Heterogeneous distribution of entanglements in a non-equilibrium polymer melt of UHMWPE; influence on crystallization without, and with, graphene oxide

In past studies have been performed to follow chain dynamics in an equilibrium polymer melt, using a low molar mass polymer. Here we show that in linear Ultra-High Molecular Weight polyethylene, entanglements formed during or after polymerization are influencing differently the overall chain topology of the polymer melt. When a disentangled UHMWPE sample is crystallized under isothermal conditions after melting, two endothermic peaks are observed. The high temperature peak is related to the melting of crystals obtained on crystallization from the disentangled domains of the heterogeneous (non-equilibrium) polymer melt, whereas the low melting temperature peak is related to the melting of crystals formed from entangled domains of the melt. On increasing the annealing time in melt, the enthalpy of the lower melting temperature peak increases at the expense of the high melting temperature peak, due to the transformation of the disentangled non-equilibrium melt into entangled equilibrium one. However, independent of the equilibrium or non-equilibrium melt state, the high melting temperature peak is observed when the disentangled samples are left to isothermally crystallize at a specific temperature, although with a decrease in bulk crystallinity. A commercial (entangled) sample instead, shows both shift in the position of the melting temperature peak and drop in crystallinity. To ascertain that entanglements are the cause for the observed difference, experiments are performed in the presence of reduced Graphene Oxide (rGON): the melting response of disentangled UHMWPE crystallized from its heterogeneous melt state remains nearly independent of the annealing time in melt. This observation strengthens the concept that in the presence of a suitable filler, chain dynamics is arrested to an extent that the non-equilibrium melt state having lower entanglement density is retained.