Isothermal Crystallization-Induced Phase Transition of Syndiotactic Polystyrene Polymorphism

Polymorphism and Multiple Melting Behavior of Bio-Based Poly(propylene 2,5-furandicarboxylate)

Furandicarboxylate-based polyesters are considered an interesting class of bio-based polymers due to their improved properties with respect to the petrol-based terephthalate homologs. An in-depth analysis of the crystal structure of poly(propylene 2,5-furandicarboxylate) (PPF), after maximum possible removal of the catalyst, was carried out. The study disclosed that purified PPF presents two different crystalline phases after crystallization from the melt. Crystallizations at temperatures lower than 120 °C lead to growth of a single crystal form (β-form), whereas two different crystal forms (α and β) were found to coexist at higher T_cs. This behavior is opposite to that previously observed for unpurified PPF. The possibility that the catalyst nucleates the α-phase, which therefore becomes the kinetically favored modification at low crystallization temperatures in the presence of a higher amount of catalyst residue, has been considered as a feasible explanation. Two concomitantly different spherulitic morphologies were observed and connected to the β- and α-phase, respectively. The association between polymorphism and melting behavior was studied. The origin of the peaks that compose the multiple melting endotherm recorded at conventional heating rates was determined by combined wide-angle X-ray scattering, differential scanning calorimetry, fast scanning chip calorimetry, and polarized light optical microscopy measurements. The higher thermal stability of the α-crystals in comparison with the β-form was thus demonstrated.

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.