Radiation and chemical crosslinking promote strain hardening behavior and molecular alignment in ultra high molecular weight polyethylene during multi-axial loading conditions

The mechanical behavior and evolution of crystalline morphology during larg e deformation of eight types of virgin and crosslinked ultra high molecular weight polyethylene (UHMWPE) were studied using the small punch test and t ransmission electron microscopy (TEM). We investigated the hypothesis that both radiation and chemical crosslinking hinder molecular mobility at large deformations, and hence promote strain hardening and molecular alignment d uring the multiaxial loading of the small punch test. Chemical crosslinking of UHMWPE was performed using 0.25% dicumyl peroxide (GHR 8110, GUR 1020 a nd 1050), and radiation crosslinking was performed using 150 kGy of electro n beam radiation (GUR 1150). Crosslinking increased the ultimate load at fa ilure and decreased the ultimate displacement of the polyethylenes during t he small punch test. Crosslinking also increased the near-ultimate hardenin g behavior of the polyethylenes. Transmission electron microscopy was used to characterize the crystalline morphology of the bulk material, undeformed regions of the small punch test specimens, and deformed regions of the spe cimens oriented perpendicular and parallel to the punch direction. In contr ast with the virgin polyethylenes, which showed only subtle evidence of lam ellar alignment, the crosslinked polyethylenes exhibited enhanced crystalli ne lamellae orientation after the small punch test, predominantly in the di rection parallel to the punch direction or deformation axis. Thus, the resu lts of this study support the hypothesis that crosslinking promotes strain hardening during multiaxial loading because of increased resistance to mole cular mobility at large deformations effected by molecular alignment. The d ata also illustrate the sensitivity of large deformation mechanical behavio r and crystalline morphology to the method of crosslinking and resin of pol yethylene, (C) 1999 Elsevier Science Ltd. All rights reserved.