12730
J. Phys. Chem. B 2007, 111, 12730-12737
Contribution of Intramolecular CdO‚‚‚H-N Hydrogen Bonding to the Solvent-Induced Reentrant Phase Separation of Poly(N-isopropylacrylamide) Yukiteru Katsumoto,*,† Takeyuki Tanaka,‡ Katsunori Ihara,§ Misao Koyama,† and Yukihiro Ozaki§ Graduate School of Science, Hiroshima UniVersity, Higashi-Hiroshima 739-8526, Japan, School of Science and Technology, Kwansei-Gakuin UniVersity, Sanda 669-1337, Japan, and Integrated Center for Science, Ehime UniVersity, Matsuyama 790-8566, Japan ReceiVed: June 28, 2007; In Final Form: August 20, 2007
Changes in the local environment around amide groups of poly(N-isopropylacrylamide) (PNiPA) during a solvent-induced reentrant phase separation have been investigated by infrared spectroscopy combined with quantum chemical calculations. The addition of methanol or tetrahydrofuran as a cosolvent to an aqueous solution of PNiPA causes spectral changes in the amide I regions. By preparing a dimer model compound for PNiPA, we can establish the assignment of the amide I bands for the polymer in solutions. Hydrogendeuterium exchange experiments of the amide protons of PNiPA and its dimer models have revealed that the amide groups of PNiPA form an intramolecular CdO‚‚‚H-N hydrogen bond even in a good solvent. The result has suggested that the change in the amide I envelope of PNiPA observed during the solvent-induced phase transition reflects the modification of the intramolecular CdO‚‚‚H-N hydrogen bond of PNiPA as well as the variation in solvation state of the amide groups. On the basis of the assignment, we have discussed contributions of the intramolecular CdO‚‚‚H-N hydrogen bond to the phase behavior of PNiPA.
1. Introduction Poly(N-isopropylacrylamide) (PNiPA) is well-known as one of stimuli-responsive polymers.1 It is considered that the stimuliresponsiveness of PNiPA arises from its characteristic phase behavior in an aqueous solution.2 The phase behavior of PNiPA has been extensively studied by turbidity, 3-5 light scattering,6-8 calorimetry,9,10 fluorescence,11,12 nuclear magnetic resonance (NMR),13 infrared (IR) spectroscopy,14,15a and dielectric relaxation.16 The lower critical solution temperature (LCST) type phase diagram for the aqueous PNiPA solution was often reported by monitoring turbidity.3-5 It has been pointed out that the LCST type phase behavior of PNiPA is caused by a characteristic interaction between PNiPA and water.6-15a,16 Ohta and co-workers13 revealed by NMR spectroscopy that the mobility of water increases with rising temperature up to the phase separation temperature (Tps), then decreases transitionally just above Tps, and increases again with a further rise of temperature. Schild and Tirrell9 found the transition enthalpy of ca. 1.5 kcal mol-1 of repeating units for PNiPA in aqueous solutions, which is consistent with the loss of ca. 1 hydrogen bond/repeating unit upon phase separation. Recently, Okada and Tanaka suggested from a theoretical viewpoint that the thermally induced phase transition of PNiPA in an aqueous solution is governed by cooperative dehydration of PNiPA.17 While many research groups have investigated the hydration state of PNiPA, much less studies focus on intramolecular interactions of PNiPA. However, several experimental results have implied that the intramolecular interaction among the side chains of PNiPA plays an important role in its phase behavior. * To whom correspondence should be addressed. Tel.: +81-82-4247408. Fax: +81-82-424-0727. E-mail:
[email protected]. † Hiroshima University. ‡ Ehime University. § Kwansei-Gakuin University.
