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1H NMR studies of poly(N-isopropylacrylamide) gels near the phase transition
European Polymer Journal 39 (2003) 1045–1050 www.elsevier.com/locate/europolj
1
H NMR studies of poly(N -isopropylacrylamide) gels near the phase transition
Pingchuan Sun a
a,*
, Baohui Li b, Yinong Wang a, Jianbiao Ma a, Datong Ding b, Binglin He a
State Key Laboratory of Functional Polymer Materials for Adsorption and Separation, Polymer Chemistry Institute, Nankai University, Tianjin 300071, PeopleÕs Republic of China b Physics Department, Nankai University, Tianjin 300071, PeopleÕs Republic of China Received 8 January 2002; received in revised form 28 August 2002; accepted 26 September 2002
Abstract The phase transition and critical phenomenon of equilibrium swollen poly(N -isopropylacrylamide) (NIPA) hydrogels were studied by 1 H NMR spectroscopy in liquid solution mode. The quantitative NMR observation shows that the peak height and line width of polymer proton and of the HOD proton, and relaxation times of HOD proton all transitionally change as the temperature approaches the transition temperature. The relaxation times of water protons are also measured quantitatively, which shows that the temperature dependence of relaxation times of HOD on temperature before the transition is not consistent with relaxation theory based on the assumption of dominated dipolar interaction between like-spin nuclei and isotropic rotational motion. To explain the surprising relaxation behavior of HOD, we suggest that the amount of bound water in gels increases gradually with temperature at the approach of the phase transition. The pulsed-gradient spin-echo NMR experiments of NIPA gel confirm this suggestion. We believe that these results have important implications concerning the mechanism of the phase transition of NIPA hydrogels. Ó 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Phase transitions and critical phenomena in polymer gels have attracted much attention because of their scientific interest and technological significance. A poly(N isopropylacrylamide) (NIPA) gel in water undergoes a volume–phase transition in response to temperature changes. Understanding the mechanism of the phase transition is complicated by the interactions between water and polymer networks, and between polymer networks. Although extensive studies of the phase transition and critical phenomena have been carried out on NIPA gel system [1–5], a few [6] has been made on the detailed state changes of water and polymer networks in
*
Corresponding author. E-mail address: [email protected] (P. Sun).
this system near the phase transition process. And the mechanism of the phase transition has not been well understood so far. The present study is an attempt to understand better the changes in state and in the dynamical properties of water in NIPA gels near the phase transition, and therefore to understand better the mechanism of the phase transition. Since the nuclear probes are very sensitive to the local chemical environment and the molecular motion, 1 H pulse NMR experiments provides useful information about the molecular motion and interactions between polymers and solvent in solutions and in gels [7–9]. NMR relaxation study provides another pathway to extract dynamic information of molecules. In the present paper, we have quantitatively investigated the behavior of equilibrium swollen NIPA gels near phase transition by measuring 1 H NMR spectrum, the relaxation times and pulsed-gradient spin-echo (PGSE) NMR of water proton.
0014-3057/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0014-3057(02)00326-9
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P. Sun et al. / European Polymer Journal 39 (2003) 1045–1050
2. Experiment The spherical beads of NIPA gels was synthesized by the standard inverse suspension polymerization of N isopropylacrylamide monomer, with a small amount of N,N 0 -methylenebisacrylamide as a cross-linking agent [10]. The monomer N -isopropylacrylamide was synthesized from acryloyl chloride and isopropylamine, and was purified by distillation followed by recrystallization from the mixture of petroleum ether and toluene. The obtained beads were washed several times with a large amount of deionized water to remove unreacted monomers and short polymers, and then dried as powder. The gel sample was prepared by allowing the known weight of dry polymer to swelling in about 0.4 ml 99.7% D2 O to its equilibrium swollen state for NMR measurements, the scanning rate of temperature was kept at 0.1 °C/min. The 1 H NMR experiments were performed on a Varian UNITY plus-400M FT-NMR spectrometer operating at a proton frequency of 400.06 MHz. Proton signals were observed through the decouple channel of a double-resonance ID probe in liquid mode. The p=2 pulse was typically 6 ls. The spin–lattice relaxation times (T1 ) for 1 H nuclei were measured by means of the inversion recovery method with [p s p=2 Acq(FID)] [11] pulse sequence. The spin–spin relaxation times (T2 ) were measured by using CPMG (Carr– Purcell–Mciboom–Gill) spin echo pulse sequence [ðp=2Þx ½s ðpÞy sn Acq(FID)] [12,13]. Data were analyzed by on-line least-square fitting method. A shortcoming of this method is that it is very time consuming due to the longer spin–lattice relaxation times of the water proton. The error range for the line width is less than 1%, and the error range for relaxation measurements is less than 5%. The pulse sequence of PGSE NMR experiments is the same as that used in our previous paper [14].
