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H MAS NMR Studies of the Phase Separation of Poly (N-isopropylacrylamide) Gel in Binary Solvents Nian Wang,†,‡ Geying Ru,†,‡ Liying Wang,† and Jiwen Feng*,†
†
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Science, Wuhan 430071, PR China, and ‡Graduate School, Chinese Academy of Science, Beijing 100029, PR China Received November 19, 2008. Revised Manuscript Received February 27, 2009
Preferential interactions of solvents with poly(N-isopropylacrylamide) (PNIPAM) gel networks in binary water/ alcohol (water/methanol and water/ethanol) mixtures have been investigated using variable-temperature high-resolution 1H MAS NMR. NMR results for PNIPAM gel in the binary solvents reveal the existence of two distinct types of water/alcohol mixtures above the LCST: confined binary solvents bound inside the gel, and free binary solvents expelled from the gel. It is interesting to find that the alcohol concentration in confined solution is significantly higher than that in free solution. Moreover, of the two alcohols, ethanol is more significantly concentrated in the confined solution. These results demonstrate that the polymer preferentially interacts with alcohol molecules over water and that the alcohol with higher hydrophobicity exhibits higher preferential absorption on PNIPAM. Our results also show that 1H NMR measurements made on two distinct types of solution provide a convenient, direct means of characterizing the preferential adsorption of solvent on polymer.
Introduction Poly(N-isopropylacrylamide) (PNIPAM) gel exhibits a very large volume change in pure water at a lower critical solution temperature (LCST) of about 33 °C.1 The PNIPAM gel network shrinks from a hydrophilic coiled state to a hydrophobic globule state as the temperature is raised above the LCST. When the temperature falls to below the LCST, the gel can return to the swollen state. Thus, the PNIPAM gel is also called a thermoreversible gel.2,3 Materials of this kind could find a wide variety of applications in intelligent microfluidic switching, controlled drug delivery, bioseparations, and biomedical fields.4-8 The PNIPAM gel also undergoes novel volume-phase transitions in water/alcohol mixtures in response to changes in temperature or solvent composition.9 The PNIPAM gel is extremely soluble in pure water and pure alcohol but insoluble in certain mixtures of them (known as co-nonsolvency).10 As a result, in PNIPAM a reentrant swelling-shrinking-swelling transition takes place as the alcohol content increases at the proper temperature. Utilizing this special property, the PNIPAM gel could be purified by a mixture of two good solvents. In fact, alcohol/water mixtures themselves are of great interest because they show various uncommon thermodynamic phenomena and have many important applications in industry and biochemistry. *Corresponding author. E-mail:
[email protected]. Tel: 86-2787197343. Fax: 86-27-87199291. (1) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163–249. (2) Hoffman, A. S.; Afrasiabi, A.; Dong, L. C. J. Controlled Release 1986, 4, 213–222. (3) Dong; L. C.; Hoffman, A. S. In Reversible Polymeric Gels and Related Systems; Russo P., Ed.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987; Vol. 350, pp 236-244. (4) Qiu, Y.; Park, K. Adv. Drug Delivery Rev. 2001, 53, 321–339. (5) Wang, Q.; Xu, H.; Yang, X.; Yang, Y. Int. J. Pharm. 2008, 361, 189–193. (6) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357–360. (7) Hoffman, A. S. Macromol. Symp. 1995, 98, 645–664. (8) Daganl, R. Chem. Eng. News 1997, 75, 26–37. (9) Hirotsu, S. J. Chem. Phys. 1988, 88, 427–431. (10) Winnik, F. M.; Ottaviani, M. F.; Bossmann, S. H.; Garciagaribay, M.; Turro, N. J. Macromolecules 1992, 25, 6007–6017.
