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LLS and FTIR Studies on the Hysteresis in Association and Dissociation of Poly(N-isopropylacrylamide) Chains in Water
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 anddissociation of linear poly(N-isopropylacrylamide) (PNIPAM) chains in dilute aqueous solutions. There exists ahysteresis in the temperature dependence of the average hydrodynamic radius (áRh?), average radius of gyration(áRg?), 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 temperaturesmuch higher than the lower critical solution temperature (LCST 32 °C) of PNIPAM in water. In the coolingprocess before the solution temperature approaches the LCST, Mw,app remains a constant and both áRg? and áRh?increase, but the ratio of áRg?/áRh? 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 thetemperature increases, the adsorption peak area related to the hydrogen bonding ?CdOaaaH-O-H decreases, butthe adsorption peak related to the hydrogen bonding ?CdOaaaH-Ná appears when the temperature is higher thanthe 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 beattributed to these additional hydrogen bonds that act as the “cross-linking” points to make the chain aggregatesbehave like a “gel”. The chain dissociation only occurs when the temperature is much lower than the LCST, atwhich water becomes a very good solvent for PNIPAM.
