Physical and mechanical properties of polyethylene for pipes in relation to molecular architecture.: II.: Short-term creep of isotropic and drawn materials

Tensile drawing and short-term creep of ethylene/alpha-olefin copolymers having bimodal (BM) molar weight distribution are studied in comparison with unimodal (UM) copolymers of similar crystallinity. The natural draw ratio and viscoelastic recovery upon unloading strongly suggest that BM copolymers have more tie chains and chain entanglements than corresponding UTV copolymers. The incorporation of co-units in the longest chains of BM copolymers is ascribed a major role on these topological changes. Creep of isotropic materials shows lower compliance for BM copolymers in parallel with higher-durability grades. This is attributed to a better "macromolecular network efficiency." The creep behavior of strain-hardened samples, which is assumed to simulate the mechanical behavior of craze fibrils at the tip of a propagating crack, reveals similar trends. The better fibril strength in BM copolymers is again ascribed to a better network efficiency. Necked samples display an odd behavior of higher compliance at low stress and lower compliance at high stress for BM copolymers compared with the behavior of UM counterparts. This is associated with the exhaustion of viscoelastic capabilities with increasing draw ratio and stress. The phenomenon is discussed in relation to cavitation. A short-term creep test is proposed for comparative prediction of long-term behavior. (C) 2002 Wiley Periodicals, Inc.