Structure Formation and Unexpected Ultrafast Re-entanglement Dynamics of Disentangled Ultrahigh Molecular Weight Polyethylene

Abstract

Because of the lack of constraints for crystallization, disentangled ultrahigh molecular weight polyethylene (UHMWPE) materials prepared by solution crystallization or low-temperature polymerization can exhibit ultrahigh drawability, making them ideal materials for producing fibers or tapes with ultrahigh modulus and strength. However, their ultrahigh drawability could vanish after a short annealing time applied above their melting temperature (T_m), hampering the aspiration of obtaining high-performance fibers using melt-spinning methods. The mechanism behind this loss of drawability has yet to be fully understood, and the time scale for reconstructing the entanglement networks is a controversial problem. In this work, we present a detailed comparison study of the structure formation of disentangled UHMWPE samples via solution-cast and low-temperature polymerization methods. All disentangled UHMWPE samples exhibit a relatively high crystallinity (above 70%) and similar lamellar stack morphologies. Constraints for forming UHMWPE crystals could be generated within a short time of melting, leading to lamellar stack structures made of widely distributed crystalline and amorphous layers. We revisit the high-temperature annealing effect (using thermal protocols proposed by Rastogi et al. Macromolecules 2016, 49 (19), 7497–7509) on disentangled UHMWPE crystals via differential scanning calorimetry (DSC). The melting enthalpies in the final heating runs remain constant and are independent of the annealing time. Combining self-nucleation and flash DSC measurements, we found that the regeneration of entanglement networks occurs in an ultrashort time scale simultaneously accompanied by partial melting. The associated times are so small that they cannot be accurately determined. Our results reveal that the recovery time of entanglements does not follow the scaling law of τ ∼ M³ proposed by the classical reptation model.