Phase transition in poly(N-isopropylacrylamide) hydrogels induced by phenols

Poly(N-isopropylacrylamide) gels swollen at 20 degreesC in aqueous solutions of weak aromatic acids (phenol and resorcinol) display a volume transition at aromatic acid concentrations close to 50 mM. Owing to selective solvation, phenol exhibits an excess equilibrium concentration inside the swollen gels that is 7% greater than that in the surrounding bath, while for resorcinol the excess is 20%. The elastic modulus G of the gels at small strains follows the expected behavior of standard network theory, i.e., G proportional to phi(1/3), where phi is the polymer volume fraction. Dynamic light scattering measurements, in which light from the thermodynamic fluctuations is heterodyned by that from the large-scale network heterogeneities, show that the collective diffusion coefficient D-c decreases at high aromatic content, while the Rayleigh ratio of the dynamic component of the scattering intensity, R-dyn, increases. No divergence of R-dyn close to the transition is observed, however, in accord with first-order nature of the transition. Within experimental error the product DcRdyn is independent of aromatic concentration. It follows that the friction coefficient of the polymer chains is not modified in the presence of the binary solvent and that the reduction in the value of D-c is due to the reduction in the osmotic modulus of the system.