Coil−Globule Transition of Poly(<i>N</i>-isopropylacrylamide):  A Study of Polymer−Surfactant Association

Abstract

Time-resolved fluorescence depolarization is applied to investigate the association of sodium n-dodecyl sulfate (SDS) micelles with poly(N-isopropylacrylamide) in aqueous solutions using an amphiphilic fluorescent probe (3-perylenedodecanoic acid) which is incorporated into the SDS micelles. First, the effect of the surfactant concentration was measured: in the presence of the polymer, above the critical aggregation concentration (CAC) of SDS, the rotational relaxation of the probe exhibits a slow and a fast component. The relaxation time of the fast component is the same as in a polymer-free solution above the CMC of SDS where, however, only one component is observed. The slower relaxation time is attributed to polymer-bound micelles which incorporate polymer segments into their core. Second, the effect of the temperature induced coil−globule transition is investigated: in the course of the transition the rotational motion slows down almost 10-fold, indicating that the probe remains firmly associated with the polymer even in its dense globular state.