FTIR Spectroscopic and Calorimetric Studies of the Phase Transitions

Matthew S. Wagner, Gabor A. Somorjai, David G. Castner, and Buddy D. Ratner ... Janire Saez , Tom Glennon , Monika Czugala , Alexandru Tudor , Jen...
0 downloads 0 Views 85KB Size
Download PDF
Langmuir 2001, 17, 7535-7539

7535

FTIR Spectroscopic and Calorimetric Studies of the Phase Transitions of N-Isopropylacrylamide Copolymers in Water Yasushi Maeda,* Tomomi Higuchi, and Isao Ikeda Department of Applied Chemistry and Biotechnology, Fukui University, Fukui 910-8507, Japan Received January 3, 2001. In Final Form: August 30, 2001

The phase transitions of copolymers of N-isopropylacrylamide (iPA) with acrylamide (PiPA-AAm) or acrylonitrile (PiPA-AN) in water have been observed by means of Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The effects of both incorporation of these comonomer units and addition of salts to their aqueous solutions on the phase behaviors have been investigated. The positions of IR bands due to iPA and the comonomer units critically shift at lower critical solution temperatures (LCST) of these copolymers. The phase transition leads shifts of the amide II, C-H stretching, and C-H bending bands to lower wavenumbers and a shift of the amide I band to a higher wavenumber. The CtN stretching band (ν(CtN)) of AN units in PiPA-AN shifts to a lower wavenumber. These shifts of the IR bands indicate that corresponding chemical groups experience dehydration to some extent upon the transition. The ν(CtN) band is composed of two components due to the CtN groups that form a hydrogen bond with water (2247 cm-1) and dehydrated CtN groups (2241 cm-1). Analysis of the band by using a curve fitting method shows that almost all CtN groups form hydrogen bonds with water in the coil state and 90% of the CtN groups are dehydrated in the globule state. The hydration of the CtN group in the globule state is quite different from that of the amide CdO group, 85% of which forms hydrogen bonds with water even in the globule state. DSC measurements show that the heat of phase transition (∆H) linearly decreases with an increase in the LCST of the copolymers, meaning that ∆H is essentially determined by the LCST of the copolymer, which in turn depends on its composition. The structure of water surrounding the polymer chains as well as the balance between hydrophilicity and hydrophobicity of the polymers as a whole has importance to the phase transition behaviors of iPA copolymers.

Introduction Poly(N-isopropylacrylamide) (PiPA) is known to show a phase separation at a critical temperature, which is accompanied by a large structural change in the polymer chain, that is, a coiled chain collapses to a globular conformation. The conformational transition of PiPA from the coil to the globule is initiated by the dehydration of the polymer chain, which makes attractive interaction between hydrophobic isopropyl groups to be dominant and induces collapse of the chain. The incorporation of comonomer units into the polymer chain1,2 and the addition of salts,3,4 cosolvents,5,6 surfactants,7,8 or other additives to the polymer solutions modify the phase transition behavior. Elucidation of factors that determine the transition temperature and heat of the transition is important in order to reveal the nature of the phase transitions of aqueous polymer solutions in molecular terms and also to predict thermal stabilities of proteins. Among many kinds of techniques that have been used to investigate the transition phenomena of PiPA and other * To whom correspondence should be addressed. Fax: +81-77627-8747. E-mail: [email protected]. (1) Hahn, M.; Go¨rnitz, E.; Dautzenberg, H. Macromolecules 1998, 31, 5616. (2) Feil, H.; Bae, Y. H.; Feijen, J.; Kim, S. W. Macromolecules 1993, 26, 2496. (3) Kunugi, S.; Yamazaki, Y.; Takano, K.; Tanaka, N.; Akashi, M. Langmuir 1999, 15, 4056. (4) Inomata, H.; Goto, S.; Otake, K.; Saito, S. Langmuir 1992, 8, 687. (5) Otake, K.; Inomata, H.; Konno, M.; Saito, S. Macromolecules 1990, 23, 283. (6) Schild, H. G.; Muthkumar, M.; Tirrell, D. A. Macromolecules 1991, 24, 948. (7) Lee, L.-T.; Cabane, B. Macromolecules 1997, 30, 6559. (8) Meewes, M.; Rie`ka, J.; Silva, M.; Nyffenegger, R.; Binkert, T. Macromolecules 1991, 24, 5811.

