Analysis of the isothermal crystallization kinetics and melting points of sustainable long-spaced aliphatic polyesters type PE-X,Y

Abstract

In this work 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, and 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. The wide range of X used enables investigating the impact of ester-ester dipolar interactions in the energy barrier for nucleation and the formation of lamellar crystals with ester-layered crystalline regions. When compared at a constant undercooling, the crystallization rates scale with the strength of dipolar interactions with polyesters with X = 2 having the fastest and those with X = 18 the slowest rates. Thus, 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.