Structure Formation\nand Unexpected Ultrafast Re-entanglement\nDynamics of Disentangled Ultrahigh Molecular Weight Polyethylene

Because of the lack of constraints for crystallization,\ndisentangled\nultrahigh molecular weight polyethylene (UHMWPE) materials prepared\nby solution crystallization or low-temperature polymerization can\nexhibit ultrahigh drawability, making them ideal materials for producing\nfibers or tapes with ultrahigh modulus and strength. However, their\nultrahigh drawability could vanish after a short annealing time applied\nabove their melting temperature (<i>T</i><sub>m</sub>),\nhampering the aspiration of obtaining high-performance fibers using\nmelt-spinning methods. The mechanism behind this loss of drawability\nhas yet to be fully understood, and the time scale for reconstructing\nthe entanglement networks is a controversial problem. In this work,\nwe present a detailed comparison study of the structure formation\nof disentangled UHMWPE samples via solution-cast and low-temperature\npolymerization methods. All disentangled UHMWPE samples exhibit a\nrelatively high crystallinity (above 70%) and similar lamellar stack\nmorphologies. Constraints for forming UHMWPE crystals could be generated\nwithin a short time of melting, leading to lamellar stack structures\nmade of widely distributed crystalline and amorphous layers. We revisit\nthe high-temperature annealing effect (using thermal protocols proposed\nby Rastogi et al. <i>Macromolecules</i> <b>2016</b>, 49 (19), 7497–7509) on disentangled UHMWPE crystals via\ndifferential scanning calorimetry (DSC). The melting enthalpies in\nthe final heating runs remain constant and are independent of the\nannealing time. Combining self-nucleation and flash DSC measurements,\nwe found that the regeneration of entanglement networks occurs in\nan ultrashort time scale simultaneously accompanied by partial melting.\nThe associated times are so small that they cannot be accurately determined.\nOur results reveal that the recovery time of entanglements does not\nfollow the scaling law of τ ∼ <i>M</i><sup>3</sup> proposed by the classical reptation model.