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Abstract
Commercial carbon fibers (CFs) are widely used as reinforcements in structural composites. However, no significant breakthrough in CF mechanical properties was reported since mid-1980s. Mechanical properties of fibers are known to substantially improve with the decrease of their diameter; however, conventional fiber manufacturing techniques can't produce fibers finer than a few microns. Continuous carbon nanofibers (CNFs) based on electrospun precursors have the potential to produce the desired breakthrough. The main goal of this dissertation was systematic analysis of CNF mechanical behavior and correlation of their properties to structure. Macroscopically long individual CNFs were tested through failure in a broad diameter range. Significant size effects in strength, modulus, and toughness were observed. Best CNFs exhibited strength on par with the best commercial CFs, while retaining significantly higher strain at failure. Observed poor graphic structure of CNFs might be responsible for the latter.
Previously, structural improvements in CNFs were realized through addition of a small amount of double wall carbon nanotubes (DWNTs). Here, mechanical properties of individual DWNT-modified CNFs were examined. Potential mechanisms for structural and properties improvements were proposed and analyzed. Internal constraint was investigated as a new mechanism and was shown to be responsible for some of the improvements.
Previous examination of polyacrylonitrile nanofibers showed simultaneous high strength and toughness. This combination of properties was explained by high degree of macromolecular alignment and low crystallinity. In this dissertation, a different polymer system (Nylon 6) with different crystallization kinetics was studied. Increase of crystallinity for smaller diameter and a corresponding decrease in failure strain were observed. However, the decrease was slight, while the increase in strength was five folds relative to the baseline of thicker nanofilaments. As a result, simultaneously high strength and toughness were still observed, although the mechanism was different.
This PhD study provides better understanding of structure-properties relations in carbon and polymer nanofiber systems. The results can be used for manufacturing and optimization of nanofibers for composites and other structural applications.
