A Atomic Understanding of the Physical Properties of Extended Chain Polymers

Extended chain polymers are of interest because of their high strength and modulus in combination with a low density. Examples are poly(p-phenylene benzobisthiazole) (PBZT), gel spun polyethylene, poly(p-phenylene terephthalamide) (PPTA), and graphite fibers. Some other common properties of these materials are a nonlinear elasticity (increasing Young's modulus with increasing 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 failure at the interface. The source of the nonlinear elasticity resides at two length scales: (1) the micron scale of the crystallites, (the reversible realignment of the crystallites along the fiber axis with increasing stress), and (2) the angstrom scale of the molecules, (the inherent nonlinear elasticity of the molecules themselves). An understanding of the source of these properties and their temperature dependence will help in the selection and design of potential replacement candidates. Recently, computer software and hardware have advanced to a state in which atomistic models can be constructed to gain a better understanding and prediction of these properties. A series of models have been used to extract the salient characteristics of the systems above and to test new methods. The modulus was studied using semi-empirical quantum mechanical methods. With a normal mode analysis on semi-empirical models of graphite and PBZT, the temperature dependence of the modulus and the thermal expansion were also modeled. Molecular dynamics have been carried out in the analysis of thermal expansion. As an extension of the usual molecular dynamics analysis a 'histogram and cumulant' method was used to extract additional temperature dependence information from a simulation at a single temperature.