Time-resolved fluorescence anisotropy studies of the cononsolvency of poly(N-isopropyl acrylamide) in mixtures of methanol and water

Abstract

Time-resolved anisotropy measurements (TRAMS) have provided invaluable information concerning the molecular interactions responsible for cononsolvency in the poly(N-isopropylacrylamide), PNIPAM, H₂O, and methanol ternary system. TRAMS have successfully monitored the intramolecular segmental dynamics of an acenaphthylene labelled sample, ACE–PNIPAM, revealing details about the conformation adopted by the polymer constituent as a function of both alcohol composition and temperature of the system. In pure aqueous solution, ACE–PNIPAM undergoes a conformational transition from an expanded solvent swollen structure to a compact globule at the lower critical solution temperature (LCST). With increasing alcohol content up to 55% v/v of methanol there is both a reduction in the magnitude and an onset temperature of the LCST of ACE–PNIPAM. From 57.5–65% v/v methanol, ACE–PNIPAM forms an extended solvent swollen structure: observation of an LCST at higher polymer concentrations (e.g., 0.1 wt%) is a consequence of intermolecular aggregation between expanded chains.