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Synthesis and Characterization of Stereocontrolled Poly(N-isopropylacrylamide) Hydrogel Prepared in the Presence of Y(OTf)3 Lewis Acid Chandra Sekhar Biswas, Vijay Kumar Patel, Niraj Kumar Vishwakarma, Avnish Kumar Mishra, Satyen Saha, and Biswajit Ray* Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221005, India
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Received October 30, 2009. Revised Manuscript Received February 2, 2010 Macroporous poly(N-isopropylacrylamide) (PNIPAM) hydrogels have been prepared in methanol-water (1:1, v/v) mixture in the presence of 0, 0.05, 0.1, 0.15, and 0.2 M Y(OTf)3 Lewis acid concentrations. Synthesis of the corresponding linear PNIPAM homopolymers in the absence of a cross-linker keeping all other conditions the same shows that the isotacticity (meso dyad, %) and the cloud point temperature of the resulted in polymers increases and decreases, respectively, with the increase in the concentration of the Lewis acid. SEM micrographs reveal that the resulted hydrogels are highly porous. Swelling ratios of all the hydrogels in water decrease with the increase in the temperature. Moreover, swelling ratios of all the hydrogels in different methanol-water mixtures pass through a minimum in the co-nonsolvency zone, and the co-nonsolvency zone shifts toward the lower methanol-content solvent mixture with gradual increase in the Lewis acid concentration. Deswelling rate of the hydrogel prepared in methanolwater (1:1, v/v) mixture is much faster than that of the conventional hydrogel prepared in water. Moreover, the deswelling rate slightly increases with the hydrogels prepared with the increasing concentrations of Lewis acid. But, the reswelling rate of the hydrogels follows almost the reverse order. All these results have been explained on the basis of the formation of highly porous hydrogels with higher isotactic PNIPAM chain segment owing to the faster polymerization rate in the methanol-water mixture in the presence of Lewis acid and the co-nonsolvency behavior of the methanolwater (1:1, v/v) mixture toward PNIPAM chain segment in the PNIPAM hydrogel.
Introduction Linear poly(N-isopropylacrylamide) (PNIPAM) homopolymer or its cross-linked gel undergoes volume phase transition in water at around 33 °C.1 This temperature is known as the lower critical solution temperature (LCST) of PNIPAM. It also undergoes volume phase transition due to the variation of the composition of water in water-miscible organic solvents like methanol,2-4 ethanol,5 tetrahydrofuran (THF),6,7 dimethyl sulfoxide,7,8 and N, N-dimethylformamide (DMF),8,9 which are good solvents for PNIPAM. This is mainly due to the co-nonsolvency phenomenon10-12 where the mixtures of two good solvents behave as a poor solvent for a linear polymer or its cross-linked gel. Rapid deswelling and reswelling of hydrogels are important for the use as actuators.13 The deswelling rate of PNIPAM hydrogels prepared in water is well-known for being very slow. Formation of porous structure is very useful in improving these properties. So far, *Corresponding author. E-mail:
[email protected].
(1) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379. (2) Amiya, T.; Hirokawa, Y.; Hiroshe, H.; Hirose, Y.; Li, Y.; Tanaka, T. J. Chem. Phys. 1987, 86, 2375. (3) Asano, M.; Winnick, F. M.; Yamashita, T.; Horie, K. Macromolecules 1995, 28, 5861. (4) Saunders, B. R.; Crowther, H. M.; Vincent, B. Macromolecules 1997, 30, 482. (5) Zhu, P. W.; Napper, D. H. J. Colloid Interface Sci. 1996, 177, 343. (6) Zhang, X.-Z.; Yang, Y.-Y.; Chung, T.-S. Langmuir 2002, 18, 2538. (7) Mukae, K.; Sakurai, M.; Sawamura, S.; Makino, K.; Kim, S. W.; Ueda, I.; Shirahama, K. Colloid Polym. Sci. 1994, 272, 655. (8) Zhu, P. W.; Napper, D. H. Chem. Phys Lett. 1996, 256, 51. (9) Tokuyama, H.; Ishihara, N.; Sakohara, S. Polym. Bull. 2008, 61, 399. (10) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Macromolecules 1990, 23, 2415. (11) Schild, H. G.; Muthukumar, M.; Tirrell, D. A. Macromolecules 1991, 24, 948. (12) Nandi, A. K.; Sen, U. K.; Bhattacharya, S. N.; Mandal, B. M. Eur. Polym. J. 1983, 19, 283. (13) Osada, Y.; Okuzaki, H.; Hori, H. Nature 1992, 355, 242.
