(180cb) How Brittle and Ductile Failures Occur: Plasticity Vs. Elasticity?

We study deformation, yielding and failure behaviors of common polymer glasses such as polystyrene, poly(methyl methacrylate) and polycarbonate.   Our work recognizes the importance to account for both the role of the intermolecular van der Waals interactions and the presence of a chain network resulting from excluded volume and chain connectivity.  It is insufficient to speak about the “entanglement network” density alone on one hand; it is equally insufficient to speak only in terms of the strength of the intermolecular forces on the other.  In other words, the important question is not why PC is ductile and PMMA is brittle because both can reverse their room temperature characteristics when examined at other temperatures.  We carry out experiments to characterize the respective contributions of plastic dissipation and elastic deformation during uniaxial extension of these glasses and to gain insight into the physics governing mechanical behavior of polymer glasses including the leveling off of the tensile force and substantial rise in the tensile force at higher strains.