Failure processes in some polymers: (A) slow crack growth in poly(vinyl chloride) and (B) breakup of poly(ethylene) nanolayers.

摘要

Failure processes in poly(vinyl chloride) and high density poly(ethylene) were studied. The effect of impact modification and molecular weight on slow crack growth in poly(vinyl chloride) (PVC) was examined in order to test a methodology for predicting long-term creep fracture from short-term tension-tension fatigue tests. In all the PVC compounds studied, the crack propagated in a stepwise manner through a crack tip craze zone. Step length was analyzed in terms of the Dugdale model for a crack tip plastic zone. The overall crack growth rate in fatigue and creep followed the conventional Paris power law with the same exponent 2.7, da/dt = AfDK2.7I and da/dt = BK2.7I , respectively. Crack growth rate was modeled as the product of a creep contribution that depended only on the maximum stress intensity factor and a fatigue contribution that depended on strain rate da/dt = BfK2.7I,max&parl0;1+C 3&d2;&parr0; , where C is a coefficient defining the strain rate sensitivity. A linear correlation allowed for extrapolation of the creep prefactor Bf from fatigue data. Decreasing molecular weight increased Bf and decreased C. Impact modification decreased Bf but had no effect on C.; The crystallization and thermal stability of high density polyethylene (HDPE) when confined to very thin layers was examined. Films with hundreds of thin HDPE layers separated by thicker amorphous PS layers were fabricated by "forced assembly" using layer-multiplying coextrusion. Characterization of the films revealed changes in the structure and properties of the HDPE layers as the thickness decreased from the microscale (>100 nm) to the nanoscale (100 nm). It is inherent to the concept of "forced assembly" that nanolayers may not be stable when they are heated into the melt state. Heating films above the melting temperature of HDPE resulted in fractionated crystallization as indicated by two crystallization exotherms in thermograms. The lower temperature exotherm at 80°C was identified with homogeneous nucleation. The droplets responsible for fractionated crystallization resulted from instability and breakup of the layers when they were taken into the melt. The number of nanodroplets formed by breakup of nanolayers was large enough that the majority did not contain an active heterogeneity and crystallization occurred primarily by homogeneous nucleation.