Investigation of the poly(N-isopropylacrylamide) sodium dodecyl sulfate complexation with viscosity, dialysis, and time-resolved fluorescence-quenching measurements

Interaction of poly(N-isopropylacrylamide) (PNIPAM) with sodium dodecyl sul fate (SDS) has been studied by viscometry, time-resolved fluorescence-quenc hing (TRFQ), and dialysis measurements. The reduced viscosity of PNIPAM in SDS aqueous solutions presents an abrupt increase in the low SDS concentrat ion region, lower than the surfactant critical micelle concentration, where as TRFQ measurements show the formation of an increasing number of polymer- surfactant aggregates in the same SDS concentration region. The aggregation number of these aggregates, determined by combining TRFQ with dialysis dat a, remains constant and equal to 7-8 surfactant molecules per polymer-bound surfactant aggregate. Moreover, the values of the inverse of the intramice llar quenching rate constant, expressing the effective microviscosity of th e aggregates, indicate that these microdomains, formed in the low SDS conce ntration region, are very rigid. After saturation of the PNIPAM chain with these aggregates, a further quantity of surfactant is adsorbed on the polym er, and this is reflected in an increase of their aggregation number. Howev er, the aggregation number of these polymer-surfactant aggregates remains m uch lower than that of free surfactant micelles.