POLYSTYRENE POLY(2-VINYL PYRIDINE) HETEROARM STAR COPOLYMER MICELLES IN TOLUENE - MORPHOLOGY AND THERMODYNAMICS

Polystyrene/poly(2-vinyl pyridine) heteroarm star copolymer (i.e. star -shaped polymers consisting of a central poly(divinyl benzene) core be aring an equal number of polystyrene and poly(2-vinyl pyridine) arms, PS(6)P2VP(6)) and the corresponding diblock copolymer were investigate d in toluene, which is a selective solvent for PS. The star copolymers are associated in polymolecular micelles following the closed associa tion model. The characteristics of micelles were determined by means o f static light scattering, viscometry and cryo-transmission electron m icroscopy. The micelles adopt a core-shell structure of spherical shap e with core radius almost equal to the corona thickness. Comparing the ir solution behaviour with that of the corresponding diblock copolymer , having the same block lengths as those of the star arms, significant differences were observed. The A(n)B(n) exhibit cmc which is three or ders of magnitude higher and an aggregation number which is about one order of magnitude lower with respect to the AB copolymer. The thermod ynamics of micellization were also studied. Delta G(0), Delta H-0 and Delta S-0 were found to be negative for both systems. Delta G(0) is le ss negative for the star copolymer implying a smaller driving force fo r micellization. This is due to a pronounced loss of the combinatorial entropy of the star polymer arms. (C) 1998 Elsevier Science Ltd. All nights reserved.