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Nanoscale Structure and Properties of High-Performance Polymer Fibers and Nanoreinforced High-Temperature Resins
High-performance polymer fibers developed in the second half of the 20th century revolutionized lightweight structural materials and composites. However, complex hierarchical features and their interactions across multiple length scales are not yet fully understood, partly due to limited experimental techniques providing access to internal structure. The main goal of this dissertation was to develop such techniques and utilize them for comparative studies of high-performance fibers. Two new sample preparation techniques using focused ion beam (FIB) notching were developed and implemented. One technique utilized shear failure and innovative specimen mounting for subsequent atomic force microscopy (AFM) topographic and mechanical property mapping. Analysis of poly-p-phenylene terephthalamide (PPTA), polybenzoxazole (PBO) and ultrahigh molecular weight polyethylene (UHMWPE) fibers showed a complex hierarchy of highly oriented fibrillar features. Advanced elastic mapping significantly enhanced nanoscale feature resolution compared to topographic mapping. A second technique using more stable, tension-assisted specimen mounting was developed and used for nanoindentation probing of intermediate-scale nanofibril bundles, which are widely observed in fiber fibrillation during failure. Interfacial separation energy measured at this scale for the first time correlated with and explained differences in fibrillation tendencies between UHMWPE and PPTA fibers. The newly measured separation energy exhibited power-law scaling with previously reported separation energies at the nano- and macro-scales in UHMWPE. Self-similar bridging of interfacial cracks by nanofilamentary features of increasing size was identified as the primary mechanism of scaling. These results provide the first critical glimpse into hierarchical mechanisms of lateral interactions in advanced fibers. Along with fiber studies, feasibility of toughening high-temperature polymer resins by continuous nanofibers was explored. Such resins are critical for future structures designed for extreme environments; however, most current resins exhibit brittle failure and microcracking. Two fabrication methods were developed and used to successfully incorporate continuous nanofibers into two different matrices. Mechanical evaluation showed significant (up to 200%) increase in energy to failure with relatively small addition of continuous nanoreinforcement. Overall, this dissertation establishes innovative experimental techniques which provide avenues for future studies of existing advanced fibers and new fiber development. The insights gained highlight the critical importance of hierarchical features and their interactions across the scales.
Stockdale, Taylor Adam, "Nanoscale Structure and Properties of High-Performance Polymer Fibers and Nanoreinforced High-Temperature Resins" (2019). ETD collection for University of Nebraska - Lincoln. AAI13865540.