Understanding The Dynamic And Mechanical Properties Of Polymers Under Nanoconfinement

Tianren Zhang, University of Pennsylvania

Abstract

The physical properties e.g. dynamics, mechanics, etc., of polymers can change drastically under nanoconfinement. For example, 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. Understanding the impact of confinement on the physical properties of polymers is helpful to guide the development of new polymer materials. For the first part of my thesis, we investigate the conformations and dynamics of polymer melts under porous-like confinement and compare the behaviors of rings with linear melts under planar confinement. By simulating linear melts confined in a diamond network geometry with two characteristic length scales mimicking porous confinement, we find chain disentanglement increases diffusivity of entangled polymers along confined channels compared to the bulk and there is competing effects between the local friction near the wall and chain disentanglement. In the study of ring melts under planar confinement, we demonstrate that the chain dynamics of rings are primarily affected by the friction from walls based on monomeric friction coefficient analysis. For the second part, 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 simulations. We demonstrate that not all entanglements carry significant load at large deformation, and our analysis allows the development of a model to describe the number of load-bearing entanglements per chain as a function of blend ratio. The film strength measured experimentally, and the simulated film toughness are quantitatively described by a new model that only accounts for load-bearing entanglements. Varying the film thickness uncovers competing effects between the reduction in entanglement density and changes in the segmental dynamics. From the mechanical study of diblock copolymer films, we notice that the toughness of the films with fingerprint morphologies is larger compared to homopolymers due to increase in the randomness of domain orientations and entanglements. Our studies of the film mechanics provide molecular insight into how segmental mobility and entanglements interplay with position and morphology to control the mechanics of thin polymer films.