thermosensitive polymers (for instance, calorimetry,9-12 NMR,13,14 fluorometry,15-17 and light scattering18-22), IR spectroscopy is a suitable method to observe changes in the conformation, interaction, and microenvironment of individual chemical groups in these polymers occurring upon the phase transition.23,24 An additional advantage of the use of IR spectroscopy is its ability to detect these changes without added probes, which may modify the phase transition by themselves. In previous studies, we have investigated the changes in the hydration state of PiPA during the phase transition by using IR spectroscopy. Analysis of the amide I band of PiPA, which is mainly due to the CdO stretching vibration, indicates that almost all (9) Tiktopulo, E. I.; Uversky, V. N.; Lushchik, V. B.; Klenin, S. I.; Bychkova, V. E.; Ptitsyn, O. B. Macromolecules 1995, 28, 7519. (10) Tiktopulo, E. I.; Bychkova, V. E.; Rie`ka, J.; Ptitsyn, O. B. Macromolecules 1994, 27, 2879. (11) Feil, H.; Bae, Y. H.; Feijen, J.; Kim, S. W. Macromolecules 1993, 26, 2496. (12) Otake, K.; Inomata, H.; Konno, M.; Saito, S. Macromolecules 1990, 23, 283. (13) Ohta, H.; Ando, I.; Fujishige, S.; Kubota, K. J. Polym. Sci., Polym. Phys. Ed. 1991, 29, 963. (14) Tokuhiro, T.; Amiya, T.; Mamada, A.; Tanaka, T. Macromolecules 1991, 24, 2936. (15) Shirota, H.; Kuwabara, N.; Ohkawa, K.; Horie, K. Macromolecules 1999, 103, 10400. (16) Walter, R.; Rie`ka, J.; Quellet, C.; Nyffenegger, R.; Binkert, T. Macromolecules 1996, 29, 4019. (17) Winnik, F. M. Macromolecules 1990, 23, 233. (18) Wang, X.; Qiu, X.; Wu, C. Macromolecules 1998, 31, 2972. (19) Qiu, X.; Kwan, C. M. S.; Wu, C. Macromolecules 1997, 30, 6090. (20) Meewes, M.; Rie`ka, J.; Silva, M.; Nyffenegger, R.; Binkert, T. Macromolecules 1991, 24, 5811. (21) Kubota, K.; Fujishige, S.; Ando, I. J. Phys. Chem. 1990, 94, 5154. (22) Fujishige, S.; Kubota, K.; Ando, I. J. Phys. Chem. 1989, 93, 3311. (23) Maeda, Y.; Higuchi, T.; Ikeda, I. Langmuir 2000, 16, 7503. (24) Maeda, Y.; Nakamura, T.; Ikeda, I. Macromolecules 2001, 34, 1391.

10.1021/la010007+ CCC: $20.00 © 2001 American Chemical Society Published on Web 11/01/2001

