Zheng-Chi Zhang

Self-reinforced polyethylene blend for artificial joint application

By means of purposeful material design and melt manipulation, we present a highly feasible approach to simultaneously improve the mechanical properties, fatigue and wear resistance of an ultrahigh molecular weight polyethylene (UHMWPE)-based self-reinforced polyethylene (PE) blend for artificial joint replacement. The fluidity of the PE blend was achieved by blending low molecular weight polyethylene (LMWPE) with radiation cross-linked UHMWPE. The use of the cross-linked UHMWPE restrained the molecular diffusion between the LMWPE and UHMWPE phases, and hence increased the content of UHMWPE up to 50 wt% under the premise of desirable fluidity for injection molding. The combination of the shear flow field and pre-additive precursors successfully induced numerous interlocking shish-kebab structures in the LMWPE phase. Mechanical reinforcement was thus attained, where the ultimate tensile strength was significantly improved from 27.6 MPa for the compression-molded UHMWPE to 81.2 MPa for the PE blend, and the impact strength was increased from 29.6 to 35.2 kJ m-2. The fatigue and wear resistance were far superior to those of the compression-molded UHMWPE. Compared to the results reported in our previous study (40 wt% UHMWPE), the increased UHMWPE content caused the LMWPE phase melt to flow faster, thus amplifying the shear rate in the interfacial region between the two phases and depressing the relaxation of oriented molecular chains. The crystalline orientation was preserved, especially in the inner layer, leading to further enhancement of the mechanical properties. These results suggest that such a self-reinforced PE blend is of benefit to lowering the risk of failure and prolonging the life span of the implant under adverse conditions.

Unexpected shear dependence of pressure-induced γ-crystals in isotactic polypropylene

Flow and pressure frequently coexist in practical polymer processing operations, but their combined influence on the microstructure of polymer parts has received very limited attention in the academic community. In the current work, we utilized a home-made pressuring and shearing device with a reliable dynamic sealing design to study the formation and microstructure of γ-form isotactic polypropylene (iPP) obtained under the coexistence of flow and pressure. We observed a strong shear dependence of pressure-induced γ-form iPP. There are three regions depending on shear flow intensity, i.e., facilitation ( 9.1 s−1) regions of the γ-form. As the shear rate is below 3.7 s−1, the pressure-induced γ-form dominates and the shear flow slightly facilitates formation of γ-form. Unexpectedly, above 3.7 s−1, the shear flow is unfavorable for γ-form growth. Even under a pressure of 100 MP, a flow field with a shear rate above 9.1 s−1 could entirely suppress the γ-form. Moreover, we did not observe any trace of the β-form in the obtained iPP that is generally generated under shear flow alone. These interesting results have never been reported, which undoubtedly help manipulate the inner structure and thus enhance the performance of final iPP products.