LLS and FTIR studies on the hysteresis in association and dissociation of poly(N-isopropylacrylamide) chains in water

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 (< R-h >) average radius of gyration (< R-g >), and apparent weight-average molar mass (M-w,M-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 similar to 32 degrees C) of PNIPAM in water. In the cooling process before the solution temperature approaches the LCST, M-w,M-app remains a constant and both < R-g > and < R-h > increase, but the ratio of < R-g >/< R-h > 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 adsorption peak area related to the hydrogen bonding > C=O center dot center dot center dot H-O-H decreases, but the adsorption peak related to the hydrogen bonding > C=O center dot center dot center dot H-N < appears when the temperature is higher than the LCST, reflecting the dehydration and the formation of some additional intersegment hydrogen bonds in the collapsed state during the heating. Therefore, the chain contraction is entropy-driven, and the hysteresis can be attributed to these additional hydrogen bonds that act as the "cross-linking" points to make the chain aggregates behave like a "gel". The chain dissociation only Occurs when the temperature is much lower than the LCST, at which water becomes a very good solvent for PNIPAM.