7536

Langmuir, Vol. 17, No. 24, 2001

CdO groups form hydrogen bonds with water molecules as a solvent in the coil state. On the other hand, about 13% of the CdO groups are bound to the N-H groups through hydrogen bonds and make intrachain or interchain cross linkages in the globule state. The C-H stretching IR bands of PiPA undergo low-wavenumber shifts during the coil-to-globule transition, which indicate dehydration of the isopropyl side chain and the main chain of PiPA. In the present study, we have further investigated IR spectral changes of PiPA copolymers upon their phase transitions in order to reveal changes in the hydration states of the comonomer units as well as the iPA moiety. Acrylonitrile (AN) is a suitable comonomer unit for this purpose because it has a characteristic IR absorption due to the CtN stretching vibration at around 2250 cm-1, whereas the iPA moiety does not have IR absorption in the region. Moreover, calorimetric measurements have also been carried out to clear the relationship between the lower critical solution temperature (LCST) of PiPA copolymers and the heat of the transition (∆H). Experimental Section Materials. N-Isopropylacrylamide (iPA, Kohjin, Tokyo, Japan) was purified by recrystallization from benzene-n-hexane. Acrylamide (AAm) from Wako Pure Chemicals (Osaka, Japan) was recrystallized. AN was from Wako and distilled before use. Copolymers were synthesized via radical polymerization with a total monomer concentration of 0.88 M in methanol (50 mL) using 2,2′-azobis(isobutyronitrile) (8.8 mM) as an initiator at 70 °C for 7 h. Poly(N-isopropylacrylamide-co-acrylonitrile) and poly(N-isopropylacrylamide-co-acrylamide) are abbreviated as PiPA-AN and PiPA-AAm, respectively, and approximate polymer compositions (mol % of comonomer units in the copolymers) are represented by the numbers at the end of these abbreviations. Feed compositions were 7.5 and 15 mol % AAm for PiPA-AAm9 and PiPA-AAm17, respectively, and 20 mol % AN for PiPA-AN20. After evaporation, these copolymers were repeatedly precipitated from acetone-n-hexane. Polymers obtained were purified by dialysis (Visking tube, exclusion limit 8000), and they were finally lyophilized. AAm contents in PiPA-AAm evaluated by elemental analysis were 9.0 and 17.5 mol % for PiPA-AAm9 and PiPAAAm17, respectively. The AN content in PiPA-AN20 was 20.4 mol %. Weight-average molecular weights and polydispersity (Mw/Mn) of the polymers were determined by gel permeation chromatography (Toso HLC-803D; column, Toso G3000PW (30 cm) + G5000PWXL (30 cm); mobile phase, THF (1.0 mL min-1). Poly(ethylene glycol)s used as standard samples were 11 000 (Mw/Mn ) 1.9), 74 000 (Mw/Mn ) 2.8), 74 000 (Mw/Mn ) 3.8), and 51 000 (Mw/Mn ) 3.4) for PiPA, PiPA-AAm9, PiPA-AAm17, and PiPA-AN20, respectively. Measurements. Methods of IR, differential scanning calorimetry (DSC), and cloud point measurements were essentially the same as those described previously.24 Here, we concisely describe the methods. IR spectra were measured by using a Jasco FTIR-620 equipped with a TGS detector. The polymer sample dissolved in D2O or H2O (typically 20 wt %, 50 µL) was placed between two CaF2 windows (diameter, 20 mm; thickness, 2 mm) with a spacer (10 µm thick). A background spectrum for one cycle of a measurement was obtained with the sample solution equilibrated at the starting temperature (T0). Then, the temperature of the solution was continuously increased or decreased with a circulating water bath (BU150S, Yamato) at a rate of ca. 1 °C/min, and timeresolved IR spectra were continuously collected at a resolution of 2 cm-1. The temperature at the cell holder was simultaneously measured by using an electronic thermometer. The IR difference spectrum given by the differences in IR absorption intensities (A) measured at T and the starting temperature (T0) is designated as ∆AT-T0. A curve fitting analysis was performed to examine the amide I and the ν(CtN) bands by using Spectra Manager software (Jasco). The positions of two Gaussian components were fixed, and their heights and widths were optimized to obtain the bestfit curve.

Maeda et al.

Figure 1. IR absorbance spectra of the solutions of PiPAAN20 (20 wt %) measured at 23.0 (solid lines) and 39.5 °C (broken lines) in (a) H2O and (b) D2O. DSC measurements were performed using a Micro Calorimetry System (MicroCal Inc.) at a scanning rate of 0.75 °C/min both in the heating and cooling processes. The polymer sample dissolved in H2O (0.5 wt %) was degassed and transferred to the sample cell (cell volume, 1.22 mL). An identical volume of the solvent of the same composition was placed in the reference cell. The turbidity of a polymer solution (0.5 wt %) was followed by the absorbance at 500 nm using a spectrophotometer (UV200100, Hitachi). The temperature of the observation cell was continuously raised at a rate of ca. 1 °C/min.

