Phase Transition in Poly(N-isopropylacrylamide) Hydrogels Induced by Phenols

Abstract

Poly(N-isopropylacrylamide) gels swollen at 20 °C 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 ∝ φ1/3, where φ 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 Dc decreases at high aromatic content, while the Rayleigh ratio of the dynamic component of the scattering intensity, Rdyn, increases. No divergence of Rdyn close to the transition is observed, however, in accord with first-order nature of the transition....