Investigation of the T u (>T g ) relaxation in homopolymers and block copolymers of styrene by torsional braid analysis

Investigation of the T u (>T g ) relaxation in amorphous polymers of styrene by the technique of torsional braid analysis is reviewed. For the most part the relaxation behaves like the glass transition ( T g ) in its dependence on molecular weight, on average molecular weight in binary polystyrene blends, and on composition in a polystyrene homogeneously plasticized throughout the range of composition. Diblock and triblock copolymers also display a T > T g relaxation above the T g , of the polystyrene phase. Two results in particular suggest that the T u relaxation is molecularly based. (1) The T u temperature is determined by the number average molecular weight for binary blends of polystyrene when both components have molecular weights below M c . (the critical molecular weight for chain entanglements). (2) Homopolymers, and diblock and triblock copolymers of styrene, have a T > T g relaxation at approximately the same temperature when the molecular weight of the styrene block is equal to that of the homopolymer.