Time-Temperature Superposition in Styrene/Butadiene/Styrene Block Copolymers

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Abstract
The dynamic mechanical response of a styrene/butadiene/styrene (SBS) block copolymer with segment molecular weights of 16,000/78,000/16,000 (designated as Shell 16/78/16) was determined at various temperatures between -83 and +86°C at frequencies between 0.1 and 1000 Hz. The data were shifted into a master curve according to a procedure developed for thermorheologically complex materials and utilizing an additive compliance model. Data on two other triblock copolymers, Kraton 102, and an SBS triblock copolymer exhibiting a continuous polystyrene phase (designated NBS 10/30/10), are also presented. A noteworthy feature of all three triblock copolymers is a long-drawn-out intertransition plateau in which the modulus changes very little for many decades of time. The modulus is high because of the presence of the polystyrene phase and trapped entanglements in the polybutadiene phase. The extent of the plateau, effectively the distance between the two glass transitions, depends on temperature because the transition for the glassy phase shifts much more rapidly with temperature than that for the rubbery phase. The data on Shell 16/78/16 and Kraton 102 appear to present evidence for an interlayer between the polystyrene and polybutadiene domains. In these materials the polystyrene domains act essentially as inert filler over the polybutadiene transition region and well into the intertransition plateau. NBS 10/30/10, containing about 40% of styrene, is very different from the other two which contain less styrene. It has a continuous polystyrene phase giving rise to a high intertransition modulus much more reflective of glassy behavior. Consequently interlayer effects are suppressed.