Discerning toughening mechanisms of filled polymers by aligning inclusions with an electric field.

Abstract

This dissertation investigates the fracture and processing of polymer matrix composites, focusing primarily on those reinforced by discontinuous fillers. The nature of toughening of glassy polymers by rigid particles was investigated in composites with glass beads either randomly distributed or aligned by an electric field. The fracture behavior was altered by the spatial arrangement of particles; most notably, the toughness was enhanced in specimens aligned perpendicular to the fracture plane. Fracture behaviors were inconsistent with those predicted by crack pinning and bowing, but they were consistent with the size of the process zones. Deformation mechanisms within the process zone included particle-matrix debonding and inelastic matrix deformation. The effects of fiber orientation on toughening of short fiber-reinforced polymers were investigated in composites with fibers randomly oriented or aligned by an electric field. Fiber pull-out (including snubbing) was predicted to give the greatest toughening for fibers angled nearly perpendicular to the fracture plane. Fiber pull-out, breakage, and debonding were predicted to give similar contributions at intermediate angles. Debonding provided minimal toughening for fibers within the fracture plane. The time over which suspensions of particles or platelets aligned by an electric field returned to a random state was examined. The time for decay of particulate strings corresponded with that for particle orientation, and the time for decay of orientation for particles and platelets was similar, except at high particle concentrations. The time for the decay of alignment was consistent with estimations from sedimentation and electrostatic repulsion between particles but disagreed with estimations from Brownian motion. Additionally, in-plane deformation of fiber preforms was investigated within a fixture that mimicked the constraints imposed by a closed mold. Preform deformation was proportional to the applied stress up to a critical value. Above this stress, the preform sustained damage in the form of localized buckling. In-plane compression was observed to vary with system parameters and was shown to be strongly dependent on friction between the preform and the fixture wall. Results were compared with a model accounting for the friction between the preform and the fixture wall.