Results IR Spectra of Aqueous Solutions of PiPA-AN. The IR absorption spectrum of PiPA-AN20 measured in H2O and D2O at 23.0 °C (below the LCST) and 39.5 °C (above the LCST) is shown in Figure 1. Although the solution separates into two phases and becomes turbid above the LCST, the difference in the profiles of the IR spectra measured below and above the LCST is not so large. Positions of bands due to the iPA unit of the copolymer are almost the same as those of the PiPA homopolymer, whose assignments were described in the previous paper.24 Important IR bands for the iPA moiety are the amide I band (1625 cm-1 in D2O) and the amide II band (1561 cm-1 in H2O and 1467 cm-1 in D2O). Hereafter, the amide bands for the deuterated species (-COND-) are denoted as amide I′ and amide II′ to distinguish them from those of the ordinal amide groups (-CONH-). AN units have a characteristic IR absorption band due to the CtN stretching vibration (ν(CtN)) at 2246 cm-1 in H2O, which overlaps with a strong O-D stretching band in D2O. A differential spectrum given by subtraction of the IR absorption spectrum of PiPA-AN20 measured at 23.0 °C (below the LCST) from that at 39.5 °C (above the LCST) is shown in Figure 2a. The difference spectrum (∆A39.5-23.0) for PiPA-AN20 in D2O is also shown in Figure 2b. The regions where strong IR absorption due to H2O or D2O exists are not shown because the difference spectra are so noisy. IR difference spectroscopy is a suitable method to observe small spectral changes because IR bands resulting from structurally altered parts are selectively enhanced.25-27 Each difference spectrum shown here is a spectrum given by the difference between two distinct (25) Ma¨ntele, W. Trends Biochem. Sci. 1993, 18, 197.

Copolymers of N-Isopropylacrylamide

Langmuir, Vol. 17, No. 24, 2001 7537

Figure 2. IR difference spectra (∆A39.5-23.0) induced by the phase transitions of the solutions of PiPA-AN20 (20 wt %) measured in (a) H2O and (b) D2O.

states of a single sample solution, that is, the coil state and the globule state. In the difference spectra, negative peaks associate with absorption by the solution at the starting temperature (23.0 °C) and positive peaks represent absorption by the solution at the temperature of acquisition of the spectra (39.5 °C). Profiles of the IR difference spectra due to the iPA moiety are almost the same as those of the PiPA homopolymer. In addition to the peaks due to the iPA unit, peaks resulting from the shift of the ν(CtN) band were clearly observed at 2250 cm-1 (negative) and 2238 cm-1 (positive). The progress of the phase transition can be observed by following the growth of the intensities of the IR difference peaks. The difference in the values of ∆A at the positive and negative peaks of a vibration mode in a difference spectrum is defined as

∆∆AT-T0(ν1,ν2) ) ∆AT-T0(ν1) - ∆AT-T0(ν2)

(1)

where ν1 and ν2 denote wavenumbers at the positive and negative peaks in the difference spectrum, respectively, and subscripts T and T0 are the temperatures of the spectral measurements. Values of ∆∆AT-25(ν1,ν2) for the C-H stretching, amide I, amide II, and C-H bending vibration mode are plotted against temperature in Figure 3. For instance, because the amide I′ mode has the positive peak at 1650 cm-1 and the negative peak at 1624 cm-1 as shown in Figure 2b, values of ∆∆AT-23.0(1650,1624) are plotted as open squares in Figure 3b. The ∆∆A-versus-T curve for the ν(CtN) band of AN units as well as those for iPA units shows onsets at the same temperature around 28 °C. The onsets in the ∆∆A-versus-T curves are almost the same as the LCST measured by turbidimetry (defined as the temperature at the onset in the turbidity-vs-T curve of the solution) and DSC (defined as the onset of the endothermic peak). To obtain information concerning the hydrogen bonding of the amide group of PiPA-AN20, the amide I′ band measured in D2O (Figure 4a) was analyzed by using the (26) Barth, A.; Hauser, K.; Ma¨ntele, W.; Corrie, J. E. T.; Trentham, D. R. J. Am. Chem. Soc. 1995, 117, 10311. (27) Barth, A.; Corrie, J. E. T.; Gradwell, M. J.; Maeda, Y.; Ma¨ntele, W.; Meier, T.; Trentham, D. R. J. Am. Chem. Soc. 1997, 119, 4149.

Figure 3. The values of ∆∆AT-23.0 for selected vibration modes of PiPA-AN20 (20 wt %) measured in (a) H2O and (b) D2O are plotted against temperature: b, ∆∆AT-23.0(2970,2989) (ν(C-H)); 2, ∆∆AT-23.0(1453,1481) (amide II); (, ∆∆AT-23.0(2238,2250) (ν(CtN)); O, ∆∆AT-23.0(2970,2989) (ν(C-H)); 4, ∆∆AT-23.0(1445,1479) (amide II′); 0, ∆∆A T-23.0(1625,1650) (amide I′).

