Molecular weight distribution of a bulk ultra‐high molecular weight polyethylene product—impax 5M+ UHMW–NAT

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Abstract
A bulk ultra‐high molecular weight polyethylene product, IMPAX 5M+ UHMW–NAT, was fractionated using an increasing‐temperature sequential‐extraction technique. In the presence of an inert atmosphere and an antioxidant, 5 g of material were methodically dissolved in decahydronaphthalene over the temperature range, 80–191°C. Initially, intrinsic viscosities were measure in decalin at 135°C by single point capillary viscometry. From this data the viscosity average molecular weights were estimated using a logarithmic expression. Results showed that the intrinsic viscosities (molecular weights) increased linearly with extraction temperature over a range from 10 to 40 dL/g (1 to 8 × 106) and that the molecular weight distribution was log normal. Mass balances both before and after extraction indicated that less than 0.3% of the material had a molecular weight less than 106 and that less than 0.1% of the material was gel. Zero shear viscometry of bulk fractionated polymer and powdered whole polymers confirmed that capillary viscometry increasingly under‐estimates the true intrinsic viscosities as the polymer chain lengths (molecular shear forces) increase. Indeed, the actual molecular weights ranged from about 2–14 × 106. Knowledge of the molecular weight distribution of bulk products and the presence of either volatiles or crosslinked networks is critical for the continued design and development of superior wearing and fatigueresistant implants.