For example, the cosolvent effects on the phase behavior of PNiPA are different from those of poly(N,N-diethylacrylamide) (PNdEA); the addition of methanol to the aqueous solutions causes the rise of Tps for PNdEA15b but the decrease of Tps for PNiPA.18 The tacticity dependence of Tps is also opposite for these two polymers: The increase of the meso diad fraction in the polymer chain causes the rise of Tps for PNdEA19 but the decrease of Tps for PNiPA.20 Since atactic PNdEA and PNiPA undergo the phase separation at a similar temperature (304 °C), it is believed that the hydrophilicities of these polymers are similar to each other. Thus, the differences in the cosolvent effect and tacticity dependence possibly arise from the intramolecular interactions of PNiPA and PNdEA. A remarkable difference in the chemical structures of these polymers is the lack of the N-H group in PNdEA. That is, the side chains of PNdEA cannot form an intramolecular CdO‚‚‚H-N hydrogen bond, whereas those of PNiPA can. For copoly(acrylic acid/Nisopropylacrylamide) in water, Bokias and co-workers have reported that the intramolecular hydrogen bonding among the side chains affects the phase behavior.21 In the previous paper,22 we have shown the possibility that some parts of the intramolecular CdO‚‚‚H-N hydrogen bonds among the side chains of PNiPA dissociate during the thermally induced phase separation. In this work, we concentrate on investigating a solvent-induced reentrant phase separation of PNiPA. Interestingly, it has been known that PNiPA undergoes a soluble-precipitation-soluble process in water-methanol mixtures.18 PNiPA does not dissolve in the mixture with 0.20.4 mole fraction of methanol at 293 K. In other words, the water/methanol mixture behaves as a poor solvent for PNiPA, although both pure water and methanol are good solvents at room temperature. The investigation of the microscopic changes around the polymer chain of PNiPA during the solvent-induced
10.1021/jp0750452 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/17/2007
Conformation Changes in PNiPA during Phase Separation
J. Phys. Chem. B, Vol. 111, No. 44, 2007 12731
Figure 1. Chemical structures of PNiPA, dNiPA-C0, dNiPA-C1, dNiPA-C2, PNdMA, NMA, and NdMA.
reentrant phase separation may give a new insight into the mechanism of the characteristic phase behavior. The first purpose of the present study is to elucidate that the amide groups in side chains of PNiPA form the intramolecular CdO‚‚‚H-N hydrogen bond even in a good solvent such as methanol and water at room temperature. The second is to discuss contributions of the intramolecular CdO‚‚‚H-N hydrogen bond to the solvent-induced reentrant phase behavior of PNiPA. For the purposes of this study, IR spectroscopy combined with quantum chemical calculations has been employed. To confirm the existence of the intramolecular CdO‚ ‚‚H-N hydrogen bond for PNiPA in solutions, we have demonstrated hydrogen-deuterium (H-D) exchange experiments on the amide protons. In general, if an amide group is stabilized by the CdO‚‚‚H-N hydrogen bond, its H-D exchange process will be delayed. For a solid assignment of the IR bands, dimer model compounds of PNiPA have been prepared, and their IR spectra have been compared with simulation results obtained by quantum chemical calculations. The solid assignments proposed here enable us to discuss contributions of the intramolecular CdO‚‚‚H-N hydrogen bond to the phase behavior of PNiPA. 2. Materials and Experimental Methods 2.1. Materials. All chemicals were purchased from Wako and TCI. N-Isopropylacrylamide (NiPA) was purified by recrystallization from a benzene/n-hexane (1:6) mixture. PNiPA was synthesized by radical polymerization in a benzene/acetone mixture using 2,2′-azobis(isobutyronitrile) as an initiator. To remove dissolved oxygen in solutions, freeze-pump-thaw cycles were applied twice, and the reactor was sealed. The reaction was carried out under vacuum at 60 °C for 1 h. The details of polymerization are described elsewhere.23 Poly(N,Ndimethylacrylamide) (PNdMA) was also prepared in the same manner. The synthesized polymer was purified by reprecipitation into n-hexane, and the precipitate was dried under vacuum for 24 h. N,N′-Di(2-propyl)-2,3-dimethylbutanediamine (dNiPA-C0), N,N′-di(2-propyl)-2-methylpentanediamine (dNiPA-C1), and N,N′-di(2-propyl)-hexanediamine (dNiPA-C2) were prepared by a similar synthetic route as described in elsewhere.24 N-