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To understand the LCST phenomenon, properties of both PNIPAM chains and the PNIPAM gel in water have been widely studied by various techniques such as rheology,11 differential scanning calorimetry (DSC),12 dynamic light scattering (DLS),13,14 small-angle neutron scattering (SANS),15 and NMR.16 The effects of temperature, pH value, ionic strength, molecular weight, surfactant additives, and cross-link density have also been investigated extensively,17-23 but the exact origin of the LCST of PNIPAM is not yet clear. There has been much debate as to whether the volume transition of the PNIPAM hydrogel is driven purely by hydrophobic interaction or hydrophilic interaction.1,24 It has also been suggested that both hydrophobic and hydrophilic interactions play an important role in the phase transition and the LCST behaviors of the PNIPAM gel result from the changes in the balance between hydrophobic and hydrophilic interactions.1,25 For the PNIPAM gel or chain in water/alcohol binary solvents, there exist more complicated interactions among water, (11) Senff, H.; Richtering, W. J. Chem. Phys. 1999, 111, 1705–1711. (12) Woodward, N. C.; Chowdhry, B. Z.; Snowden, M. J.; Leharne, S. A.; Griffiths, P. C.; Winnington, A. L. Langmuir 2003, 19, 3202–3211. (13) Shibayama, M.; Norisuye, T.; Nomura, S. Macromolecules 1996, 29, 8746– 8750. (14) Wu, C.; Zhou, S. Q. Macromolecules 1996, 29, 1574–1578. (15) Kratz, K.; Hellweg, T.; Eimer, W. Polymer 2001, 42, 6631–6639. (16) Ohta, H.; Ando, I.; Fujishige, S.; Kubota, K. J. Mol. Struct. 1991, 245, 391– 397. (17) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379–6380. (18) Inomata, H.; Goto, S.; Otake, K.; Saito, S. Langmuir 1992, 8, 687–690. (19) Varga, I.; Gilanyi, T.; Meszaros, R.; Filipcsei, G.; Zrinyi, M. J. Phys. Chem. B 2001, 105, 9071–9076. (20) Zhu, X.; DeGraaf, J.; Winnik, F. M.; Leckband, D. Langmuir 2004, 20, 10648–10656. (21) Plunkett, K. N.; Zhu, X.; Moore, J. S.; Leckband, D. E. Langmuir 2006, 22, 4259–4266. (22) Zhu, X.; Yan, C.; Winnik, F. M.; Leckband, D. Langmuir 2007, 23, 162– 169. (23) Karg, M.; Pastoriza-Santos, I.; Rodriguez-Gonzalez, B.; von Klitzing, R.; Wellert, S.; Hellweg, T. Langmuir 2008, 24, 6300–6306. (24) Ilmain, F.; Tanaka, T.; Kokufuta, E. Nature (London) 1991, 349, 400–401. (25) Otake, K.; Inomata, H.; Konno, M.; Saito, S. Macromolecules 1990, 23, 283–289.
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methanol, and the PNIPAM gel.9,10,17,26-32 Therefore, a mechanism for the LCST should include ternary interactions among the polymer, water, and alcohol, but there have also been efforts to explain the above LCST behavior simply in terms of the formation of stoichiometric compounds between the solvent molecules.30,31 A new theoretical model based on the competitive polymer-water and polymer-methanol interactions without considering the interaction between solvent molecules has been developed recently to explain the equilibrium swelling behavior of the PNIPAM gel in water/methanol.32 This new model predicts the preferential adsorption of solvent molecules on polymers. Electronic paramagnetic resonance (EPR) and solution-depletion studies reveal the preferential adsorptions of methanol or ethanol on PNIPAAM.10,26 As a powerful experimental technique, NMR has been applied mainly to study the PNIPAM gel transition in pure water33,34 and rarely to study the PNIPAM gel transition in binary mixtures.10 Utilizing NMR, one can study not only the dynamics and transitions of gel networks directly but also the dynamics of water and its interaction with polymers. In particular, it was previously reported that when a hydrogel with pure water was heated above the LCST some water molecules were expelled from the polymer network to form so-called free water (FW) and the rest were still confined in the polymer network and are usually called confined or bound water (CW).35 Intrigued by this remark, we design the present simple NMR experiments to characterize the preferential interactions of solvent with gel networks in binary water/alcohol solvents. (See the description below.) In the present study, the volume-phase transition of the PNIPAM gel in binary water/alcohol solvents has been investigated using variable-temperature high-resolution 1H MAS NMR. It is found that when PNIPAM gel in binary water/alcohol solvents was heated above the LCST only part of the water/ alcohol solvents was expelled from the polymer networks, namely, the free binary solvents, and the other part remained inside the polymer networks (called the confined binary solvents), resulting in two distinct types of solution as in the case of a pure water solution. Then, we directly compare the compositions of two distinct solutions and therefore obtain information about preferential interactions of solvent with polymer.
Experimental Section Materials. The PNIPAAM gel used in this study was synthesized according to the method described in ref 23. The solvents (99.9% deuteron water purchased from Cambridge Isotope Laboratory and the normal analytical-grade methanol and ethanol was obtained from Shanghai Zhenxing Chemical Company), used without further purification, were bubbled with nitrogen to remove oxygen before use. The swollen samples with a PNIPAM/solvent ratio of 1:3 (w/w) were left overnight at 4 °C (26) Mukae, K.; Sakurai, M.; Sawamura, S.; Makino, K.; Kim, S. W.; Ueda, I.; Shirahama, K. J. Phys. Chem. 1993, 97, 737–741. (27) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Macromolecules 1990, 23, 2415– 2416. (28) Rao, J. Y.; Xu, J.; Luo, S. Z.; Liu, S. Y. Langmuir 2007, 23, 11857–11865. (29) Schild, H. G.; Muthukumar, M.; Tirrel, D. A. Macromolecules 1991, 24, 948–952. (30) Zhang, G. Z; Wu, C. J. Am. Chem. Soc. 2001, 123, 1376–1380. (31) Cheng, J. H.; Chen, H. H.; Chang, Y. X.; Chuang, P. Y.; Hong, P. D. J. Appl. Polym. Sci. 2008, 107, 2732–2742. (32) Tanaka, F.; Koga, T.; Winnik, F. M. Phys. Rev. Lett. 2008, 101, 28302. (33) Sun, P. C.; Li, B. H.; Wang, Y. N.; Ma, J. B.; Ding, D. T.; He, B. L. Eur. Polym. J. 2003, 39, 1045–1050. (34) Sierra-Martin, B.; Romero-Cano, M. S.; Cosgrove, T.; Vincent, B.; Fernandez-Barbero Colloids Surf., A 2005, 270, 296–300. (35) Diez-Pena, E.; Quijada-Garrido, I.; Barrales-Rienda, J. M.; Wilhelm, M.; Spiess, H. W. Macromol. Chem. Phys. 2002, 203, 491–502.
