Thermoplastic biodegradable polyurethanes: The effect of chain extender structure on properties and in-vitro degradation

Biodegradable polyurethanes are typically prepared from polyester polyols, aliphatic diisocyanates and chain extenders. We have developed a degradable chain extender (DCE) based on dl-lactic acid and ethylene glycol to accelerate hard segment degradation. Three series of polyurethane elastomers were synthesised to investigate the effect of incorporating DCE on synthesis, mechanical and thermal properties and in-vitro degradation. Polyurethane soft segments were based on poly(ε-caprolactone) (PCL) polyol. The hard segment was based on either ethyl lysine diisocyanate or hexamethylene diisocyanate in combination with ethylene glycol or DCE. Polyurethanes were characterised by gel permeation chromatography, tensile testing (Instron) and differential scanning calorimetry. Polymer degradation in-vitro (phosphate buffered saline) was tested by measuring mass loss, change in molecular weight and amine concentration in degradation products at three different time points over a 1 year period. Incorporation of DCE did not affect thermal or mechanical properties but had an influence on the in-vitro degradation. All polyurethanes exhibited considerable molecular weight decrease over the test period, and DCE-based polyurethanes showed the highest mass loss. The presence of the DCE and the initial molecular weight of the polyurethane are the key factors responsible for high mass losses. Differential scanning calorimetry, amine group analysis and the observation that mass loss was directly proportional to hard segment weight percentage, strongly supported that the polyurethane hard segment is the most susceptible segment to degradation in these polyurethanes. The PCL-based soft segment appears to undergo little or no degradation under these test conditions.