Methylacetamide (NMA), N,N-dimethylacetamide (NdMA), perdeuterated methanol (CD3OD), methanol (CH3OH, Infinity pure grade: Wako), and tetrahydrofuran (THF, Infinity pure grade: Wako) were used as received. Figure 1 shows the chemical structures of PNiPA, NMA, PNdMA, NdMA, and dNiPAs. The resistance of water used was greater than 18.1 MΩ. For all sample solutions, the concentration of PNiPA was fixed to 2 wt %. 2.2. IR Spectroscopy and Hydrogen-Deuterium Exchange. IR spectra were measured with a resolution of 4 cm-1 using a Nicolet Magna 760 Fourier-transform IR spectrometer equipped with a liquid-nitrogen-cooled MCT detector. An attenuated total reflection (ATR) technique was employed for the IR measurements. A total of 512 scans were co-added for each spectrum. The ATR cell used was made of a horizontal ZnSe crystal (the refractive index is 2.403) with an incident angle of 45°. The temperature of the ATR cell was kept at 297 K (23.8 °C) by a CN4400 thermoelectric device (OMEGA) with an accuracy of (0.1 °C throughout the entire experiment. Data processing, such as the subtraction of a background solvent spectrum, the calculation of derivative, and the nonlinear least-square method for curve fitting, was performed by software composed by one of the authors (Y.K.).25 The pretreatment procedure for the ATR/IR spectra of polymers in solutions including the subtraction for a background solvent spectrum was described in the previous paper.22 A second derivative of IR spectrum was calculated using the Savitzky-Golay method26 with the number of smoothing points being equal to 7. The quasi-Marquardt algorithm27 with a nonlinear least-square method was employed for curve fitting. The position and number of the peaks used in the curve-fitting procedure were estimated by a third derivative, and the band shape with a linear combination of Gaussian and Lorentzian functions was assumed. For H-D exchange studies, PNiPA, dNiPA-C0, dNiPA-C1, dNiPA-C2, and NMA were dissolved in CD3OD, and the time dependence of the integral intensity of the amide II band was recorded. The concentration of solute was fixed to 2 wt %. The time when PNiPA was immersed in CD3OD was defined as t ) 0. The solution was kept in a vial for 15 min with gentle stirring. Then the sample solution was placed onto an ATR cell. A spectrum was collected every 15 min from t ) 55 to 420
12732 J. Phys. Chem. B, Vol. 111, No. 44, 2007
Figure 2. IR spectra in the amide I and II regions for PNiPA in watermethanol mixtures (A) and those in water-THF mixtures (B) at 297 K (23.8 °C). The curve-fitting results are represented by solid lines.
min. A toggle-clamp type sealing cover on the ATR cell was used for preventing the evaporation of solvent. The timedependent IR spectra were analyzed after the following pretreatments. First, the solvent spectrum was subtracted by employing a linear least-square.22 Second, the baseline was corrected by assuming the three minimum points around 1700, 1500, and 1400 cm-1 to be zero. 2.3. Quantum Chemical Calculations. All quantum chemical calculations were performed using the B3LYP functional with the 6-31G(d) basis set.28 Computations were carried out with GAUSSIAN 98 program29 at the Information Processing Center of Kobe University. Structures of dimer models for PNiPA were optimized, and their vibrational wavenumbers were calculated. The single scale factor, 0.9613, was multiplied to the calculated wavenumbers.30 In order to compare the calculated wavenumbers and intensities with the observed ones, IR spectra were simulated from the calculation results assuming a Lorentzian band shape with a bandwidth of 7 cm-1. The vibrational assignments of the dimer models for PNiPA were carried out from the view of Cartesian normal coordinates. 3. Results and Discussion 3.1. Spectral Changes during Solvent-Induced Reentrant Phase Separation of PNiPA. Panels A and B of Figure 2 show IR spectra in the 1760-1475 cm-1 region for PNiPA dissolved in water-methanol mixtures and those in water-THF mixtures, respectively, together with the curve-fitting results. The mole fraction of cosolvent is represented by xm for methanol and by xt for THF. Note that PNiPA precipitates in the ranges of 0.13 < xm < 0.51 and 0.05 < xt < 0.47 at 297 K. The IR
