Synthesis and Study of the Properties of Stereocontrolled Poly(N

Publication Date (Web): April 4, 2012 ... The isotacticity (meso dyad (m), %) and cloud-point temperature of these homopolymers were gradually increas...
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Synthesis and Study of the Properties of Stereocontrolled Poly(Nisopropylacrylamide) Gel and Its Linear Homopolymer Prepared in the Presence of a Y(OTf)3 Lewis Acid: Effect of the Composition of Methanol−Water Mixtures as Synthesis Media Chandra Sekhar Biswas, Niraj Kumar Vishwakarma, Vijay Kumar Patel, Avnish Kumar Mishra, Satyen Saha, and Biswajit Ray* Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221005, India S Supporting Information *

ABSTRACT: Poly(N-isopropylacrylamide) (PNIPAM) hydrogels and the corresponding linear homopolymers were synthesized in different methanol−water mixtures (xm = 0, 0.13, 0.21, 0.31, 0.43, 0.57, and 0.76, where xm is the mole fraction of methanol) in the presence of 0.1 M Y(OTf)3 Lewis acid. The isotacticity (meso dyad (m), %) and cloud-point temperature of these homopolymers were gradually increased and decreased, respectively, with the increase in the xm values of the synthesis solvent mixtures. Moreover, the corresponding linear PNIPAM homopolymers prepared in the absence of Y(OTf)3 showed an almost constant isotacticity of m = 45% and a cloud-point temperature of 33.0 °C. A SEM study revealed that the resulting hydrogels were highly porous except for the gels prepared at xm = 0 and 0.76. The swelling ratios of these hydrogels in water at different temperatures and in different methanol−water mixtures at 20 °C and the deswelling rate and the reswelling rate of these hydrogels were studied. All of these swelling results were compared with that of the corresponding gels prepared in the absence of a Lewis acid (Biswas, C. S.; Patel, V. K.; Vishwakarma, N. K.; Mishra, A. K.; Bhimireddi, R.; Rai, R.; Ray, B. J. Appl. Polym. Sci. 2012, DOI: 10.1002/app.36318) and explained on the basis of the porosity of the gel, the state of aggregation and isotacticity of the PNIPAM chain segment, and the cononsolvency of the methanol−water mixture toward the PNIPAM chain segment.

1. INTRODUCTION Poly(N-isopropylacrylamide) (PNIPAM) homopolymer and its cross-linked gel generally show a volume phase transition in water at its lower critical solution temperature (LCST) of around 33 °C.1 PNIPAM also shows a volume phase transition in the specific composition of water and its water-miscible good organic solvents such as methanol,2−4 ethanol,5 tetrahydrofuran (THF),6,7 dimethyl sulfoxide,7,8 and N,N-dimethylformamide (DMF).8,9 This is essentially due to the cononsolvency phenomenon10−14 wherein mixtures of two good solvents become a poor solvent. Recently, Okamoto et al. reported the synthesis of isotacticity-rich PNIPAM homopolymers in methanol15 or a methanol−toluene (1:1 v/v) mixture16−18 in the presence of rare earth Lewis acids such as Y(OTf)3, Yb(OTf)3, and Sc(OTf)3. They showed that the isotacticity of the resultant polymers increased with the increase in the concentration of the Lewis acid loading, and the solubility of such PNIPAMs in water decreased with the increase in the isotacticity of the polymers. They also showed that the volume phase transition temperature of the resultant polymer decreases with increases in its isotacticity.18 Later, Hietala et al. reported in detail the thermal association properties of A-B-A-type stereoblock PNIPAM copolymers in water.19,20 Very recently, © 2012 American Chemical Society

Nakano et al. reported the thermoreversible gelation of isotactic (m = 64%) PNIPAM in water.21 Recently, we reported the synthesis and study of the swelling properties and morphology of the isotactic-rich PNIPAM gels prepared in the presence of different concentrations of Lewis acid Y(OTf)3 in a 1:1 v/v methanol−water mixture.22 We also showed there that the variation in the isotacticity and volume phase transition (cloudpoint) temperature of the corresponding PNIPAM homopolymers with the Lewis acid loading followed the same trend as reported earlier (Figure S1, Supporting Information).18 Very recently, we have also reported the synthesis and study of the swelling properties and morphology of PNIPAM hydrogels prepared in different ethanol−water mixtures23 and methanol− water mixtures.24 Here, we report the synthesis of a series of stereocontrolled PNIPAM gels in different compositions of methanol−water mixtures in the presence of 0.1 M Y(OTf)3 and the study of their morphology, swelling ratios in water at different temperatures and in different methanol−water mixtures at 20 °C, deswelling kinetics in water when swiftly Received: January 26, 2012 Revised: April 4, 2012 Published: April 4, 2012 7014

