Structure, processing and performance of ultra-high molecular weight polyethylene (IUPAC Technical Report). Part 1: characterizing molecular weight
AbstractThis is the first of four reports from IUPAC Sub-Committee 4.2.1: Structure and Properties of Commercial Polymers, which in 2010 set up a Task Group to evaluate the effectiveness of available methods of quality control (QC) and quality assurance (QA) of ultra-high molecular weight polyethylene (UHMWPE) mouldings and to find improvements where possible. This was seen as an important investigation, because prosthetic hip and knee joints are among the most demanding applications of synthetic polymers and UHMWPE is the polymer of choice for this purpose: it is biocompatible, durable, and has robust mechanical properties. In this context, the term ‘ultra-high’ indicates a weight average relative molar mass, ¯¯¯¯Mw, greater than 106. In practice, the minimum ¯¯¯¯Mw for orthopaedic grades of polyethylene (PE) is about 5 × 106, which for linear PE means that chains have a contour length of at least 45 μm. The authors note that the more familiar and historical name of relative molecular mass is ‘molecular weight’, which features in the title of this series of reports. That name will be used throughout this series, except where numerical values are involved. Because it is impossible to process materials with such large molecules using conventional injection moulding or screw extrusion, reactor powder (fine particulate material obtained directly from the polymerization plant) is usually either ram-extruded or compression moulded under elevated pressure over extended periods of time to allow adequate levels of reptation to take place across inter-particle boundaries. Relaxation times can be reduced by raising processing temperatures, but this option has strict limits. Thermally induced chain scission must be avoided as much as possible. A related problem is that high pressures raise melting points and therefore narrow the processing window. The effect of temperature on relaxation times is less pronounced in PE than it is in other thermoplastics, because of its high crystallinity and low glass transition temperature Tg.