Abstract
Improving the processability of ultrahigh molecular weight polyethylene (UHMWPE) and understanding the effect of the polymeric chain mobility has long been a challenging task. Herein, we show that UHMWPE without any processing aids can be processed at a lower temperature of 180 °C compared to conventional processing temperatures (~250 °C) under a continuous elongational flow (CEF) by using an eccentric rotor extruder (ERE). By probing the effect of the residence time of UHMWPE samples under a CEF on the morphology, rheological behavior and molecular orientation, we find that the long polymer chains of UHMWPE are apt to orientate under a consecutive volume elongational deformation, thereby leading to a higher residual stress for the extruded sample. Meanwhile, the residence time of samples can regulate the polymeric chain mobility, giving rise to the simultaneous decrease of the melting defects and residual stress as well as Hermans orientation function with increasing residence time from 0 to 60 s. This also engenders the enhanced diffusion of UHMWPE segments, resulting in a defect-free morphology and higher entanglement with lower crystallinity but without causing obvious thermal oxidative degradation of UHMWPE. This interesting result could originate from the fast chain entanglement and particle welding enabled by a desirably short residence time, which could be explained by the empirical, entropy-driven melting explosion mechanism.
Highlights
Ultrahigh molecular weight polyethylene (UHMWPE), a well-known high performance engineering thermoplastic polymer with a molecular weight greater than 1,000,000 g/mol exhibited excellent properties such as high toughness, abrasion resistance, and chemical resistance
We demonstrate that UHMWPE without any processing aids can be processed at the temperature of 180 ◦C, far below conventional processing temperatures for UHMWPE (~250 ◦C), under a continuous elongational flow (CEF) by using an eccentric rotor extruder (ERE)
We show that desirably varying residence time can modulate the chain mobility of UHMWPE, leading to simultaneous decrease of the melting defects, residual stress, as well as Herman’s orientation function while enhancing the diffusion of UHMWPE segments to enable a defect-free morphology and higher entanglement with lower crystallinity but without obvious molecular degradation
Summary
Ultrahigh molecular weight polyethylene (UHMWPE), a well-known high performance engineering thermoplastic polymer with a molecular weight greater than 1,000,000 g/mol exhibited excellent properties such as high toughness, abrasion resistance, and chemical resistance. The low processability of UHMWPE via conventional processing techniques hampers the large-scale applications of UHMWPE materials [4]. Dissolving UHMWPE in a suitable solvent or using unconventional polymerization techniques/conditions (such as using a homogeneous single-site catalyst [7]) can lead to the disentanglement of UHMWPE chains. These methods have shown some advantages over conventional processing techniques, they cannot serve as a universal solution to meet the challenges of UHMWPE melt processing. The noncontinuity of HVC process and small products size have limited its large-scale practical applications in UHMWPE processing
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