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different methods were reported for the synthesis of porous PNIPAM hydrogels, e.g., by incorporation of surfactants,14-16 silica,17,18 poly(ethylene glycol),19 glucose,20 etc., during hydrogel synthesis; by synthesis above the LCST of the hydrogel;21-23 by synthesis using freeze-drying and hydration processes;24 by synthesis in different solvent/solvent mixtures like acetone,25 ethanol,25 DMF,25 1,4-dioxane,26 tert-butyl alcohol,10 water-DMF mixture,27-29 water-ethanol mixture,25 wateracetone mixture,25,30,31 water-THF mixture,6 etc. Only Erbil et al.31 reported the synthesis of a cross-linked PNIPAM gel (14) Antonietti, M.; Caruso, R. A.; G€oltner, C. G.; Weissenberger, M. C. Macromolecules 1999, 32, 1383. (15) Gemeinhart, R. A.; Chen, J.; Park, H.; Park, K. J. Biomater. Sci., Polym. Ed. 2000, 11, 1371. (16) Baek, N.; Park, K.; Park, J. H.; Bae, Y. H. J. Bioact. Compat. Polym. 2001, 16, 47. (17) Serizawa, T.; Wakita, K.; Akashi, M. Macromolecules 2002, 35, 10. (18) Serizawa, T.; Uemura, M.; Kaneko, T.; Akashi, M. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 3542. (19) Zhang, X.-Z.; Yang, Y.-Y.; Chung, T.-S.; Ma, K.-X. Langmuir 2001, 17, 6094. (20) Zhang, J.-T.; Cheng, S.-X.; Zhuo, R.-X. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 2390. (21) Kabra, B. G.; Gehrke, S. H. Polym. Commun. 1991, 32, 322. (22) Wu, X. S.; Hoffman, A. S.; Yager, P. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 2121. (23) Zhang, X.; Zhuo, R. Langmuir 2001, 17, 12. (24) Kato, N.; Takahashi, F. Bull. Chem. Soc. Jpn. 1997, 70, 1289. (25) Lee, W. F.; Yen, S. H. J. Appl. Polym. Sci. 2000, 78, 1604. (26) Tokuyama, H.; Ishihara, N.; Sakohara, S. Eur. Polym. J. 2007, 43, 4975. (27) Mukae, K.; Sakurai, M.; Sawamura, S.; Makino, K.; Kim, S. W.; Ueda, I.; Shirahama, K. J. Phys. Chem. 1993, 97, 737. (28) Asano, M.; Horie, K.; Yamashita, T. Polym. Gels Networks 1995, 3, 281. (29) Ito, K.; Ujihara, Y.; Yamashita, T.; Horie, K. J. Polym. Sci., Part B: Polym. Phys. 1998, 36, 1141. (30) Zhang, X.-Z.; Zhuo, R.; Yang, Y. Biomaterials 2002, 23, 1313. (31) Erbil, C.; Yildz, Y.; Uyanik, N. Polym. Int. 2000, 49, 795.
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Table 1. Synthesis of Poly(N-isopropylacrylamide) Gels in the Presence of Y(OTf)3 Lewis Acid in Methanol-Water (1:1, v/v) Mixturea run ID 1
2
3
4
5
6
54 108 162 216 Y(OTf)3 (mg) MeOH (mL) 1 1 1 1 1 water (mL) 1 95 72 92 88 88 95 conversion (%)b appearance transparent opaque opaque opaque opaque opaque 11.4 16.7 21 15.5 10.6 9.6 swelling ratio (Ws/Wd) at 20 °Cc c 1.8 1.8 1.6 1.5 1.5 1.4 swelling ratio (Ws/Wd) at 40 °C a NIPAM = 160 mg; BIS = 8 mg; TEMED = 0.5 mL aqueous solution of concentration 107 mmol/dm3; APS = 0.5 mL aqueous solution of concentration 42 mmol/dm3; polymerization temperature = 5 °C, polymerization time = 12 h. b Determined gravimmetrically after drying under vacuum at 50 °C for 72 h after dialysis. c Ws = weight of the swelled gel at a specified temperature after 24 h swelling; Wd = weight of the dry gel.
sample in 60% (v/v) methanol-water mixture and studied only its temperature dependence of volume swelling ratios. On the other hand, recently Okamoto et al. reported that the isotacticity of the PNIPAM polymer prepared in methanol solvent32 or methanoltoluene (1:1, v/v) mixture33-35 in the presence of rare earth Lewis acids, like Y(OTf)3, Yb(OTf)3, Sc(OTf)3, etc., increases with the increase in the concentration of the Lewis acid, and the solubility of such PNIPAM polymers in water decreases with the increase of the isotacticity of the polymers. So far, to our knowledge, there is no report of the synthesis of PNIPAM gels in the presence of rare earth Lewis acids in the water-methanol mixture and on the study of the morphology, swelling, deswelling, and reswelling properties of such PNIPAM gels. Here, we report the synthesis of a series of PNIPAM gels in the presence of different concentrations of Y(OTf)3 Lewis acid in (1:1, v/v) water-methanol mixture, their morphology, variation of their swelling ratios in water at different temperatures, variation of their swelling ratios in different methanol-water mixtures, their deswelling kinetics in water in swift changing the temperature from 20 to 40 °C, and their reswelling kinetics in water at 20 °C. In order to understand clearly the effect of the Lewis acid on the properties of the PNIPAM chain segment in the formed gel, we have also synthesized the corresponding linear PNIPAM homopolymers in the absence of cross-linker and determined their tacticities and cloud point temperatures.