Fig. 1. 1 H NMR spectra of (a) NIPA gel in an equilibrium swollen state in D2 O at 25 °C and (b) NIPA gel at collapse state of 36 °C. (1) Methyl proton of the N -isopropyl group: –CH3 ; (2) methylene proton: CH2 @; (3) methylene proton: CH@; (4) lone proton of the N -isopropyl group: –H<.
3. Experimental results 1
H NMR spectrum of equilibrium swollen NIPA gels in D2 O in the ‘‘liquid mode’’ at room temperature (25 °C) is shown in Fig. 1(a). The spectrum has sufficient resolution to identify four distinct peaks of the polymer chain protons and a high peak of the HOD proton. The chemical shifts of these peaks are 1.2, 1.6, 2.0, 3.9 and 4.8 ppm in reference to TMS respectively, which is nearly the same as the results obtained with MAS NMR technique [3]. The assignment of the chemical groups is given in the caption of Fig. 1. In order to make a comparison with the proton spectrum in the swollen state, Fig. 1(b) shows the proton spectrum of NIPA gels in a collapsed state above the transition temperature. Fig. 2 plots the peak height and line width of the HOD peak (at 4.8 ppm) as a function of temperature.
Fig. 2. Peak height and line width of the HOD peak as a function of temperature.
These curves were obtained as the temperature was raised from 25 to 38 °C, as well as the temperature was lowered backward. There was no evident difference in spectra between the two directions. The peak height of the HOD peak decreases slowly and the line width broadens slowly as the temperature is increased up to 34.4 °C. Above 34.4 °C, the peak height and the line width of the HOD peak transitionally increases and narrows, respectively. As the temperature is further in-
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creased, the peak height decreases a little and the line width broadens a little. Fig. 3 plots the peak height and line width of –CH3 proton and –CH< proton peaks as a function of temperature in the same temperature range as in Fig. 2. Below 34.4 °C, the peak height of these protons increases slowly where the line width, instead of the ‘‘motional narrowing’’ as usually expected, remains almost unchanged. Above 34.4 °C, a sharp decreasing of peak heights and an abrupt broadening of the line widths are shown. Fig. 4 shows the temperature dependence of the spin– lattice and spin–spin relaxation times (T1 and T2 ) of HOD proton. The T1 value increases slowly whereas the T2 value decreases slowly as the temperature increases up to 34.4 °C. Above 34.4 °C, the T1 and T2 values transitionally increase. As the temperature increases further, they decrease again. Fig. 4. Relaxation times T1 and T2 of HOD protons in NIPA gel as a function of temperature.
4. Discussion From Figs. 2–4, we notice that the peak height and line width of polymer proton and of the HOD proton peaks, and relaxation times of HOD proton all transitionally change as the temperature passing 34.0–35.0 °C. The transition temperature is around 34.4 °C. Shibayama and Tanaka [5] observed macroscopically that the phase transition temperatures are around 33.6 °C for NIPA gels in water and 34.6 °C for those in heavy water (D2 O), and it is insensitive to the cross-linking density or polymer concentration. The difference between these values and that obtained here may be due to the residue H2 O in our solvent. When the temperature rises gradually at the approach of the phase transition point, an obvious volume contraction of the gel beads can be observed macroscopi-
cally. Tanaka group [4] suggests that a portion of ice-like structure water is expelled from gel and becomes free water upon the bead contraction. If it is so, the line width of the HOD peak should narrow or remain unchanged as the temperature increases at the approach of the transition temperature. Nevertheless, this speculation is not consistent with the above surprising result that the line width of the HOD peak broadens as the temperature increases up to 34.4 °C. The surprising result apparently is not the result of the ‘‘motional narrowing’’ effect either. And it suggests that the mobility of HOD proton is reduced and elucidates that the state of water has changed. From the viewpoint of mobility, the state of water in gels has been classified into three-type [15]: bound water, intermediate water, and free water. Since the line width of the bound water
Fig. 3. (a) Peak height and (b) line width of –CH3 proton and –CH< proton peaks as a function of temperature.
