Unique properties of flow-drawn epoxy fibers

Epoxy micro and nano fibers prepared from a solution by extensional flow, using newly-developed mechanical and electrical drawing techniques, were recently shown to exhibit remarkable mechanical stiffness. In the current study, we investigate the ultimate strength and ductility of these fibers. The flow-induced high extension is shown to increase these properties well beyond those of bulk epoxy because of the molecular orientation and rearrangement of branching polymer clusters. At the same time, due to volume conservation and tighter molecular packing, when the extension is increased the fiber diameter gets progressively smaller. The mechanical properties are predicted to correlate with the fiber diameter via inverse-square power laws, reflecting the simultaneous molecular alignment and diameter shrinkage. This size-dependence phenomenon starts at a critical diameter, unique to the resin composition and drawing conditions, leading to a steep rise in strength and stiffness for fiber diameters thinner than the critical diameter. The strength and ductility analysis is corroborated by experimental testing of epoxy fibers prepared by extensional flow and may apply to other thermoset polymers, demonstrating the potential to manipulate mechanical properties by controlling the curing and drawing processes.