TEMPERATURE-INDUCED MICELLE FORMATION OF A DIBLOCK COPOLYMER OF STYRENE AND TERT-BUTYLSTYRENE IN N,N-DIMETHYLACETAMIDE

Static and dynamic light-scattering studies are presented on the temperature-induced associating behavior of a polystyrene-poly(tert-butylstyrene) block copolymer in a selective solvent N,N-dimethylacetamide. On cooling from 65-degrees-C down to room temperature, three temperature regions of unimer, transition, and micellar particles appeared sequentially. The molecular weight and the hydrodynamic radius of the polymeric micelles formed at 25-degrees-C were found to be 1 X 10(7) g mol-1 and 50 nm, respectively, whereas those of unimers examined at 60-degrees-C were 2 x 10(5) g mol-1 and 10 nm, respectively. In both cases, a narrow size distribution was observed. In the intermediate transition region, dynamic light-scattering measurements have clearly demonstrated a temperature- and concentration-dependent equilibrium between single polymer chains and polymer micelles. From the concentration dependence of the critical micelle temperature, the standard enthalpy of micellization was estimated to be -240 kJ mol-1. A closed association model with the enthalpy factor as the driving force is discussed. On the basis of the mass action law model of micelle formation and the time-averaged scattered intensity results or the results of bimodal relaxation rate distributions in the transition region obtained by the CONTIN analysis, we were able, for the present system, to estimate the critical micelle concentration and related standard thermodynamic functions of micellization.