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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2015

Month Degree Awarded

September

First Advisor

Alan Lesser

Abstract

This thesis focuses on unconventional methods to improve the mechanical properties of glassy polymers such as PMMA, Tritan™ Copolyester, and epoxy-based thermosets by influencing the intrinsic mechanical behavior through solid-state processing. The solid-state processing includes pre-stress, mechanical rejuvenation, and mechanical work hardening that cause changes in microscopic conformation structure and dynamics of glassy polymers and enhance properties such as non-linear deformation behavior and low and high velocity fracture. To utilize such unconventional techniques, one needs to understand fundamental origins of dynamics in polymeric glasses where the chain segments are not completely frozen and the segmental diffusion displays high degree of intermolecular cooperativity. We demonstrate how fundamental principles of polymer physics can be applied to improve fracture toughness of polymeric glasses. Emphasis is placed on structure-process-property relationships of these systems.

In Chapter 2 long-term effects of physical aging and solid state processing are monitored through dynamic mechanical properties of an amorphous glassy polymer. These phenomena are investigated through dynamic mechanical testing that evaluates in-situ the evolution of the storage modulus with time during annealing and physical aging. Comparisons are made on samples with different thermal histories and mechanical treatment. The results are discussed in context to an aging rate obtained from the various thermal and mechanical treatments. We demonstrate that there is apparent work hardening of glassy polymers. The effect of strain rate, dwell time and material are compared and the permanence of the processing is investigated.

In Chapter 3, we investigate the effect mechanical rejuvenation on the fracture toughness of epoxy-based thermosets and correlate the kinetics of the recovery of fracture toughness to compression based ductility parameters and dynamic mechanical analysis (DMA)-based aging rates. We also investigate the effect of molecular additives, antiplasticizers, on the structural recovery rate of the epoxy after mechanical rejuvenation.

Chapter 4 studies the optimization of prestressed poly (methyl methacrylate) (PMMA), through equibiaxial compression with increasing amounts of shear and simple shear. To suppress inherent large radial crack growth associated with simple shear prestress, orientation is superimposed to minimize crack growth. These prestressed states are compared at both low velocity and ballistic rates. To investigate the low velocity impact dependence on rate, a strain energy density term is used to remove the dependence of geometry. Lastly, to reduce scatter in ballistic date, a master curve is developed to collapse all data regardless of boundary conditions, rate of impact and materials.

In Chapter 5 we examine the correlation between ductility parameters based on dynamic mechanical data and fracture toughness and other non-linear mechanical properties. This chapter focuses on the model system poly (methyl methacrylate) and the relationship of these ductility parameters to other engineering properties for a range of temperatures and strain rates.

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