CBE PhD Dissertation Defense | Understanding the Dynamic and Mechanical Properties of Polymer under Nanoconfiements

Abstract: 

The dynamic properties of glassy polymers are well-known to change up confinement to the nanoscale. Confinement to free-standing thin films leads to an enhancement in the segmental dynamics, and changes in the chain conformation lead to changes in the 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 find that entanglements near the chain ends are unable to 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 the blending ratio. We find an exponential scaling between the film toughness and number of effective entanglements per chain, which also agrees well with the trends observed in experimental measurements of film strength. Varying the film thickness uncovers competing effects between the reduction in entanglement density and changes in the segmental dynamics. 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 significant 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.