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Change in Hydration State during the Coil-Globule Transition of Aqueous Solutions of Poly(N-isopropylacrylamide) as Evidenced by FTIR Spectroscopy† Yasushi Maeda,* Tomomi Higuchi, and Isao Ikeda Department of Applied Chemistry and Biotechnology, Fukui University, Fukui 910-8507, Japan Received February 3, 2000. In Final Form: June 15, 2000 Coil-globule transition of poly(N-isopropylacrylamide) (PIPA), followed by intermolecular association in H2O and D2O, was investigated by Fourier transform infrared (FTIR) spectroscopy. IR spectra of the solutions were measured as a function of temperature, and spectral changes induced by the transition were observed. The intensities of the difference IR bands due to the vibrational modes of isopropyl and amide groups critically increased at the lower critical solution temperature (LCST). Heating of the PIPA solutions above the LCST led shifts of the amide II, C-H-stretching, and C-H-bending bands to lower wavenumbers during a shift of the amide I band to a higher wavenumber. The amide I band of the PIPA observed below the LCST could be fitted with a single component centered at 1625 cm-1, whereas two components (1625 and 1650 cm-1) were necessary to fit the band above the LCST. These components may be assigned to the CdO group which is bound to water molecules as the solvent (1625 cm-1) and to the N-H in the side chain (1650 cm-1) via hydrogen bonding. About 13% of the CdO group is estimated to form the intra- or interchain hydrogen bonding, and the remaining CdO group forms a hydrogen bond with water in the globule state. Red shifts of the antisymmetric and symmetric C-H-stretching bands for the isopropyl group also indicate dehydration of the hydrophobic moiety during the transition. A mechanistic hypothesis of the coil-globule transition, insisting that above the LCST the polymer chain is dehydrated and hydrophobic interaction between isopropyl groups induces the collapse of the chain, is strongly supported. Though the presence of metal halides (NaCl, KCl, KBr, KI) lowered the LCST, the profiles of IR difference spectra of the PIPA in those solutions were similar to those measured in pure water. Specific interactions between the amide groups on the polymer chain and the ions were unlikely. Importance of the structure and properties of water in the solution to determine the LCST of the polymer solutions is suggested.
Introduction Aqueous solutions of poly(N-isopropylacrylamide) (PIPA) have a lower critical solution temperature (LCST), and above the temperature, the solutions become turbid. The transition can also be induced by an addition of salts or cononsolvents. Gels of N-isopropylacrylamide (IPA) also exhibit volume-phase transition; that is, the gels discontinuously collapse above the transition temperature. In the last two decades, both basic and applicational1 studies of the typical temperature-responsive polymer have been extensively carried out. Phenomena occurring at the transition of the polymer have been investigated by a wide variety of techniques including turbidimetry,2 calorimetry,3 NMR,4 fluorescence,5 light scattering,6 neutron † Presented at the 48th Annual Meeting of the Society of Polymer Science, Japan, in May 1999. * To whom correspondence should be addressed. Tel: +81-77627-8638.Fax: +81-776-27-8747.E-mail:
[email protected].
(1) (a) Liang, L.; Feng, X.; Liu, J.; Rieke, P. C.; Fryxell, G. E. Macromolecules 1998, 31, 7845-7850. (b) Stayton, P. S.; Shimoboji, T.; Long, C.; Chilkoti, A.; Chen, G.; Harris, J. M.; Hoffman, A. S. Nature 1995, 378, 472-474. (c) Nozaki, T.; Maeda, Y.; Ito, K.; Kitano, H. Macromolecules 1995, 28, 522-524. (d) Nagayama, H.; Maeda, Y.; Shimasaki, C.; Kitano, H. Macromol. Chem. Phys. 1995, 196, 611-620. (e) Kitano, H.; Maeda, Y.; Takeuchi, S.; Ieda K.; Aizu, Y. Langmuir 1994, 10, 403-406. (2) Fujishige, S.; Kubota, K.; Ando, I. J. Phys. Chem. 1989, 93, 33113313. (3) (a) Tiktopulo, E. I.; Bychkova, V. E.; Rie`ka, J.; Ptitsyn, O. B. Macromolecules 1994, 27, 2879-2882. (b) Feil, H.; Bae, Y. H.; Feijen, J.; Kim, S. W. Macromolecules 1993, 26, 2496-2500. (c) Inomata, H.; Goto, S.; Otake, K.; Saito, S. Langmuir 1992, 8, 687-690. (d) Schild, H. G.; Tirrell, D. A. J. Phys. Chem. 1990, 94, 4352-4356.