Experimental Section Materials. N-Isopropylacrylamide (NIPAM, Aldrich) was purified by recrystallization from n-hexane. N,N0 -Methylenebis(acrylamide) (BIS, Aldrich), ammonium persulfate (APS, Loba Chemie), N,N,N0 ,N0 -tetramethylethylenediamine (TEMED, Aldrich), and yttrium trifluoromethanesulfonate (Y(OTf)3, Aldrich)) were used as received. Methanol (Loba Chemie) was dried and distilled over ignited calcium oxide. Deionized water was prepared by redistillation of the double distilled water in an all-glass distillation apparatus. Synthesis of Poly(NIPAM) Hydrogels. Two stock solutions were prepared: (i) solution of TEMED in water having concentration of 107 mmol/dm3; (ii) solution of APS in water having concentration of 42 mmol/dm3. At first the required amount (as specified in Table 1) of NIPAM, BIS, Y(OTf)3, and TEMED solution and the solvent were taken in a small borosilicate glass tube (6 mm i.d. 100 mm length) fitted with a rubber (32) Ishobe, Y.; Fujioka, D.; Habaue, S.; Okamoto, Y. J. Am. Chem. Soc. 2001, 123, 7180. (33) Ray, B.; Isobe, Y.; Morioka, K.; Habaue, S.; Okamoto, Y.; Kamigaito, M.; Sawamoto, M. Macromolecules 2003, 36, 543. (34) Ray, B.; Isobe, Y.; Matsumoto, K.; Habaue, S.; Okamoto, Y.; Kamigaito, M.; Sawamoto, M. Macromolecules 2004, 37, 1702. (35) Ray, B.; Okamoto, Y.; Kamigaito, M.; Sawamoto, M.; Seno, K.; Kanaoka, S.; Aoshima, S. Polym. J. 2005, 37, 234.
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Table 2. Preparation of Poly(N-isopropylacrylamide) Homopolymer in the Absence of Cross-Linker BIS, Keeping Other Experimental Conditions the Same Like Runs 1-6 of Table 1 run
Y(OTf)3 (M)
yielda (%)
tacticityb (m/r)
cloud pointc (°C)
l0 20 30 40 50 60
0.00 93.6 43/57 33.5 0.00 81.2 45/55 33.3 0.05 90.4 48/52 32.8 0.10 95.0 55/45 31.0 0.15 95.6 57/43 29.5 0.20 96.4 58/42 28.0 a Determined gravimmetrically from the hot-water-insoluble part. b Determined by 1H NMR in DMSO-d6 at 130 °C. c Determined by measuring the transmittance of the 500 nm light through 1% (w/v) aqueous polymer solution with 0.5 °C/min heating and cooling rate.
septum. Both the pregel mixture (of NIPAM, BIS, Y(OTf)3, and TEMED) and the APS stock solution in water were purged with N2 gas for 30 min. These two mixtures were dipped into an isothermal bath maintained at 5 ( 0.1 °C under N2 purge conditions for 30 min. Then, the nitrogen-purged APS stock solution was added to the pregel mixture of NIPAM, BIS, Y(OTf)3, and TEMED through a rubber septum by a degassed syringe, mixed immediately by tilting the reaction tube up and down, and allowed to react at 5 ( 0.1 °C for 12 h. The prepared gels were cut into small disk type pieces of 3 mm thickness 6 mm i.d. and dipped into deionized water for dialysis in order to remove the unreacted chemicals, and water was changed twice a day until the conductance of water used in dialysis became equal to that of freshly distilled deionized water. After the dialysis, the gels were dried under vacuum at 50 °C for 72 h. The conversion (%) was determined gravimetrically.
Synthesis of Linear Poly(NIPAM) Homopolymers.
Linear poly(PNIPAM) homopolymers (runs 10 -60 , Table 2) were synthesized in the absence of the cross-linker BIS, keeping other experimental conditions the same as mentioned in the synthesis of poly(NIPAM) hydrogels (runs 1-6, Table 1). For each run, polymerization was stopped by freezing the reaction mixture at liquid N2 temperature. The resulting polymerization mixture was dissolved in 5 mL of methanol and precipitated from 50 mL of hot water. The separated polymer was purified by repeated dissolution in methanol, precipitated twice from hot water, and dried under vacuum at 50 °C for 72 h. Finally, polymer yield was determined gravimetrically. 1 H NMR spectra of the resulting polymers were recorded at 130 °C on a JEOL AL300 FTNMR (300 MHz) in DMSO-d6 solvent and are reported in parts per million (δ) from residual solvent peak. Observed 1H NMR spectra are shown in Figure 1. The diad tacticities of polymer samples were calculated from the backbone methylene proton peaks of the polymers as discussed under the Results and Discussion section.32 Figure 2 shows the corresponding plot of the tacticity (meso dyad, %) of the resulting homopolymers against the respective concentration of Y(OTf)3 used for the synthesis. Langmuir 2010, 26(9), 6775–6782
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Figure 1. 1H NMR (300 MHz, DMSO-d6, 130 °C) spectra of the linear poly(NIPAM) homopolymers prepared in water (run 1) and in methanol-water (1:1, v/v) mixture (run 2) in the absence of Lewis acid, and the linear poly(NIPAM) homopolymers prepared in methanol-water (1:1, v/v) mixture in the presence of 0.05 M Lewis acid (run 3), 0.1 M Lewis acid (run 4), and 0.15 M Lewis acid (run 5).