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Table 1. Synthesis of Poly(N-isopropylacrylamide) Gels in the Presence of a 0.1 M Y(OTf)3 Lewis Acid in Different Compositions of Methanol−Water Mixturesa run ID X0 MeOH (mL) water (mL) solution of TEMED (107 mmol/dm3) in water (mL) solution of TEMED (107 mmol/dm3) in methanol (mL) conversion (%)b appearence swelling ratio (Ws/Wd) at 20 °Cc swelling ratio (Ws/Wd) at 40 °Cc

X0.13

X0.21

X0.31

X0.43

1.25 0.5

0.5 0.75 0.5

0.75 0.5 0.5

1.0 0.25 0.5

1.25 0.5

89 transparent 11.9 2.0

93 opaque 18.3 1.7

91 opaque 25.7 1.7

89 opaque 15.5 1.5

86 opaque 26.5 1.5

X0.57

X0.76

1.0 0.25

1.25

0.5 92 transparent 17.0 1.4

0.5 96 transparent 5.4 1.7

a NIPAM = 160 mg; BIS = 8 mg; Y(OTf)3 = 108 mg; APS = 0.25 mL of an aqueous solution with a concentration of 84 mmol/dm. Polymerization temperature = 5 °C, polymerization time = 12 h. bDetermined gravimmetrically after drying under vacuum at 50 °C for 72 h after dialysis. cWs = weight of the swelled gel at a specified temperature after 24 h of swelling, Wd = weight of the dry gel.

Table 2. Synthesis of Poly(N-isopropylacrylamide) Homopolymer in the Presence of a 0.1 M Y(OTf)3 Lewis Acid in Different Compositions of Methanol−Water Mixturesa run ID MeOH (mL) water (mL) solution of TEMED (107 mmol/dm3) in water (mL) solution of TEMED (107 mmol/dm3) in methanol (mL) conversion (%)b appearence tacticityc cloud pointd (°C)

X0′

X0.13′

X0.21′

X0.31′

X0.43′

X0.57′

X0.76′

0.75 0.5 0.5

1.0 0.25 0.5

1.25

1.0 0.25

1.25

1.25 0.5

0.5 0.75 0.5

90 transparent 45 33.3

94 opaque 50 31.5

92 opaque 53 30.7

95 opaque 55 30.3

97 opaque 62 28

0.5 91 transparent 71

0.5 97 transparent 81

e

e

0.5

NIP AM = 160 mg, Y(OTf)3 = 108 mg, APS = 0.25 mL solution in water of 84 mmol/dm concentration, polymerization temperature = 5 °C, polymerization time = 12 h. bDetermined gravimmetrically after drying under vacuum at 50 °C for 72 h after dialysis. cDetermined by 1H NMR in DMSO-d6 at 130 °C. dDetermined by measuring the transmittance of 500 nm light through a 1% (w/v) aqueous polymer solution with a 0.5 °C/ min heating and cooling rate. eInsoluble in water.

a

3

changing the temperature from 20 to 40 °C, and reswelling kinetics in water at 20 °C. To understand the effect of the solvent composition in the presence 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 and presence of 0.1 M Y(OTf)3 and determined their tacticities and cloud-point temperatures. We have also compared these results with the reported results obtained from the similar gel systems prepared in the absence of the Lewis acid.24

with a rubber septum. The rest of the procedure is the same as reported earlier.22 2.3. Synthesis of Linear PNIPAM Homopolymers. A series of linear PNIPAM homopolymers (runs X0′−X0.76′, Table 2) were synthesized in the absence of the BIS cross-linker by keeping other experimental conditions the same as mentioned in the synthesis of PNIPAM hydrogels (runs X0−X0.76, Table 1). The polymerization and purification procedures are the same as reported earlier.22 Another series of linear PNIPAM homopolymers (runs X0′′−X0.76′′, Table S1, Supporting Information) were synthesized in the absence of the BIS cross-linker and Lewis acid Y(OTf)3 by keeping other experimental conditions the same as mentioned in the synthesis of PNIPAM hydrogels (runs X0−X0.76, Table 1). 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 were reported in parts per million (δ) from residual solvent peaks. The diad tacticities of polymer samples were calculated from the methylene proton peaks of the polymers as reported earlier in the literature.22 Cloud-point temperature of the polymer was determined using a Cary 100Bio UV−vis spectrophotometer (Varian) equipped with a Peltier series II thermostatic cell holder by the method reported earlier.22 2.4. Surface Morphology. Equilibrium-swollen hydrogels in deionized water at 20 °C for 24 h were freeze-dried under vacuum to remove water completely. FEI-SEM Quanta 200F (Philips) at an acceleration voltage of 5 kV was used to study the surface morphology of these freeze-dried samples. 2.5. Swelling Ratios in Water at Different Temperatures. The preweighed dried gels were immersed in deionized water for 24 h at the desired temperature (20, 22.5, 27.5, 30, 32.5, 35, 38, and 40 °C). Then, these equilibrium-swollen gels were taken out, the surface water was soaked with moistened filter paper, and their weights were taken.

