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synopsisAs a model for the vulcanizates of block copolymers, mixtures of polyisoprene and polyethylene vulcanized both with a peroxide and with sulfur were investigated by stress birefringence. It was found that the polyethylene dispersion showed a reinforcing effect only in the peroxide vulcanizates. On the other hand, the stress birefringence was decreased with increased polyethylene content except at high polyethylene content.In the latter case, the slope of the line in the birefringence-stress plot was almost equal to that for the pure polyisoprene vulcanizate at high stress levels. However, at lower stresses significant optical creep was observed, i.e., the stresa increased without birefringence. Such an optical creep as this exists also in styrene-butadiene block copolymer. These facts are interpreted by the assumption that polyethylene dispersion, when it is linked chemically with polyisoprene matrix, acts as a reinforcing agent by forming physical crosslinks similar to the hard domains in block copolymers. Such physical crosslinks can slip during elongation, resulting in the observed optical creep. These phenomena disappear at the elevated temperature. -INTRODUCTIONIn the preceding paper' on the properties of a styrene-butadiene block copolymer, an optical creep (i.e., stress increase without increased birefringence) was observed in the early stage of elongation. This phenomenon was interpreted by the assumption that the hard polystyrene domains acted as crosslinks, although some slippage in these domains may take place on stretching. In similar vulcanized block copolymers, two kinds of crosslinks occur, i.e., chemical crosslinks with sulfur and physical ones produced by the polystyrene domains. The stress birefringence of the latter is lower than that of the former, because the deformation of block copolymer involves not only the orientation of polybutadiene matrix, but also the slippage of polystyrene blocks. At higher temperatures such physical crosslinks become ineffective and under these conditions the stress birefringence increases to a high value characteristic of pure polybutadiene vulcanizates. In this paper we wish to describe a stress birefringence experiment carried out on a blend of polyisoprene rubber and polyethylene dispersion, which is regarded as a model of block copolymer composed of a soft matrix and hard * Present address: Japan