The cloud point temperature was calculated as the mean of the temperatures at 50% transmission of the heating and cooling scans. Observed results are included in Table 2. Surface Morphology. Hydrogels were swollen in deionized water at 20 °C for 24 h in order to reach the equilibrium swelling condition. These equilibrium swollen gels were freeze-dried under vacuum to remove water completely. The surface morphology of the freeze-dried samples were analyzed with FEI-SEM Quanta 200F (Philips) at an accelerated voltage of 5 kV. Observed morphologies are shown in Figure 3. Swelling Ratios at Different Temperatures. Swelling ratios of the different gels at 20, 22.5, 27.5, 30, 32.5, 35, 38, and 40 °C temperatures were measured gravimetrically. The preweighed dried gels were immersed in deionized water for 24 h at the desired temperature in order to get the equilibrium swollen gels. These equilibrium swollen gels were then taken out, the surface water was soaked with moistened filter paper, and their weights were taken. The swelling ratio (Ws/Wd) was calculated as the ratio of the weight of the equilibrium swollen gel (Ws) to that of the dried gel (Wd). Results are shown in Figure 4. Figure 2. Plot of the tacticity (meso dyad) of the resulting poly(NIPAM) homopolymers against the respective concentration of Y(OTf)3 used for their synthesis (vide Table 2).
For the cloud point temperature determination, aqueous polymer solution of 1% (w/v) concentration was used. The transmission of a 500 nm light beam was monitored through a 1 cm quartz sample cell at the rate of 0.5 °C/min during the heating and cooling scans using Cary 100Bio UV-vis spectrophotometer (Varian) equipped with a Peltier series II thermostatic cell holder. Langmuir 2010, 26(9), 6775–6782
Swelling Ratios in Different Methanol-Water Mixtures at 20 °C. Swelling ratios of different gels in 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.8, and 1.0 mole fractions of methanol (xm) in the methanol-water mixtures at 20 °C were measured gravimetrically using the same method as described above. Results are shown in Figure 5. Deswelling Kinetics at 40 °C. Deswelling kinetics in water at 40 °C of the equilibrium swollen gels obtained after immersing in water at 20 °C for 24 h were measured gravimetrically. The preweighed equilibrium swollen gels at 20 °C were immersed quickly in the water at 40 °C. At the definite time intervals, the DOI: 10.1021/la9041259
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Figure 3. SEM micrographs of the conventional hydrogels prepared (a) in water (run 1), (b) in methanol-water (1:1, v/v) mixture (run 2), and hydrogels prepared in methanol-water (1:1, v/v) mixture in the presence of (c) 0.05 M Lewis acid, (run 3), (d) 0.1 M Lewis acid, (run 4), (e) 0.15 M Lewis acid, (run 5), (f) 0.2 M Lewis acid (run 6), and (g) the collapsed state of hydrogel prepared in the presence of 0.05 M Lewis acid after its deswelling study.
Figure 4. Swelling ratios of the hydrogels prepared in water (0) (run 1) and methanol-water (1:1, v/v) mixture (O) (run 2), in the presence of 0.05 M (2) (run 3), 0.1 M (1) (run 4), 0.15 M (]) (run 5), and 0.2 M (g) (run 6) concentrations of Lewis acids in methanol-water mixtures (1:1, v/v) measured in the temperature range from 20 to 40 °C. gels were taken out, and the surface water was soaked with moistened filter paper, their weights were taken, and then the gels were quickly immersed back in the water at 40 °C. Water retention (%) was calculated as the weight percentage of the water retained (Wt - Wd) by the swollen gel (Wt) at any definite time interval (t) 6778 DOI: 10.1021/la9041259
Figure 5. Equilibrium swelling ratios of all PNIPAM hydrogels in 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.8, and 1.0 mole fractions of methanol in water mixtures at 20 °C. with respect to that (Ws - Wd) by the equilibrium swollen hydrogel (Ws) at 20 °C. Results are shown in Figure 6. Reswelling Kinetics at 20 °C. Reswelling kinetics in water at 20 °C of the equilibrium swollen gels obtained after immersing in water at 40 °C for 24 h were measured gravimetrically. The preweighed equilibrium swollen gels at 40 °C were immersed quickly in the water at 20 °C. At the definite time intervals, the gels were taken out, and the surface water was soaked with moistened filter paper, their weights were taken, and then the gels Langmuir 2010, 26(9), 6775–6782
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Figure 6. Deswelling kinetics of the hydrogels prepared in water (0) (run 1) and methanol-water (1:1, v/v) mixture (b) (run 2) and hydrogels prepared in the presence of 0.05 M (g) (run 3), 0.1 M (1) (run 4), 0.15 M (]) (run 5), and 0.2 M (O) (run 6) concentrations of Lewis acids in methanol-water mixtures (1:1, v/v) measured in the deionized water at 40 °C.
