Published online by Cambridge University Press: 21 February 2011
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.
Dedicated to the memory of Donald R. Ulrich, colleague and friend.