Fluorescence investigations of the conformational behaviour of Poly( N-vinylcaprolactam)

Abstract

Fluorescence techniques, particularly time-resolved anisotropy measurements, TRAMS, have confirmed that poly( N-vinylcaprolactam), PNVCap, undergoes a conformational transition from an expanded coil to a compact globule at its lower critical solution temperature, LCST. TRAMS and cloud point measurements have revealed the onset of the LCST as 38 °C. The nature of the conformational change for PNVCap is distinct from that of another thermoresponsive polymer, poly( N-isopropylacrylamide), PNIPAM: the discontinuity in the thermal dependence of the rate of macromolecular motion occurs over a range of ca. 15 °C for PNVCap while that for PNIPAM takes place between 31 and 33 °C. Furthermore, there is a reduction in the magnitude of the conformational change of PNVCap when compared to that of PNIPAM. Pyrene solubilisation experiments provide support for a model for PNVCap above the LCST, consisting of a globular structure with a hydrophobic core which is stabilised by more hydrophilic units on the surface.