Figure 7. Reswelling kinetics of the hydrogels prepared in water (0) (run 1) and methanol-water (1:1, v/v) mixture (b) (run 2) and hydrogels prepared in the presence of 0.05 M (2) (run 3), 0.1 M (r) (run 4), 0.15 M ([) (run 5), 0.2 M (tilted 4) (run 6) concentrations of Lewis acids in methanol-water mixtures (1:1, v/v) measured in the deionized water at 20 °C. were quickly immersed back in the water at 20 °C. Water uptake (%) was calculated as the weight percentage of water absorbed (Wt - Wd) by the swollen hydrogel at any definite time interval (Wt) with respect to that (Ws - Wd) by the equilibrium swollen hydrogel (Ws) at 20 °C. Results are shown in Figure 7. Swelling Kinetics of Dried Gel at 20 °C. Swelling kinetics in water at 20 °C of the dried gels were measured gravimetrically using the same method as described above for the reswelling study. Results are shown in Figure 8.
Results and Discussion Synthesis of PNIPAM Hydrogels in the Presence of Y(OTf)3 Lewis Acid. The preparation conditions and the characterization of porous PNIPAM hydrogels are summarized Langmuir 2010, 26(9), 6775–6782
Figure 8. Swelling kinetics of the dry hydrogels prepared in water (9) (run 1) and methanol-water (1:1, v/v) mixture (O) (run 2) and dry hydrogels prepared in the presence of 0.05 M (4) (run 3), 0.1 M (r) (run 4), 0.15 M ([) (run 5), and 0.2 M (tilted 4) (run 6) concentrations of Lewis acids in methanol-water mixtures (1:1, v/ v) measured in the deionized water at 20 °C.
in Table 1. In run 1, water was used as the medium. In runs 2-6, a methanol:water (1:1, v/v) mixture was used as the medium. In runs 1 and 2, no Lewis acid Y(OTf)3 was used. In runs 3-6, the concentration of the Lewis acid Y(OTf)3 was increased gradually through 0.05, 0.1, 0.15, and 0.2 M, respectively. Yields of the PNIPAM hydrogels were within 72-95%. Appearance of all the hydrogels was opaque except the one prepared in the run 1, which was transparent. The observed transparency of the hydrogel prepared in water (run 1) is due to the highly solvated coiled conformation of PNIPAM chain segment in the gel owing to the strong interaction of water with the PNIPAM chain segment. On the other hand, the observed opacity of the hydrogels prepared in runs 2-6 is due to the formation of less solvated aggregated globular PNIPAM chain segment owing to the co-nonsolvency of the 1:1 (v/v) methanol-water mixture toward PNIPAM chain segment in the gel. The swelling ratio (Ws/Wd) in water at 20 °C is larger for the gel prepared in methanol-water mixture (run 2) than that in water (run 1). This is due to (i) the formation of more porous gel (vide Figure 3a vs Figure 3b) in methanol-water mixture (1:1 (v/v)) owing to the faster polymerization rate in the presence of methanol because of the faster decomposition of the initiator ammonium persulfate36 and (ii) the occurrence of the solid (swollen) phase polymerization in the co-nonsolvency mixture of 1:1 (v/v) methanol-water mixture owing to the collapse of the PNIPAM chain segment into its globular state (runs 1 and 2). Swelling ratio increases for the gel prepared at 0.05 M concentration of Y(OTf)3 (run 3). Then, it gradually decreases with the increase in the concentration of Y(OTf)3 from 0.1 M through 0.15-0.2 M (runs 4, 5, and 6, respectively). In the runs 3-6, in the presence of Lewis acid Y(OTf)3, Lewis acid can affect the PNIPAM gel in two ways: first, by increasing the polymerization rate, which will lead to the increase in the porosity of the gel, and second, by increasing the isotacticity of the resulted PNIPAM chain of the hydrogel, which will lower its hydrophilicity. The increase in swelling ratio values at 20 °C due to the increase of the Y(OTf)3 concentration from 0 to 0.05 M (runs 2 and 3) may be due to predominance of the first factor over the second one. (36) Ray, B.; Mandal, B. J. Polym. Sci., Part A: Polym. Chem. Ed. 1999, 37, 493.
