Ultrahigh molecular weight polyethylene (UHMWPE) fibers are considered the most preferred high-performance fibers for use in body armors, nautical ropes, medical implants due to their high-specific tensile properties, high-toughness and low friction coefficient. Current processing methods for manufacturing UHMWPE fibers use solution gel-spinning and swell-drawing processes. There are two major unresolved issues limiting the attainment of high-mechanical properties. First, elongation of gel filaments prior to complete UHMWPE crystallization during gel spinning leads to reduced fiber mechanical strength. Second, heating of dried gel precursor above the resin’s melting temperature results in rapid loss of drawability in the semi-solid state, an essential step for inducing molecular alignme...[ Read more ]

Ultrahigh molecular weight polyethylene (UHMWPE) fibers are considered the most preferred high-performance fibers for use in body armors, nautical ropes, medical implants due to their high-specific tensile properties, high-toughness and low friction coefficient. Current processing methods for manufacturing UHMWPE fibers use solution gel-spinning and swell-drawing processes. There are two major unresolved issues limiting the attainment of high-mechanical properties. First, elongation of gel filaments prior to complete UHMWPE crystallization during gel spinning leads to reduced fiber mechanical strength. Second, heating of dried gel precursor above the resin’s melting temperature results in rapid loss of drawability in the semi-solid state, an essential step for inducing molecular alignment. This thesis used a food grade petrolatum, a low molecular weight polyethylene, as an athermal solvent to model the gel-spinning process aiming to tackle the above two critical issues. It identified that these issues are both related to the time scale of molecular alignment and chain relaxation in the molten state. A phenomenological model was thus proposed to relate the orientation induced order formation prior to gel crystallization and at the onset of crystal melting. Specifically, the following are the major findings:

1. Simulated gel-fiber spinning at different temperatures shows that drawing at temperatures at the onset of gel-crystallization leads to the formation of Shish-Kebab crystalline morphology in the crystalline gel, and these structures become the bottle neck for the ultimate drawability in the semi-solid state. This limits the attainable maximum mechanical strength to be less than ¼ of that drawn directly from the isotropic gel fibers.

2. Annealing at temperatures above that of the “Shish” crystal melting temperature is effective in recovering drawability, and consequently, the ultimate tensile strength becomes identical to those from the isotropic gel fibers. It should be emphasized that this is the first study that uses high boiling point petrolatum as the athermal solvent for simulating the gel-spinning process. Hence, all studies including differential scanning calorimetry were carried out in the presence of the solvent. This contrasts with all previous studies where low boiling point solvents were used and annealing of fibers at high temperatures in the presence of solvents was impossible.

3. Gel fibers containing 10 wt% UHMWPE with tensile strengths ~600 MPa and moduli ~20 GPa were developed in this study. These fibers exhibit excellent biocompatibility and have potential applications for use in sutures.

4. Systematic non-linear viscoelastic studies were carried out. The relaxation behavior was observed to be highly dependent upon melt structures. Reduced relaxation modulus was modelled successfully.

5. A new relaxation phenomenon was observed, and modelled as a time scale that correlates to flow-induced orientation in the molten state. As the time scale for the structural relaxation is of the order of seconds, it is postulated that the loss of drawability is due to the explosive growth of such local alignment structures due to crystal melting induced volumetric expansion.