Aspects of environmental degradation and fracture in polymer films and fibers
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.
0794: Materials science