AbstractThe dependency of the mechanical properties (Young's modulus, maximum load, breaking strain, and breaking energy) of polystyrene (PS) and high‐impact polystyrene (HIPS) on the tensile deformation speeds was examined without changing the mode of deformation or the shape of the test specimen. It was found that HIPS has an excellent mechanical balance compared with PS for both low (1.7 × 10−4to 2.9 × 10−2m/sec) and high (1.3–16m/sec) speeds. This is due to the following two mechanisms ( which have different time responses) originating from the dispersed rubber particles: (1) at low speeds, the generation of large numbers of microcrazes, and (2) at high speeds, tensile‐orientation hardening of the rubber and cold‐drawing of the PS matrix resulting from the rise in temperature accompanied by the abrupt eleongation of the
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