Crystallization Rate Minima of Poly(ethylene brassylate) at Temperatures Transitioning between Quantized Crystal Thicknesses

Poly(ethylene tridecanodiate), also known as poly(ethylene brassylate) (PEB), is a short–long aliphatic polyester obtained from a renewable source. Cooled from the melt by differential scanning calorimetry or by fast scanning calorimetry, PEBs in a range of molar mass between 27000 and 188000 Da display single crystallization exotherms. On heating, PEBs exhibit two major melt-recrystallization events at ∼40 and ∼60 °C prior to their final melting at ∼70 °C. WAXD patterns collected in situ during heating rule out any polymorphic transition, but SAXS patterns collected at the isothermal crystallization temperatures below and above the highest melt-recrystallization event (∼60 °C) indicate a step increase by one repeating unit of the crystal thickness. The overall isothermal crystallization rate displays two minima at the same temperatures where melt-recrystallization was observed on heating. The minima correspond to the transition between 2–3 repeats (Tc = 40 °C) and between 3–4 (Tc = 60 °C) monomer repeats in the crystal thickness. A minimum also occurs at the same temperature in the isothermal linear growth rates and is accompanied by a minimum in nucleation density. The observed rate minima at the transition between crystals differing by a quantized thickness are equivalent to the behavior of n-alkanes and low-Mw PEO fractions and are also explained by the manifestation of self-poisoning. At Tc approaching a rate minimum from above, PEB stems with noninteger repeats attach temporarily to the integer lateral growing surface halting productive growth until they detach or expand to complete the layer. Hence, for this type of polyester, it is the length of the stem approaching the growing surface rather than stems with a different conformation that drives self-poisoning.