Physical aging, viscoelasticity and their impact on performance of polymers and composites

When a glass forming liquid, such as a polymer melt or rubber, is cooled there is some temperature at which the molecular mobility decreases enough that the material's thermodynamic state or structure cannot attain equilibrium in the time scale of the experiment (determined by the cooling rate). Below this temperature, generally referred to as the glass transition temperature T_g, glassy materials are unstable with the result that their properties continuously evolve toward a temporally distant equilibrium. Efficient design with and use of polymeric materials requires an understanding of the physics underlying the structural recovery and the impact of the changing thermodynamic state on the mechanical properties of the material. In this work we attempt to put into the interrelationships between the physics of glasses, how it influences material performance and how we model material response using nonlinear constitutive equations. The effect of aging on such engineering properties as yield strength and time to rupture or craze will also be discussed. Additionally, we will describe some of the implications of structural recovery in polymers for composite materials performance. Finally, we examine the importance of developing methods to combine computer codes with nonlinear constitutive law modeling in order to enhance material processing and performance prediction.