Advances in the Development of Polymeric Fibers with High Elastic Energy Storage Capacity

Polymer fibers such as Nylon, Polyester, Polyethylene, Kevlar, and Spectra have wide range of industrial applications which go beyond the realm of traditional composites. From light weight armor to automotive bumpers, tires, air bags, to drug delivery, tissue engineering to wound dressing, polymeric fibers have ever increasing demands. In most of these applications, the elastic energy storage capacity, Ω of these fibers would be an important parameter to gauge their performances. The cubic root of Ω, i.e., Ω 3 , also known as normalized velocity of the fiber essentially determines as to how much energy can be stored [1-3]. That is: ⎜ ⎜ ⎜ ⎝ ⎛ ⎟ ⎟ ⎠ ⎞ Ω = 1 / 3 max max 3 2 y y y y y E ρ ρ σ ε where y max y max σ ,ε , ρy and Ey are strength, fracture strain, density, and modulus of the fiber, respectively It is obvious that to enhance fiber performance, one has to increase y max y max σ ,ε , Ey and reduce ρy . 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 dissipation, Ω 3 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 will not be enough to increase normalized velocity; hence infusion of CNTs is also considered. This strategy of coupling nanoscale inclusion with polymer blending is expected to increase Ω 3 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. In the three phase system, the loading of UHMWPE, Nylon 6 and SWCNT was 78 wt%, 20 wt% and 2 wt%, respectively. 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 was observed. This processing also resulted in 441% and 88% increase in toughness and normalizing velocity. Nylon 6 in the blend increased intercrystalline amorphism inducing plasticity, while SWCNTs shared the load and co-continuously deformed – both contributing to the improvement 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 at the interfaces were responsible for such enhancement Details of fiber processing, thermal and mechanical characterizations and elastic energy evaluation are described in the paper.