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When the concentration of Lewis acid was further increased in runs 4-6, the second factor predominates over the first one, which led to the gradual decrease in the swelling ratio at 20 °C. Thus, the swelling ratio values of all hydrogels in water at 20 °C varied in the following order: run 3 > run 2 > run 4 > run 1 > run 5 > run 6. Swelling ratios (Ws/Wd) of the hydrogels at 40 °C are almost the same (2) for all runs. This is due to the complete collapse of the coiled conformation of the PNIPAM chain segment into its slightly solvated globular form. In order to get the idea about the effect of Y(OTf)3 on the properties of the PNIPAM chain segment in the formed gel, we have prepared linear poly(PNIPAM) homopolymers (runs 10 -60 , Table 2) in the absence of the cross-linker BIS keeping other experimental conditions the same as mentioned in runs 1-6 in Table 1. Polymer yields are varied between 81 and 96%. Higher yields are obtained in the presence of the Lewis acid. Figure 1 compares the 300 MHz 1H NMR spectra of linear PNIPAM homopolymers (runs 10 - 50 ) prepared in Table 2. The degree of isotacticity of the polymer is estimated by calculating the proportion of meso (m) and racemo (r) dyads from the region of the backbone methylene protons (Ha) at ∼1.2-1.8 ppm (2H). Two methylene protons of meso dyad are not equivalent and appear as two broad peaks of equal intensity centered at ∼1.3 and ∼1.7 ppm. But the two methylene protons of the racemo dyads are equivalent and appear as a single broad peak centered at ∼1.5 ppm. The meso peak at ∼1.7 ppm is well separated. But another meso peak at ∼1.3 ppm overlaps with the racemo dyads peak at ∼1.5 ppm. So, the fraction of meso dyads is equal to the twice of its peak area divided by the peak areas of all the three peaks in the region ∼1.2-1.8 ppm. In the absence of Lewis acid, atactic polymers with m = 43% and 45% are formed in water and methanol-water (1:1 v/v) mixture, respectively. But the polymers with m = 48%, 55%, 57%, and 58% are formed in the presence of 0.05, 0.1, 0.15, and 0.2 M Y(OTf)3, respectively. Thus, isotacticity (meso dyad, m) of the resulting polymers gradually increases from 45 to 58 with the gradual increase in the loading of Y(OTf)3 from 0 to 0.2 M (vide Table 2 and Figures 1 and 2). Therefore, the higher is the concentration of Y(OTf)3 Lewis acid used for gel synthesis, the higher is the isotacticity of the poly(NIPAM) chain segment in the formed gel. With the increase in the isotacticity, the number of side-chain functional groups on the same side of the poly(NIPAM) chain segment of the gel increases. Consequently, the interactions among the side-chain functional groups (hydrogen bonding among amide groups and hydrophobic bonding among isopropyl groups) become stronger. The stronger interactions of side-chain functional groups lead to the lesser interaction of water with the side-chain amide groups. Thus, the hydrophilicity (solubility) of the PNIPAM chain segment of the gel decreases with the increase of its isotacticity. Moreover, with the increase in the temperature, the interactions among the side-chain functional groups of the isotactic PNIPAM chain segment of the gel easily become stronger than that of the atactic PNIPAM chain segment of the conventional gel at the cost of the destruction of hydrogen bondings between the side-chain amide groups and water molecules. This leads to the lowering of the cloud point temperature. Indeed, the observed cloud point temperatures of the linear PNIPAM homopolymers prepared in water and methanol-water (1:1, v/v) mixture are almost the same and close to 33.5 °C (runs 10 and 20 , Table 2); it gradually decreased with increase in the isotacticity of the linear PNIPAM homopolymers (runs 30 -60 , Table 2). Surface Morphology of the PNIPAM Hydrogels. Surface morphology of the freeze-dried samples is shown in Figure 3. Gels 6780 DOI: 10.1021/la9041259
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prepared in the methanol-water mixture are more porous than the conventional gel prepared in water (Figure 3, a vs b) as discussed in the Synthesis section. Macroporous gels were formed in the presence of Y(OTf)3 in methanol-water (1:1) mixture (Figure 3c-f) owing to the further enhancement of the rate of polymerization in the presence of Y(OTf)3 Lewis acid.34 Swelling Ratios of PNIPAM Hydrogels in Water at Different Temperatures. Swelling ratios of the all PNIPAM hydrogels observed at different temperatures are shown in the Figure 4. Below volume phase transition temperature, swelling ratios of conventional gel prepared in water (run 1) are lower than that prepared in methanol-water (1:1) mixture (run 2). Moreover, swelling ratios of the hydrogels prepared in the presence of Y(OTf)3 (runs 4-6) except the one prepared with 0.05 M Y(OTf)3 (run 3) are lower than that of the gel prepared in the absence of Y(OTf)3 in methanol-water (1:1) mixture (run 2). In general, swelling ratios of all hydrogels in water at 20 °C varied in the following order: run 3 > run 2 > run 4 > run 1 > run 5 > run 6. These observations are as per the discussion in the Synthesis section. In general, for all hydrogels, swelling ratio decreases with the increase in the temperature due to the release of water owing to the gradual collapse of the PNIPAM segment in the gel. But the rate of decrease of swelling ratio with the increase in the temperature is faster with the gel prepared using lesser amount of Y(OTf)3. This is because more water is absorbed in the gel network with highly porous