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to equilibrate in (A) D2O (PNIPAM-water), (B) D2O + methanol (water/methanol 95:5 and 97.5:2.5 mol/mol) (PNIPAM-water/methanol), and (C) D2O + ethanol (water/ethanol 95:5 and 97.5:2.5 mol/mol) (PNIPAM-water/ethanol). NMR Measurements. The 1H MAS NMR spectra and diffusion experiments were performed on a Varian UNITYINOVA 600 MHz spectrometer operating at a proton frequency of 599.186 MHz. Proton signals were observed through the NANO probe with a relatively low spinning rate of 2 kHz to reduce the phase-separation effects of the centrifugal force caused by sample spinning.36 The π/2 pulse length was typically 5.4 μs. The diffusion experiments were made using the bipolar gradient longitudinal eddy-current delay (LED-BPP) sequence.37 The diffusion time is 80 ms, the gradient pulse is 2 ms, and the gradient value is 16 G/cm. The spin-lattice relaxation time (T1) for 1H nuclei were measured by means of the inversion recovery method with a [π-τ-π/2-Acq (FID)] pulse sequence. The relaxation delay time is 60 s, which is longer than 5T1 of protons. A variable-temperature NMR experiment was carried in the temperature range from 17 to 37 °C with an accuracy of (0.1 °C. The sample was sealed in the rotor, and the HOD signal intensity was monitored before and after experiments to ensure no obvious change in water content.
Results and Discussion Figure 1a,b shows two typical 1H NMR spectra of the PNIPAM gel in D2O acquired at 21 and 37 °C, respectively, which represent a low-temperature equilibrium swollen state and a high-temperature collapse state, respectively. The spectral region of the HOD resonance around 4.7 ppm is magnified in the insets of the corresponding Figures. In the low-temperature equilibrium swollen state as shown in Figure 1a, the HOD signal appears as a strong single peak at 4.79 ppm, and the proton signals of PNIPAM are also distinguishably observable. (The assignment of peaks is given in Figure 1.) Our variable-temperature NMR experiment shows that a coil-to-globule transition occurs at around 33 °C on heating, which is characterized by a sudden decrease in the intensity of the 1H resonance of PNIPAM. This transition temperature is in good agreement with the LCST reported in the literature.27 When the temperature is increased to 37 °C, which is above the LCST, the proton resonances of polymer PNIPAM are broadened almost beyond detection, and only HOD signals can be seen (Figure 1b). From the inset in Figure 1b, it is obvious that two HOD signals appear at 37 °C, a narrow one at 4.65 ppm and a broad one at 4.58 ppm. This implies that there are two different types of water in the collapsed-state PNIPAM at high temperature. A similar result has already been reported.35 As explained before, when the gel network shrinks on heating, some water molecules are expelled from the polymer network to form free water (FW), and the rest are confined in the polymer network as confined water (CW). Here, a sharp resonance at higher resonance frequency is attributed to free water (FW), and the low-frequency broad resonance is assigned to confined water (CW). Figure 2a,b shows two 1H NMR spectra of the PNIPAM gel in a binary water/methanol mixture (95:5) acquired at 21 and 37 °C. The spectral regions containing water and methanol-CH3 resonances are separately magnified in the corresponding insets. As shown in Figure 2a, when the polymer network is in a lowtemperature equilibrium swollen state in water/methanol binary solvents, a single sharp HOD peak at 4.82 ppm, a single sharp proton peak of methanol-CH3 at 3.36 ppm, and four proton (36) Jeong, S. Y.; Han, O. H. Bull. Korean Chem. Soc. 2007, 28, 662–666. (37) Wu, D. H.; Chen, A. D.; Johnson, C. S. J. Magn. Reson., Ser A 1995, 115, 260–264.
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Figure 1. H MAS NMR spectra of the PNIPAM gel (a) in an equilibrium swollen state in water at 21 °C and (b) a collapse state at 37 °C. (1) Methyl proton of the N-isopropyl group, -CH3; (2) methylene proton, CH2d;(3) methyne proton, CHd; (4) lone proton of the N-isopropyl group, -CH
