The role of polymorphic crystal structure and morphology in enzymatic degradation of melt-crystallized poly(butylene adipate) films

The enzymatic degradation of melt-crystallized poly(butylene adipate) (PBA) films corresponding to the α crystal structure, β crystal structure, and the two mixed crystal forms were studied by using the lipase from Pseudomonas sp. The results indicate that the degradation of each crystal form depends on its crystal size, while the comparison of degradation between polymorphic crystals indicates that there is no relationship between thermodynamic stability and biodegradability. The films with α crystal structure were found to have faster degradation rate than the films with β crystal structures, although the α crystal is a thermodynamically stable phase with larger crystal size. The films with mixed α and β crystal structures showed a lowest degradation rate. In order to study the degradation mechanism, the morphologies of melt-crystallized PBA films before and after enzymatic degradation were studied by means of polarized optical microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). Different spherulite morphologies have been found in the PBA films corresponding to the different crystal structures. The PBA films with mixed α and β crystal structures showed spherulites with banded rings, while the films with either α or β crystal structure did not show banded spherulites. In addition, the oriented “lines” which are crystalline structures induced by the Teflon films during melt-crystallization were only observed on the film surface with α crystal structure. The influences of surface structure, morphology, and chain mobility of polymorphic crystals were discussed for understanding the role of crystal structure and morphology in determining the biodegradability of polymorphic crystals.