Aspects of environmental degradation and fracture in polymer films and fibers.

摘要

This thesis is focused in three areas: An investigation of a thermodynamic criterion for failure by environmental stress cracking using observations of the wetting behavior of stress-cracking liquids on glassy polymer substrates; Determination of the dominant chemical and physical degradation mechanisms associated with exposure of poly-p-phenylenebisbenzoxazole fiber to moisture moisture and UV-Vis spectrum light; And finally, the effect of constraint on fracture at a bi-material interface is investigated using a model epoxy-metallic adherend specimen.; The wetting behavior of an ESC liquid on polycarbonate substrates has been evaluated as a function of substrate stress using a variation of Contact Adhesion Testing, a novel method of measuring small contact angles by refraction and conventional goniometry. The inelastic and elastic strain condition and time to the onset of crazing were also observed. A normalization of the time to onset of crazing using stress state, solubility difference and diffusion coefficients was shown to collapse the kinetic observations.; A comprehensive study of the degradation mechanisms of PBO AS fiber exposed in a controlled manner to challenging chemical environments, moisture and UV-Visible spectrum light was undertaken. Fibers were characterized using a broad range of mechanical and physical tests including tensile testing, Elemental Analysis, scanning electron microscopy, small angle X-ray diffraction, wide angle X-ray diffraction and attenuated total reflectance infrared spectroscopy. Degradation by moisture is found to be primarily due to a loosening of the fiber's fibrillar structure. Degradation by UV-Visible spectrum light is found to be chemical in nature involving hydrolytic disruption of the oxazole ring and possible subsequent conversion to an amide bond.; Approaches to alleviation of PBO AS fiber degradation were studied including super-critical carbon dioxide extraction of residual acid, the use of UV-Vis blocking coatings, compaction of the fiber microstructure and PBO AS/Siloxane composites prepared in super-critical carbon dioxide.; Finally, the effect of constraint on fracture at the interface between a polymer and adherend having orders of magnitude larger stiffness was studied using a model epoxy/metallic adherend system. Fracture energy was measured using an Elastic Wedge Opened Double Cantilevered Beam test and the process zone imaged using photoelastic methods.