structure having lesser hydrophobicity owing to the lesser isotactic PNIPAM chain segment. So, for the same temperature change, a larger amount of water is coming out from such gels. It is clear from Figure 4 that there is a trend of gradual decrease in the volume phase transition of hydrogels prepared with increasing concentration of Lewis acid. The decrease of the cloud point temperature of PNIPAM homopolymers with the increase of its isotacticity (vide Table 2) supports this observation. Thus, swelling and thermoresponsiveness properties of the gels are changed due to the use of different concentrations of Y(OTf)3. Swelling Ratios of PNIPAM Hydrogels in Different Methanol-Water Mixtures at 20 °C. The changes of the swelling ratio values of all PNIPAM gels in different mole fractions of methanol at 20 °C are shown in Figure 5. As per discussion in the Synthesis section, swelling ratio values of all hydrogels in water at 20 °C varied in the following order: run 3 > run 2 > run 4 > run 1 > run 5 > run 6. On increasing the methanol content (xm) from 0 to 0.15 mole fraction, the swelling ratio values of the hydrogels prepared in water (run 1) decrease sharply owing to the onset of the co-nonsolvency and become almost equal to the minimum value of about 2 at around xm = 0.15. With further increase of the methanol contents from 0.15 to 0.2, swelling value decreases slightly. After this point, swelling ratio values of this gel increase slightly with the increase of xm value from 0.2 to around 0.35. This indicates that the complete collapse of the PNIPAM chain segment of the gel occurs at around xm = 0.2 in the co-nonsolvency region, and then their swelling ratio values increase very slowly due to very slow redissolution of the aggregated PNIPAM chain segment with the increase of the methanol content. On further increase of the methanol content from 0.35 to 1 mole fraction, the swelling ratio values of this hydrogel gradually increase due to the redissolution of the aggregated PNIPAM chain segment outside the co-nonsolvency region. Finally, the observed swelling ratio value of this hydrogel in methanol is lower than that in water. This is due to the lower density and larger molecular volume of methanol with respect to that of water. Similar swelling ratio variation trend is also observed for the PNIPAM hydrogel prepared in 1:1 (v/v) Langmuir 2010, 26(9), 6775–6782
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methanol-water mixture (run 2), but it showed slightly higher swelling ratio values in all methanol-water compositions and consequently the faster deswelling and reswelling rates with the variation of the methanol-water compositions outside the cononsolvency zone. This is due to the macroporous character of this gel, which can accommodate relatively larger amount of solvent inside its cross-linked network. PNIPAM hydrogel prepared in 1:1 (v/v) methanol-water mixture in the presence of 0.05 M Y(OTf)3 (run 3) showed almost the similar swelling ratio variation like the hydrogel prepared in run 2. But the observed swelling ratios of this gel (run 3) are higher than that of run 2 in all methanol-water mixtures except in the xm range of 0.1-0.35. The almost comparable swelling ratio values in the xm range of 0.1-0.35 are due to the co-nonsolvency effect, and the higher swelling ratio values outside the xm range of 0.1-0.35 is due to the predominance of the larger macroporosity over its higher isotacticity property (m = 48). The minimum swelling ratio of this gel (run 3) was observed at xm = 0.2 like run 2. On further increase of the Y(OTf)3 content from 0.05 M (run 3) to 0.1 M (run 4), the resulting gel showed slightly lower swelling ratio values than that of run 3 in almost all methanol-water mixtures due to the increase in the tacticity of the PNIPAM chain segment in the gel from m = 48 to m = 55. Moreover, the minimum swelling ratio of this gel was shifted to xm = 0.15, which was lower than that observed in cases of run 2 and run 3. On further increase of the Y(OTf)3 content from 0.1 M (run 4) to 0.15 M (run 5), the resulting gel showed the lower swelling ratio values than that of run 4 in almost all methanol-water mixtures due to the further increase in the tacticity of the PNIPAM chain segment in the gel from m = 55 to m = 57. With further increase of the Y(OTf)3 content from 0.15 M (run 5) to 0.2 M (run 6), the resulting gel showed slightly lower swelling ratio values than that of run 5 in almost all methanol-water mixtures due to further increase in the tacticity of the PNIPAM chain segment in the gel from m = 57 to m = 58. An early onset of the co-nonsolvency zone was observed for both the gels prepared in runs 5 and 6. This may be due to gradual decrease in the solubility of the PNIPAM segment in the gel in the water-rich zone because of the increase of its tacticity. In addition, the minimum swelling ratios of these two gels were also observed at xm = 0.15 like run 4. Deswelling Rates of PNIPAM Hydrogels. Time-dependent water retention (%) of all the hydrogels when the temperature was increased instantly from 20 to 40 °C is shown in the Figure 6. Deswelling rate of PNIPAM hydrogel prepared in water (run 1) is very slow. It is drastically increased for the gel prepared in a water-methanol (1:1, v/v) mixture (run 2). This is due to the formation of macroporous gel in the latter system (vide Figure 3 and discussion in the Synthesis section). Moreover, the PNIPAM chain segment of the gel prepared in the co-nonsolvent methanol-water (1:1, v/v) mixture will be in relatively aggregated chain forms, while the