Regulating the polymorphism behaviour and crystal transformation of poly(butylene adipate) by incorporating butylene fumarate units into the crystal lattice

Abstract

Poly(butylene adipate-co-butylene fumarate) (PBAF) copolyesters with different butylene fumarate (BF) contents were synthesized, and the effect of BF units on the polymorphism, crystallization behavior and crystal transformation were studied using various methods. BF units are found to cocrystallize into both α- and β-form crystal lattices, and the β-form lattice shows a better accommodation degree for BF units than the α-form. Incorporation of BF units alters the formation temperature ranges for polymorphism behavior, and facilitates the β-form crystal at high temperature due to their stronger restriction effect on primary and secondary nucleation of the α-form crystal than the β-form crystal. Furthermore, BF units retard β to α solid–solid crystal transformation, and a novel triple melting phenomenon emerges in the DSC curve of β-form PBAF, which is reasonably attributed to the melting of the original β-form crystal and two types of newly-formed α-form crystals during heating. The β to α transformation takes place through both solid–solid and solid–liquid–solid (melt–recrystallization) routes. A special hydrogen-bonding interaction between neighbouring chain stems in the crystal phase is suggested to be responsible for the restriction of the solid–solid route and the induction of the solid–liquid–solid route for β to α transformation.