Figure 4. (a) IR absorption spectra in the amide I′ region of PiPA-AN20 measured at 27.9, 30.1, 31.1, 31.7, 32.2, and 33.7 °C in D2O. (b) The molar fractions of the CdO groups of PiPA-AN20 (O) and PiPA (b) that form hydrogen bonds with D2O (fCdO‚‚D-O) are plotted against temperature.

curve fitting method. The amide I′ band of PiPA-AN20 measured below the LCST consists of a single Gaussian component centered at 1625 cm-1 that can be assigned to the CdO group bound to water through a hydrogen bond (CdO‚‚‚H-O).23 The second components centered at 1650 cm-1 appeared above the LCST, which is assigned to the CdO‚‚‚H-N species. The behavior is almost identical to that of the amide I′ band of the PiPA homopolymer. If we assume a 1:1 conversion of the CdO species, molar fractions of the CdO‚‚‚H-O and CdO‚‚‚H-N species can be estimated as shown in Figure 4b. About 15% of the CdO groups form hydrogen bonds with the amide N-H, and the remaining CdO groups form hydrogen bonds with water even in the globule state. Though the LCSTs of PiPA-AN20 and the PiPA homopolymer are different, hydrogen bonding about the CdO groups in these polymers is almost the same. After all, the CdO‚‚‚H-O species are dominant even in the globule states. Information about the hydration of the AN moiety is obtained from the IR spectra in the ν(CtN) region (Figure 5a). The ν(CtN) band measured in the transition temperature region can be separated into two components centered at 2241 and 2247 cm-1. Reimers and Hall have

7538

Langmuir, Vol. 17, No. 24, 2001

Figure 5. (a) IR absorption spectra in the ν(CtN) region of PiPA-AN20 measured at 22.9, 27.8, 29.5, 31.0, 32.8, 34.4, and 36.3 °C in D2O. (b) Relative areas of the 2247 cm-1 component of the ν(CtN) band (the hydrogen-bonding CtN) of PiPA-AN20 (20 wt %, in H2O) at different temperatures (from 35.9 to 40.6 °C) are plotted against areas of the 2241 cm-1 component (free CtN). The slope of the least-squares line gives the ratio of the molar absorption coefficient of these components (2247/2241) as 0.96. (c) Molar fractions of the hydrogen-bonding (b) and the free (O) CtN species are plotted against temperature.

investigated the hydration of acetonitrile (ACN) by Raman spectroscopy. They have shown that the ν(CtN) band for ACN that forms a hydrogen bond with water and for free ACN appears at 2256-2260 and 2253.2 cm-1, respectively.28 The lowest energy complex of ACN and water has a linear H-bonding structure and is calculated to produce a shift of 11.8 cm-1 from free ACN by using MP2. Thus, the 2241 and 2247 cm-1 components of PiPA-AN can be assigned to free and hydrogen-bonding CtN species, respectively.11 Further discussion will be given in the following section. These components can be separated from the profiles of the ν(CtN) band by using the curve fitting method, and relative areas of the 2247 cm-1 component are plotted against those of the 2241 cm-1 component (Figure 5b). The slope of the line connecting the data points gives the ratio of the molar absorption coefficients of the components as 2247/2241 ) 0.96. By using the value, molar fractions of the hydrogenbonding and free CtN species could be calculated and plotted against temperature in Figure 5c. Below 26 °C (the coil state), almost all CtN groups form hydrogen bonds with water, and above 40 °C (the globule state) only 10% of the CtN groups form hydrogen bonds. This is quite different from the change in hydration states of the CdO group of iPA units, 85% of which form hydrogen bonds with water even in the globule state. Effect of Comonomer Contents and Salt Concentration on the LCST and IR Spectra of PiPA Copolymers. Next, effects of ions on the LCST of PiPA copolymers were investigated. Figure 6 shows changes in the values of ∆∆A for the amide II′ mode and the C-H deformation mode of the -C(CH3)2 group (δ(i-Pr)) of PiPA-AAm9 measured in the presence and absence of NaCl. Although the curves shift toward lower temperatures with increasing NaCl concentration, the magnitudes of the spectral changes (∆∆AT-T0(1365,1374) and ∆∆AT-T0(1448,1479)) are similar. In Figure 7a, the LCSTs of PiPA, PiPA-AAm, and PiPA-AN determined (28) Reimers, J. R.; Hall, L. E. J. Am. Chem. Soc. 1999, 121, 3730.

Maeda et al.

