Hydrolytic degradation of double crystalline PPDX-b-PCL diblock copolymers

The degradation behaviour of the diblock copolymers PPDX-b-PCL was studied under in vitro conditions, in samples with high PPDX content. Molded films were immersed in phosphate buffer solution at pH = 7.4 and 37 degrees C for 9 months. The samples were periodically extracted, dried and evaluated by weighing, SEC H-1 NMR, DSC, and POM. The results point out that an increase in PCL content reduced the weight loss in the diblock polymers. H-1 NMR and DSC analysis showed that degradation occurred almost exclusively in the PPDX block during the first 9 months of hydrolysis. POM results for the diblock copolymer with high PPDX content (77%) indicated the presence of some typical homo-PPDX spherulites in the 0.8 months degraded sample when no weight loss was detected. This result demonstrated that random chain scission during the early stages of degradation can produce homo-PPDX chains that cannot be dissolved in the hydrolysis medium because their molecular weight is still too high. It was found that a small increase in PCL content in the diblock copolymers produced a synergistics increase in the PPDX block degradation stability. This is a direct result of the inter-digitized lamellar morphology present in the copolymers where PCL and PPDX lamellae are alternated within mixed spherulites. In view of its much higher resistance to hydrolysis, the PCL lamellae offer a barrier-type protection to the PPDX within the copolymer. A schematic morphological model is proposed to explain the observed changes during the different degradation stages encountered by the diblock copolymers.