scattering,7 Raman spectroscopy,8 and EPR.9 Calorimetric study revealed cooperativity of the phase transition3a and a relationship between the LCST and the heat of transition.3b Motional and dimensional changes of the polymer chain were investigated by NMR and light scattering. NMR studies revealed that not only mobility of the main and side chains of the polymer but also mobility of the surrounding water molecules changed significantly at the phase transition.4 Measurements in an extremely dilute solution or in the presence of a surfactant prevent interchain aggregation of the polymer and made it possible to observe the transition of a single chain by light (4) (a) Ohta, H.; Ando, I.; Fujishige, S.; Kubota, K. J. Polym. Sci., Polym. Phys. Ed. 1991, 29, 963-968. (b) Tokuhiro, T.; Amiya, T.; Mamada, A.; Tanaka, T. Macromolecules 1991, 24, 2936-2943. (5) (a) Walter, R.; Rie`ka, J.; Quellet, C.; Nyffenegger, R.; Binkert, T. Macromolecules 1996, 29, 4019-4028. (b) Winnik, F. M. Macromolecules 1990, 23, 233-242. (6) (a) Wang, X.; Qiu, X.; Wu, C. Macromolecules 1998, 31, 29722976. (b) Wu, C.; Wang, X. Phys. Rev. Lett. 1998, 80, 4092-4094. (c) Qiu, X.; Kwan, C. M. S.; Wu, C. Macromolecules 1997, 30, 6090-6094. (d) Wu, C.; Zhou, S. Macromolecules 1995, 28, 8381-8387. (e) Wu, C.; Zhou, S. Macromolecules 1995, 28, 5388-5390. (f) Meewes, M.; Rie`ka, J.; Silva, M.; Nyffenegger, R.; Binkert, T. Macromolecules 1991, 24, 5811-5816. (g) Kubota, K.; Fujishige, S.; Ando, I. J. Phys. Chem. 1990, 94, 5154-5158. (7) Lee, L.-T.; Cabane, B. Macromolecules 1997, 30, 6559-6566. (8) Terada, T.; Inada, T.; Kitano, H.; Maeda, Y.; Tsukida, N. Macromol. Chem. Phys. 1994, 195, 3261-3270. (9) (a) Winnik, F. M.; Ottaviani, M. F.; Bossman, S. H.; Pan, W.; Garcia-Garibay, M.; Turro, N. J. J. Phys. Chem. 1993, 97, 12998-13005. (b) Winnik, F. M.; Ottaviani, M. F.; Bossman, S. H.; Pan, W.; GarciaGaribay, M.; Turro, N. J. Macromolecules 1993, 26, 4577-4585. (c)Vesterinen, E.; Dobrodumov, A.; Tenhu, H. Macromolecules 1997, 30, 1311-1316.
