Polymerization of Low-Entangled Ultrahigh Molecular Weight Polyethylene: Analytical Model and Computer Simulations

Effects of Diffusing Squalene on the Plastic Deformation of Ultrahigh-Molecular-Weight Polyethylene─Insights from Molecular Dynamics Simulations

Ultrahigh-molecular-weight polyethylene (UHMWPE) stands out as a popular artificial joint material. However, wear limits its service life, which is mainly caused by accumulation of plastic deformation. The plastic deformation on the frictional interface reflects the early wear of UHMWPE. To investigate the effect of squalene, a typical component in the body fluid, on the tribological properties of UHMWPE at microscopic scale, the diffusion behavior of squalene into polyethylene and its influence on the plastic deformation of polyethylene are discussed using the molecular dynamics (MD) simulation. The lubrication model shows that polyethylene reconstructed from the interface to lower substrate, with refactor gaps between polyethylene chains. This promotes squalene molecules to gradually diffuse into polyethylene from these gaps and causes the polyethylene structure to become loose. On the other hand, in the diffused model, squalene in polyethylene substrates increases the plastic deformation of polyethylene. The separation of squalene reduces the interaction strength between adjacent polyethylene chains and accelerates the disentanglement of polyethylene. The flexibility of "C═C" bonds in squalene allows the continuous adjustment of its spatial structures to adapt the space between polyethylene chains. The squalene fragments will not hinder the plastic flow of polyethylene.

Comparison of gels synthesized by controlled radical copolymerization and free radical copolymerization: molecular dynamics simulation

The structures of gels synthesized by controlled radical copolymerization (CRP) and conventional free radical copolymerization (FRP) were studied by a coarse-grained molecular dynamics simulation. It was confirmed that the CRP gel has a larger number of elastically effective chains and fewer cyclic structures and entanglements than the FRP gel, i.e., the network structure of the CRP gel is more uniform than that of the FRP gel. However, the difference in the shear modulus between the two gels was small due to the opposing changes in the number of elastically effective chains and that of entanglements. The relatively uniform structure of the CRP gel is attributed to the suppression of intramolecular cross-linking by the fast initiation and slow propagation, and the development of cross-linked structures in the post-gel region due to the limited termination. The effects of these CRP characteristics were studied in detail. From the results, it was found that all of these characteristics of CRP cooperatively act to improve the homogeneity of the structure of the CRP gel.

Analysis of Stress Relaxation in Bulk and Porous Ultra-High Molecular Weight Polyethylene (UHMWPE)

The reported study was devoted to the investigation of viscoelastic behavior for solid and porous ultra-high molecular weight polyethylene (UHMWPE) under compression. The obtained experimental stress curves were interpreted using a two-term Prony series to represent the superposition of two coexisting activation processes corresponding to long molecular (~160 s) and short structural (~20 s) time scales, respectively, leading to good statistical correlation with the observations. In the case of porous polymer, the internal strain redistribution during relaxation was quantified using digital image correlation (DIC) analysis. The strongly inhomogeneous deformation of the porous polymer was found not to affect the relaxation times. To illustrate the possibility of generalizing the results to three dimensions, X-ray tomography was used to examine the porous structure relaxation at the macro- and micro-scale levels. DIC analysis revealed positive correlation between the applied force and relative density. The apparent stiffness variation for UHMWPE foams with mixed open and closed cells was described using a newly proposed three-term expression. Furthermore, in situ tensile loading and X-ray scattering study was applied for isotropic solid UHMWPE specimens to investigate the evolution of internal structure and orientation during drawing and stress relaxation in another loading mode.

A possible strategy for generating polymer chains with an entanglement-free structure

We propose a possible strategy that may experimentally generate long polymeric chains with an entanglement-free structure. The basic idea is designing the conditions to restrict polymer chains from growing along the surface with an obviously concave curvature. This strategy is proved to effectively reduce the chance of forming both inter- and intra-molecular entanglements, which is quite similar to the self-avoiding random walking of chains on a two dimensional plane. We believe that this kind of chain growth strategy may supply a kind of possible explanation on the formation of the entanglement-free structure of chromosomes, which also have tremendously large molecular weight. Besides, this study also guides experimentalists on synthesizing specific entanglement-free functional polymeric or biological materials.

Elongational Flow Field Processed Ultrahigh Molecular Weight Polyethylene/Polypropylene Blends with Distinct Interlayer Phase for Enhanced Tribological Properties

Herein, we produced a series of ultrahigh molecular weight polyethylene/polypropylene (UHMWPE/PP) blends by elongational-flow-field dominated eccentric rotor extruder (ERE) and shear-flow-field dominated twin screw extruder (TSE) respectively and presented a detailed comparative study on microstructures and tribological properties of UHMWPE/PP by different processing modes. Compared with the shear flow field in TSE, the elongational flow field in ERE facilitates the dispersion of PP in the UHMWPE matrix and promotes the interdiffusion of UHMWPE and PP molecular chains. For the first time, we discovered the presence of the interlayer phase in blends with different processing modes by using Raman mapping inspection. The elongational flow field introduces strong interaction to enable excellent compatibility of UHMWPE and PP and induces more pronounced interlayer phase with respect to the shear flow field, eventually endowing UHMWPE/PP with improved wear resistance. The optimized UHMWPE/PP (85/15) blend processed by ERE displayed higher tensile strength (25.3 MPa), higher elongation at break (341.77%) and lower wear loss of ERE-85/15 (1.5 mg) compared to the blend created by TSE. By systematically investigating the microstructures and mechanical properties of blends, we found that with increased content of PP, the wear mechanism of blends varies from abrasive wear, fatigue wear, to adhesion wear as the dominant mechanism for two processing modes.