Structural analysis of poly(butylene adipate) banded spherulites from their biodegradation behavior

Abstract

Poly(butylene adipate) (PBA) has been reported to have polymorphic crystal structures and only the mixture of two crystal modifications (α and β crystals) forms banded spherulites. However, how the two crystals coexist in banded spherulites remains to be solved. In this work, the morphological structure of PBA banded spherulites was analyzed from their biodegradation behavior by means of polarized optical microscopy (POM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and wide-angle X-ray diffraction (WAXD). It was found that the banded spherulites were made up of alternate flat-on and edge-on domains in lamellar crystals along the radiating direction. To determine the distribution of two crystal modifications in banded spherulites and the interrelationships between the alternate domains with polymorphic crystal structures, the relative contents of two crystal modifications in banded spherulites were first quantified, and their changes were then correlated to the morphological changes of banded spherulites in the course of biodegradation in terms of the difference in biodegradation kinetics of two crystal modifications. It was found that the flat-on domains show a slower crystal growth rate but a faster biodegradation rate than the edge-on domains. The analysis indicates that the flat-on domains in spherulites are composed of α crystals, while the edge-on domains are composed of β crystals. The primary growth mechanism of PBA banded spherulite was proposed based on the difference in crystallization heat and kinetics of two crystal modifications.