Strain-induced crystallization of crosslinked polyethylene

The kinetics of strain-induced crystallization of ultra high molecular weight polyethylene (UHMWPE) in both the dry and swollen states have been studied using a digital imaging technique coupled with dynamometry. Polymer fibers obtained by spinning a gel of polyethylene in paraffin oil were lightly crosslinked with dicumyl peroxide to provide samples with different amounts of chemical crosslinks and physical trapped entanglements. For experiments in the dry state, the sample was surrounded by silicon oil while in the swollen state experiments the sample was swollen in xylene. Samples were drawn to different draw ratios (1.2 $\leq$ $\alpha$ $\leq$ 1.7) at elevated temperatures and then quenched to the crystallization temperature. The birefringence and retractive force of the sample during crystallization were measured and converted to volume fraction crystallinity. An Avrami analysis indicated that in both the dry and swollen states the crystallization followed a heterogenous nucleation with one-dimensional linear growth mechanism regardless of draw ratio and degree of undercooling. The crystal growth rate exhibited a normal exponential decay with the degree of undercooling as predicted by theories. The size of the critical growth nucleus is found to increase exponentially with the concentration of chemical crosslinks while the concentration of physical entanglements was held constant. In the limit of zero chemical crosslink concentration, the size of the critical growth nucleus falls between the range of 5 to 8 A for different physical entanglement concentrations. It is concluded that when the concentration of chemical crosslinks is low, their role in aiding the orientation and crystallization is indistinguishable from that of physical entanglements. Furthermore, there may exist a threshold of entanglement concentration which is needed to preserve the structure formed by the orienting force so that crystallization can take place.U of I OnlyETDs are only available to UIUC Users without author permissio