Katsumoto et al. spectrum of PNiPA in this wavenumber region consists of two major bands; the amide I band is mainly due to the CdO stretching vibration, and the amide II band arises from the coupling between the N-H bending and C-N stretching vibrations. The amide I band appears above 1600 cm-1, whereas the amide II band is located below 1600 cm-1. As seen in Figure 2, spectral variations in the amide I band are more significant than those in the amide II band. Thus, we concentrate on analyzing the amide I band hereafter. A unimodal amide I band is found at 1624 cm-1 for PNiPA in an aqueous solution. When methanol or THF is added to the aqueous solution of PNiPA, a new band appears at 1652 cm-1. The relative intensity of the band at 1652 cm-1 increases with increasing xm or xt. At xm ) 1.00 or in the range of 0.67 e xt e 1.00, the band around 1650 cm-1 is dominant. Spectral changes of PNiPA induced by the addition of cosolvent are almost identical for both mixtures especially in a lower xm or xt region, although the IR spectrum of PNiPA in pure methanol is different from that in pure THF. During the spectral changes, the wavenumber shift of each amide I band is not significant (1∼3 cm-1). The results clearly show that the local environment around the amide groups of PNiPA changes during the solventinduced reentrant phase separation. 3.2. Comparison of the Amide I Bands for PNiPA and Model Compounds in Water and Alcohols. Several research groups have proposed the assignment for the amide I bands for PNiPA in aqueous solutions; the band around 1625 cm-1 is due to a hydrated CdO group, and the band near 1650 cm-1 is associated with the CdO‚‚‚H-N interaction that is formed when the PNiPA chain collapses after the phase separation.14,15a However, these assignments cannot explain the spectral changes in the amide I bands in the region of 0.51 e xm e 1.00. In 0.51 e xm, PNiPA redissolves into the mixture and the PNiPA chain is re-swelled,31 though the band around 1650 cm-1 increases in intensity as shown in Figure 2. That is, the band around 1650 cm-1 unlikely arises from the amide groups in a collapsed chain of PNiPA. The relative intensity of the band around 1625 cm-1 at xt ) 0.81 is too strong to consider that the band arises from the hydration in terms of the stoichiometry. Therefore, we must reconsider the assignment of the amide I bands for PNiPA. Figure 3 shows IR spectra of NdMA, PNdMA, NMA, and PNiPA in alcohols and water. A CdO‚‚‚H-N interaction does not occur in the NdMA and PNdMA systems, because these compounds have no N-H group. NMA does not form an intramolecular CdO‚‚‚H-N hydrogen bond. NdMA shows a unimodal CdO stretching (νCdO) band at 1603 cm-1 in water, which should have the same origin as the νCdO band at 1605 cm-1 for PNdMA in water. Since the band around 1605 cm-1 does not appear in IR spectra for the alcohol solutions, it should be assigned to the CdO group solvated by water.32 For NdMA and PNdMA in protic solvents, the bands around 1614-28 and 1632-45 cm-1 are assignable to the di-hydrogen-bonded and mono-hydrogen-bonded CdO groups, respectively.32 For NMA in water, the amide I band is observed at 1618 cm-1, whereas the amide I band with the lowest wavenumber for NMA in alcohols appears at 1635 cm-1. On the basis of the assignment for NdMA, the band at 1618 cm-1 for NMA in water is assigned to the hydrated CdO groups. The bands at 1660 and 1635 cm-1 are due to the di-hydrogen-bonded and monohydrogen-bonded CdO groups, respectively.33 Note that the bands at 1660 and 1635 cm-1 may also be associated with the intermolecular CdO‚‚‚H-N hydrogen bond among NMAs when the concentration of NMA increases in a solution.
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J. Phys. Chem. B, Vol. 111, No. 44, 2007 12733
Figure 3. IR spectra in the 1700-1500 cm-1 region for PNdMA, NdMA, NMA, and PNiPA in water, methanol, ethanol, and 1-propanol. The concentration of solute in each solution was fixed to 2 wt %.