2. EXPERIMENTAL SECTION 2.1. Materials. N-Isopropylacrylamide (NIPAM, Aldrich, St. Louis, MO) was purified by recrystallization from n-hexane. N, N′Methylenebisacrylamide (BIS, Aldrich, St. Louis, MO), ammonium persulfate (APS, Loba Chemie, Mumbai, India), N,N,N′,N′-tetramethylethylenediamine (TEMED, Aldrich, St. Louis, MO), and yttrium trifluoromethanesulfonate [Y(OTf)3, Aldrich, St. Louis, MO] were used as received. Methanol (Loba Chemie, Mumbai, India) was dried and distilled over anhydrous calcium oxide. Deionized water was prepared by the redistillation of doubly distilled water in an all-glass distillation apparatus. 2.2. Synthesis of PNIPAM Hydrogels. Three stock solutions were prepared: (i) a solution of TEMED in water having a concentration of 107 mmol/dm3; (ii) a solution of TEMED in methanol having a concentration of 107 mmol/dm3; and (iii) a solution of APS in water having a concentration of 84 mmol/dm3. At first, the required amounts (as specified in Table 1) of NIPAM, Y(OTf)3, BIS, and TEMED solutions and solvents were taken in a small borosilicate glass tube (i.d. = 6 mm, length = 100 mm) fitted 7015

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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). 2.6. Swelling Ratios in Different Methanol−Water Mixtures at 20 °C. The swelling ratios of the different gels in the methanol− water mixtures containing 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 mol fractions of methanol (xm) at 20 °C were measured gravimetrically using the same method as described above. 2.7. Deswelling Kinetics in Water at 40 °C. The preweighed equilibrium-swollen gels in water at 20 °C for 24 h were immersed quickly in water at 40 °C. At definite time intervals, the gel was taken out, the surface water was soaked with moistened filter paper and its weight was taken, and then the gel was quickly re-immersed 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) with respect to that (Ws − Wd) by the equilibrium-swollen hydrogel (Ws) at 20 °C. 2.8. Reswelling Kinetics in Water at 20 °C. The preweighed equilibrium-swollen gels at 40 °C for 24 h were immersed quickly in the water at 20 °C. At definite time intervals, the gel was taken out, the surface water was soaked with moistened filter paper and its weight was taken, and then the gel was quickly re-immersed in the water at 20 °C. The water uptake (%) was calculated as the weight percentage of water absorbed by the swollen hydrogel at any definite time interval t (Wt − Wd) with respect to that by the equilibrium-swollen hydrogel (Ws − Wd) at 20 °C.

(Supporting Information), and the corresponding plot of the observed isotacticities (meso dyad (m), %) of the resulting polymers against the corresponding xm values is shown in Figure 1.

Figure 1. Plot of the mole fraction of methanol (xm) of the synthesis solvent vs the tacticity (meso %) and cloud point (°C) of the homopolymer and the relative change in the swelling ratio in water at 20 °C (%) of the gel prepared in the presence of the Lewis acid with respect to that of the same prepared in the absence of the Lewis acid.

3. RESULTS AND DISCUSSION 3.1. Synthesis of PNIPAM Hydrogels in Different Methanol−Water Mixtures in the Presence of a 0.1 M Y(OTf)3 Lewis Acid. The synthesis conditions and the characterization of PNIPAM hydrogels are summarized in Table 1. In runs X0−X0.76, the methanol−water mixtures with xm values of 0, 0.13, 0.21, 0.31, 0.43, 0.57, and 0.76 were used as synthesis media in the presence of a 0.1 M Y(OTf)3 Lewis acid. Yields of the PNIPAM gels were within 86−96%. The appearances of the as-prepared hydrogels changed from transparent (run X0) to opaque (runs X0.13, X0.21, X0.31, and X0.43) and then changed back to transparent (runs X0.57 and X0.76). The observed transparency of the hydrogels prepared at xm = 0 (run X0), 0.57 (run X0.57), and 0.76 (run X0.76) was due to the highly solvated coiled conformation of the PNIPAM chain segment in these gels owing to the stronger interaction of water or such methanol−water mixtures with the corresponding PNIPAM chain segment. However, the observed opacity of the gels prepared at xm = 0.13 (run X0.13), 0.21 (run X0.21), 0.31 (run X0.31), and 0.43 (run X0.43) was indicative of the formation of the less-solvated aggregated globular PNIPAM chain segment due to the cononsolvency of the corresponding methanol−water mixtures toward the PNIPAM chain segment in the gels. To obtain an idea of the effect of the methanol−water composition in the presence of the 0.1 M Y(OTf)3 Lewis acid on the tacticity of the PNIPAM chain segment in the synthesized gels, we have prepared a series of corresponding linear PNIPAM homopolymers (runs X0′−X0.76′, Table 2) in the absence of the BIS cross-linker while keeping other experimental conditions the same as mentioned for runs X0− X0.76 in Table 1. The appearance of the as-prepared homopolymerization mixtures (runs X0′−X0.76′) was similar to that of the corresponding hydrogels (runs X0−X0.76, respectively), as discussed above. Polymer yields were within 90− 97%. 1H NMR spectra of PNIPAM homopolymers prepared in runs X0′−X0.76′ (Table 2) are shown in Figures S2 and S3

