Isothermal Crystallization and Melting of Sustainable Long-Spaced Aliphatic Polyesters. Role of Ester-Ester Interactions on Nucleation

We have analyzed the isothermal crystallization kinetics and melting behavior after isothermal crystallization of two series of sustainable aliphatic polyesters type PE-X,Y; where X and Y denote the number of carbon atoms in the diol and diacid respectively. The two series comprise polyesters PE-X,18 and PE-X,12 with 2 ≤ X ≤ 18. The isothermal crystallization rates of most PE-X,18 and odd-X PE-X,12 are unusual, exhibiting a temperature dependence with local rate minima. At the crystallization temperature where a minimum in the crystallization rate is observed, the melting point increases stepwise by 4 – 5 °C due to quantized crystal thickening. Conversely, the levels of crystallinity are lower than expected even after prolonged annealing. The crystallinity level recovers outside the temperature range corresponding to the rate minimum. While polyesters with the longest X crystallize at the highest temperatures, when compared at a constant undercooling, they exhibit the slowest crystallization rate. A detailed analysis of the isothermal crystallization kinetics using nucleation theory reveals that, unlike the dominant van der Waals interactions in longer X polyesters, polyesters with X < 4 exhibit energetically favored nucleation due to a 5–10-fold reduction in the energy barrier, facilitated by stronger ester–ester interactions.