the addition of a small amount of aqueous HCl or NaOH solutions.
Experimental Section
Results and Discussion
Materials. N-Isopropylacrylamide (iPA; Kohjin, Tokyo, Japan) was purified by recrystallization. N,N-Diethylacrylamide (dEA, Kohjin), N-vinylcaprolactam (VCL, Aldrich), and VIm (Wako, Osaka, Japan) were purified by vacuum distillation. Copolymers were synthesized with radical polymerization in methanol at 70 °C using 2,2′-azobis(isobutyronitrile) as an initiator. After evaporation, these polymers were precipitated from acetone-nhexane. Polymers obtained were purified by dialysis (Visking tube, exclusion limit 8000), and they were finally lyophilized. VIm contents (mol %) were evaluated to be 4.9, 10.2, 5.1, 10.4, and 5.2 for PiPA-VIm5, PiPA-VIm10, PdEA-VIm5, PdEAVIm10, and PVCL-VIm5, respectively, by using 1H NMR. Measurements. Methods of turbidity, DSC, and Fourier transform infrared measurements have been described previously.28 IR spectra were measured by using an FTS-3000 IR spectrometer (Bio-Rad). To substitute dissociable protons of the polymers by deuterons and to replace trace amounts of adsorbed H2O by D2O, freeze-dried polymers were dissolved in D2O (EURISTOP, 99.9%) and lyophilized again before IR measurements. The process minimizes the contamination of H2O (exist as HOD in
Dependences of Tp and ∆H of the Copolymers on pH. Figure 1a,b represents temperature-dependent turbidity changes of PiPA-VIm10 and PdEA-VIm10 measured in H2O at various pH values. With decreasing pH, the turbidity-vs-temperature curves move toward higher temperatures. The slopes of these curves in the transition regions are lowered with decreasing pH, partially because electrostatic repulsion between positive charges on ionized VIm units prevents an aggregation of the globules formed from polymer chains. DSC thermograms of PiPA-VIm10 and PdEA-VIm10 measured at different pH values are shown in Figure 1c,d. The endothermic peaks for PiPAVIm solutions are much narrower than those for PdEAVIm solutions. The endothermic peaks shift toward higher temperatures and reduce their areas with decreasing pH. The endothermic peaks for PiPA-VIm broaden with decreasing pH because the sizes of cooperative domains in a molecule that undergo phase transition all at once become smaller due to the division of the domains by the charged groups.2 The values of Tp of PiPA-VIm, which are defined as onsets of the turbidity-vs-temperature curves or DSC thermograms, are plotted against pH in Figure 2a. The values of Tp for PiPA-VIm5 and PiPA-
(29) Schild, H. G.; Muthkumar, M.; Tirrell, D. A. Macromolecules 1991, 24, 948. (30) Maeda, Y.; Nakamura, T.; Ikeda, I. Macromolecules, in press. (31) Maeda, Y. Langmuir 2001, 17, 1737.
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Figure 2. pH dependencies of Tp of (a) PiPA-VIm5 (0, 9) and PiPA-VIm10 (O, b) and (b) PdEA-VIm5 (0), PdEA-VIm10 (O), and PVCL-VIm5 (4) determined by turbidimetry (open marks) and DSC (closed marks). Curves are drawn by using a least-squares regression with eq 2 and pKa values listed in Table 1. Broken and dotted lines indicate the Tp for the corresponding homopolymers.
Figure 3. (a) Enthalpies of the phase transitions (∆H) for PiPA-VIm solutions measured at various pH values are plotted against their Tp. VIm contents (4.9, 10.2, 20.5 mol %), pH (4.5-7.1), and NaCl concentration (0, O; 0.5, y; 1.0 M, b) were varied. (b) The values of ∆H for the solutions of PdEA-VIm (0) and PVCL-VIm (4) measured at various pH values are plotted against their Tp.
