Degradable Polymers. IV. Degradation of Aliphatic Thermoplastic Block Copolyesters

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Abstract
Three block copolymers of poly(ethylene succinate) and poly(tetramethylene glycol) with about 20, 54, and 59 mol% polyether have been prepared and subjected to hydrolytic degradation at 37°C. The sample containing 59 mol% showed drastic changes in the properties after 3 months of degradation, whereas the other samples exhibited only minor changes. The tensile strength was completely lost, the molecular weight had decreased to 7% of the original value, and the crystallinity (measured as heat of fusion) had more than doubled. IR and 1H-NMR analyses showed that the rates of release of the different polymeric blocks varied throughout the period of hydrolytic degradation. Fibers of the block copolymer poly(ethylene succinate)/poly(tetramethylene glycol) with poly(tetramethylene glycol) fractions ranging from 20 to 50 mol% have been analyzed by 13 C NMR to determine the molecular weights of the PES blocks and by reflection IR, ESCA, and SEM to investigate the surface composition. The molecular weights of the polyester blocks were inversely proportional to the mol% of polyether, and the values were in agreement with theoretical calculated values. The surface concentration of the polyether was found to be higher than that in the bulk and also independent of the mol% polyether in the range of study. A degradation mechanism is proposed which involves a combined effect of surface erosion and hydrolytic attack on the ester linkages connecting the amorphous polyether and the crystalline polyester blocks.

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