Study on Melting and Recrystallization of Poly(butylene succinate) Lamellar Crystals via Step Heating Differential Scanning Calorimetry

Differential scanning calorimetry (DSC) has been widely applied to study crystallization and melting of materials.However,for polymeric lamellar crystals,the melting thermogram during heating process usually exhibits a broad endothermic peak or even multiple endotherms,which may result from changes of metastability via recrystallization process.Sometimes,the recrystallization exotherm cannot be observed due to its overlapping with the melting endotherm.In this work,we employed a step heating procedure consisting of successive heating and temperature holding stages to measure the metastability of isothermally crystallized poly(butylene succinate) (PBS) crystals.With this approach we could gain the fraction of crystals melted at different temperature ranges and quantitatively detect the melting-recrystallization behavior.The melting-recrystallization behavior depends on the polymer chain structure and the crystallization temperature.For instance,PBS block copolymer hardly shows recrystallization behavior while PBS oligomer and high molecular weight PBS homopolymer demonstrate remarkable melting-recrystallization phenomenon.High molecular weight PBS isothermally crystallized in the low temperature range shows multiple melting-recrystallization while those isothermally crystallized at elevated temperatures do not exhibit observable recrystallization behavior.Furthermore,the melting endotherms were fitted via the melting kinetics equations.The original isothermally crystallized lamellae demonstrate quite different melting kinetics from the recrystallized lamellar crystals that melt at the highest temperature range,which is attributed to the different degrees of stabilization.Finally,the mechanism of melting-recrystallization is briefly discussed.We propose that apparent meltrecrystallization phenomenon be observed when melting of preformed lamellar crystals and recrystallization of thicker lamellae have similar free energy barrier.