VIm10 decrease with increasing pH up to pH 6, and the values are almost constant above pH 6. The values of Tp for PdEA-VIm and PVCL-VIm decrease with increasing pH in the pH ranges of 3-6 and 6-8, respectively, and Tp is almost independent of pH at pH values out of the ranges. The values of Tp for these polymers seem to depend on the degree of ionization (R) of VIm units. Here, we assume that the phase transition temperature of each copolymer depends on R of VIm units and that R is expressed as a function of pH with the following equation
Table 1. Values of pKa and ∆Tp of the VIm Units in PiPA-VIm, PdEA-VIm, and PVCL-VIm
R ) 10-pH/(10-pKa + 10-pH)
(1)
where Ka is a dissociation constant for the protonated VIm unit (VImH+ h VIm + H+) and pKa ) -log Ka. Then, Tp as a function of pH can be expressed by the following equation
Tp(pH) ) (1 - R)Tp,R)0 + RTp,R)1 ) Tp,R)0 + (Tp,R)1 - Tp,R)0)10-pH/(10-pKa + 10-pH) (2) where Tp,R)0 and Tp,R)1 are the values of Tp at R ) 0 and 1, respectively. Curves in Figure 2 are calculated by using a nonlinear least-squares regression analysis with eq 2. The values of pKa for PiPA-VIm, PdEA-VIm, and PCVL-VIm are estimated to be 5.2, 4.5, and 6.7, respectively. The differences in pKa values of VIm units clearly indicate that the local concentration of protons around VIm units and/or the interaction between VIm units and other parts of the polymers are different among these copolymers. The highest value of pKa for PVCL-VIm indicates that PVCL most effectively stabilizes the ionized form of VIm. Next, the efficiencies of incorporated VIm units on Tp of these copolymers are compared. The difference between Tp for the copolymer and Tp for the corresponding homopolymer is defined as ∆Tp,R)1 ) (Tp,R)1 - Tp,homopolymer)/x for the ionized form and ∆Tp,R)0 ) (Tp,R)0 Tp,homopolymer)/x for the neutral form. The values of ∆Tp,R)1 and ∆Tp,R)0 are compiled in Table 1. Efficiency of ionized VIm units on Tp is the highest in PdEA and is the lowest in PVCL. The nonionic form of the VIm units slightly raises the Tp of PiPA and PVCL but lowers the Tp of PdEA. Each of these copolymers is surrounded by complex hydration shells consisting of different local water structures.32 The (32) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; Wiley: New York, 1980.
PiPA PdEA PVCL
pKa
∆Tp,R)0
∆Tp,R)1
Tp,homopolymer
5.2 4.5 6.7
0.13 -0.04 0.06
1.7 2.2 1.5
32.0 31.2 33.3
Tp of the copolymer is determined by the balance between an unfavorable decrease in the entropy of water in the hydrophobic hydration shell and a favorable interaction between polar groups and water. Because incorporated charged groups perturb the hydrogen-bonding structure of water33 and induce an increase in the entropy of water, Tp of the copolymer with ionized VIm units is higher than that with neutral VIm units. Figure 3a represents the relationship between the temperature at the maximum of a DSC endothermic peak and the enthalpy of the phase transition (∆H) as calculated by integrating the endothermic peak. The open circles in Figure 3a are data points for PiPA-VIm measured in the absence of added salt at various pH values. These points lie on a single line, suggesting that ∆H is essentially determined by the Tp of the solution, which in turn depends on the composition (x) and the degree of ionization (R) of the copolymer. Different dependences of ∆H on Tp are observed with PiPA-VIm solutions when measurements were performed in the presence of NaCl (broken and dotted lines). The values of ∆H and Tp for PdEA-VIm and PVCLVIm also show linear relationships. The slopes of ∆Hvs-Tp lines for PdEA-VIm and PVCL-VIm are similar to each other, and they are lower than that for PiPA-VIm. As shown in our previous studies, the breakage of hydrogen bonds between water molecules that surround the hydrophobic moieties of polymer chains mainly contributes to ∆H.28,34 An increasing Tp reduces ∆H because the number of the hydrogen bonds at high temperatures is lower than that at low temperatures. Although the addition of NaCl to the solutions alters the Tp of the solution as well as changes in x or R of the copolymers, the relationship between Tp and ∆H for the polymer solutions containing NaCl is different from that for the solutions without ions. Because Na+ and Cl- tend to subtract water molecules from the polymer chain,35,36 they strengthen hydrophobic interactions, which stabilize the (33) Maeda, Y.; Tsukida, N.; Kitano, H.; Terada, T.; Yamanaka, J. J. Phys. Chem. 1993, 97, 13903. (34) Otake, K.; Inomata, H.; Konno, M.; Saito, S. Macromolecules 1990, 23, 283. (35) McBain, J. W. Colloid Science; Heath: Boston, 1950. (36) Robinson, D. R.; Jencks, W. P. J. Am. Chem. Soc. 1965, 87, 2470.
