Influence of Molecular Weight and Entanglement Density on the Creep Response of the Uniaxially Drawn Tapes of dis-UHMWPE

Polyethylene with a molecular weight greater than a million g/mol, is of increasing commercial interest because it provides a route to make fibers and films that have unprecedented mechanical properties. The unique properties are attributed to the chain alignment of the long molecules packed in the orthorhombic unit cell. The anisotropic orthorhombic unit cell, which has weak secondary interaction planes, is prone to slip under a constant load and increasing temperature, thereby restricting the life expectancy of a product. Recent advances in polymer synthesis have allowed us to tailor the molecular weight, molecular weight distribution, and the entangled state between crystalline regions of the semicrystalline polymer using a single-site catalytic system via homogeneous and heterogeneous routes. The decrease in the entanglement network in the polymer has opened the possibility of solid-state processing of the nascent powder, thus providing an economical and sustainable route to make films with oriented chains in the draw direction. In this paper, we aim to link the creep response of the solid-state processed tapes made from low-entangled ultrahigh molecular weight polyethylene (UHMWPE) with the entangled state and molecular weight. Our observations are that compared with the Ziegler-Natta-synthesized UHMWPE polymer, and all polymers synthesized using a single-site catalytic system show significantly low creep rate. The commercial tape prepared using entangled UHMWPE shows a higher creep rate in comparison with the tapes made of the synthesized low-entangled UHMWPE, which indicates the adverse effect of the higher entangled state in addition to the low M_n. The higher creep rate of the commercial tape can be attributed to higher segmental mobility of the noncrystalline domain, as determined by solid-state NMR, whereas the tapes made from low-entangled UHMWPE showed a constrained noncrystalline domain. When either catalytic system is used, the creep rate is found to decrease as a function of the molecular weight. Strong dependence of the creep rate is found for polymers having M_n values below 1.5 × 10⁶ g/mol, while above this threshold, it is not significantly affected. A (linear) dependence of the creep rate on the M_w and M_z of the polymer used to make the tapes is found.