AbstractA study of the influence of processing conditions and structure on engineering mechanical properties was conducted in bulk isotactic polypropylene. The influence of one processing parameter, undercooling, defined so as to account for both pressure and temperature effects, was particularly studied.Improved mechanical properties were found with increased undercooling. At low undercoolings, brittle failure without yield occurred, presumably the result of a sparsity of intercrystalline links under these conditions. As undercooling was increased, failure occurred after yielding as failure stress elevated dramatically, apparently because of greater link density. A modest improvement in yield stress with increased undercooling was attributed to the increasingly crosshatched lamellar structure produced at higher undercoolings, a structural trend confirmed by electron microscopy.Spherulite size, varied by altering melt history (melt temperature and time at melt temperature) at constant undercooling, was found to have no effect on engineering yield stress. This result indicates that apparent yield stress–spherulite size effects found by several earlier investigators were probably caused by structural variations other than spherulite siz