Effect of polyol molecular weight on the physical properties and haemocompatibility of polyurethanes containing polyethylene oxide macroglycols.

The physical properties and haemocompatibility of polyurethanes containing polyethylene oxide (PEO) of varying molecular weights but constant weight fraction of hard segment are investigated. The PEO molecular weights studied were 600, 1450 and 8000. Analysis of polyurethane phase separation and crystallinity using dynamic-mechanical analysis and differential scanning calorimetry show that the degree of phase separation and crystallinity increase with polyol molecular weight, but level off at the highest molecular weights. The degree of water absorption increases substantially with increasing PEO molecular weight, levelling off at the highest molecular weight. Tensile data show a maximum in extensibility at a polyethylene glycol (PEG) molecular weight of 1450, while ultimate strength increases with increasing segment length. When the materials are hydrated, there is a significant drop in the modulus, ultimate stress and ultimate elongation. Dynamic contact angle measurements show that surface hydrophobicity decreases as the soft segment molecular weight increases. Using electron spectroscopy for chemical analysis (ESCA) to determine the surface composition of these polyurethanes, it was found that the hard segment content at the surface increases as the polyol block length decreases. The haemocompatibility of these polyurethanes was investigated in an ex vivo canine blood-contacting model. Only for the shortest block length studied, PEG-600, are differences in blood compatibility observed. This material was found to be the most thrombogenic. The PEG-1450 sample shows comparable blood compatibility to PEG-8000.