10.1021/la0001575 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/12/2000
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scattering. Fluorescence studies were carried out to investigate the process of the aggregation of polymers and to evaluate the microenvironment in the aggregates. Effects of the comonomer on the transition10 and pressureresponsive behavior11 have also been examined. The phase transition of aqueous solutions of PIPA has been explained by a theory of coil-globule transition,12 which insists that dehydration of the polymer chain above the LCST induces the segmental interactions of the polymer to be attractive and to collapse the coiled polymer chain into a globular conformation. However, the mechanism has not been fully examined. How does the hydration state of the polymer chain change? How do the interaction and microenvironment of the amide and isopropyl groups of the polymer change? These problems remain to be solved. IR spectroscopy is quite suitable to answer these questions because of the high sensitivity of IR spectra not only to conformational changes of a molecule but also to the local environment of a molecule and the interaction between molecules, especially the vibrations of an amide group. That is, the amide I mode (mainly the CdO-stretching vibration) and the amide II mode [a combination of N-H-bending (60%) and C-N-stretching (40%) vibrations] are sensitive to the strength of hydrogen bonding. Therefore, amide bands of proteins have been frequently used to investigate their secondary structures. However, as far as we know, no systematic IR spectroscopic studies on the coil-globule transition of PIPA solutions have been carried out. Such a study is worth doing to explain what takes place in the polymer chain and the surrounding water molecules in the course of the phase transition. In the present study, we observed changes in the IR spectra of PIPA during the transition in aqueous solutions (H2O and D2O). In addition, the effects of some metal halides on the LCST and profiles of the IR spectra were investigated. Experimental Section Materials. IPA (Koujin, Tokyo, Japan) was purified by recrystallization from benzene-n-hexane. PIPA was synthesized by radical polymerization in methanol at 70 °C for 7 h using 2,2′-azobis(isobutyronitrile) as an initiator. After evaporation, the polymers were precipitated from acetone-n-hexane. Polymers obtained were purified by dialysis (Visking tube, exclusion limit 8000), and they were lyophilized. The molecular weight and polydispersity of the polymer determined by gel permeation chromatography [Toso HLC-803D, column Toso G3000PW (30 cm) + G5000PWXL (30 cm), mobile phase water (1.0 mL‚min-1)] were 11000 and 1.9, respectively. Poly(ethyleneglycol)s (molecular weights: 2000, 5000, 18000, 39000, 86000, 250000, and 590000) were used as standard samples. To prepare deuterated PIPA {([-CH2-CH(CONDiPr)-n, PIPA-d}, the polymer was dissolved in D2O (EURIS-TOP, 99.9%) and lyophilized (the cycle was repeated 2 times), deuterating the N-H into N-D. Estimation by IR (intensity of the amide II band) and 1H NMR indicated that >98% of the N-H was deutarated. To distinguish it from PIPA-d, the undeuterated counterpart [(-CH2-CH(CONHiPr)]n is denoted as PIPA-h. Turbidimetry. The turbidity of the polymer solution was followed by the absorbance at 500 nm by using a spectrophotometer (S2010, Hitachi, Japan). The observation quartz cell was thermostated by a thermoelectric cell holder (SDR-30, (10) (a) Suzuki, H.; Wang, B.; Yoshida, R.; Kokufuta, E. Langmuir 1999, 15, 4283-4288. (b) Ito, S.; Ogawa, K.; Suzuki, H.; Wang, B.; Yoshida, R.; Kokufuta, E. Langmuir 1999, 15, 4289-4294. (11) (a) Otake, K.; Karaki, R.; Ebina, T.; Yokoyama, C.; Takahashi, S. Macromolecules 1993, 26, 2194-2197. (b) Kunugi, S.; Yamazaki, Y.; Takano, K.; Tanaka, N.; Akashi, M. Langmuir 1999, 15, 4056-4061. (c) Zhang, Y.; Li, M.; Fang. Q.; Zhang, Y.-X.; Jiang, M.; Wu, C. Macromolecules 1998, 31, 2527-2532. (12) Ptitsyn, O. B.; Kron, A. K.; Eizner Y. Y. J. Polym. Sci. C 1968, 16, 3509-3517.
Maeda et al.
