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Wang, Wenjie

Materials Science Program

2010

Ph. D.

Polymeric fibers are highly oriented semi-crystalline materials. High performance fibers with smaller diameter and fewer defects have higher strength than bulk materials. Carbon nanotubes (CNTs) or nanofibers (CNFs) have exceptionally high tensile strength modulus. Polymer fibers reinforced with CNTs or CNFs as nano-fillers are one of the most promising nanocomposites. To fully benefit from the unique mechanical properties of these nano-fillers, a better understanding of reinforcing mechanisms, especially load-transfer between matrix and fillers, is critical. X-ray scattering/diffraction is an ideal tool to look into the structure and morphology of materials over the length scale 1 nm to 100 nm. Synchtrotron x-ray radiation with its high brilliance enables in-situ dynamic measurement and is an effective method to monitor the structural and morphological chances in materials with hierarchical structure. Two-dimensional (2D) scattering data that are obtained from fibers need to be properly analyzed to fully characterize the hierarchical structure. In the work described in this dissertation, in situ small-angle x-ray scattering (SAXS) and wide-angle x-ray scattering (WAXS) were obtained during tensile deformation of solution-spun and gel-spun polyacrylonitrile (PAN) fibers with or without nano-reinforcement. 2D elliptical analyses methods were developed to extract the significant features of 2D x-ray scattering data and correlated to structure and morphology of the fiber. The WAXS showed PAN to have zigzag and helical conformation that are intimately associated to the crystal lattice. It was found that the presence of nano-fillers alter the structure of polymer matrix at the molecular scale. At 10-100 nm length scale, the PAN filament can be modeled as fibrils with crystalline, amorphous domains and voids. These voids provide the electron density contrast for SAXS. PAN has non-lamellar structure that results in diffuse SAXS patterns. It was found that the voids were surrounded by fibrils and oriented the same manner as crystals. Processing conditions and fillers affect the distribution, size and behaviors of voids. These structural details enhance our understanding of the factors that control the ultimate mechanical properties of nano-reinforced polymer fibers.