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J. Phys. Chem. B 1999, 103, 9530-9532
Effect of Polymer-Metal Complexation on the Phase Transition of Thermoreversible Copolymer Gels Shyni Varghese,† Ashish K. Lele,*,† D. Srinivas,† and Raghunath A. Mashelkar‡ Chemical Engineering DiVision, Catalysis DiVision, National Chemical Laboratory, Pune 411008, India, and Council of Scientific and Industrial Research, Anusandhan BhaVan, 2-Rafi Marg, New Delhi 110001, India ReceiVed: July 21, 1999
We have investigated the effect of transition metal complexation on the swelling behavior in water of a new terpolymer gel made from N-tert-butylacrylamide (N-t-BAm) as the hydrophobic monomer, 2-acrylamido2-methyl-1-propanesulfonic acid (AMPS) as the hydrophilic monomer, and N,N′-methylenebisacrylamide (BisAm) as the cross-linker. At a critical balance of hydrophilicity and hydrophobicity, the terpolymer exhibited a sharp volume transition at 15 °C. A shift in the transition temperature of the terpolymer was observed after complexation with trace metal ions. Electron paramagnetic resonance spectroscopy (EPR) suggested a tetrahedral structure for the complexes. We propose that the shift in volume transition temperature of the gel is due to subtle changes in the hydrophilic-hydrophobic balance resulting from the complexation of hydrophilic -SO3H groups of the gel with trace amounts of the metal ions.
Introduction Smart hydrogels undergo a swelling collapse volume transition by a small change in external parameters such as temperature,1 pH,2 light,3 electric field,4 etc. It is well-known that nonionic hydrogels such as poly(N-isopropylacrylamides) [PNIPAm] exhibit a first-order volume transition. The macroscopic volume transition of PNIPAm has been shown to occur by a rearrangement of hydrogen bonds in the polymer-water mixture5,6 and by hydrophobic interactions.7 A balance of hydrophilic and hydrophobic interactions is crucial for the occurrence of discontinuous volume transition in thermoreversible gels.8 The hydrophilic-hydrophobic balance can be altered by subtle changes in the hydrophobic groups or by copolymerization of hydrophilic and hydrophobic monomers.9 Katakai et al.10 have experimentally shown that a pinpoint variation in the hydrophobic nature of the monomer introduces a change in the lower critical solution temperature (LCST) of the gel. Feil et al.11 have observed that the copolymerization of poly(NIPAmco-butyl methacrylate-co-X) with hydrophilic comonomers increased the LCST while copolymerization with hydrophobic comonomers decreased the LCST. In this work we report the effect of complexation of metal ions on the volume transition temperature of a terpolymer gel containing N-tert-butylacrylamide (N-t-BAm), 2-acrylamido-2methyl-1-propanesulfonic acid (AMPS), and N,N′-methylenebisacrylamide (BisAm). We show that complexation of the gel with trace amounts of metal ions can shift the transition temperature significantly. We propose that the complexation of metal ions with the sulfonic acid groups of the macromolecular chains results in a reduction in the overall hydrophilicity of the gel, which decreases the volume transition temperature. Experimental Section The gel was prepared by dissolving 1.272 g (0.01g mol) of N-t-BAm and 0.207 g (0.001g mol) of AMPS in 23 mL of * To whom correspondence should be sent. E-mail:
[email protected]. † National Chemical Laboratory. ‡ Anusandhan Bhavan.
DMSO, along with 0.462 g (0.003 g mol) of N,N-methylenebisacrylamide as the cross-linker and 0.08 g azo-bis-isobutyronitrile as the initiator with constant stirring under nitrogengas bubbling. The reaction mixture was poured into test tubes and sealed. The polymerization was carried out at 70 °C for 24 h. After polymerization, the gel rod was washed with water for 72 h to remove the unreacted reactants. During this time, fresh water was added frequently. The complexation of the gel with the metal ions Cu(II), Co(II), and Cr(III) was done by swelling dried gels in the respective aqueous metal salt solutions for 72 h followed by washing the equilibrated gel until all the metal ions except the ones that are bound to the network were leached out. Washings were done for at least 12 h at 6 °C until the UV-visible spectra of the washed solutions showed no peaks corresponding to the metal ions. About 10 such washings were required for complete leaching. The washed complexed gels were dried in an oven at 50 °C to constant weight. The dried Cr(III) complexed gels showed green color, the Co(II) complexed gels showed pink color, and the Cu(II) complexed gels showed blue color. Swelling measurements were done in doubly distilled water by immersing the dried gels for typically 72 h, by which time the gels reached equilibrium swelling. The swollen gels showed only a trace of color due to dilution with water. The complexed gels in swollen as well as collapsed states were characterized by electron paramagnetic resonance (EPR) using a Bruker EMX spectrometer at ν ) 9.7 GHz. Results and Discussion The terpolymer gel exhibited a volume phase transition from a swollen to a collapsed state at 15 °C within a narrow composition range of around 1.0:0.1 mol of N-t-BAm to AMPS. The volume transition is discontinuous, as evidenced from the observation that at the transition temperature of 15 °C, the swollen and collapsed phases coexisted in the gel. The discontinuous nature of the volume transition is lost with a slight increase in the AMPS content as shown in Figure 1. This shows that a critical balance of hydrophilicity and hydrophobicity is
10.1021/jp992510p CCC: $18.00 © 1999 American Chemical Society Published on Web 09/29/1999
Complexation of Copolymers
J. Phys. Chem. B, Vol. 103, No. 44, 1999 9531
Figure 2. EPR spectra for the Cu(II)-complexed (N-t-BAm-co-BisAmco-AMPS) gel in swollen and dried states.