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Figure 4. (a) IR absorption spectrum (1700-1100 cm-1) of PiPA-VIm10 (R ) 0) measured at 30 (solid line) and 46 °C (broken line) in D2O. (b) IR absorption spectra (1120-1050 cm-1) of PiPA-VIm10 measured at R ) 0 (solid line, 30 °C; broken line, 46 °C) and R ) 1 (35 °C) in D2O. (c) IR difference spectrum (1700-1100 cm-1) of PiPA-VIm10 (R ) 0) measured in D2O solution (∆A46-30). (d) IR difference spectra (1120-1050 cm-1) of PiPA-VIm10 measured at R ) 0 (∆A46-30) and R ) 1 (∆A58.8-35) in D2O.
globular conformation of the polymer and lower Tp. The ions have effects not only on the hydration states of the polymers but also on the structure of the bulk water. Parts of water molecules that are removed from polymer chains upon the phase transition will hydrate these ions, which accompany exothermic heat and reduce the endothermic heat of the transition as a whole. This may be a reason why ∆H in the presence of NaCl is lower than the value of ∆H expected from Tp and the solid line shown in Figure 3a. IR Spectra of PiPA-VIm, PdEA-VIm, and PVCL-VIm. Figure 4a,b shows IR absorption spectra of deuterated PiPA-VIm10 measured below (solid line, 30 °C) and above Tp (broken line, 46 °C). D2O was used as a solvent instead of H2O in order to prevent the O-H bending band of water (ca. 1640 cm-1) from overlapping with the amide I band of the polymer. The positions of IR bands due to iPA units of PiPA-VIm10 are almost the same as those of PiPA homopolymer, and assignments of these bands have been described in the previous paper.27 Prominent IR bands are the amide I′ band (mainly CdO stretching vibration of the deuterated amide group) and the amide II′ band (combination of N-D bending and C-N stretching vibrations of the deuterated amide group) observed at 1624 and 1469 cm-1, respectively. The IR bands that can be attributed to VIm moiety are C-N stretching bands (ν(C-N)VIm) that are observed at 1112 and 1084 cm-1 at R ) 0 and 1096 cm-1 at R ) 1 (Figure 4b). IR difference spectra (∆AT-To) that are obtained by subtracting the IR absorption spectra of PiPA-VIm10 at To (Tp) are shown in Figure 4c,d. In these IR difference spectra, negative peaks associate with IR absorption by the solution below Tp and a positive peak represents absorption above Tp. Profiles of the IR difference bands due to iPA moiety are almost the same as those of PiPA homopolymer. Important features are red shifts of the C-H stretching (ν(C-H)) and the amide II′ bands and a blue shift of the amide I′ band. These shifts in IR bands indicate the dehydration of the alkyl groups (C-H bond contracts and its vibrational frequency undergoes a blue shift upon interaction with water)37 and the reduction of the strength of hydrogen (37) Gu, Y.; Kar, T.; Scheuner, S. J. Am. Chem. Soc. 1999, 121, 9411.
Maeda et al.
Figure 5. (a) Differences in IR absorbance at a positive peak and a negative peak (∆∆AT-To(ν1, ν2)) of selected vibration modes (ν(C-H), amide I′ and amide II′) for PiPA-VIm10 (open symbols, R ) 0; closed symbols, R ) 0.5) are plotted against temperature: (9, 0) ∆∆AT-To(2970, 2991); (b, O) ∆∆AT-To(1625, 1650); (2, 4) ∆∆AT-To(1448, 1479). (b) The molar fractions of the CdO groups of PiPA-VIm5 (b) and PiPA-VIm10 (2) that form a hydrogen bond with the N-D group (CdO‚‚‚D-N) in D2O solution at R ) 0 are plotted against temperature.
