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

The degradation behavior 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 °C for 9 months. The samples were periodically extracted, dried and evaluated by weighing, SEC, ¹H NMR, DSC, and POM. The results point out that an increase in PCL content reduced the weight loss in the diblock copolymers. ¹H 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 synergistic 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 PCX lamellae offer a barrier-type protection to the PPDX within the copolymer. A schematic morpholog ical model is proposed to explain the observed changes during the different degradation stages encountered by the dibiock copolymers.