Microscopic insights into the phase transition of poly(N-isopropylacrylamide) in aqueous media: Effects of molecular weight and polymer concentration

Abstract

We study a temperature (T)-induced phase transition of semi-dilute poly(N-isopropylacrylamide) (pNIPAm) in H2O and D2O, focusing on effects of molecular weight and polymer concentration. We use simultaneous small- and wide-angle x-ray scattering (SWAXS) and dielectric relaxation spectroscopy (DRS). The hydration number Zhyd of pNIPAm is determined by DRS. We observe a gradual decrease of Zhyd with increasing T below TS − 6 K, where TS is the spinodal temperature, and its subsequent steep decrease on approaching TS. This eccentric two-mode dehydration behavior implies strong cooperative nature of hydration/dehydration of the pNIPAm chains. Furthermore, the observed dehydration is coupled to the critical fluctuations manifested by a power-law divergence of the correlation length ξ. The emergence of a low-q interference peak provides evidence that high-density microglobules are formed even far below Ts, where a swollen coil state of polymer chains is generally considered to be predominant. The local number density of the microglobules is almost unchanged below Ts, but then exhibits a step-like increase just above Ts, where most pNIPAm chains are already collapsed. This observation suggests that in addition to the ordinary (macroscopic) density order parameter, the local number density of the microglobules should serve as an additional order parameter monitoring the phase transition of aqueous pNIPAm. The correlation length significantly decreases at higher polymer concentration because of the stronger interpenetration effect between different chains. At higher concentration, ξ converges to similar values independent of molecular weight, which indicates an already congested state of the pNIPAm chains. Lowering averaged molecular weight leads to a stronger tendency of inter-chain aggregation and interface formation especially at high T.