The isotacticity (m) of the resulting polymers increased gradually from 45 to 81% with the gradual increase in the xm values from 0 to 0.76, respectively. Initially, it slowly increased from 45 to 55% with the increase in the xm values from 0 to 0.31 (runs X 0 −X 0.31 ), respectively. Then, it increased significantly from 55 to 81% with further increases in the xm value from 0.31 to 0.76 (runs X0.31−X0.76), respectively. It is to be noted here that the isotacticity of the PNIPAM homopolymer prepared in water in the absence of the Lewis acid is 43%.22 These results clearly indicate the roles of both the Lewis acid and the xm values of the polymerization medium with respect to the tacticity of the PNIPAM chain segment in the resulting polymers. The presence of the Lewis acid is necessary to increase the isotacticity, as reported earlier.15−22 Here, for a specific concentration of Lewis acid, the higher the xm value of the synthesis solvent, the higher the isotacticity of the PNIPAM chain segment. In this regard, the observed slow increase in tacticity of PNIPAMs in close proximity to the cononsolvency zone (xm = 0.13, 0.21, and 0.31) may have been due to a small increase in the interaction of Y(OTf)3 with the amide groups of the active propagating PNIPAM chain-end radical species and the incoming NIPAM monomers owing to the cononsolvency of such methanol−water mixtures. This again clearly confirms the role of methanol in producing isotactic PNIPAM in the presence of the Y(OTf)3 Lewis acid.15−21 Therefore, the higher the xm value of the synthesis solvent, the higher the isotacticity of the PNIPAM chain segment in the formed gel. Moreover, the higher the isotacticity of the PNIPAM chain segment in the formed gel, the lower the hydrophilicity of the PNIPAM chain segment in the gel.22 The observed cloud-point temperature of these linear PNIPAM homopolymers gradually decreased from 33.3 to 28 °C with the increase in xm values of the polymerization media from 0 to 0.43, respectively, due to the gradual increase in the tacticity (Figure 1) (vide Figure S4 (Supporting Information) and Table 7016

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Figure 2. SEM images of the hydrogels synthesized in the presence of the 0.1 M Y(OTf)3 Lewis acid in methanol−water mixtures with xm values of (a) 0 (X0), (b) 0.13 (X0.13), (c) 0.21 (X0.21), (d) 0.31(X0.31), (e) 0.43 (X0.43), (f) 0.57 (X0.57), and (g) 0.76 (X0.76).

2). Similar types of results were also reported earlier.18,22 The determination of the cloud-point temperature for the linear PNIPAM homopolymers prepared with xm = 0.57 and 0.76 (runs X0.57′ and X0.76′, respectively) was not possible because of their insolubility in water owing to their very high isotacticity. To get an idea of the effect of only the methanol−water composition (xm) on the tacticity of the PNIPAM chain segment in the formed gel, we have also prepared a series of linear PNIPAM homopolymers (runs X0″−X0.76″, Table S1 (Supporting Informations)) in the absence of the BIS crosslinker and the Y(OTf)3 Lewis acid by keeping other experimental conditions the same as mentioned for runs X0− X0.76 in Table 1. The appearance of the as-prepared homopolymerization mixtures (runs X0″−X0.76″) was similar to that of the corresponding hydrogels (runs X0−X0.76, respectively) as discussed above. Polymer yields were within 81−93%. The isotacticities (m) of these resulting polymers are close to ∼45% (vide Table S1 (Supporting Information), Figure 1). Moreover, the observed cloud-point temperatures of these polymers are close to 33.0 °C (vide Figure 1, Table S1 and Figure S5 (Supporting Information)). These results clearly indicate the almost negligible effect of the xm value of the polymerization media on the isotacticity and cloud-point temperature of the formed polymers in the absence of the Lewis acid. 3.2. Surface Morphology. The SEM images (magnification ×6000) of all of the freeze-dried hydrogels are shown in Figure 2. The gels prepared at xm = 0.76 (Figure 2g, run X0.76) are macroscopically homogeneous, as evident from the absence of any apparently visible pores. This is due to the stronger interaction of PNIPAM chain in the gel with the corresponding synthesis solvent mixture. It is to be noted here that methanol or methanol-rich methanol−water mixtures are good solvents for isotactic (m)-rich PNIPAM chain segments.17,22 However, the gels prepared at other xm values of the synthesis solvent (runs X0.13−X0.57) have macroporous morphology (Figure 2b− f, respectively). This is due to (i) the increase in the polymerization rate in the presence of methanol and Lewis acid Y(OTf)3 owing to the faster decomposition of the ammonium persulfate initiator25 and (ii) the decrease in the solvency of the PNIPAM chain segment owing to its isotacticity and the cononsolvency of the synthesis medium. Moreover, the

