The Isothermal Melting Kinetics of Ultra‐High Molecular Weight Polyethylene Crystals

Abstract

The isothermal melting behaviors of ultrahigh molecular weight polyethylene (UHMWPE) with different entangled states (i.e., nascent and melt‐crystallized samples) have been studied. For two kinds of UHMWPE samples, our result shows that the relative content of survived crystals (Xs) exponentially decreases with time and reaches a constant value. We suggest that such a melting behavior is related to the observed nonlinear growth of crystals induced by the kinetically rejected entanglements accumulated at the growth front. Additionally, the exponential decay of Xs with time provides a characteristic melting time (τ) for the melting process. Compared to the melt‐crystallized UHMWPE, the τ value of nascent UHMWPE is generally longer even in a higher temperature range, which is mainly because the former has a larger entanglement density difference. Furthermore, our observations demonstrated that UHMWPEs with different entangled states have an analogous melting mechanism since they exhibit a similar melting activation energy (∼1300 kJ/mol).This article is protected by copyright. All rights reserved

Crystallization of Polymers with a Reduced Density of Entanglements

“…Ultra high molecular weight polyethylene (UHMWPE) is practically the only polymer synthesized while preventing the entanglement of the growing macromolecule chains. The nascent polymer obtained from the reactor was compared with another fully entangled one [108,109]. Pandey et al [108] tested thermal properties when heating with different rates.…”

“…This can be explained by the lower entanglement density occurring in the amorphous region of the higher Mw samples, supporting the growth of more perfect crystals. Xue et al [109] focused on another issue related to the properties of UHMWPE, namely melting under isothermal conditions. This polymer studied by DSC showed two melting peaks at temperatures of 132 °C and 144 °C.…”

“…The exponential decay of Xs with time provided a characteristic melting time τ that was much higher for the disentangled UHMWPE, suggesting a dependence on the difference in entanglement density in the polymer. Xue et al [109] focused on another issue related to the properties of UHMWPE, namely melting under isothermal conditions. This polymer studied by DSC showed two melting peaks at temperatures of 132 • C and 144 • C. When the sample temperature was maintained at 140 • C, an increase in the first peak and a decrease in the second peak with annealing time were observed.…”