Abstract
Reducing greenhouse gas emissions in the transportation sector is crucial to slowing the progress of global climate change. This may be accomplished in part by improving the energy efficiency of the vehicle through weight reduction. Polymer-matrix composite materials reinforced with high-performance fibers, such as carbon fibers and ultra-high molecular weight polyethylene (UHMWPE) fibers, or nanoparticles, such as graphene, can be used to replace dense metals thereby reducing mass. However, producing them is costly and environmentally damaging due to extensive use of non-renewable petrochemical solvents, and much remains to be learned about the effects of using more sustainable materials and methods to make these fibers and composites.
The aim of this dissertation is to advance our understanding of the relationship between the processing and resultant microstructure and properties of these materials. UHMWPE fibers were selected as the material of interest because its extremely long molecular chains present significant opportunities and challenges. It has been shown that these fibers can be converted into carbon fibers, and it is hypothesized that the long-range order in these fibers may make an ideal framework for highly graphitic carbon fiber production. However, the effect of tension applied during the stabilization of these fibers in preparation for carbonization is heretofore unexplored. Probing this processing parameter revealed the microstructural evolution of UHMWPE during conversion to carbon fibers and offered key insights for optimizing the conversion of all PE-grades.
UHMWPE fibers possess remarkable properties thanks to their long molecular chains, but the deep entanglement of these chains inhibits the use of low-cost melt processing methods. Instead, non-renewable petrochemical solvents are used at high concentration (as much as 98% by weight) to disentangle these chains through dissolution thereby enabling extrusion into fibers. A bio-derived solvent called orange terpenes has been previously demonstrated as a renewable replacement for solution spinning of UHMWPE fibers, but scant information on the microstructure and properties of these fibers makes it difficult to assess its potential. Single-filament tensile testing coupled with thermal and X-ray microstructural characterizations revealed that these fibers develop the fundamental crystalline structures and mechanical properties indicative of high-performance potential given parametric optimization of processing conditions. This result inspired the search for a solvent suitable for dissolving UHMWPE and stably dispersing graphene nanoparticles simultaneously to form high-performance polymer nanocomposite fibers. Another terpene, 1,4-cineole, was selected and shown to be capable of highly stable graphene suspensions. UHMWPE-graphene fibers were produced and characterized to understand the effect of the nanoparticles on the microstructure and properties of the fibers. It was revealed that the nanoparticles were well-dispersed but suffer poor interfacial adhesion in the polymer matrix. At high concentration, they impede the formation and orientation of the load-bearing crystalline microstructure of the fibers, but a percolation threshold was found whereby the particles reinforce the matrix.
The findings presented in this dissertation expand the current understanding of the process-structure-property relationships underlying the production of UHMWPE fibers, polymer nanocomposites, and polyethylene-derived carbon fibers. These represent new steppingstones towards more environmentally friendly production of materials that are needed to improve the sustainability of human mobility. Recommendations for future work are included in the final chapter of this dissertation.
