Tensile yield of isotactic polypropylene in terms of a lamellar-cluster model

In this study, the structural factors controlling the yield in isotactic po lypropylene materials were theoretically investigated. To describe the yiel ding behavior of spherulitic polypropylenes, we introduced a new structural unit, lamellar clusters, which are several stacked lamellae bound by tie m olecules. It was shown that tie molecules between adjacent lamellar cluster s produce a concentrated load acting on the cluster surface, leading to the bending deformation of the lamellar dusters. The yielding behavior can be explained if one assumes that the disintegration of the lamellar clusters o ccurs when the elastic-strain energy stored by the bending deformation reac hes a critical value. By applying the fracture theory of composites to a sy stem consisting of lamellar clusters and tie molecules, we found the yield stress sigma(y) to be proportional to root 2E(Y)U(y), in which E-Y is the Y oung's modulus and U-y is the yield energy. The proportional coefficient be tween sigma(y) and root 2E(Y)U(y) depends only on the cluster size and tie- molecule density, so this proportionality is expected to be true for other spherulitic semicrystalline polymers such as polyethylenes, being independe nt of temperature and tensile rate. (C) 2000 John Wiley & Sons, Inc.