Stress–strain behavior, hardness, and thermomechanical properties of butadiene–styrene block copolymers as a function of processing technique

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Abstract

Stress–strain properties of star-shaped butadiene–styrene block copolymers were carried out on extruded sheet and injection molded samples. A striking “plastic-like” behavior was observed during the first extension, with a marked yield point, a drawing process, and an important hysteresis. These phenomena disappeared on the second elongation, but annealing below the polystyrene glass transition temperature gave again the initial behavior. A marked processing technique dependence was observed in the stress–strain experiments. An important variation in hardness was observed before and after stretching, and annealing experiments permitted the study of hardness recovery. A logarithmic relation between recovered hardness and the annealing time at constant temperature was deduced from the experimental data. Thermomechanical analysis curves show singular transitions between 20° and 85°C, which disappear on stretching. An important discrepancy occurs in TMA curves of extruded and injected samples. A comparison is made of thermomechanical behavior of linear and star-shaped SBS block copolymers. A modified model is proposed for a reversibly deformable structure composed of polybutadiene and polystyrene linked ends at the diffuse interfacial region between the PS domains and the PB matrix.

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