bonds of the amide groups38,39 as described in our previous paper.28 In addition to the IR bands due to iPA units, IR difference peaks due to the ν(C-N)VIm mode are also clearly observed at 1114 cm-1 (negative), 1107 cm-1 (positive), 1088 cm-1 (negative), and 1077 cm-1 (positive) at R ) 0 (top of Figure 4d). In contrast, IR difference peaks for the ν(C-N)VIm mode of the ionized VIm groups cannot be observed at R ) 1 (bottom of Figure 4d). The result indicates that change in the hydration states of charged VIm groups upon the phase transition is small probably because they exist at the surface of the globules and are exposed to water even above Tp. The differences in the absorbance at the positive and negative peaks (∆∆AT-To(ν1, ν2)) of three selected vibration modes (ν(C-H), amide I′, and amide II′) of PiPA-VIm10 at R ) 0 (open marks) and R ) 0.5 (closed marks) are plotted against temperature in Figure 5a. Onsets of ∆∆Avs-temperature curves for PiPA-VIm10 at R ) 0 and 0.5 are located at around 36 and 46 °C, respectively. Gradual changes in ∆∆A observed both below and above the transition temperature region are due to a simple temperature effect, which weakens the strength of the hydrogen bond between the amide and the water and causes a blue shift of the amide I band and a red shift of the amide II band.40 At R ) 0, the ∆∆A curve for the ν(C-N)VIm mode as well as the ∆∆A curves due to iPA units start to increase at the same temperature, suggesting that the dehydration of nonionic VIm units and iPA units proceeds simultaneously in the course of the phase transition of the copolymer. To obtain information concerning the hydrogen bond of the amide group, the amide I′ bands of PiPA-VIm are analyzed by using a peak separation method. In the previous study, we have shown that the amide I′ band of PiPA homopolymer consists of two components centered at 1625 and 1650 cm-1. These components have been assigned to the CdO groups that bind to water (CdO‚‚‚D-O, 1625 cm-1) and to an amide N-D group of the polymer (CdO‚‚‚D-N, 1650 cm-1) through a hydrogen bond.27 The amide I′ band of PiPA-VIm also consists of a single component at 1625 cm-1 below Tp, and it contains an additional 1650-cm-1 component above Tp. (38) Jackson, M.; Mantsch, H. H. Crit. Rev. Biochem. Mol. Biol. 1995, 30, 95. (39) Parker, F. S. Application of Infrared, Raman, and Resonance Raman Spectroscopy in Biochemistry; Plenum Press: New York, 1983. (40) Manas, E. S.; Getahun, Z.; Wright, W. W.; DeGrado, W. F.; Vanderkooi, J. M. J. Am. Chem. Soc. 2000, 122, 9883.
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IR difference spectra for PdEA-VIm10 and PVCLVIm5 are shown in Figure 6a,b. Profiles of the IR difference spectra due to dEA and VCL moieties are similar to those for the corresponding homopolymers. The values of ∆∆A for selected vibration modes of these copolymers measured at R ) 0 and 0.5 are plotted against temperature in Figure 6c,d. The ∆∆A-vs-temperature curves for PdEA-VIm10 and PVCL-VIm5 also shift toward higher temperatures with an increase in R. The onset temperatures of these curves are close to Tp determined by turbidimetry and DSC measurements, indicating that changes in hydration states of these polymers proceed during the phase separations. Because amide I bands of PdEA and PVCL contain more than four components and their assignments are ambiguous at this moment, further analysis is not done here. Conclusion Figure 6. (a and b) IR difference spectra of (a) PdEA-VIm10 (∆A43.6-33.8) and (b) PVCL-VIm5 (∆A44.1-34.2) measured at R ) 0 in D2O. Arrows indicate the positive and negative peaks that are used to calculate values of ∆∆AT-To(ν1, ν2) shown in c and d. (c) The values of ∆∆AT-To(ν1, ν2) for the amide I (b, O) and amide II (2, 4) modes of PdEA-VIm10 (open symbols, R ) 0; closed symbols, R ) 0.5) are plotted as a function of temperature. (d) The values of ∆∆AT-To(ν1, ν2) for the amide I (b, O) and ν(C-H) (9, 0) modes of PVCL-VIm5 (open symbols, R ) 0; closed symbols, R ) 0.5) are plotted against temperature.
We have shown that about 13% of the CdO groups of PiPA form hydrogen bonds with the amide N-D, and the remaining CdO groups form hydrogen bonds with water in the globule state (>Tp).27 The molar fractions of the CdO‚‚‚D-N species (polymer-polymer hydrogen bond) for PiPA-VIm5 and PiPA-VIm10 at R ) 0 are plotted against temperature in Figure 5b. The molar fractions of the CdO‚‚‚D-N species for PiPA-VIm5 and PiPA-VIm10 at R ) 0 are slightly higher than that for PiPA homopolymer.
The phase transition temperatures (Tp) of PiPA-VIm, PdEA-VIm, and PVCL-VIm increase with the degree of ionization (R) of VIm units. Values of pKa for the VIm units determined from the pH dependencies of Tp are different from one another (PiPA-VIm, 5.2; PdEA-VIm, 4.5; PVCL-VIm, 6.7). Both DSC and IR measurements show that transition temperature regions for PiPA-VIm broaden with increasing R, indicating that the domains that cooperatively undergo structural changes and dehydration in a macromolecule are divided by charged VIm units. Linear decreases in ∆H with increasing Tp suggest that the hydrogen-bonding structure of water around hydrophobic moieties of the polymer is gradually broken with increasing temperature and that the energy consumed to break the water structure upon the dehydration of the polymer chain is reduced at higher Tp. Acknowledgment. This work was supported by Grant-in-Aid for Encouragement of Young Scientists (12750795) from the Ministry of Education, Science and Technology, Japan. LA0106438
