Crystallization Rate Minima of Aliphatic Polyesters Type PE-X,Y in a Wide Range of Undercooling : Role of CH₂ Sequence Length and Layered Crystallites

Abstract

Herein, we use three series of aliphatic polyesters, designated as PE-X,Y─where X and Y denote the number of carbon atoms in the diol and diacid, respectively─as model systems to ascertain the molecular and structural factors that govern the manifestation of minima in the temperature dependence of the isothermal crystallization rate. We demonstrate that a required structural factor to observe rate minima is the formation of layered crystallites (lamellar crystals with ester-layered crystalline regions). Up to X ≅ Y/2, polyesters of series PE-X,18 develop layered crystals, and many display up to three rate minima. Unlayered polyesters such as PE-10,18 do not show rate minima. Such correspondence supports the notion of frustration or “self-poisoning” to explain the retardation of the rate observed in a narrow range of temperatures at the transition between quantized crystal thicknesses, thus indicating that the layer is a marker to detect rate anomalies that may point out specifics of polymer crystal growth. The aliphatic CH2 length between esters is an additional factor limiting the manifestation of rate minima. Despite forming layered crystals with quantized crystal thicknesses, as documented by X-ray measurements, the lack of rate minima in the even-spaced polyesters PE-X,12 is explained as a fast thickening at the transition between quantized crystal thicknesses. Furthermore, we also point out that the rate minima observed at temperatures close to the glass transition occur at the convergence of bimodal crystallization kinetics that differs between heterogeneous (high temperature) and homogeneous (low temperature) nucleation modes. Polymorphism, which is observed in some polyesters with a short odd-spaced diol, is uncorrelated with the observed rate minima.