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in gels is wider than that of free waterÕs, the above surprising result elucidates that more bound water are formed before the phase transition. This conclusion is in agreement with that showed in Ref. [16]. In the previous work on NIPA gels, Tanaka group measured the spin–lattice relaxation times (T1 ) of polymer protons, and made a perfect interpretation on the dynamics of the side chain and backbone chain through the phase transition process [4]. In this paper, instead of measuring the polymer proton relaxation times, we focus our study on the dynamics of water molecules. As seen from the observed T1 and T2 plots, they are very sensitive to the phase transition. Therefore they can provide dynamic information about the phase transition. In order to extract the dynamic information of the molecular motion in terms of the relaxation times of HOD proton, a simple but fundamental relaxation theory based on the assumption of dominated dipolar interaction between like-spin nuclei and isotropic rotational motion is used [17]. 1 2 l0 2 4 2 IðI þ 1Þ sc 4sc þ ¼ c h 1 þ x2 s2c 1 þ 4x2 s2c T1 5 4p r6 1 1 l0 2 4 2 IðI þ 1Þ 5sc 2sc þ ¼ c h 3s þ c 1 þ x2 s2c 1 þ 4x2 s2c T2 5 4p r6 ð1Þ where r is the distance between the protons in the water molecule, l0 is the magnetic field constant in vacuum (4p 107 H m1 ), c is the nuclear gyromagnetic ratio (2:675 108 rad T1 s1 for protons), I is the nuclear spin (1/2 for protons), x=2p is the spectrometer frequency, and h is equal to h=2p with PlankÕs constant h ¼ 6:626 1034 J s. The relation between the correlation time sc and absolute temperature T is described as sc ¼ s0 expðEa =RT Þ
ð2Þ
where Ea is the activation energy, R ¼ 8:31 J mol1 K the gas constant, and T is the absolute temperature in Kelvin. The temperature dependence of relaxation times is shown schematically in Fig. 5. In order to make a direct qualitative comparison between experimental and theoretical results of the relaxation times in Figs. 4 and 5, the horizontal axis in Fig. 5 is scaled directly by temperature T , instead of the commonly used 1=T scale in the literature. In this study, x is a fixed value, and thus the shift of the curves toward the high- or low-temperature side depends solely on the nature of the thermal motion represented by Ea (when s0 is fixed). For lower Ea (¼ 23 kJ/mol), T1 almost equals to T2 , while for higher Ea (¼ 32 kJ/mol), T1 much larger than T2 . The later case is at our observation temperature range. T1 goes through a minimum; T2 always increases with increasing temperature. Considering the temperature dependence of relaxation times of the HOD proton in Fig. 4, T1 value increases
Fig. 5. Theoretical temperature dependence of proton relaxation times T1 and T2 for intra-molecular dipole–dipole interaction. They are calculated using Eqs. (1) and (2) with the , s0 ¼ 5 1015 s, and x ¼ 2p 400 parameters r ¼ 1:6 A MHz.
with temperature up to 34.4 °C, which qualitatively corresponds to curve A in Fig. 5. Whereas T2 just shows an opposite temperature dependence in contrast to the theoretical T2 of curve A in Fig. 5. This special spin–spin relaxation behavior is significantly different from the previous study of water in NIPA polymer solution [18]. It is also different from the relaxation behavior of HOD in NIPA gels obtained by Tanaka et al. [6]. This difference may be raised from the differences in the rates of heating and cooling, and in the degree of swelling of samples. The rates of heating and cooling of ours is much slower than theirs, while the degree of swelling of our sample is more than 20. We suppose that the Ea value for the bound water protons is larger than that for the free water protons (actually it is reasonable). If the temperature dependence of relaxation times of free water protons can be quantitatively delineated as curve A in Fig. 5, the temperature dependence of relaxation times of bound water protons will quantitatively correspond to curve B in Fig. 5. The observed relaxation times of HOD protons in gels are the mean value of relaxation times of free and bound water protons according to the following equation [19]. 1 pf pb ¼ þ Tn Tnf Tnb
n ¼ 1; 2
ð3Þ
where Tnf and Tnb are the intrinsic relaxation times of the free and bound water components, and pf and pb are the corresponding fractions.
