ASSOCIATION CHARACTERISTICS OF COPOLYMER MICELLES IN A SOLVENT SELECTIVE FOR THE MIDDLE BLOCK

A combination of static and dynamic light-scattering studies provides a clear picture concerning the temperature-dependent association behav ior of a long chain triblock copolymer-(t-BS)(385)S-770(t-BS)(385)(S a nd t-BS represent styrene and tert-butylstyrene, respectively) in N,N- dimethylacetamide, a selective solvent for the middle block. The so-ca lled anomalous micellization has been consistently observed between th e unimer and micelle regions, which can be ascribed to the copolymer c omposition heterogeneity. The self-assembly process is enthalpy-driven , yielding closed association structures in the form of flower-like mi celles, but only at or below room temperatures. A much lower critical micelle temperature or a much higher critical micelle concentration as well as the reduced association number is characteristic of the tribl ock micelles as compared with the diblock copolymer micelles of the sa me composition. The entropy penalty associated with the loop formation largely reduces the micellization ability.