Chiroptical Differentiation of Twisted Chiral and Achiral Polymer Crystals

Charles Mauguin, in his groundbreaking 1911 paper on mechanically twisted, nematic liquid crystal cells, speculated on the possibility of measuring the effects on polarized transmitted light along a sufficiently plastic crystalline material with a preserved twist. This experiment is realized in crystalline, spherulitic polymers wherein twisted, fibrous lamellae emanating from the cores are directed along the wave vector of the incident light. Twisted lamellae of synthetic polymers are found in optically banded spherulites of both chiral and achiral polymers. The material's chirality (the mesoscopic twist sense) is established at the cores where crystals nucleate. However, determining the twist sense directly by microstructural analyses (e.g., scanning probe or electron microscopy) is possible only in favorable cases. An optical assay, of the kind suggested by Mauguin, would be universal. The differential transmission of circularly polarized light in the cores of polymer spherulites was imaged by complete or Mueller matrix polarimetry. In this way, chiral and achiral polymers were distinguished as having optically homochiral and heterochiral cores, respectively. The sign of circular retardance, in the regime where the operating wavelength is much shorter than the pitch, indicates directly the sign of the twist and distinguishes the left and right sides of enantiopolar spherulites.