The rigid-rod polymers poly(paraphenylne benzobisthiazole) (PBZT) and poly(paraphenylene benzobisoxazole) (PBZO) can be spun from lyotropic liquid-crystalline solutions into solid fibers with extraordinary mechanical properties. However, these fibers are comparatively weak in compression, with deformation occurring by strain localization into kink bands. Here, we examine the ultrastructure of PBZT and PBZO fibers as a function of processing condition. In particular, High Resolution Electron Microscopy (HREM) is used to directly image structural features such as grain boundaries, dislocations, and the molecular level details of deformation processes. HREM images of PBZT and PBZO enable the quantitative determination of crystallite size, shape, orientation, and internal perfection as a function of processing condition. The nature of the disorder present within and between PBZT and PBZO crystallites is analyzed, modeled, and compared to analyzed, modeled, and compared to experimental Wide Angle X-ray Scattering (WAXS) and Selected Area Electron Diffraction (SAED) data. HREM images in and around PBZT and PBZO kink bands reveal that local, sharp bending and/or breaking of covalently bonded molecules is involved in compressive failure. A model for kinking is proposed which involves the nucleation and growth of a region of shear deformation bounded by partial edge dislocations. A stress analysis using this model is used to correlate systematic morphological features of kinks with local material instabilities. A quantitative analysis of the linear density of kinks with a fiber as a function of applied plastic strain enables the energy required to form a kink to be determined. The geometry and energetics of grain boundaries in extended chain polymer solids is discussed. Possible mechanisms for grain boundary motion are presented. Comparisons between different grain boundary structural models and experimental HREM data are shown.
| Identifer | oai:union.ndltd.org:UMASS/oai:scholarworks.umass.edu:dissertations-4955 |
| Date | 01 January 1990 |
| Creators | Martin, David Charles |
| Publisher | ScholarWorks@UMass Amherst |
| Source Sets | University of Massachusetts, Amherst |
| Language | English |
| Detected Language | English |
| Type | text |
| Source | Doctoral Dissertations Available from Proquest |
Page generated in 0.0013 seconds