Polymorphism and isomorphism in biodegradable polyesters

Aliphatic polyesters, which can be obtained from bacterial or chemical synthesis, are emerging as an important class of biodegradable and biocompatible polymers for applications as biomedical and conventional materials. The last two decades have witnessed explosive growth in this subject. Most biodegradable polyesters are crystallizable. The crystallization and thermal behavior of biodegradable polyesters have been extensively studied in recent years. The physical properties, such as the mechanical, thermal properties, and biodegradability (of biodegradable polymers) of a semicrystalline polymer generally depend on the crystalline structure. The polymorphic crystallization of biodegradable polyesters, including poly(3-hydroxybutyrate) [P(3HB)], poly(lactide) (PLA), poly(3-hydroxypropionate) (PHP), and poly(butylene adipate) (PBA), as well as isomorphic crystallization in many kinds of random copolyesters, such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)], have been drawing more and more attention from both academic and industrial viewpoints. This article aims to provide a comprehensive review of the current understanding on (i) polymorphic crystallization and phase transition of biodegradable polyesters, (ii) isomorphic crystallization of biodegradable random copolyesters, where the effects of comonomer composition and crystallization conditions are highlighted. The relationships between the solid-state structure and physical properties as well as the biodegradability of biodegradable polyesters are also addressed.