P. Sun et al. / European Polymer Journal 39 (2003) 1045–1050
In order to explain the above observations, we think that when the temperature rises up to the critical point, the state of a portion of water molecules is changed. Some of water is gradually bound to hydrate sites in polymer while some others are expelled out of the gel beads. It is the former that contributes to the observed temperature dependence of relaxation times. Although the T2 of the bound water protons is increases with temperature (see Fig. 5), its value is much smaller than that of free water protons. Therefore the observed T2 value of HOD protons in gels will decrease with increasing temperature up to the critical point. At the same time although the T1 of the bound water protons decreases with rising temperature, its value is larger than that of free water protons when the temperature is lower than the transition temperature. Therefore the observed T1 value of HOD protons in gels will increase with increasing temperature up to the critical point. Just above the transition temperature the observed T1 and T2 values correspond to curve A in Fig. 5 because the bound water are largely transformed into free water. They all increase sharply because the relaxation times of free water protons are larger than the corresponding values of bound water protons. As the temperature increases further, the observed relaxation times of HOD protons are again the mean value of relaxation times of free and bound water protons because a little portion of free water is bounded to the polymer networks again. Which makes that they all decrease a little. Therefore the transitional changes of T1 and T2 at the phase transition temperature is the direct result of the changes of water states. On the other hand, the relationship between the line width Dm1=2 and the effective transverse relaxation rate ðT2 Þ1 can be written as [20] Dm1=2 / ðpT2 Þ1 . The ðT2 Þ1 value includes the contribution of both inhomogeneous magnetic field and the ðT2 Þ1 value measured by spin echo technique. With our 400 MHz high-resolution spectrometer, the changing of inhomogeneous magnetic field in the experimental temperature range can be neglect. A direct comparison between the temperature dependence of both the Dm1=2 and ðT2 Þ1 in Figs. 2 and 4 qualitatively confirmed the above relationship. This analysis conclude that the temperature dependence of the line width confirm the same hypothesis as proposed above from the analysis of relaxation times of HOD proton. We also examined the relaxation times T1 and T2 of HOD 2 H in NIPA gel at different temperature. The results are shown in Table 1. From Table 1, it is seen that the change of T2 with temperature seems normal, whereas the change of T1 with temperature seems abnormal according to the above discussion. In order to confirm the above suggestions that the amount of bound water in gels increases gradually with temperature at the approach of the phase transition, we made PGSE NMR experiments at different tempera-
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Table 1 Relaxation times T1 and T2 of HOD 2H in NIPA gel at different temperature Temperature (°C) T1 (s) T2 (s)
25
31
36
0.29 0.27
0.42 0.30
0.14 0.51
Fig. 6. Logarithm of the proton NMR signal intensity [logðA=A0 Þ] versus d2 (D d=3) for absorbed water in gels with different temperatures.
tures. Fig. 6 shows the plot of logðA=A0 Þ versus the factor d2 ðD d=3Þ for absorbed water in gels with different temperatures. The curve at temperature of 36 °C in Fig. 6 is straight line, which indicate that the diffusion of absorbed water at this temperature is in the one phase state. The curves at temperature of 25 and 31 °C in Fig. 6 are not straight lines, which indicate that the diffusion of absorbed water at those temperatures is in the form of multiphase states. The analysis of self-diffusion coefficients and relative contents of bound and free water are the same as those described in our previous paper [14]. The results are shown in Table 2. From Table 2, it is obvious that the relative content of bound water increases with temperature before the transition. Table 2 Self-diffusion coefficient, D ( 109 m2 /s) and relative content, PI (%), of bound and free water in PNIPA gels at different temperature Temperature (°C) 25
Bound water Free water
31
36
PI
D
PI
D
PI
D
40 60
0.93 1.71
46 54
1.05 2.35
– 100
– 2.72
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5. Conclusions We have applied 1 H NMR technique in the quantitative study of critical phenomenon and phase transition exhibited by temperature-sensitive NIPA gels. The evolution of spectral structure of protons both in polymer and in HOD and the evolution of the relaxation times of HOD proton with temperature near the phase transition provides detailed microscopic information for study the transition behavior. Acknowledgements Supported by the foundation of visiting scholar in State Key Laboratory of China. References [1] Hirokawa Y, Tanaka T. J Chem Phys 1984;81:6379. [2] Hitotsu S, Hirokawa Y, Tanaka T. J Chem Phys 1987;87:1392. [3] Badiger MV, Rajamohanan PR, Kulkarni MG, Ganapathy S, Mashelkar RA. Macromolecules 1991;24:106.
[4] Tokuhiro T, Amiya T, Mamada A, Tanaka T. Macromolecules 1991;24:2936. [5] Shibayama M, Tanaka T. J Chem Phys 1992;97:6829. [6] Tanaka N, Matsukawa S, Kurosu H, Ando I. Polymer 1998;39:4703. [7] Ricka J, Meewes M, Nyffenegger R, Binkert T. Phys Rev Lett 1990;65:657. [8] Schild HG, Tirrell DA. Langmuir 1991;7:1319. [9] Winnik FM, Ringsorf H, Venzmer J. Langmuir 1991;7: 912. [10] Matsuo ES, Tanaka T. J Chem Phys 1988;89:1695. [11] Vold RL. J Chem Phys 1968;48:3831. [12] Carr HY, Purcell EM. Phys Rev 1954;94:630. [13] Meiboom S, Gill D. Rev Sci Instr 1958;29:688. [14] Li B, Ding D, Sun P, Wang Y, Ma J, He B. J Appl Polym Sci 2000;77:424. [15] Quinn FX, Kampff E, Smyth G, Mcbrierty VJ. Macromolecules 1988;21:3191. [16] Osada Y, Ross-Murphy SB. Sci Am 1993;5:42. [17] Abragam A. The principles of nuclear magnetism. Oxford: Oxford University Press; 1967 [Chapter 8]. [18] Ohta H, Ando I, Fujishige S, Kubota K. J Polym Sci, Part B 1991;29:963. [19] Zimmerman JR, Brittin WE. J Phys Chem 1957;61:1328. [20] Becker ED. High resolution NMR theory and chemical applications. New York: Academic Press; 1980.
