High Energy Fibers through Nanoparticle Reinforcement
Polymer fibers such as Nylon, Polyester, Polyethylene, Kevlar, and Spectra have wide range of industrial applications. From light weight armor to automotive bumpers, tires, air bags, to drug delivery, tissue engineering, and wound dressing, polymeric fibers have ever increasing demands. In most of these applications, the elastic energy storage capacity of these fibers would be an important property. A key parameter to quantify this energy is the normalized velocity of the fiber which depends on the strength, fracture strain, density and modulus of the fiber. Normalized velocity is essentially the cubic root of the product of toughness and tensile wave speed of the fiber. Normalized velocities for commercial fibers such as Nylon-6, Spectra, Kevlar, and Dyneema lie between 500 and 800 m/sec. To revolutionize energy absorption and subsequent dissipation, normalized velocity must be doubled, tripled, or even quadrupled. Nanoparticle reinforcement and polymer hybridization offer a unique opportunity to accomplish such goals. The strength and modulus of Nylon – a polyamide based fiber, is one order lower than that of Spectra – a polyethylene based fiber. On the other hand, fracture strain of Nylon is one order higher than that of Spectra. Molecular structures of Nylon and Spectra are such that one provides higher elongation while the other contributes to strength and modulus. If these two polymers can be blended into one precursor, fibers with very high elastic energy will be a reality. From quantum energy concept, this exchange of molecular features is possible since both polymers transition from liquid to solid over a wide range of temperatures allowing an opportunity to exchange such features. However, blending alone is not enough to increase normalized velocity; hence infusion of CNTs is also considered. This strategy of coupling nanoscale inclusion with polymer hybridization is expected to increase normalized velocity substantially.

In this investigation, we have blended Nylon-6 with ultrahigh molecular weight polyethylene (UHMWPE) to develop a hybrid polymer precursor. To enhance strength and modulus further, we have infused single-walled carbon nanotubes (SWCNTs) into the blended polymer. Hybridized fibers were processed using a solution spinning method coupled with melt mixing and extrusion. A phenomenal increase in strength, modulus, and fracture strain of UHMWPE fiber by 103%, 219%, and 108%, respectively has been observed. This processing also resulted in 441% and 88% increase in toughness and normalized velocity. Nylon-6 caused increase in intercrystalline amorphism inducing plasticity, while SWCNTs shared the load and co-continuously deformed – both contributing to significant improvement in toughness and strength that we have observed. Differential scanning calorimetry (DSC), x-ray diffraction (XRD), and scanning electron microscopy (SEM) studies have shown that changes in percent crystallinity, rate of crystallization, crystallite size, alignment of nanotubes, and sliding between polymer interfaces were responsible for such enhancement. Details of nanoparticle infusion, fiber processing, thermal and mechanical characterization, and elastic energy evaluation will be presented.