Figure 1. (a) IR absorption spectrum of PIPA-h in H2O and PIPA-d in D2O at 25.0 °C. (b) IR difference spectra induced by the phase transition of PIPA-h in H2O and PIPA-d in D2O. To obtain the difference spectra, IR absorption spectra measured at 25.0 °C were subtracted from those measured at 40.0 °C. Hitachi). Concentration of the polymers in the sample solutions was 0.5 wt %. FTIR Measurements. IR spectra were measured using a Jasco FTIR-620 (single-beam spectrometer) equipped with a TGS detector. To measure IR spectra, 10 µL of the polymer solution (16.7 wt %, preincubated >10 h below phase-transition temperature to equilibrate) was pipetted into an IR cell with two CaF2 windows (φ 20 × 2 mm) and a spacer (10 µm thick). The IR cell was attached to a metal cell holder and thermostatted with a circulating water bath (BU150S, Yamato, Tokyo, Japan). The temperature of the cell holder was measured by an electronic thermometer (SK-L200T, Sato Keiryoki Mfg, Tokyo, Japan) with a thermistor sensor. The IR spectra reported here were an average of 16 scans recorded at a resolution of 2 cm-1. The background spectrum for one cycle of the measurement was obtained using a sample solution in the IR cell equilibrated at starting temperature (T0). The temperature of the bath was then raised or lowered and maintained for 5 min to make the sample solution equilibrate at that temperature (T). Measurement of the IR spectrum at T gave an IR difference spectrum. The difference in IR absorption intensities (A) is designated as ∆AT-T0. Analyses of IR spectra, such as Fourier self-deconvolution, curve-fitting, and baselinesubtraction, were performed using Spectra Manager software (Jasco). Differential Scanning Calorimetry (DSC). DSC measurements were performed using a Micro Calorimetry System (MicroCal Inc.). PIPA-h was dissolved in H2O (0.5 wt %) and scanned at a rate of 0.75 °C/min, in both the heating and the cooling processes. Sample solutions were degassed and transferred to the sample cell (1.22-mL) using a syringe. An identical volume of pure H2O was placed in the reference cell. Data analyses were carried out using Origin software (MicroCal).
Results IR Spectra of Aqueous Solutions of PIPA. Figure 1a shows IR absorbance spectra of PIPA-h in H2O solution and PIPA-d in D2O solution, measured at 25 °C. Prominent IR bands are C-H-stretching (2900-3000 cm-1) bands, amide bands, and C-H-bending bands. IR frequencies and assignments of observed peaks are compiled in Table 1. The major advantage of the use of D2O as a solvent is
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Table 1. Observed IR Frequencies (in cm-1) and Assignments of PIPA-h and PIPA-d PIPA-h H2O soln
difference IR band in H2O soln
2982 2940 2880
2991(-), 2971(+) 2947(-), 2926(+) 2886(-), 2973(+)
1561 1463 1390 1371 1286 1249 1173 1156 1133
1567(-), 1535(+) 1464(-), 1457(+) 1392(-), 1387(+) 1373(-), 1367(+) 1291(-), 1275(+) 1253(-), 1233(+) unclear unclear 1135(-), 1128(+)
neat 2973 2935 2876 1647 1550 1460 1388 1368 1279 1242 1172 1157(sh) 1131
CHCl3 soln 2973a 2932a 2875a 1651 1544 1459 1386 1369a 1171a 1157(sh) 1130
THF soln
1650 1543 1459 1386 1365 1290 1237 1175 1130
assignment antisymmetric C-H stretching of -C(CH3)2 antisymmetric C-H stretching of -CH2symmetric C-H stretching of -C(CH3)2 amide I amide II symmetric deformation of -C(CH3)2 antisymmetric deformation of -C(CH3)2 antisymmetric deformation of -C(CH3)2 amide III skeletal vibration of -C(CH3)2 skeletal vibration of -C(CH3)2
PIPA-d D2O soln 2981 2939 2882 1625 1467 1390 1372 1160 1134 a
difference IR band in D2O soln 2991(-), 2970(+) 2947(-), 2927(+) 2887(-), 2872(+) 1650(+), 1624(-) 1479(-), 1448(+) 1393(-), 1386(+) 1374(-), 1365(+) unclear 1136(-), 1128(+)
neat 2972 2934 2876 1638 1456 1387 1367 1162 1132
CHCl3 soln
1645 1441 1387 1368 1161 1132
THF soln
assignment
1641 1450 1385 1366 1165 1132
antisymmetric C-H stretching of -C(CH3)2 C-H stretching of -CH2symmetric C-H stretching of -C(CH3)2 amide I′ amide II′ deformation of -C(CH3)2 deformation of -C(CH3)2 skeletal vibration of -C(CH3)2 skeletal vibration of -C(CH3)2
Observed in CDCl3.
