The effect of salts on the conformation and microstructure of Poly(N-isopropylacrylamide) (PNIPA) in aqueous solution

We present an experimental investigation of the conformation and microstructure of Poly(N-isopropylacrylamide) (PNIPA) in aqueous solution in the presence of salts. As a model, a strong salting-out salt (Na2SO4) and a strong salting-in salt (NaSCN) were chosen. Light scattering measurements show that Na2SO4 decreases the radius of gyration of PNIPA compared to its value in water, whereas NaSCN increases it. Moreover, the NaSCN solution was found to be a better solvent for PNIPA compared to water, whereas Na2SO4 solution is worse. Small-angle neutron scattering measurements of semidilute PNIPA solutions, at temperatures well below the phase-transition temperature, exhibit the behavior predicted by the model of dynamic concentration fluctuations characterized by a single correlation length. Excess scattering at low angles is observed in salt solutions at temperatures that are near, yet below, the phase-transition temperature. This may indicate intrachain heterogeneities on the scale of 6 - 8 nm. (C) 2004 Wiley Periodicals, Inc.