TABLE 1: Assignments for the Amide I Bands of NMA, dNiPA-C1, and PNiPA in THF, Methanol, and Water NMA ν/cm-1 1618 1635 1660
dNiPA-C1
solvents
ν/cm-1
water alcohols
1629
alcohols
1651
methanol
1673
methanol
PNiPA
solvents
ν/cm-1
solvents
assignments
methanol
1624 1627 1651 1649 1673
water methanol methanol THF methanol, THF
hydrated CdO groups in waters di-hydrogen-bonded CdO groups; CdO‚‚‚H-N and/or CdO‚‚‚H-O
By comparing the amide I envelope of PNiPA in water with that in an alcohol, we find that there is no amide I band due to the hydrated CdO group. The two amide I bands are observed around 1650 and 1625 cm-1 for PNiPA in alcohols, whereas the amide I band for PNiPA in water is located at 1624 cm-1. That is, the amide I wavenumber for PNiPA in water is very similar to that with lower wavenumber in alcohols. The result is remarkably different from those for NdMA, PNdMA, and NMA. The wavenumber of the amide I band for PNiPA in water implies that the CdO groups of the polymer are not fully hydrated in the aqueous solution. It seems likely that the intramolecular CdO‚‚‚H-N hydrogen bond among the amide groups gives rise to the amide I wavenumber of PNiPA in water. 3.3. Comparison of the Amide I Bands for PNiPA with Those for dNiPA-C1. Figure 4 shows IR spectra of PNiPA and dNiPA-C1 in methanol. Two bands around 1650 and 1625 cm-1 were found in the amide I major region. The amide I
Figure 4. IR spectra in the 1750-1475 cm-1 region for PNiPA and dNiPA-C1 in methanol. The concentration of each solution was 2 wt %.
mono-hydrogen-bonded CdO groups; CdO‚‚‚H-O or CdO‚‚‚H-N weakly solvated CdO groups in an organic solvent
wavenumbers of PNiPA are very similar to those of dNiPAC1, although their relative intensities are different. Hence, the assignment of the amide I bands for both PNiPA and dNiPAC1 should be similar to each other. On the basis of the assignment for NMA, the amide I bands around 1650 and 1625 cm-1 are assignable to the di-hydrogen-bonded and monohydrogen-bonded CdO groups, respectively.33 Note that the amide I wavenumber of N-isopropylacetamide in vacuum or low-temperature matrix is lower by about 10 cm-1 than that of NMA because of the isopropyl group.34 For dNiPA-C1 in methanol, the OH group of solvent and the NH group in the neighboring amide group can be a proton donor in hydrogen bonding of the CdO groups. However, it is generally very difficult to identify proton donors by monitoring the shift in the amide I wavenumber. Thus, the spectral simulations for a dimer model compound have been performed by using quantum chemical calculations. Two optimized structures of the NiPA dimer are shown in Figure 5. Although both of the conformers form the intramolecular CdO‚‚‚H-N hydrogen bond, the conformations of the main chain are different from each other: trans for Conformer 1 and gauche for Conformer 2. Figure 6 illustrates the optimized structures of the NiPA dimer solvated by water or methanol. The solvent molecules form a hydrogen bond with the CdO or N-H group that is not involved in the intramolecular CdO‚‚‚H-N hydrogen bond. The simulated IR spectra corresponding to these optimized structures of the NiPA dimer are shown in Figure 7. Figure 7a represents the IR spectra of the bare NiPA dimer shown in Figure 5. The amide I bands around 1660 cm-1 are due to the CdO group involved in the intramolecular CdO‚‚‚H-N hydrogen bond, whereas the bands around 1690 cm-1 arise from the free CdO group. Correspondingly, two bands also appear in the amide II region; the higher wavenumber band (around
12734 J. Phys. Chem. B, Vol. 111, No. 44, 2007
Katsumoto et al.
Figure 5. Optimized structures for the NiPA dimer calculated by the B3LYP/6-31G(d) method.
Figure 7. (a) Simulated IR spectra for Conformer 1 (solid line) and Conformer 2 (dotted line) of the NiPA dimer displayed in Figure 5. (b) Simulated IR spectra of the NiPA dimer hydrogen bonding with methanol molecules shown in Figure 6. (c) Simulated IR spectra of the NiPA dimer hydrogen bonding with water molecules shown in Figure 6. IR bands marked by an asterisk indicate the bands due to the OH bending vibration of water.
Figure 6. Optimized structures for the NiPA dimer hydrogen bonding with water or methanol molecules calculated at the B3LYP/6-31G(d) level.