pore sizes of the gels are maximized in the cononsolvency zone (at xm = 0.21, 0.31, and 0.43; runs X0.21, X0.31, and X0.43, respectively). A similar trend was also observed for PNIPAM gels prepared in different methanol−water mixtures in the absence of the Lewis acid.24 The gel prepared in water (xm = 0) (Figure 2a, run X0) has very small pores owing to the relatively slower polymerization rate in the absence of methanol. 3.3. Swelling Ratios in Water at Different Temperatures. The swelling ratios (Ws/Wd) in water at different temperatures within 20−40 °C for all of the hydrogels (runs X0−X0.76, Table 1) are shown in Figure 3. The swelling ratio

Figure 3. Equilibrium swelling ratios of all of the PNIPAM hydrogels (runs X0−X0.76, Table 1) in water at 20, 22.5, 27.5, 30, 32.5, 35, 38, and 40 °C.

values in water at 20 °C of all of these gels are separately shown in Figure 4. Here, the equilibrium swelling ratio of these gels in water at 20 °C varied in the following order: X0.43 ≈ X0.21 > X0.13 > X0.57 > X0.31 > X0 > X0.76 (Table 1 and Figures 3 and 4). Similar order was also observed for the gels prepared in the absence of the Lewis acid except for the gel prepared at xm = 0.76.24 The observed very low swelling ratio (5.4) for the gel prepared in run X0.76 (at xm = 0.76) was due to the formation of 7017

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to the faster rate of polymerization (cross-linking) (Figure S1, Supporting Information). Here, although the tacticity is slightly increased in the presence of the Lewis acid, the porosity factor also dominates the tacticity factor. Then, it decreases exponentially because of the exponential increase in the tacticity of the PNIPAM chain segment in the formed crosslinked gel. The swelling ratio (Ws/Wd) in water at 40 °C for all of the hydrogels was close to 2 (Table 1 and Figure 3). This was due to the complete collapse of the coiled conformation of the PNIPAM chain segment into its slightly solvated globular form at this temperature. In general, below the LCST, the swelling ratio values gradually decreased with the increase in the temperature because of the release of water due to the gradual collapse of the PNIPAM chain segment in the gel (Figure 3). Similar types of results have also been reported for PNIPAM gels in the literature for mixtures of water and different water-miscible solvent systems.6,9,26−31 The comparative results of the swelling ratio in water at different temperatures of the gels prepared in different methanol−water mixtures in the absence24 and presence of 0.1 M Lewis acid are shown in Figure 5. The swelling ratios of the gels prepared at xm = 0 and 0.13 in the presence of the Lewis acid are higher mainly because of the predominant effect of the porosity over the tacticity. Apart from this, in general, swelling ratios of the gels prepared at higher xm values in the presence of the Lewis acid are lower because of the gradual increase in the isotacticity. More or less, these values are comparable for all of the gels prepared in the cononsolvency zone (xm = 0.21, 0.31, and 0.43). This clearly indicates the predominant effect of the cononsolvency of methanol−water mixtures. Above xm = 0.43, these values deviate gradually from each other mainly because of the increase in the isotacticity of the PNIPAM chain and are maximized at xm = 0.76. 3.4. Swelling Ratios in Different Methanol−Water Mixtures at 20 °C. The changes in the swelling ratio (Ws/Wd) of all PNIPAM gels in different methanol−water mixtures (xm) at 20 °C are shown in Figure 6. The swelling ratios of all of the hydrogels in different methanol−water mixtures at 20 °C passed through a minimum in the cononsolvency zone. Such an observation was also made for PNIPAM hydrogels prepared in the absence of the Lewis acid by keeping the other conditions the same.24 Interestingly, the PNIPAM hydrogel prepared at xm = 0.76 (run X0.76) showed a very early onset of cononsolvency at xm = 0.05, and

Figure 4. Plot of the swelling ratio of the hydrogels in water at 20 °C against the mole fraction of methanol (xm) in the synthesis solvent in the presence and absence (ref 24) of Lewis acid Y(OTf)3.