Figure 6. The values of ∆∆AT-T0 of selected vibration modes of PiPA-AAm9 (20 wt %) in pure D2O (b, O), 5 wt % NaCl/D2O (2, 4), and 10 wt % NaCl/D2O (9, 0) were plotted against temperature. O, 4, 0: ∆∆AT-T0(1365,1374) (δ(i-Pr)); b, 2, 9: ∆∆AT-T0(1448,1479) (amide II′).

Figure 7. (a) NaCl concentration dependence of the LCSTs of PiPA (b, O), PiPA-AAm9 (2, 4), PiPA-AAm17 (9, 0), and PiPA-AN20 ((, )). (b) The LCSTs of PiPA (b, O), PiPA-AAm9 (2, 4), PiPA-AAm17 (9, 0), and PiPA-AN20 in aqueous solutions of KCl, KBr, and KI (0.9 M) are plotted against the viscosity B coefficient of anions. Solid and open marks represent the LCSTs determined by IR measurements (polymer concentration: 20 wt %, in D2O) and turbidimetry (polymer concentration: 0.5 wt %, in H2O), respectively.

by turbidimetry (0.5 wt % in H2O) and IR spectroscopy (20 wt % in D2O) are plotted as a function of NaCl concentration. The LCSTs determined by the IR measurements were close to those determined by turbidimetry. The LCSTs of each polymer solution linearly decreased with an increase of NaCl concentration. Figure 7b shows the relationship between the LCSTs of PiPA copolymers in aqueous solutions containing one of the potassium halides (KI, KBr, and KCl) and viscosity B coefficients (Bη) of anions resulting from the salts, which represent the strength of interaction between the ions and water.29,30 The integration of an endothermic peak in a DSC thermogram of a polymer solution gives the heat of phase transition (∆H). The relationship between ∆H and the LCST of the solution is shown in Figure 8. Open circles in Figure 8 are data for several PiPA-AAm and PiPA-AN copolymers of different polymer compositions measured in the absence of added salts. These data lie on a single line, suggesting that ∆H is essentially determined by the LCST, which in turn depends on the composition of the copolymer. Solid triangles and solid squares in Figure 8 are data points obtained in the presence of KBr and KCl, (29) Jones, G.; Dole, M. J. Am. Chem. Soc. 1929, 51, 2950. (30) Kaminsky, M. Discuss. Faraday Soc. 1957, 24, 171.

Copolymers of N-Isopropylacrylamide

Figure 8. The enthalpies of the phase transitions (∆H) for the solutions of PiPA copolymers of different polymer compositions are plotted against their LCSTs. Open circles represent data for PiPA-AN20, PiPA, poly(N-isopropylacrylamide-co-N-vinylpyrrolidone) (N-vinylpyrrolidone: 10.4 mol %, Mw ) 45000, Mw/Mn ) 3.6), PiPA-AAm9, and PiPA-AAm17 (from lower LCST to higher) measured in the absence of added salt. Closed marks represent data measured in 0.9 M KBr (2) and 0.9 M KCl (9). The polymers used are PiPA, PiPA-AAm9, and PiPA-AAm17 (from lower LCST to higher).

respectively. Each of the lines connecting the data for KBr or KCl solutions is parallel to the line for the system without added salt and is shifted to lower LCST. Discussion There exists a local heterogeneity in the structure of water around iPA copolymers. Polar groups will form hydrogen bonds with water, whereas nonpolar sites are encapsulated in hydrophobic hydration shells. The present IR spectroscopic study on PiPA-AN revealed that the hydration states of the AN unit and the iPA unit change upon the coil-to-globule transition. The direction of shift of the amide I band (CdO stretching mode) is opposite to that of the ν(CtN) band, although both the CdO and the CtN groups act as hydrogen bond acceptors. Shifts in the ν(CtN) band of ACN induced by interactions with water, organic solvents, and ions have been extensively studied.31-33 The ν(CtN) band of ACN shows a blueshift in the presence of a molecule that is a stronger Lewis acid than ACN itself. ACN donates an electron to the Lewis acid through a dipole-dipole interaction. The molecular orbital involved in the charge donation has CtN and C-C antibonding contributions as well as substantial nitrogen lone pair character. Thus, as charge is removed from the orbital by coordination with the Lewis acid, the net bond order along the C-CtN axis is increased, and a blueshift is observed in the ν(CtN) band.13 The wavenumber shift of the ν(CtN) band of the AN unit might be explained in a similar manner, and the redshift of the band upon the coil-to-globule transition indicates dehydration of the AN moiety. The blueshift of the amide I band and the redshift of the amide II band upon the phase transition indicate the reduction of the number or the strength of hydrogen bonds between the amide groups and water molecules.23 The redshifts of the C-H stretching bands are also signs of the dehydration of the alkyl group upon the transition. After all, the alkyl, amide, and nitrile groups are more or less dehydrated upon the phase transition. The degree of dehydration of the amide CdO group of PiPA-AN20 in (31) Fawcett, W. R.; Liu, G.; Kessler, T. E. J. Phys. Chem. 1993, 97, 9293. (32) Fawcett, W. R.; Liu, G.; Faguy, P. W.; Foss, A. A., Jr.; Motheo, A. J. J. Chem. Soc., Faraday Trans. 1993, 89, 811. (33) Fawcett, W. R.; Liu, G. J. Phys. Chem. 1992, 96, 4231.