1530 cm-1) is due to the N-H group involved in the intramolecular CdO‚‚‚H-N hydrogen bond. The simulated IR spectra for the NiPA dimer/solvent complexes are shown in Figure 7b,c. The results indicate that both of the amide I bands shift to lower wavenumbers compared with those estimated for the bare NiPA dimer. In Figure 7b,c, the bands around 1645 cm-1 are due to the intramolecular CdO‚‚‚H-N hydrogen bond, whereas the bands around 1670 cm-1 arise from intermolecular CdO‚‚‚H-O hydrogen bonding with the solvent molecules. The following three points are revealed by the spectral simulation on the NiPA dimer forming the intramolecular Cd O‚‚‚H-N hydrogen bond: (1) Two amide I bands appear, (2) the solvation of the NiPA dimer causes the lower wavenumber shift of both the amide I bands, and (3) the lower wavenumber band always contains the contribution from the intramolecular CdO‚‚‚H-N hydrogen bond. Therefore, we assume that the amide I band around 1625 cm-1 for dNiPA-C1 and PNiPA in methanol contains the contribution from the intramolecular Cd O‚‚‚H-N hydrogen bond. The assignments of the amide I bands for dNiPA-C1 based on the simulation are listed in Table 1. 3.4. H-D Exchange Experiment for PNiPA and dNiPAC1 in Methanol Solutions. To confirm the existence of the intramolecular CdO‚‚‚H-N hydrogen bond in PNiPA and dNiPAs in methanol, we carried out H-D exchange experiments. Since the chemical structure changes from -CONH-
Figure 8. Time dependence of IR spectra for PNiPA in a CD3OD solution at 297 K (solid line). The spectra were measured from t ) 55 to 520 min at an interval of 15 min after the exposure of PNiPA to CD3OD. t ) 0 is set when PNiPA is immersed in CD3OD. The dotted line represents the IR spectra of PNiPA in a CH3OH solution at 297 K. “Arrow” marks indicate the direction of band intensity changes with time.
to -COND- during the H-D exchange process, the H-D exchange rate is expected to reflect the hydrogen-bonding strength for compounds forming an intramolecular hydrogen bond. Figure 8 shows time-dependent IR spectra in the 1700-1400 cm-1 region of PNiPA in CD3OD together with the IR spectrum of PNiPA in CH3OH. The dominant bands are the amide I bands located around 1650 and 1625 cm-1, the amide II band near 1555 cm-1, and the amide II′ band near 1460 cm-1.35 The amide II and II′ bands are concerned with the NH and ND bending mode, respectively. As shown in Figure 8, the relative intensity of the amide II′ band increases with time, while that of the amide
Conformation Changes in PNiPA during Phase Separation
J. Phys. Chem. B, Vol. 111, No. 44, 2007 12735
Figure 9. Time profiles of the normalized integrated intensity of the amide II envelope (1516-1577 cm-1 region) for PNiPA, dNiPA-C0, dNiPA-C1, dNiPA-C2, and NMA. The solid line indicates the bestfitted curve for each profile.
II band decreases. Figure 9 depicts the temporal change in the integrated band intensity of the amide II envelope of PNiPA (1577-1516 cm-1) together with those of NMA and dNiPAs. In order to estimate the H-D exchange rate, the time-dependent profile is fitted to the following equation assuming a singleexponential decay:36
I(t) ) I0 exp(-kt) + I∞
Figure 10. Temporal changes in the amide I wavenumbers near 1650 and 1620 cm-1.
(1)
where I0, I∞, and k are the integrated band intensity of the amide II envelope at t ) 0, that at equilibrium, and the hydrogen exchange rate, respectively. The solid line in Figure 9 illustrates the best fitting curve. The k value was estimated to be 7.5 × 10-5 s-1 (4.5 × 10-3 min-1) for PNiPA in CD3OD. In the same manner, the k value for NMA, dNiPA-C0, dNiPA-C1, and dNiPA-C2 were determined to be 1.6 × 10-3 s-1, 5.7 × 10-4 s-1, 4.7 × 10-4 s-1, and 2.7 × 10-4 s-1, respectively. The k value for NMA is 2.8-5.9 times larger than that of dNiPAs, suggesting the stabilization of the amide groups of dNiPAs by the intramolecular CdO‚‚‚H-N hydrogen bond. The intermolecular CdO‚‚‚H-N hydrogen bond of dNiPA-C1 is negligible in this concentration, because the amide I envelope of dNiPAC1 does not change in the concentration range of 2.9 × 10-4∼4.4 × 10-1 M (about 0.007∼10 wt %).37 Interestingly, the k value decreases with increasing the number of methylene groups between the amide groups. The result suggests that the methylene “spacer” between the amide groups of the dimer hinders the formation of the intramolecular CdO‚‚‚H-N hydrogen bond. The k value for PNiPA is 21 times smaller than that for NMA and is 3.6-7.6 times smaller than that for dNiPAs. This result also indicates the existence of the