the gel with an invisible pore size and a very highly isotactic (m = 81%) PNIPAM chain segment. Interestingly, the corresponding relative change (%) in the swelling ratio in water at 20 °C of these gels prepared in the presence of Y(OTf)3 with respect to that of the gel prepared in the absence of the Lewis acid24 initially increases, decreases slowly, passes through an almost flat region in the cononsolvency zone, and finally decreases steeply with the increase in the xm value of the synthesis media (Figure 1). Initial observed increments (+ve) in the relative swelling ratio at xm = 0 and 0.13 are due to the predominant effect of the porosity (vide Figure 2a,b) over the isotacticity. Then, it follows that there is a slight decrease (becomes negative (−ve)) at xm = 0.21 due to the predominant effect of isotacticity over porosity. The observed almost slow decrease in the cononsolvency zone might be due to the combined effects of the increases in isotacticity (and insolubility) of the PNIPAM chains and the porosity of the formed gels. Beyond the cononsolvency zone, it decreases sharply because of predominant effects of the increment of isotacticity. It is to be mentioned here that the relative change (%) in the swelling ratios in water at 20 °C of the cross-linked PNIPAM gels prepared in a 1:1 v/v methanol−water mixture (xm = 0.31) in the presence of different Y(OTf)3 concentrations22 initially increases because of the formation of a highly porous gel owing

Figure 5. Comparative results of the swelling ratio in water at different temperatures for the gels prepared in different methanol−water mixtures in the absence (ref 24) and presence of 0.1 M Lewis acid. 7018

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Figure 6. Equilibrium swelling ratios of all of the PNIPAM hydrogels (runs X0−X0.76, Table 1) in methanol−water mixtures with xm values of 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 at 20 °C.

Figure 8. Deswelling kinetics of the PNIPAM hydrogels synthesized in the presence of 0.1 M Y(OTf)3 Lewis acid in methanol−water mixtures with xm values of 0 (run X0), 0.13 (run X0.13), 0.21 (run X0.21), 0.31 (run X0.31), 0.43 (run X0.43), 0.57 (run X0.57), and 0.76 (run X0.76).

the minimum swelling ratio (∼2) was observed at xm = 0.1. This is due to the very poor interaction between its highly isotactic PNIPAM chain segment and such water-rich methanol−water mixtures; consequently, the solvent uptake tendency decreased. Then, the swelling ratio of this gel (run X0.76) increased gradually with the increase in the methanol content, as in other gels. However, unlike the other gels, the observed swelling ratio was much higher in pure methanol than in water. This was due to the greater solvency of the highly isotactic PNIPAM chain segment of the gel in methanol. The comparative results of the swelling ratio of the gels prepared in the absence24 and presence of 0.1 M Lewis acid are shown in Figure 7. It is clear from the figure that swelling ratios of all of the gels for both systems are comparable, except for the gels prepared at xm = 0.57 and 0.76. The observed discrepancies for xm = 0.57 and 0.76 gel systems (runs X0.57 and X0.76) are mainly due to the higher isotacticity of the PNIPAM chain segment in the formed gel. 3.5. Deswelling Kinetics in Water at 40 °C. The timedependent water retention (%) of all of the hydrogels when the temperature was increased instantly from 20 to 40 °C is shown in Figure 8. The deswelling rate of these hydrogels decreased in

the following order: X0.43 > X0.31 > X0.21 > X0.57 > X0.13 > X0.76 > X0. Here, this rate expectedly depends on the isotacticity of the PNIPAM chain segment apart from other factors such as the porosity, state of aggregation of the PNIPAM chain segment, and so forth. The greater all of these factors, the faster the deswelling of the gels. The deswelling rate was observed to be slowest with the hydrogel prepared at xm = 0 (run X0). The observed, considerably faster deswelling rate for the gel prepared at xm = 0.13 (run X0.13) was due to its higher porosity (Figure 2b), relatively more aggregated PNIPAM chains owing to the onset of cononsolvency, and more highly isotactic PNIPAM chains. This rate increased further for run X0.21 because of the further increment in these factors at xm = 0.21 (vide Figures 1 and 2c). It increased further gradually but slowly for gels prepared at xm = 0.31 (run X0.31) and 0.43 (run X0.43) presumably because of the increase in the isotacticity. However, the observed slower rate of the gel prepared at xm = 0.57 (run X0.57) may possibly be due to the formation of macroporous gel morphology (Figure 2f) containing less aggregated and more highly isotactic (m = 71%) PNIPAM chain segments. The observed further slower deswelling rate of the gel prepared at xm = 0.76 (run X0.76) was due to the formation of a macroscopically homogeneous gel (Figure 2g)

Figure 7. Comparative results of the swelling ratios in different methanol−water mixtures at 20 °C for all of the gels prepared in different methanol− water mixtures in the absence (ref 24) and presence of 0.1 M Lewis acid. 7019

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Figure 9. Comparative results of the deswelling kinetic in water at 40 °C of the gels prepared in different methanol−water mixtures in the absence (ref 24) and presence of 0.1 M Lewis acid.