Langmuir, Vol. 17, No. 24, 2001 7539

the globule state is estimated to be 15%, whereas that of the nitrile group is 90%. Because the C-H stretching bands of the isopropyl group of the iPA unit and the main chain overlap each other, precise estimation of the degrees of the dehydration of these groups are difficult. However, a curve fitting analysis on the symmetric C-H stretching band of the cyclopropyl group in poly(N-cyclopropylacrylamide), which is isolated from other bands, has shown that the alkyl group is almost fully dehydrated above the LCST.24 Moreover, the methyl group of poly(vinyl methyl ether) is also fully dehydrated upon the phase transition.34 After all, more hydrophobic groups on polymer chains seem to be dehydrated to a greater extent upon the phase transition. The dehydration of the polymer chain is a major driving force of the phase transition. Below the LCST, hydrogen bonds between polar groups and water molecules energetically overcome the unfavorable decrease in entropy of the structured water in the hydrophobic shell and, therefore, the polymer is soluble in water. The hydrogen bonds are weakened with increasing temperature, and the situation is reversed above the LCST. Thus, the ratios of hydrophobicity/hydrophilicity of the copolymers as a whole are a main determinant of their LCST. Incorporation of hydrophobic comonomers increases the ratio of the hydrophobicity and lowers the LCST, and incorporation of hydrophilic comonomers vice versa. The linear relationship between ∆H and the LCST of the copolymers also suggests the importance of changes in the structure of water surrounding the hydrophobic groups. The breaking of hydrogen bonds between water molecules around the hydrophobic moieties is mainly responsible for ∆H,11 and therefore ∆H is reduced at elevated LCST because of relatively lower extents of water-water hydrogen bonds. Although the addition of salts to the solutions modifies the LCST behaviors of these polymers as well as incorporation of comonomer units, the ∆H-LCST relationship for the systems containing added salts is different from that for the system without added salt. The effect of ions on the phase transition of aqueous polymer solutions is indirect and can be explained by changes in the structure and properties of water.23 The effectiveness of ions on the denaturation of proteins is known to follow the lyotropic series (F-, 4.8; Cl-, 10; Br-, 11.1; I-, 12.5).35,36 Because anions with higher Bη or lower lyotropic numbers tend to subtract water molecules from polymers more strongly, they strengthen hydrophobic interactions that induce the collapse of polymers into the globule states. In contrast, anions with lower Bη or higher lyotropic numbers break the structure of bulk water and stabilize the hydration of polymers, and then the coil states are favored. Thus, the LCSTs of aqueous polymer solutions are influenced by the presence of ions. In addition, parts of water molecules that are removed from polymer chains upon the phase transition will hydrate ions, which accompany exothermic heat and reduce the total endothermic heat of the transition. This may be a reason ∆H in the presence of ions is lower than that expected for a solution of a copolymer that has the same LCST without added salt. Acknowledgment. This work was supported by a Grant-in-Aid (10750645) from the Ministry of Education, Science and Culture, Japan. LA010007+ (34) Maeda, Y. Langmuir 2001, 17, 1737. (35) McBain, J. W. In Colloid Science; Heath: Boston, 1950. (36) Robinson, D. R.; Jencks, W. P. J. Am. Chem. Soc. 1965, 87, 2470.