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 (Na ₂SO₄) 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.