Phase transition of aqueous poly-( N-isopropylacrylamide) solutions: a study by non-radiative energy transfer

Abstract

The thermoreversible phase separation of aqueous solutions of poly-( N-isopropylacrylamide) (PNIPAM) was investigated by fluorescence experiments that probe transfer of electronic excitation between donor and acceptor chromophores attached to the same polymer chain. Doubly labelled polymers were prepared by reaction of a copolymer of N-isopropylacrylamide and N-acryloxysuccinimide first with 4-(1-pyrenyl)-butylamine, then with 1-naphthylethylamine. Samples were prepared and characterized. They differ in total chromophore incorporation, on average 1 chromophore per 170 and 416 N-isopropylacrylamide units, and in pyrene to naphthalene molar ratio, 3.8 and 7.3, respectively. Non-radiative energy transfer between excited naphthalene and pyrene occurred in solutions of both polymers in water, but not in methanol. The energy transfer efficiency was monitored as a function of solution temperature for the more lightly labelled polymer. The value increased gradually from c. 15% at 15°C to c. 75% for solutions heated above their lower critical solution temperature (LCST). Results are interpreted in terms of a mechanism of phase separation initiated by a gradual shrinking of solvated polymer coils into a collapsed state, followed by aggregation of individual chains into larger particles.