Hierarchy in Extended Chain Polymers 1
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HIERARCHY IN EXTENDED CHAIN POLYMERS' P.
E. Klunzinger and R. K. Eby The University of Akron Department and Institute of Polymer Science Akron OH, 44325-3909, USA W. W. Adams Wright Research and Development Center Dayton OH 45433-6533, USA ABSTRACT Rigid rod polymers are of potential use in fibers for composites because of their high specific tensile strength and modulus as well as their high thermal stability. Some other common properties of systems meeting these criteria are a nonlinear elasticity (increasing Young's modulus with increasing tensile stress) and a negative coefficient of thermal expansion along the fiber axis. In composites, these two properties can combine to cause an increase of Young's modulus with increasing temperature and can lead to yielding or interfacial failure. The source of the nonlinear elasticity resides at two length scales: 1) the scale of the crystallites, (the reversible realignment of the crystallites along the fiber axis with increasing stress), and 2) at the scale of the molecules, (an inherent nonlinear elasticity of the molecules themselves) . The former effect is reviewed and computational results for the latter are presented along with computational results for the molecular origin of the negative coefficient of thermal expansion. INTRODUCTION Extended chain polymers are of interest because of their high strength and modulus in combination with a low density. Examples of these are poly (p-phenylene benzobisthiazole) (PBZT), poly (p-phenylene benzobisoxazole) (PBO), and graphite (carbon fibers). The chemistry which is used to make such rigid rod polymers often gives these systems high temperature resistance. However, the combination of these properties into a part requiring high modulus and strength over a large temperature range requires a detailed understanding of the modulus and thermal expansion. For example, one must not only understand that the modulus is high but also how the modulus changes with applied stress and temperature. A common characteristic of the above polymers is the nonlinear elasticity (NLE-a reversible change of Young's modulus with increasing stress or strain)'. Particularly, the modulus increases as tensile stress is applied and decreases as the temperature increases. Another important property of well-oriented rigid-rod polymers is the highly anisotropic nature of isobaric expansivity (this is more commonly known as the coefficient of thermal expansion which we shall abbreviate as CTE) . In fact, highly oriented fibers of rigid-rod polymers 1 Dedicated to the memory of Donald R. friend.
Ulrich,
colleague and
Mat. Res. Soc. Symp. Proc. Vol. 255. ©1992 Materials Research Society
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contract along the axis upon heating. This has been observed as a negative CTE. The combination of negative CTE and increasing modulus of the fiber due to increasing stress created by the mismatch of thermal expansion with the matrix can be used to tailor a composite's modulus-temperature profile if the interface does not fail', 3 . OBSERVATIONS OF NONLINEAR ELASTIC
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