A Model for Predicting the Ultimate Strength of Styrene-Diene Thermoplastic Elastomers Based on the Failure Processes at
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A Model for Predicting the Ultimate Strength of Styrene-Diene Thermoplastic Elastomers Based on the Failure Processes at the Molecular Level Qiumei Zeng and Jeremy W. Leggoe Chemical Engineering Department, Texas Tech University, Lubbock, TX, 79409 ABSTRACT The objective of this investigation is to formulate a model to predict the theoretical strength for styrene-diene thermoplastic elastomers (TPEs) that takes into account the failure processes occurring at the molecular level. Styrenic TPEs may fail via either chain pull-out, in which the polystyrene (PS) end-blocks are pulled out of the glassy PS domains, or chain scission, in which the C-C bonds in the elastomer mid-blocks are ruptured. By relating the microscopic deformation of an individual chain to the macroscopic strain rate, the maximum force a chain can sustain is obtained. The theoretical strength of the material is then computed by determining the force sustained by the PS domains and matrix chain sections intersecting a planar unit area at the onset of failure. The model has been used to investigate the effect of PS molecular weight, PS content, and strain rate on the ultimate strength. INTRODUCTION Styrene-diene thermoplastic elastomers are a class of A-B-A tri-block copolymers with intermediate engineering properties between those of plastic and rubbers [1]. The polystyrene "A" end-blocks are incompatible with the polydiene "B" mid-blocks, causing the TPE to exhibit a phase separated-structure in which glassy PS domains are dispersed in an elastomeric polydiene matrix [2]. As the PS content increases, the glassy PS domains may adopt a spherical, cylindrical, or lamellar morphology, the size of the PS domains usually being in the range of 2030 nm [3, 4]. Incorporating TPEs in consumer goods often requires subjecting the materials to a variety of cutting and separation processes as part of the manufacturing process, prompting interest in developing models to predict the failure properties of these materials under varying manufacturing conditions. Failure in this (or indeed any) class of materials is a complex multiscale process, in which the spatial distribution of flaws plays an important role. Predicting the ultimate strength of macroscale specimens accordingly requires the development of models that account for flaws and the "theoretical" strength of the undamaged material in the bulk of the specimen. Tensile strength data reported in the literature typically is derived from testing macroscale specimens, and due to the influence of flaws must underestimate the theoretical strength. The purpose of this investigation is to propose a model to predict the theoretical strength of styrene-diene TPEs based on failure processes occurring at the molecular level. At the molecular level, styrene-diene TPEs fail via one of two "chain failure" mechanisms; chain pull-out, in which the force acting in the chain is sufficient to pull the PS end-block out of the PS domain, and chain scission, in which the chain ruptures via scission of a C-C bond along the p
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