Effect of multi-step annealing above the glass transition temperature on the crystallization and melting kinetics of semicrystalline polymers

Fast scanning calorimetry (FSC) measurements were carried out for multi-step, isothermal, crystallized poly (butylene terephthalate) to quantify the effect of single-step to triple-step annealing above Tg (40 degrees C interval below the previous annealing temperature) on the crystallization/melting kinetics. In the case of double-step annealing, two endothermic peaks were observed on the FSC curves due to the melting of the crystal formed during the 1st step (base crystal; BC) and 2nd step (thermal history crystal; THC). The crystallization rate of the THC decreased due to the spatial restriction of the pre-existing BC. The melting kinetics of the THC were not affected by the time of 1st step crystallization. If the sample was re-heated to the 1st step temperature (T-c1) after multiple annealing, the thermal history of the previous annealing was completely erased. Moreover, in the re-annealing process at T-c1, secondary crystallization showed the same behavior as that expected from the result of single-step annealing.