AbstractCrosslinked networks of butyl rubber polyisobutylene (PIB) with 2 mol unsaturation were swelled with various amounts of styrene containing 0.25–4.6 wt benzoyl peroxide as initiator. Polymerization of the styrenein situ, by increase in temperature, gave novel elastomeric composites. Scanning electron micrographs suggest phase separation which is locked in at the microscopic level, with no evidence for the particulate domains seen in the similarly prepared polystyrene‐polydimethylsiloxane system. The wt polystyrene (PS) extractible increased with increase in initiator concentration, which is consistent with decrease in the lengths of the PS chains being formed and grafted onto the PIB. Mechanical properties obtained in continuous extension showed maxima in their dependence on composition. Specifically, the ultimate strength (f/A*)m, maximum extensibility εr, energyErrequired for rupture, and permanent set ΔL/L0generally showed maxima in the vicinity of 50 wt PS. The shortest‐chain PS samples generally showed the largest values of (f/A*)m, εr, andEr, and the smallest values
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