intramolecular CdO‚‚‚H-N hydrogen bond among the amide groups of PNiPA in methanol. Since the k value for PNiPA is significantly smaller than those for dNiPAs, it is likely that the intramolecular CdO‚‚‚H-N hydrogen bond is stabilized in PNiPA more than in dNiPAs. For the stabilization of the CdO‚‚‚H-N hydrogen bond in the polymer, the following factors can be considered: the steric hindrance to the solvation of the amide groups of PNiPA,11,21 the stabilization of the amide group by the cooperative CdO‚ ‚‚H-N hydrogen bonding along the polymer chains,38-40 and the restriction on the mobility of the polymer main chain.41 3.5. Wavenumber Shifts of the Amide I and II Bands upon the Deuteration. During the H-D exchange process the two major bands shift from 1651 and 1627 cm-1 to lower wavenumbers by about 7 and 17 cm-1, respectively. Figure 10 shows the time dependence of the amide I wavenumber estimated by
Figure 11. (a) Simulated IR spectra for the bare NiPA dimers shown in Figure 5: Conformer 1 (solid line) and Conformer 2 (dotted line). (b) Simulated IR spectra for the dimers in which only the amide proton in the free NH group is replaced by deuterium. (c) Simulated IR spectra for the dimers in which all the amide protons are replaced by deuterium.
the third derivatives of IR spectra. The time profiles of the wavenumber shift for the two bands are different from each other; the wavenumber shift of the band around 1625 cm-1 precedes that of the band around 1650 cm-1. The spectral simulation was carried out in order to reveal the origin of the wavenumber shift in the amide I bands during the H-D exchanges. Figure 11a represents the simulated IR spectra of the bare NiPA dimer, whereas Figure 11c shows those of the dimer in which all the amide protons (-CONH-) are replaced by deuterium (-COND-). In Figure 11c, the amide II′ bands are observed near 1400 cm-1 instead of the amide II bands. The simulation result has revealed that the wavenumbers of both the amide I bands of the deuterated dimer are lower by about 5 cm-1 than those of the undeuterated one. That is to say, the H-D exchange of the amide proton modifies not only the amide II mode but also the amide I mode.
12736 J. Phys. Chem. B, Vol. 111, No. 44, 2007 The simulated IR spectra shown in Figure 11b are assumed to represent intermediate states in the H-D exchange process for the NiPA dimer. The amide proton in the free NH group is replaced by deuterium, whereas another amide proton is not. In Figure 11b, therefore, both the amide II′ and II bands appear. By comparing Figure 11b with Figure 11a, we found that the band around 1660 cm-1 shifts to a lower frequency by ∼5 cm-1, whereas the amide I band around 1690 cm-1 does not. That is, the amide I band around 1660 cm-1 due to the intramolecular hydrogen bond shows the wavenumber shift when the amide proton in the free NH group is exchanged to the deuterium. The spectral simulation can explain the reason why the wavenumber shift of the band around 1625 cm-1 precedes that of the band around 1650 cm-1. This is because the amide protons of PNiPA that are not directly associated with the intramolecular CdO‚‚‚H-N hydrogen bond are precedently exchanged. The result indicates again that the band around 1625 cm-1 contains the contribution from the intramolecular CdO‚ ‚‚H-N hydrogen bond. The assignments for the amide I bands of PNiPA proposed here are compiled in Table 1. 3.6. Changes in the Intramolecular CdO‚‚‚H-N Hydrogen Bond of PNiPA during the Solvent-Induced Phase Separation. The changes in the amide bands of PNiPA in the range of 0 < xm < 0.15 or 0 < xt < 0.05 are very similar to the spectral changes observed during thermally induced phase separation of the polymer in water.22 As revealed in this study, the amide I band at 1625 cm-1 contains the contribution from both the intramolecular CdO‚‚‚H-N hydrogen bonding and solvation. Therefore, it is very likely that some parts of the changes in the amide I bands during the phase separation processes of PNiPA reflect the breakage and formation of the intramolecular CdO‚‚‚H-N hydrogen bond. Of course, some parts of the spectral changes may also be owing to the changes in the solvation. Our interpretation of the IR result seems to be consistent with the 1H NMR results for PNiPA aqueous solutions. Ohta et al.13 reported that the 1H spin-spin relaxation time for the side chains remains constant below Tps and increases transitionally at Tps, suggesting that the mobility of the side chains of PNiPA increases just above Tps. This transitional change in the molecular