same gel prepared in water will be in the wellexpanded form. So, the ejection of water during deswelling will obviously not be easier in the gel prepared in water rather than that prepared in methanol-water (1:1, v/v) mixture.10,11,30 Deswelling rate is increased slightly further for the gels prepared in the methanol-water mixture in the presence of Lewis acid Y(OTf)3 (runs 3-6). Moreover, there is a gradual increase in the deswelling rate for the gels prepared with the increasing concentration of Lewis acid. This may be due to the lowering in the hydrophilicity of the resulting PNIPAM gels owing to the formation of the higher isotactic PNIPAM segment in the formed hydrogel as well as due to the formation of more porous network structure, which helps water in diffusing out of the gels relatively quickly. Langmuir 2010, 26(9), 6775–6782
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Reswelling Rates of PNIPAM Hydrogels. Reswelling of all the hydrogels in water at 20 °C, after shrinking at 40 °C for 12 h, is shown in Figure 7. The fastest reswelling rate is observed for the gel sample prepared in water (run 1) followed by the gel prepared in methanol-water mixture (1:1, v/v) (run 2). This is because the PNIPAM chain segment of the gel prepared in the co-nonsolvent methanol-water (1:1, v/v) mixture is present in relatively aggregated chain forms owing to the phase separation, while the same of the gel prepared in water is present in well-expanded form. So, the penetration of water during reswelling will obviously be easier in the gel prepared in water as compared to that prepared in methanol-water (1:1, v/v) mixture. Reswelling rate gradually decreases for the gels prepared with the increased concentration of Lewis acid from 0.1 M to 0.15-0.2 M Y(OTf)3 (runs 4, 5, and 6), respectively. This may be due to the increase in the isotacticity of the PNIPAM chain segment present in the hydrogels, which eventually lowers its hydophilicity. But the hydrogel prepared in the presence of 0.05 M Y(OTf)3 (run 3) showed the reswelling rate close to that of the run 6 up to 70% water retention, and after that it remained almost constant. It may be due to the mechanical breakage of the cross-linked gel network during the deswelling process. Figure 8 shows the swelling kinetics of all the corresponding dried PNIPAM gels at 20 °C. All the hydrogels except the one prepared with 0.05 M Y(OTf)3 Lewis acid concentration show the similar swelling trend as was observed during the reswelling studies after their deswelling studies (Figure 6). The observed swelling rate is fastest with the hydrogel prepared with 0.05 M Y(OTf)3 (run 3). Moreover, the SEM (Figure 3h) of the hydrogel prepared using 0.05 M Y(OTf)3 (run 3) after its deswelling study at 40 °C shows the presence of the collapsed gel network structure. All these results support the mechanical breakage of its cross-linked gel network structure during the deswelling process.
Conclusion Macroporous poly(N-isopropylacrylamide) (PNIPAM) hydrogels are prepared in water-methanol (1:1, v/v) mixture in the presence of 0.05, 0.1, 0.15, and 0.2 M Y(OTf)3 Lewis acid concentration. Synthesis of the corresponding linear PNIPAM homopolymers in the absence of the cross-linker keeping all other conditions the same reveals that the isotacticity (meso dyad, %) and the cloud point temperature of the resulting polymers increase and decrease, respectively, with the increase in the concentration of the Lewis acid. SEM micrographs reveal that the resulting hydrogels are highly porous. Swelling ratios of all the hydrogels decrease with the increase in the temperature. Moreover, swelling ratios of all the hydrogels in different methanolwater mixtures pass through a minimum in the co-nonsolvency zone, and the co-nonsolvency zone shifted toward the lower methanol content solvent mixture with gradual increase in the Lewis acid concentration. Deswelling rate of the hydrogel prepared in methanol-water (1:1, v/v) mixture is much faster than that of the conventional hydrogel prepared in water. Moreover, the deswelling rate slightly increases for the hydrogels prepared with the increasing concentrations of Lewis acid. The reswelling rate gradually decreases with the hydrogels prepared with the increasing concentrations of the Lewis acid. All these results are explained on the basis of the formation of the highly porous hydrogels with the higher isotactic PNIPAM chain segment because of the faster polymerization rate in methanol-water mixture in the presence of the Lewis acid and of the co-nonsolvency behavior of the methanol-water (1:1, v/v) mixture toward PNIPAM chain segment in the PNIPAM hydrogel. DOI: 10.1021/la9041259
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Acknowledgment. The authors gratefully acknowledge the financial support from the Council of Scientific and Industrial Research, Government of India, through Grant 01(2099)/07/ EMR-II and Department of Science and Technology, Government of India, through Grant SR/S1/PC-25/2006. V.K.P. and N.K.V. acknowledge CSIR, Government of India, for the Junior Research Fellowships (09/013(0123)/2007-EMR-I and 09/013(0192)/
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2008-EMR-I respectively). The authors acknowledge the financial support and others extended by Banaras Hindu University. Supporting Information Available: Results of the cloud point temperature determination of the linear poly(NIPAM) homopolymers. This material is available free of charge via the Internet at http://pubs.acs.org.
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