Understanding the Failure and Mechanical Properties of Glassy Polymer Thin Films

The dynamic and mechanical properties of glassy polymers are known to change up confinement to the nanoscale. Confinement to free-standing thin films leads to an enhancement in segmental dynamics, and changes in chain conformation lead to changes in entanglement density in confined polymers. In this study, we investigate the role of both segmental dynamics and changes in entanglement density on the mechanical response of glassy polymer films under uniaxial tension using molecular dynamics (MD) simulations. We show that not all entanglements can carry significant stress at large deformation, and this leads to the development of a simple model to describe the number of effective entanglements per chain as a function of blending ratio. The film toughness and the strength measured experimentally can be characterized in terms of the effective entanglement density. In well-ordered diblock copolymer thin films, we find that failure tends to occur near the center of the block copolymer domains due to the high concentration of chain ends that are unable to support stress. Our studies of the thin film mechanics provide molecular insight into how segmental mobility and entanglements interplay with position and morphology to control the mechanics of thin polymer films.