Biodegradable poly(ethylene succinate) (PES). 2. Crystal morphology of melt-crystallized ultrathin film and its change after enzymatic degradation

Poly(ethylene succinate) (PES) ultrathin films with an initial thickness of similar to 100 nm were prepared by the solution cast method on either cover glass or freshly cleaved mica as the substrate. The ultrathin films were then melt-crystallized at a given temperature for a certain period of time. The surface morphologies of these films on the substrates were observed by an atomic force microscope (AFM) and an optical microscope (OM) under ambient conditions. Two different crystal morphologies having fibril-like structure and flat-on lamellar crystals with a certain width were formed, and their growth mechanisms were discussed in association with previous kinetic data. It has been shown that at a higher crystallization temperature such as 85 degreesC (smaller degree of undercooling) the crystal aggregates tend to form lozenge-shaped hedrites which evolved from a single crystal. The enzymatic degradation of PES crystals on the ultrathin films was carried out by using a PHB depolymerase from Pseudomonas stutzeri at room temperature. The crystal morphologies before and after enzymatic degradation were examined by AFM. The lamellar crystals were hydrolyzed into many small fragments, and these fragments had the same thickness as that of the lamellar crystals before enzymatic degradation. The analysis of morphological results for PES lamellar crystals has revealed the existence of many defects on the surface of melt-crystallized lamellar crystals. These defects were preferentially attacked by the enzyme molecules. Hydrolysis starts from the chains folding in crystal defect area and proceeds along the lateral edges, i.e., along the direction perpendicular to the folding chain.