mobility of the side chains may originate from the dissociation of the intramolecular CdO‚‚‚ H-N hydrogen bond of PNiPA. When a cosolvent is added to the aqueous solution of PNiPA, the amide I band around 1650 cm-1 appears with increasing xm or xt, indicating that the intramolecular CdO‚‚‚H-N hydrogen bonds among the side chains of the polymer are partially broken. The relative intensity of the amide I band around 1650 cm-1 keeps increasing as the concentration of the cosolvent increases in xm > 0.51 or xt > 0.47. Finally, the amide I envelope of PNiPA in methanol or THF shows a different feature from that in water. It is likely that the intra- and intermolecular hydrogenbonding pattern of the amide groups of PNiPA in THF or methanol is not similar to that in water. Light scattering studies of dilute PNiPA solutions have indicated that the PNiPA chain is in an extended coil state in bulk water, methanol, and THF.6,7,31 The IR result obtained here indicates that the local conformation of PNiPA in these solvents are different from each other, even if the polymer has a similar conformation at the light scattering level. It should be emphasized that the thermally induced phase separation does not occur in the THF or methanol solutions of PNiPA. That is, the analysis on the molecular interaction at the functional group level is very important for understanding the phase behavior of PNiPA in solutions.
Katsumoto et al. The spectral changes observed here suggest that the addition of a cosolvent promotes the changes in the intramolecular Cd O‚‚‚H-N hydrogen bond among the amide groups of PNiPA. The amide I envelope of PNiPA changes monotonically and never shows a backward change as increasing xm or xt. Thus, the local conformation and solvation state around the amide groups of PNiPA keeps monotonically changing upon the addition of a cosolvent. These results drive us to the question of whether there is a contribution of the intramolecular CdO‚‚‚H-N hydrogen bond of PNiPA to the phase behavior of the polymer or not. The amide I envelope of PNiPA in water above Tps is very similar to that in the water-methanol solution at xm ) 0.13 and to that in the water-THF solution at xt ) 0.05. This implies that the local environments around the amide groups under these conditions are similar to each other. That is to say, when a part of the intramolecular CdO‚‚‚H-N hydrogen bonds of PNiPA dissociates, the polymer may start precipitating in the aqueous solution. The intramolecular CdO‚‚‚H-N hydrogen bond of the precipitated PNiPA keeps dissociating by further addition of the cosolvent (xm g 0.51 or xt g 0.47). When the concentration of the cosolvent becomes high, the local conformation of PNiPA may be transformed to adjust for dissolving into the cosolvent. It is therefore likely that the stimuli responsiveness of PNiPA originate from the changes in the local conformations of the polymer as well as the solvation and desolvation of the amide groups. 4. Conclusion The present work has revealed that the side chains of PNiPA form the intramolecular CdO‚‚‚H-N hydrogen bond in a good solvent such as methanol and cold water. The IR spectroscopy combined with the quantum chemical calculations was employed to investigate the local environment around the amide groups of PNiPA. The IR studies and the quantum chemical calculations on the dimer model compounds have enabled us to assign the amide I bands of PNiPA in solutions. The amide I band around 1625 cm-1, which is observed in water and methanol, contains the contribution from the intramolecular CdO‚‚‚H-N hydrogen bond among the side chains. Some of the intramolecular Cd O‚‚‚H-N hydrogen bonds of PNiPA are dissociated during the solvent-induced phase separation process. Comparing the microscopic observation with the macroscopic one, we have suggested that the changes in the intramolecular CdO‚‚‚H-N hydrogen bond around the amide groups of PNiPA plays an important role in the phase behavior of the polymer, as well as the solvation and desolvation of the amide groups. Acknowledgment. This research was supported in part by the Iketani Science and Technology Foundation No. 0181090-A and by the Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aid for Scientific Research, 16205003, 2004. Supporting Information Available: Concentration dependence of the amide I bands for dNiPA-C1 in methanol; the optimized geometries and absolute energies for the calculated structures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (2) (3) (4)
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