Helical single-lamellar crystals thermotropically formed in a synthetic nonracemic chiral main-chain polyester

Phase structures and transformation mechanisms of nonracemic chiral biologi cal and synthetic polymers are fundamentally important topics in understand ing their macroscopic responses in different environments. It has been know n for many years that helical structures and morphologies can exist in low- ordered chiral liquid crystalline (LC) phases. However, when the chiral liq uid crystals form highly ordered smectic liquid crystal phases, the helical morphology is suppressed due to the crystallization process. A double-twis ted morphology has been observed in many liquid crystalline biopolymers suc h as dinoflaggellate chromosomes (in Prorocentrum micans) in an in vivo arr angement. Helical crystals grown from solution have been reported in the ca se of Bombyx mori silk fibroin crystals having the beta modification. This study describes a synthetic nonracemic chiral main-chain LC polyester that is able to thermotropically form helical single lamellar crystals. Flat sin gle lamellar crystals can also be observed under the same crystallization c ondition. Moreover, flat and helical lamellae can coexist in one single lam ellar crystal, within which one form can smoothly transform to the other. B oth of these crystals possess the same structure, although translational sy mmetry is broken in the helical crystals. The polymer chain folding directi on in both flat and helical lamellar crystals is determined to be identical , and it is always along the long axis of the lamellae. This finding provid es an opportunity to study the chirality effect on phase structure, morphol ogy, and transformation in condensed states of chiral materials. [S0163-182 9(99)01042-5].