Influence of hydrostatic recrystallization on the microstructure and mechanical properties of ultra-high molecular weight polyethylene

Abstract
The influence of high-pressure recrystallization on the semi-crystalline rnicrostructure and on the mechanical behavior of ultra-high molecular weight polyethylene (UHMWPE, M_γ5.10⁶ g/mol) is investigated. This process involves polymer re-melting at 250°C under 2800 bars in pressurized water. The unmodified and modified UHMWPE materials are compared with a reference high-density polyethylene (HDPE, M_γ2.10⁵ g/mol). It is found that UHMWPE normally exhibits much more plastic hardening than HDPE at large strains, while the behavior at small strains (Young's rnodulus and yield stress) is weaker. The reason for this poor resistance in the elastic regime and at yield is that the degree of crystallinity of UHMWPE is 20 % lower than for HDPE, due to the extremely high viscosity of the rnelt at the crystallization temperature. Since this work proves that recrystallization under pressure increases considerably the degree of crystallinity without affecting chain length, this treatment enhances rigidity and plastic resistance at a level similar to the HDPE, while keeping the advantages of higher hardening. These optimized properties are controlled by the favorable combination of microstructural features : i) higher crystallinity ii) bimodal distribution of crystallite sizes and, iii) large density of very long amorphous macromolecules forming "tie molecules" between the crystallites. Thus, pressure recrystallization induces higher microstructural cohesion and avoids chain slippage or cavitation.