1
H NMR studies of poly(N -isopropylacrylamide) gels near the phase transition
Pingchuan Sun a
a,*
, Baohui Li b, Yinong Wang a, Jianbiao Ma a, Datong Ding b, Binglin He a
State Key Laboratory of Functional Polymer Materials for Adsorption and Separation, Polymer Chemistry Institute, Nankai University, Tianjin 300071, PeopleÕs Republic of China b Physics Department, Nankai University, Tianjin 300071, PeopleÕs Republic of China Received 8 January 2002; received in revised form 28 August 2002; accepted 26 September 2002
Abstract The phase transition and critical phenomenon of equilibrium swollen poly(N -isopropylacrylamide) (NIPA) hydrogels were studied by 1 H NMR spectroscopy in liquid solution mode. The quantitative NMR observation shows that the peak height and line width of polymer proton and of the HOD proton, and relaxation times of HOD proton all transitionally change as the temperature approaches the transition temperature. The relaxation times of water protons are also measured quantitatively, which shows that the temperature dependence of relaxation times of HOD on temperature before the transition is not consistent with relaxation theory based on the assumption of dominated dipolar interaction between like-spin nuclei and isotropic rotational motion. To explain the surprising relaxation behavior of HOD, we suggest that the amount of bound water in gels increases gradually with temperature at the approach of the phase transition. The pulsed-gradient spin-echo NMR experiments of NIPA gel confirm this suggestion. We believe that these results have important implications concerning the mechanism of the phase transition of NIPA hydrogels. Ó 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Phase transitions and critical phenomena in polymer gels have attracted much attention because of their scientific interest and technological significance. A poly(N isopropylacrylamide) (NIPA) gel in water undergoes a volume–phase transition in response to temperature changes. Understanding the mechanism of the phase transition is complicated by the interactions between water and polymer networks, and between polymer networks. Although extensive studies of the phase transition and critical phenomena have been carried out on NIPA gel system [1–5], a few [6] has been made on the detailed state changes of water and polymer networks in
*
Corresponding author. E-mail address: [email protected] (P. Sun).
this system near the phase transition process. And the mechanism of the phase transition has not been well understood so far. The present study is an attempt to understand better the changes in state and in the dynamical properties of water in NIPA gels near the phase transition, and therefore to understand better the mechanism of the phase transition. Since the nuclear probes are very sensitive to the local chemical environment and the molecular motion, 1 H pulse NMR experiments provides useful information about the molecular motion and interactions between polymers and solvent in solutions and in gels [7–9]. NMR relaxation study provides another pathway to extract dynamic information of molecules. In the present paper, we have quantitatively investigated the behavior of equilibrium swollen NIPA gels near phase transition by measuring 1 H NMR spectrum, the relaxation times and pulsed-gradient spin-echo (PGSE) NMR of water proton.
0014-3057/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0014-3057(02)00326-9
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P. Sun et al. / European Polymer Journal 39 (2003) 1045–1050
2. Experiment The spherical beads of NIPA gels was synthesized by the standard inverse suspension polymerization of N isopropylacrylamide monomer, with a small amount of N,N 0 -methylenebisacrylamide as a cross-linking agent [10]. The monomer N -isopropylacrylamide was synthesized from acryloyl chloride and isopropylamine, and was purified by distillation followed by recrystallization from the mixture of petroleum ether and toluene. The obtained beads were washed several times with a large amount of deionized water to remove unreacted monomers and short polymers, and then dried as powder. The gel sample was prepared by allowing the known weight of dry polymer to swelling in about 0.4 ml 99.7% D2 O to its equilibrium swollen state for NMR measurements, the scanning rate of temperature was kept at 0.1 °C/min. The 1 H NMR experiments were performed on a Varian UNITY plus-400M FT-NMR spectrometer operating at a proton frequency of 400.06 MHz. Proton signals were observed through the decouple channel of a double-resonance ID probe in liquid mode. The p=2 pulse was typically 6 ls. The spin–lattice relaxation times (T1 ) for 1 H nuclei were measured by means of the inversion recovery method with [p s p=2 Acq(FID)] [11] pulse sequence. The spin–spin relaxation times (T2 ) were measured by using CPMG (Carr– Purcell–Mciboom–Gill) spin echo pulse sequence [ðp=2Þx ½s ðpÞy sn Acq(FID)] [12,13]. Data were analyzed by on-line least-square fitting method. A shortcoming of this method is that it is very time consuming due to the longer spin–lattice relaxation times of the water proton. The error range for the line width is less than 1%, and the error range for relaxation measurements is less than 5%. The pulse sequence of PGSE NMR experiments is the same as that used in our previous paper [14].