PNIPAM chain segment in gel matrix, and so forth. It increases with increase in porosity and decrease in both the state of aggregation and isotacticity of the PNIPAM chain segment. Therefore, the observed fastest rate for run X0 is mainly due to its less aggregated and lowest isotacticity PNIPAM chain (Figure 1). The observed gradual significant decrease in the rate from runs X0 through X0.13 to X0.21 is due to the predominance of the increase in the state of aggregation and the isotacticity of the PNIPAM chain segment (Figure 1) over the porosity of the gel (Figure 2a−c, respectively). The observed unusual faster reswelling rate for run X0.31 may have been due to its macroporous morphology with a highly cross-linked network structure (Figure 2d). Such behavior was also observed for the PNIPAM gel prepared in the absence of the Lewis acid.24 The observed slowest reswelling rate for run X0.43 is due to the predominance of the higher isotacticity and state of aggregation of its PNIAM chain segment over the porosity factor. A slight increase in the reswelling rate for run X0.57 is presumably due to the predominance of the decrease in the state of aggregation of the PNIPAM chain owing to its higher solvency over the tacticity and porosity factors. The observed faster reswelling rate for run X0.76 is also due to the same reasons as for run X0.57. It is to be mentioned here that the observed swelling ratio of this gel is very low (vide Figure 4) because of the very high isotacticity of the PNIPAM chain segment and the macroscopically homogeneous surface morphology. The comparative reswelling rate in water at 20 °C of the gels prepared in different methanol−water mixtures in the absence24 and presence of 0.1 M Lewis acid is shown in Figure 11. It is clear from the figure that the reswelling rates are comparable for all gels and therefore are almost independent of the isotacticity of the PNIPAM chain.

containing relatively less aggregated and more highly isotactic PNIPAM chain segments (Figure 1). The comparative results of the deswelling kinetics of the gels prepared in the absence24 and presence of 0.1 M Lewis acid are shown in Figure 9. It is clear from the figure that the deswelling rates of all of the gels prepared in the presence of the Lewis acid are faster with respect to those prepared in the absence of the Lewis acid. Interestingly, this rate is very fast and comparable for the gels prepared in the cononsolvency zone (xm = 0.21− 0.43). This result indicates that this rate is almost independent of the isotacticity of the PNIPAM chain in the gels prepared in this zone. However, for the gels prepared in the precononsolvency zone (xm = 0 and 0.13), the porosity factor mainly induced such a faster rate. However, the isotacticity of the PNIPAM chain of the gels predominantly induced such a faster rate for the gels prepared in the postcononsolvency zone (xm = 0.57 and 0.76). 3.6. Reswelling Kinetics in Water at 20 °C. The reswelling rate of all of the hydrogels at 20 °C in water, after shrinking at 40 °C for 24 h, is shown in Figure 10. This rate decreased in the following order: X0 > X0.31 > X0.76 > X0.13 > X0.57 > X0.21 > X0.43. Here, this rate expectedly depends on the porosity, the state of aggregation and isotacticity of the

4. CONCLUSIONS PNIPAM hydrogels and the corresponding homopolymers were synthesized in different methanol−water mixtures (xm = 0−0.76) in the presence of 0.1 M Y(OTf)3 Lewis acid. The observed isotacticity (m, %) and the cloud-point temperature of the resulting homopolymers gradually increased and decreased, respectively, with the increase in the xm value of the synthesis solvent. The corresponding linear PNIPAM homopolymers prepared in the absence of Y(OTf)3 showed an almost constant isotacticity (m, %) (45%) and cloud-point temperature (33 °C). The SEM study revealed that the resulting hydrogels were highly porous except for the gels prepared at xm = 0 and 0.76. The swelling ratios of all of the hydrogels (prepared in the presence and absence of 0.1 M Lewis acid) in water decreased

Figure 10. Reswelling kinetics of the PNIPAM hydrogels synthesized in the presence of 0.1 M Y(OTf)3 Lewis acid in methanol−water mixtures with xm values of 0 (run X0), 0.13 (run X0.13), 0.21 (run X0.21), 0.31 (run X0.31), 0.43 (run X0.43), 0.57 (run X0.57), and 0.76 (run X0.76). 7020

