On the Influence of Chain Morphology on the Shock Response of Three Thermoplastics
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INTRODUCTION
POLYMERS and composites (with polymers as their matrix) are of great importance in a range of engineering applications. It is thus a pressing need to understand their mechanical response. Whereas there have been great developments in applying microstructural understanding to the continuum response of metals, such advances have been less apparent in polymers and composites containing them. This is partly due to the paucity of data collected in the shock regime. Early static work at high pressure was augmented by shock studies by Carter and Marsh and Champion, but this is nevertheless a field with relatively few published studies.[1–5] It is the aim of this work to understand the effect of microstructural order upon the dynamic response of polymeric materials. These materials do not have the microstructure to couple with the traditional techniques used in metals and crystals to understand their response at the smallest scales. Polymers may be ordered into two broad groups: those that consist of chains packed in an ordered or amorphous manner (thermoplastics) and those having a three-dimensional network (thermosets or rubbers). Thermoplastics may have some cross-linking, but in the study presented here, the three materials do not. The three compared are polyethylene (PE), the simplest hydrocarbon, with hydrogen atoms substituted onto a simple carbon backbone; polyvinylchloride (PVC), which N.K. BOURNE, Professor and Distinguished Scientist, and J.C.F. MILLETT, Senior Scientist, are with AWE, Aldermaston, Reading, RG7 4PR, Berkshire, United Kingdom. Contact e-mail: neil.bourne@ mac.com This article is based on a presentation made in the symposium entitled ‘‘Dynamic Behavior of Materials,’’ which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee. Article published online December 19, 2007 266—VOLUME 39A, FEBRUARY 2008
has a chlorine atom substituted onto every other carbon; and polymethylmethacrylate (PMMA). Polyethylene is a commodity thermoplastic extensively used in consumer products. It is classified into at least nine different categories based primarily on its density, molecular weight, and branching. The current work focuses on high density polyethylene with a density of 950 kg m-3. It has a low degree of branching and thus greater intermolecular forces and tensile strength.[6] The second material is PVC, which represents halogenation of the chain with a chlorine atom substituted onto every other carbon atom. Other work has studied the effect of fluorination of the chain.[7] Polyvinylchloride is an industrially important polymer that finds widespread application in the building industry. These two polymers are semicrystalline, comprising crystalline domains exhibiting a high degree of order both along the polymer backbone and between the polymer chains. These domains are surrounded by an amorphous fill
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