Unlayered–Layered Crystal Transition in Long-Spaced Aliphatic Polyesters

A crystal structure in which functional groups are present in layers is characteristic of many linear polymers with regularly spaced moieties. We report the first known instance where layered crystal formation is bypassed by rapid quenching. Long-spaced aliphatic polyesters with CH₂ spacer lengths between 18 and 48 form orthorhombic, highly symmetric, layered crystals on slow crystallization. Though crystals are still lamellar and orthorhombic on rapid quenching, ester layer periodicity disappears from X-ray patterns, indicating formation of unlayered crystals. Development of unlayered crystals requires a larger quenching depth with decreasing CH₂ spacer length. Unlayered crystals transform to the layered type on heating. We posit that on fast crystallization, lamellar crystals form via random chain folding and fast staggering of polyester segments, while on slow crystallization ester layering is facilitated by maximizing packing of the full length of CH₂ units and intermolecular dipole interactions of esters. At the same undercooling, enhanced contribution of esters as defects in unlayered crystals limits long-range packing symmetry, leading to crystals which are thinner and metastable with respect to their layered counterparts.