Correlation between Thermal and Mechanical Response of Nascent Semicrystalline UHMWPEs

In spite of the abstract nature, entanglements are the folklore of polymer science, and their presence is realized in the semicrystalline and amorphous solid state as well as in the molten state (melt) of polymers. For example, the entangled state in the noncrystalline region is known to be the determining factor in melting kinetics and also in uniaxial as well as biaxial deformation of linear ultrahigh molecular weight polyethylenes (UHMWPEs). In this paper we aim to investigate a possible correlation between the melting of crystals and the ease of deformation of the semicrystalline polymer. Considering that the entanglement density, and the associated topological constraints residing in the noncrystalline region, can be influenced by the polymerization conditions, a series of UHMWPEs have been synthesized. For a set of polymers synthesized at fixed pressure, the characteristic melting time of the as-synthesized nascent crystals (tau(1)) measured by temperature-modulated DSC (TM-DSC) is found to decrease with an increase in molar mass. The decrease in tau(1) with molar mass is indicative of the decrease in the entanglement density. The characteristic melting time of the same set of samples crystallized from their melt state (tau(2)) is considered to be the reference point, viz. entangled crystals. The characteristic melting time ratio between the two characteristic times (tau(2)/tau(1)) is found to increase linearly with molar mass. The linear increase suggests an increasing deviation between the entangled states of the as-polymerized and melt-crystallized samples. The adopted approach helps in estimating the entangled fraction created during polymerization. The drawing tension of these polymers, in their nascent semicrystalline state, increases linearly with increasing draw ratio. The slope defines the ease in processing, processability index PI. The index is found to increase with the ratio tau(2)/tau(1). The relationship between the characteristic melting time ratio and the ease in solid state processing indicates the common role of the entangled state TM-DSC is shown to be a convenient tool to establish the processability of UHMWPE in the solid state. TM-DSC measurements focus on the intracrystalline topology (arrangement of methylene units in the noncrystalline region) whereas in the total drawing process, further along the lines, other factors play a role like intercrystalline entanglements and intermolecular van der Waals forces (slippage of chains). Consequently TM-DSC assesses the indispensable intracrystalline topology required for solid-state processing of intractable UHMWPE.