On the morphology of some irradiated ultra high molecular weight polyethylenes

The morphology of various grades of ultra high molecular weight polyethylene (UHMWPE), prepared for use in orthopaedic implants, has been examined using differential scanning calorimetry (DSC), wide and small angle X-ray diffraction (WAX and SAX) and Raman spectroscopy. Preparation included gamma irradiation at various dose rates and mechanical annealing, and post-irradiation changes were of particular interest. The experimental results are interpreted in terms of previous proposals that UHMWPE is best considered as a three phase material, fully amorphous, all-trans amorphous and fully crystalline. The all-trans amorphous material is thought to be interfacial. The phase analysis shows that the age related increase in crystallinity occurs through conversion of all-trans material to fully crystalline, and there is little change in the total amorphous content of the polymers. SAX patterns show a change in the sharpness of the main diffraction peak and the emergence of a second diffraction peak at a higher q value, and this is considered to arise from crystallisation of all-trans amorphous material. Increasing the irradiation dose rate has a similar effect on the crystallography as does ageing the material. Mechanically annealed polymer also shows a similar trend towards a bimodal crystal population, accompanied by a reduction in interfacial material.