a shift in the deformation band of water to around 1200 cm-1, which prevents the band from overlapping the amide I band (see below). Because the amide II band contains a contribution from an N-H-bending vibration, the band is sensitive to deuteration and shifts from 1561 cm-1 for PIPA-h to 1470 cm-1 for PIPA-d. To distinguish the amide bands of PIPA-h, the bands for the deuterated species (-COND-) are denoted with a prime, such as amide I′ and amide II′. Weak bands observed at 1286 cm-1 in PIPA-h might be assigned to the amide III mode, because no bands appeared in this region in PIPA-d (may shift to 1000-900 cm-1 region). IR frequencies of vibrational modes concerning the main chain and the isopropyl group are insensitive to deuteration of an amide group. A differential spectrum given by the difference in IR absorption intensities (A) of PIPA-h in H2O measured at 40 °C (above the LCST) and 25 °C (below the LCST) are shown at the top of Figure 1b. The IR difference spectrum is designated as ∆A40-25, where the subscript denotes temperatures for the spectral measurements. The difference spectrum (∆A40-25) for PIPA-d in D2O is also shown at the bottom of Figure 1b. IR difference spectroscopy is a convenient method to visualize structural changes in molecules, because structurally altered parts are selectively observed with the absorption arising from the unchanged parts’ being removed.13 Each difference spectrum shown here is a spectrum given by the difference between two distinct states of a single sample solution, that is, the coil state and the globule state. In the difference spectra, negative peaks are associated with absorption by the solution at the starting temperature (25 °C), and positive peaks represent absorption by the solution at the temperature of acquisition of the spectra (40 °C). A pair of positive and negative peaks is observed for each vibration mode of PIPA, meaning that a shift of the IR (13) (a) Ma¨ntele, W. Trends Biochem. Sci. 1993, 18, 197-202. (b) Barth, A.; Hauser, K.; Ma¨ntele, W.; Corrie, J. E. T.; Trentham, D. R. J. Am. Chem. Soc. 1995, 117, 10311-10316. (c) 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, 4149-4159.
band associated with changes in the conformation and/or the interaction of the polymer takes place. For example, a pair of difference peaks observed at 2991 cm-1 (negative) and 2971 cm-1 (positive) is due to a red shift of the C-Hstretching band of the isopropyl group at the higher temperature. Most IR bands shift to the lower-frequency side during the phase transition, with an exception of the amide I band, which shows a blue shift. 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(at ν1) - ∆AT-T0(at ν2) where ν1 and ν2 denote wavenumbers at the positive and negative peaks in the difference spectrum, respectively, and T and T0 are the temperatures of the spectral measurements. Values of ∆∆AT-25(ν1-ν2) for the C-Hstretching, amide I and II, and C-H-bending vibration mode are plotted against temperature in Figure 2. For instance, because amide I′ mode has the positive peak at 1650 cm-1 and the negative peak at 1624 cm-1, as shown at the bottom of Figure 1b, values of ∆∆AT-25(1650-1624) are plotted with closed squares in Figure 2a. By using ∆∆AT-25(ν1-ν2) instead of the value for ∆AT-25 at ν1 or ν2, the effect of baseline drift can be canceled. Deviations of the values of ∆∆AT-25(ν1-ν2) measured at the same temperature in two separate runs were