Figure 1. Effect of comonomer composition on swelling behavior of (N-t-BAm-co-BisAm-co-AMPS) gels.
required for the thermoreversible LCST behavior in this gel. The addition of slightly greater amounts of the highly hydrophilic AMPS comonomer to the gel disturbs the hydrophilichydrophobic balance and qualitatively changes the swelling behavior. Details of the swelling behavior of this terpolymer gel as a function of the comonomer composition will be discussed in a separate publication. Figure 2 shows the EPR spectra for the swollen and the dried Cu(II)-complexed gels. Similar spectra were observed for the Cr(III)- and the Co(II)complexed gels. For the swollen and the dried gels, the EPR measurements were done at 5 and 25 °C, respectively. The spin Hamiltonian parameters for the collapsed state (gII ) 2.385, g⊥ ) 2.045, and AII(Cu) ) 122.6 G) correspond to a tetrahedrally distorted square planar geometry for Cu(II) as shown in the inset of Figure 2. For the Cu(II) ions in a free mobile state one could observe an isotropic resonance at giso ) (gII + 2g⊥)/3. However, the anisotropic spectrum observed for the swollen state (gII ) 2.241 and g⊥ ) 2.147) clearly suggests that the Cu(II) ions in the swollen gels are complexed with the polymer probably through SO3H groups. The lesser “g” anisotropy (∆g ) gII g⊥) for swollen gels compared to the collapsed state is consistent with the restricted mobility of the paramagnetic unit in the swollen state. The effect of the complexation of metal ions on the swelling behavior of the washed terpolymer gel is shown in Figure 3. All metal-complexed gels showed lower equilibrium swelling capacity than the uncomplexed gels. More important, the volume transition temperature of the complexed gel was reduced by 4-6 °C. The volume transition observed for the complexed gels was very sharp and could be discontinuous in nature. However, coexisting phases were not observed for these gels at a single temperature. The decrease in the swelling capacity can be attributed to the increase in the overall cross-link density of the gel introduced owing to complexation with metal ions. It is
Figure 3. Volume transition of complexed and uncomplexed (N-tBAm-co-BisAm-co-AMPS) gels.
important to note that complexation exists even in the swollen state, as indicated by the EPR spectra. We propose that for the metal-complexed gel, the hydrophilic sulfonic acid groups on the macromolecular chains are engaged in complexation with the metal ions and are not freely available to the water molecules. This results in a decrease in the effective hydrophilicity of the network and indirectly enhances the overall hydrophobicity. Hence, the volume transition temperature of the complexed gels decreases. Tanaka et al.12 have observed a similar shift in the volume transition temperature of N-isopropylacrylamide-co-acrylic acid (NIPAm-co-AAc) gel immersed in copper chloride solution. The
9532 J. Phys. Chem. B, Vol. 103, No. 44, 1999 shift in the transition temperature was attributed to an extra attraction created by the copper ions between the polymers. The NIPAm-co-AAc gel absorbed the copper ions in the collapsed state and released the copper ions in the swollen state. Our experimental data and interpretations differ considerably from the work of Tanaka et al.12 First, we have studied the swelling behavior of complexed gels in pure water, whereas Tanaka et al.12 have studied the volume transition of gels in salt solutions. Further, we have shown here that our gel complexes with the metal ions in the collapsed state as well as in the swollen state. Our data show that a subtle change in the hydrophilichydrophobic balance caused by complexation of hydrophilic -SO3H groups with trace amounts of metal ions can result in a significant shift in volume transition temperature of N-t-BAmAMPS-BisAm terpolymer gels. Finally, our study shows that trace amounts of metal ions complexed with the polymer provide yet another stimuli for inducing volume transition in gels. A gel, which can undergo volume transition on complexation and which can change its color in the presence of ppm levels of a transition metal ion, can find application as sensors for trace metal ion impurities.
Varghese et al. Acknowledgment. Ms. Shyni Varghese thanks the Council of Scientific and Industrial Research for the SRF grant and the Young Scientist Award Grant of Dr. A. K. Lele. References and Notes (1) Tanaka, T. Phys. ReV. lett. 1978, 40, 820. (2) Katchalsky, A.; Kunzle, O.; Kuhn, W. J. Polym. Sci. 1950, 5, 283. (3) Irie, M. Macromolecules 1986, 19, 2890. (4) Tanaka, T. Science 1982, 218, 467. (5) Prange, M. M.; Hooper, H. H.; Prausnitz, J. M. AIChE J. 1989, 35, 803. (6) Walker, J. S.; Vause, C. A. Sci. Am. 1987, 256, 90. (7) Otake, K.; Inomata, H.; Konno, M.; Saito, S. J. Chem. Phys. 1989, 91, 1345. (8) Lele, A. K.; Badiger, M. V.; Hirve, M. M.; Mashelkar, R. A. Chem. Eng. Sci. 1995, 50, 3535. (9) Badiger, M. V.; Lele, A. K.; Bhalerao, V. S.; Varghese, S.; Mashelkar, R. A. J. Chem. Phys. 1998, 109, 1175. (10) Katakai, K.; Yoshida, M.; Hasegawa, S.; Iijima, Y.; Yonezawa, S. Macromolecules 1996, 29, 1065. (11) Feil, H.; Bae, H.; Feijen, J.; Wan Kim, S. Macromolecules 1993, 26, 2496. (12) Tanaka, T.; Wang, C.; Pande, V.; Gosberg, A. Y.; English, A.; Masamune, S.; Gold, H.; Levy, R.; King, K. Faraday Discuss. 1996, 102, 201.
