LLS and FTIR Studies on the Hysteresis in Association and Dissociation of Poly(N-isopropylacrylamide) Chains in Water

Abstract

Using a combination of static and dynamic laser scattering, we examined the association and dissociation of linear poly(N-isopropylacrylamide) (PNIPAM) chains in dilute aqueous solutions. There exists a hysteresis in the temperature dependence of the average hydrodynamic radius (〈Rh〉), average radius of gyration (〈Rg〉), and apparent weight-average molar mass (Mw,app) in one heating-and-cooling cycle. In the heating process, the chains first undergo intrachain contraction before interchain association to form stable aggregates at temperatures much higher than the lower critical solution temperature (LCST ∼ 32 °C) of PNIPAM in water. In the cooling process before the solution temperature approaches the LCST, Mw,app remains a constant and both 〈Rg〉 and 〈Rh〉 increase, but the ratio of 〈Rg〉/〈Rh〉 decreases. In other words, the aggregates undergo an unevenly swelling; namely, the periphery swells more than the center, and there is no chain dissociation. FTIR spectra reveal that as the temperature increases, the ...