The melting behavior, crystal structure, and spherulitic morphology of melt-crystallized poly(butylene adipate) (PBA) at a temperature range from 25 to 35 °C have been investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and optical microscopy. Two distinct melting behaviors with double peaks have been observed after melt-crystallizing at different temperatures. X-ray analysis proves that these two behaviors arise from two forms of PBA crystal structures. The β-form crystals are formed at temperatures below 31 °C, while the α-form crystals are formed above 29 °C. When the crystallization temperature is 30 ± 1 °C, a mixture of α-form and β-form crystals has been formed. The influences of annealing treatment and sample preparation method on the two forms of PBA crystal structures have been discussed. The results indicate that the low temperature-formed β-form crystals are very sensitive to the preparation conditions.
The melting curves at different heating rates for poly(butylene adipate) after melt-crystallized at 25 °C and 33 °C, respectively.