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Document Type

Campus-Only Access for One (1) Year

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2018

Month Degree Awarded

May

First Advisor

Alan J. Lesser

Subject Categories

Mechanics of Materials | Polymer and Organic Materials | Structural Materials

Abstract

Polymer-based composite systems have been developed for a wide variety of applications ranging from aerospace to electronics. My work has focused on the structure-process-property relationships of anisotropic polymeric materials and composites, aimed primarily for structural applications. Anisotropic materials such as fibers have superior mechanical properties along the axial direction and this property can be exploited to engineer exceptionally strong and light materials. In the first chapter, we discuss the physics of degradation of Poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers. PBO, a fiber of extraordinary tensile modulus and strength has been found to degrade rapidly under moderate conditions of humidity and heat. Solid-state NMR was utilized as a screening tool to study the long-term aging performance of these fibers under systematic exposure to environmental conditions. These studies provide direct evidence to confirm the proposed contribution of the residual P to the hydrolytic degradation mechanisms reported in the literature. In the second chapter, we discuss new methods to improve composite strength by modifying the critical fiber-matrix interphase to improve fiber-matrix adhesion. The work described here is based on an aramid fiber (poly (paraphenylene terephthalamide))and natural rubber matrix system, however this approach is universal and should be applicable to any composite system. Fiber pre-treatments and coupling agents are utilized to create new morphologies on the fiber surface to enable enhanced mechanical interlocking between the fiber and rubber matrix. and create an interphase region with a gradient in properties from the fiber to the matrix. In the third chapter, a method to template kinetically trapped composite foams from anisotropic semi-crystalline media is presented. Supercritical carbon dioxide (ScCO2) was used as a solvent to transport styrene monomer mixed with a radical initiator into the template. This phase was allowed to polymerize and foam to create a microcellular structure. Interesting cell structures such as a radial gradient and a sheet-like structure were observed due to the processing conditions. Initial mechanical tests showed an improvement in specific modulus and specific strength.

Available for download on Saturday, May 11, 2019

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