Fig. 1. 1 H NMR spectra of (a) NIPA gel in an equilibrium swollen state in D2 O at 25 °C and (b) NIPA gel at collapse state of 36 °C. (1) Methyl proton of the N -isopropyl group: –CH3 ; (2) methylene proton: CH2 @; (3) methylene proton: CH@; (4) lone proton of the N -isopropyl group: –H<.
3. Experimental results 1
H NMR spectrum of equilibrium swollen NIPA gels in D2 O in the ‘‘liquid mode’’ at room temperature (25 °C) is shown in Fig. 1(a). The spectrum has sufficient resolution to identify four distinct peaks of the polymer chain protons and a high peak of the HOD proton. The chemical shifts of these peaks are 1.2, 1.6, 2.0, 3.9 and 4.8 ppm in reference to TMS respectively, which is nearly the same as the results obtained with MAS NMR technique [3]. The assignment of the chemical groups is given in the caption of Fig. 1. In order to make a comparison with the proton spectrum in the swollen state, Fig. 1(b) shows the proton spectrum of NIPA gels in a collapsed state above the transition temperature. Fig. 2 plots the peak height and line width of the HOD peak (at 4.8 ppm) as a function of temperature.
Fig. 2. Peak height and line width of the HOD peak as a function of temperature.
These curves were obtained as the temperature was raised from 25 to 38 °C, as well as the temperature was lowered backward. There was no evident difference in spectra between the two directions. The peak height of the HOD peak decreases slowly and the line width broadens slowly as the temperature is increased up to 34.4 °C. Above 34.4 °C, the peak height and the line width of the HOD peak transitionally increases and narrows, respectively. As the temperature is further in-
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creased, the peak height decreases a little and the line width broadens a little. Fig. 3 plots the peak height and line width of –CH3 proton and –CH< proton peaks as a function of temperature in the same temperature range as in Fig. 2. Below 34.4 °C, the peak height of these protons increases slowly where the line width, instead of the ‘‘motional narrowing’’ as usually expected, remains almost unchanged. Above 34.4 °C, a sharp decreasing of peak heights and an abrupt broadening of the line widths are shown. Fig. 4 shows the temperature dependence of the spin– lattice and spin–spin relaxation times (T1 and T2 ) of HOD proton. The T1 value increases slowly whereas the T2 value decreases slowly as the temperature increases up to 34.4 °C. Above 34.4 °C, the T1 and T2 values transitionally increase. As the temperature increases further, they decrease again. Fig. 4. Relaxation times T1 and T2 of HOD protons in NIPA gel as a function of temperature.
4. Discussion From Figs. 2–4, we notice that the peak height and line width of polymer proton and of the HOD proton peaks, and relaxation times of HOD proton all transitionally change as the temperature passing 34.0–35.0 °C. The transition temperature is around 34.4 °C. Shibayama and Tanaka [5] observed macroscopically that the phase transition temperatures are around 33.6 °C for NIPA gels in water and 34.6 °C for those in heavy water (D2 O), and it is insensitive to the cross-linking density or polymer concentration. The difference between these values and that obtained here may be due to the residue H2 O in our solvent. When the temperature rises gradually at the approach of the phase transition point, an obvious volume contraction of the gel beads can be observed macroscopi-
cally. Tanaka group [4] suggests that a portion of ice-like structure water is expelled from gel and becomes free water upon the bead contraction. If it is so, the line width of the HOD peak should narrow or remain unchanged as the temperature increases at the approach of the transition temperature. Nevertheless, this speculation is not consistent with the above surprising result that the line width of the HOD peak broadens as the temperature increases up to 34.4 °C. The surprising result apparently is not the result of the ‘‘motional narrowing’’ effect either. And it suggests that the mobility of HOD proton is reduced and elucidates that the state of water has changed. From the viewpoint of mobility, the state of water in gels has been classified into three-type [15]: bound water, intermediate water, and free water. Since the line width of the bound water
Fig. 3. (a) Peak height and (b) line width of –CH3 proton and –CH< proton peaks as a function of temperature.
