Emergence of Morphological Chirality from Twinned Crystals

摘要

Helical and spiral architectures are fundamental shapes that have chirality and are observed in various environments and scales in nature. Their unique morphology is fascinating; the formation mechanisms for many kinds of helical forms are still a mystery. Biological polymers such as DNA, collagen, and agar produce helical chains from the nanometer to the submicron scale with a specific interaction through hydrogen bonding. Artificially produced chiral organic molecules, polymer chains, and macromolecules have been reported to induce twisted aggregates on various scales. In general, the formation of these helices was ascribed to the chirality of the components and hydrogen bonding originating from their asymmetric molecular structure. However, helical morphologies are not always attributable to only microscopic chirality at a molecular level. For example, macroscopic helical morphologies have also been observed with organic molecules, polymers and inorganic materials without microscopic chirality. Helical whiskers, produced by vapor-phase deposition techniques, originate from the asymmetric behavior of a growing site at the top of each whisker and dislocations in the crystal phase. Well-known twisted ribbons in spherulites of polymer crystals without chirality have been characterized by X-ray diffraction and by modeling. It was reported in 1929 that that several organic and inorganic crystal systems, except for cubic systems, induce twisted shapes. However, the details of the shapes and growth conditions have not been previously described in the literature. In recent years, new families of helical shapes were discovered for barium sulfate, calcium, barium and strontium carbonates, potassium dichromate, manganese oxide, and silica-surfactant composite. Consequently, a novel concept is required to understand the emergence of these morphological chirality from achiral components.