Hydrolytic Degradation of Phase-Segregated Multiblock Copoly(ester urethane)s Containing Weak Links

Abstract

The hydrolytic degradation of DegraPolTM/btcg multiblock copolymers has been investigated in buffer solutions of different pH at 37 and 70°C. With 1H NMR spectroscopy it was demonstrated that the degradation rate of these copoly(ester urethane)s is predominantly controlled by the number and distribution of easily hydrolyzable glycolyl-glycolate ester bonds in the soft segments of the polymers. The degradation rate is largely independent of the elastic mechanical properties of the block copolymers, which are dominated by the weight fraction of hard segments. The degradation of DegraPolTM/btcg occurs in several steps. At the beginning, the molar mass decreases due to hydrolytic cleavage of the main chain in the soft segments, while the sample mass remains unaltered. In the second stage, the material looses its elasticity and the elongation at break becomes dependent on the decreasing molar mass. While the molar mass continuously decreases, the experimental contact angle in advance of water on polymer films also decreases due to chemical changes of the film surface and increasing surface roughness. The loss of mass starts at the next stage, where the degradation products predominantly come from the soft segment. The molar mass changes only very little. In the fifth stage of degradation, a decrease in sample dimensions can be observed. With prolonged degradation times, the sample mass and the average molar mass achieve nearly constant values. The remaining material then consists essentially of poly[(3-(R)-hydroxybutyrate)-co-(3-(R)-hydroxyvalerate)] hard segments. The time interval to achieve this state can be varied from a few days up to several months.