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in gels is wider than that of free waterÕs, the above surprising result elucidates that more bound water are formed before the phase transition. This conclusion is in agreement with that showed in Ref. [16]. In the previous work on NIPA gels, Tanaka group measured the spin–lattice relaxation times (T1 ) of polymer protons, and made a perfect interpretation on the dynamics of the side chain and backbone chain through the phase transition process [4]. In this paper, instead of measuring the polymer proton relaxation times, we focus our study on the dynamics of water molecules. As seen from the observed T1 and T2 plots, they are very sensitive to the phase transition. Therefore they can provide dynamic information about the phase transition. In order to extract the dynamic information of the molecular motion in terms of the relaxation times of HOD proton, a simple but fundamental relaxation theory based on the assumption of dominated dipolar interaction between like-spin nuclei and isotropic rotational motion is used [17]. 1 2 l0 2 4 2 IðI þ 1Þ sc 4sc þ ¼ c h 1 þ x2 s2c 1 þ 4x2 s2c T1 5 4p r6 1 1 l0 2 4 2 IðI þ 1Þ 5sc 2sc þ ¼ c h 3s þ c 1 þ x2 s2c 1 þ 4x2 s2c T2 5 4p r6 ð1Þ where r is the distance between the protons in the water molecule, l0 is the magnetic field constant in vacuum (4p 107 H m1 ), c is the nuclear gyromagnetic ratio (2:675 108 rad T1 s1 for protons), I is the nuclear spin (1/2 for protons), x=2p is the spectrometer frequency, and h is equal to h=2p with PlankÕs constant h ¼ 6:626 1034 J s. The relation between the correlation time sc and absolute temperature T is described as sc ¼ s0 expðEa =RT Þ
ð2Þ
where Ea is the activation energy, R ¼ 8:31 J mol1 K the gas constant, and T is the absolute temperature in Kelvin. The temperature dependence of relaxation times is shown schematically in Fig. 5. In order to make a direct qualitative comparison between experimental and theoretical results of the relaxation times in Figs. 4 and 5, the horizontal axis in Fig. 5 is scaled directly by temperature T , instead of the commonly used 1=T scale in the literature. In this study, x is a fixed value, and thus the shift of the curves toward the high- or low-temperature side depends solely on the nature of the thermal motion represented by Ea (when s0 is fixed). For lower Ea (¼ 23 kJ/mol), T1 almost equals to T2 , while for higher Ea (¼ 32 kJ/mol), T1 much larger than T2 . The later case is at our observation temperature range. T1 goes through a minimum; T2 always increases with increasing temperature. Considering the temperature dependence of relaxation times of the HOD proton in Fig. 4, T1 value increases
Fig. 5. Theoretical temperature dependence of proton relaxation times T1 and T2 for intra-molecular dipole–dipole interaction. They are calculated using Eqs. (1) and (2) with the , s0 ¼ 5 1015 s, and x ¼ 2p 400 parameters r ¼ 1:6 A MHz.
with temperature up to 34.4 °C, which qualitatively corresponds to curve A in Fig. 5. Whereas T2 just shows an opposite temperature dependence in contrast to the theoretical T2 of curve A in Fig. 5. This special spin–spin relaxation behavior is significantly different from the previous study of water in NIPA polymer solution [18]. It is also different from the relaxation behavior of HOD in NIPA gels obtained by Tanaka et al. [6]. This difference may be raised from the differences in the rates of heating and cooling, and in the degree of swelling of samples. The rates of heating and cooling of ours is much slower than theirs, while the degree of swelling of our sample is more than 20. We suppose that the Ea value for the bound water protons is larger than that for the free water protons (actually it is reasonable). If the temperature dependence of relaxation times of free water protons can be quantitatively delineated as curve A in Fig. 5, the temperature dependence of relaxation times of bound water protons will quantitatively correspond to curve B in Fig. 5. The observed relaxation times of HOD protons in gels are the mean value of relaxation times of free and bound water protons according to the following equation [19]. 1 pf pb ¼ þ Tn Tnf Tnb
n ¼ 1; 2
ð3Þ
where Tnf and Tnb are the intrinsic relaxation times of the free and bound water components, and pf and pb are the corresponding fractions.
P. Sun et al. / European Polymer Journal 39 (2003) 1045–1050
In order to explain the above observations, we think that when the temperature rises up to the critical point, the state of a portion of water molecules is changed. Some of water is gradually bound to hydrate sites in polymer while some others are expelled out of the gel beads. It is the former that contributes to the observed temperature dependence of relaxation times. Although the T2 of the bound water protons is increases with temperature (see Fig. 5), its value is much smaller than that of free water protons. Therefore the observed T2 value of HOD protons in gels will decrease with increasing temperature up to the critical point. At the same time although the T1 of the bound water protons decreases with rising temperature, its value is larger than that of free water protons when the temperature is lower than the transition temperature. Therefore the observed T1 value of HOD protons in gels will increase with increasing temperature up to the critical point. Just above the transition temperature the observed T1 and T2 values correspond to curve A in Fig. 5 because the bound water are largely transformed into free water. They all increase sharply because the relaxation times of free water protons are larger than the corresponding values of bound water protons. As the temperature increases further, the observed relaxation times of HOD protons are again the mean value of relaxation times of free and bound water protons because a little portion of free water is bounded to the polymer networks again. Which makes that they all decrease a little. Therefore the transitional changes of T1 and T2 at the phase transition temperature is the direct result of the changes of water states. On the other hand, the relationship between the line width Dm1=2 and the effective transverse relaxation rate ðT2 Þ1 can be written as [20] Dm1=2 / ðpT2 Þ1 . The ðT2 Þ1 value includes the contribution of both inhomogeneous magnetic field and the ðT2 Þ1 value measured by spin echo technique. With our 400 MHz high-resolution spectrometer, the changing of inhomogeneous magnetic field in the experimental temperature range can be neglect. A direct comparison between the temperature dependence of both the Dm1=2 and ðT2 Þ1 in Figs. 2 and 4 qualitatively confirmed the above relationship. This analysis conclude that the temperature dependence of the line width confirm the same hypothesis as proposed above from the analysis of relaxation times of HOD proton. We also examined the relaxation times T1 and T2 of HOD 2 H in NIPA gel at different temperature. The results are shown in Table 1. From Table 1, it is seen that the change of T2 with temperature seems normal, whereas the change of T1 with temperature seems abnormal according to the above discussion. In order to confirm the above suggestions that the amount of bound water in gels increases gradually with temperature at the approach of the phase transition, we made PGSE NMR experiments at different tempera-
1049
Table 1 Relaxation times T1 and T2 of HOD 2H in NIPA gel at different temperature Temperature (°C) T1 (s) T2 (s)
25
31
36
0.29 0.27
0.42 0.30
0.14 0.51
Fig. 6. Logarithm of the proton NMR signal intensity [logðA=A0 Þ] versus d2 (D d=3) for absorbed water in gels with different temperatures.
