Metastability and transformation of polymorphic crystals in biodegradable poly(butylene adipate)

Polymorphism phenomenon of melt-crystallized poly(butylene adipate) (PBA) has been studied by wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC). It has been found that the isothermal crystallization leads to the formation of PBA polymorphic crystals, simply by changing the crystallization temperature. The PBA alpha crystal, beta crystal, and the mixture of two crystal forms grow at the crystallization temperatures above 32 degrees C, below 27 degrees C, and between these two temperatures, respectively. The relationship between PBA polymorphism and melting behaviors has been analyzed by the assignments of multiple melting peaks. Accordingly, the equilibrium melting temperatures Tm degrees of both alpha and beta crystals were determined by Hoffman-Weeks and Gibbs-Thomson equations for the purpose of understanding the structural metastability. The Tm degrees of the PBA alpha crystal was found to be higher than that of the beta crystal, indicating that the PBA alpha crystal form is a structurally stable phase and that the beta crystal form is a metastable phase. The analysis of growth kinetics of PBA polymorphic crystals indicates that the metastable PBA beta crystal is indeed the kinetically preferential result. Based on the thermal and kinetic results, the phenomenon of stability inversion with crystal size in melt-crystallized PBA was recognized, in terms of the growth mechanisms of PBA alpha and beta crystals and the transformation of beta to alpha crystals. The PBA beta --> alpha crystal transformation takes place at a sufficiently high annealing temperature, and the transformation has been evident to be a solid-solid-phase transition process accompanied by the thickening of lamellar crystals. The molecular motion of polymer chains in both crystalline and amorphous phases has been discussed to understand the thickening and phase transformation behaviors.