Influence of Solvent Quality and Crystallization Conditions on the Drawability of Ultra-High Molecular Weight Polyethylene Cast from Solution

The standard route toward high-performance polyethylene fibers involves "ultra-drawing": large-strain solid-state uniaxial plastic deformation at elevated temperatures of precursor ultra-high molecular weight polyethylene (UHMWPE) fibers to achieve crucial near-to-perfect orientation of macromolecular chains. It is well accepted that the drawability (and therewith orientation) of UHMWPE is largely determined by the entanglement density. This research evaluates the effect of solvent quality and crystallization conditions on ultra-drawability of UHMWPE by several independent experimental methods. Rheological and mechanical evaluation of the elastic properties of UHMWPE solutions and the resulting gels cast from good and poor solvents show that at a given UHMWPE concentration, the entanglement density in solution and the connectivity in the resulting gels are similar if not identical, suggesting that the entanglement density is not directly influenced by solvent quality. Nevertheless, at a given concentration, dried films obtained from poor solvents depict a twofold higher drawability, resulting in improved fiber properties. The observed correlation of the maximum draw ratio with lamellar thickness of films cast from poor and good solvents at a given polymer concentration suggests that polymer crystals formed at low undercooling have a higher maximum draw ratio. In addition, this study presents two new methods that, by solely changing the crystallization conditions of UHMWPE solutions at a given polymer concentration, result in an increase in the maximum draw ratio of the produced UHMWPE precursor films and ultimately leading to improved fiber properties.