tures. Fig. 6 shows the plot of logðA=A0 Þ versus the factor d2 ðD d=3Þ for absorbed water in gels with different temperatures. The curve at temperature of 36 °C in Fig. 6 is straight line, which indicate that the diffusion of absorbed water at this temperature is in the one phase state. The curves at temperature of 25 and 31 °C in Fig. 6 are not straight lines, which indicate that the diffusion of absorbed water at those temperatures is in the form of multiphase states. The analysis of self-diffusion coefficients and relative contents of bound and free water are the same as those described in our previous paper [14]. The results are shown in Table 2. From Table 2, it is obvious that the relative content of bound water increases with temperature before the transition. Table 2 Self-diffusion coefficient, D ( 109 m2 /s) and relative content, PI (%), of bound and free water in PNIPA gels at different temperature Temperature (°C) 25
Bound water Free water
31
36
PI
D
PI
D
PI
D
40 60
0.93 1.71
46 54
1.05 2.35
– 100
– 2.72
1050
P. Sun et al. / European Polymer Journal 39 (2003) 1045–1050
5. Conclusions We have applied 1 H NMR technique in the quantitative study of critical phenomenon and phase transition exhibited by temperature-sensitive NIPA gels. The evolution of spectral structure of protons both in polymer and in HOD and the evolution of the relaxation times of HOD proton with temperature near the phase transition provides detailed microscopic information for study the transition behavior. Acknowledgements Supported by the foundation of visiting scholar in State Key Laboratory of China. References [1] Hirokawa Y, Tanaka T. J Chem Phys 1984;81:6379. [2] Hitotsu S, Hirokawa Y, Tanaka T. J Chem Phys 1987;87:1392. [3] Badiger MV, Rajamohanan PR, Kulkarni MG, Ganapathy S, Mashelkar RA. Macromolecules 1991;24:106.
[4] Tokuhiro T, Amiya T, Mamada A, Tanaka T. Macromolecules 1991;24:2936. [5] Shibayama M, Tanaka T. J Chem Phys 1992;97:6829. [6] Tanaka N, Matsukawa S, Kurosu H, Ando I. Polymer 1998;39:4703. [7] Ricka J, Meewes M, Nyffenegger R, Binkert T. Phys Rev Lett 1990;65:657. [8] Schild HG, Tirrell DA. Langmuir 1991;7:1319. [9] Winnik FM, Ringsorf H, Venzmer J. Langmuir 1991;7: 912. [10] Matsuo ES, Tanaka T. J Chem Phys 1988;89:1695. [11] Vold RL. J Chem Phys 1968;48:3831. [12] Carr HY, Purcell EM. Phys Rev 1954;94:630. [13] Meiboom S, Gill D. Rev Sci Instr 1958;29:688. [14] Li B, Ding D, Sun P, Wang Y, Ma J, He B. J Appl Polym Sci 2000;77:424. [15] Quinn FX, Kampff E, Smyth G, Mcbrierty VJ. Macromolecules 1988;21:3191. [16] Osada Y, Ross-Murphy SB. Sci Am 1993;5:42. [17] Abragam A. The principles of nuclear magnetism. Oxford: Oxford University Press; 1967 [Chapter 8]. [18] Ohta H, Ando I, Fujishige S, Kubota K. J Polym Sci, Part B 1991;29:963. [19] Zimmerman JR, Brittin WE. J Phys Chem 1957;61:1328. [20] Becker ED. High resolution NMR theory and chemical applications. New York: Academic Press; 1980.