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Figure 11. Comparative results of the reswelling rate in water at 20 °C of the gels prepared in different methanol−water mixtures in the absence (ref 24) and presence of 0.1 M Lewis acid. (2) Amiya, T.; Hirokawa, Y.; Hiroshe, H.; Hirose, Y.; Li, Y.; Tanaka, T. Reentrant phase transition of N-isopropylacrylamide gels in mixed solvents. J. Chem. Phys. 1987, 86, 2375−2379. (3) Asano, M.; Winnick, F. M.; Yamashita, T.; Horie, K. Fluorescence studies of dansyl-labeled poly(N-isopropylacrylamide) gels and polymers in mixed water/methanol solutions. Macromolecules 1995, 28, 5861−5866. (4) Saunders, B. R.; Crowther, H. M.; Vincent, B. Poly[(methyl methacrylate)-co-(methacrylic acid)] microgel particles: swelling control using pH, cononsolvency, and osmotic deswelling. Macromolecules 1997, 30, 482−487. (5) Zhu, P. W.; Napper, D. H. Coil-to-globule type transitions and swelling of poly(N-isopropylacrylamide) and poly(acrylamide) at latex interfaces in alcohol−water mixtures. J. Colloid Interface Sci. 1996, 177, 343−352. (6) Zhang, X.-Z.; Yang, Y.-Y.; Chung, T.-S. Effect of mixed solvents on characteristics of poly(N-isopropylacrylamide) gels. Langmuir 2002, 18, 2538−2542. (7) Mukae, K.; Sakurai, M.; Sawamura, S.; Makino, K.; Kim, S. W.; Ueda, I.; Shirahama, K. Swelling of poly(N-isopropylacrylamide) gels in water-aprotic solvent mixtures. Colloid Polym. Sci. 1994, 272, 655− 663. (8) Zhu, P. W.; Napper, D. H. Volume phase transitions of poly(Nisopropylacrylamide) latex particles in mixed water-N,N-dimethylformamide solutions. J. Chem. Phys. Lett. 1996, 256, 51−56. (9) Tokuyama, H.; Ishihara, N.; Sakohara, S. Porous poly(Nisopropylacrylamide) gels polymerized in mixed solvents of water and N,N dimethylformamide. Polym. Bull. 2008, 61, 399−405. (10) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Methanol-water as a cononsolvent system for poly(N-isopropylacrylamide). Macromolecules 1990, 23, 2415−2416. (11) Schild, H. G.; Muthukumar, M.; Tirrell, D. A. Cononsolvency in mixed aqueous solutions of poly(N-isopropylacrylamide). Macromolecules 1991, 24, 948−952. (12) Nandi, A. K.; Sen, U. K.; Bhattacharya, S. N.; Mandal, B. M. Cononsolvent systems for poly(ε-caprolactone) and poly(methyl methacrylate). Eur. Polym. J. 1983, 19, 283−286. (13) Koga, T.; Tanaka, F.; Motokawa, R.; Koizumi, S.; Winnik, F. M. Theoretical modelling of hierarchically associated structures in hydrophobically modified PNIPAM aqueous solutions on the basis of a neutron scattering study. Macromol. Symp. 2010, 291, 177−185. (14) Tanaka, F.; Koga, T.; Kojma, H.; Xue, N.; Winnik, F. M. Preferential adsorption and co-nonsolvency of thermoresponsive polymers in mixed solvents of water/methanol. Macromolecules 2011, 44, 2978−2989. (15) Ishobe, Y.; Fujioka, D.; Habaue, S.; Okamoto, Y. Efficient Lewis acid-catalyzed stereocontrolled radical polymerization of acrylamides. J. Am. Chem. Soc. 2001, 123, 7180−7181. (16) Ray, B.; Isobe, Y.; Morioka, K.; Habaue, S.; Okamoto, Y.; Kamigaito, M.; Sawamoto, M. Synthesis of isotactic poly(Nisopropylacrylamide) by RAFT polymerization in the presence of Lewis acid. Macromolecules 2003, 36, 543−545.

with the increase in temperature. The swelling ratios of all of the gels (prepared in the presence and absence of 0.1 M Lewis acid) in different methanol−water mixtures at 20 °C passed through a minimum in the cosolvency zone and are observed to be comparable except for the gels prepared at xm = 0.57 and 0.76. The deswelling rates of the hydrogels decreased in the following order: X0.43 > X0.31 > X0.21 > X0.57 > X0.13 > X0.76 > X0. In general, the deswelling rates of these gels are faster with respect to the rates of those prepared in the absence of the Lewis acid. The observed faster rates for the gels prepared in the precononsolvency (xm = 0 and 0.13), cononsolvency (xm = 0.21 to 0.43), and postcononsolvency zones (xm = 0.57 and 0.76) are predominantly due to the porosity, cononsolvency, and isotacticity of the PNIPAM chain, respectively. The reswelling rate of the hydrogels decreased in the order X0 > X0.31 > X0.76 > X0.13 > X0.57 > X0.21 > X0.43, and this rate is almost independent of the isotacticity of the PNIPAM chain.

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ASSOCIATED CONTENT

S Supporting Information *

Plot of the Lewis acid concentration versus the tacticity and cloud point of the PNIPAM homopolymers. Relative change in the swelling ratio in water at 20 °C of the PNIPAM gels. Synthesis and characterization data of linear PNIPAM homopolymers in the absence of the Y(OTf)3 Lewis acid. Results of 1H NMR and the cloud-point determination. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Council of Scientific and Industrial Research, Government of India, through grant no. 01(2099)/07/EMR-II. C.S.B. also acknowledges the University Grant Commission for financial support. N.K.V., V.K.P., and A.K.M. acknowledge CSIR, Government of India, for research fellowships.

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REFERENCES

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