Unlayered–Layered Crystal Transition in Recyclable Long-Spaced Aliphatic Polyesters

We report the first known instance where the formation of layered crystals in long-spaced polyesters is bypassed on rapid quenching. Aliphatic polyesters spaced by 18–48 carbons in both the diol and diacid components of the repeating unit form orthorhombic, highly symmetric, layered crystals on relatively slow or isothermal crystallization. Though the unit cell is maintained on rapid quenching to 0 °C and lamellar crystals still form, the X-ray reflection of the ester layer disappears in PE-48,48 and weakens in the shorter-spaced polyesters. Since all crystal thicknesses are larger than the distance between the two consecutive esters, the esters must be inside the crystals in a random distribution. On heating, such unlayered crystals transform into the layered type at temperatures between 45 and 60 °C, which further melt at 98–115 °C with an increasing methylene spacer in the polyester. Rapidly quenched PE-48,48 develops only the unlayered structure, while shorter-spaced polyesters form mixed unlayered and layered crystals, indicating that a larger depth of quenching is required for the development of the unlayered form with decreasing CH2 spacer length. We posit that on fast crystallization, metastable lamellar crystals form via staggering of chain segments and random chain folding, locking a structure where the ester groups are unlayered, while on slower crystallization, ester layering is facilitated by maximizing packing of the full length of CH2 units via van der Waals interactions and intermolecular dipolar interactions of ester units. The discovery of unlayered, metastable structures of polyethylene-like materials developed under fast cooling from the melt is important for applications that mimic those which currently utilize commercial polyethylenes and that would benefit from sustainable monomer sources and material recyclability.