Banding in spherulites of polymers having uncompensated main-chain chirality

The study of twisting crystalline orientation in banded polymer spherulites has recently been revitalized by availability of optically active macromolecules that crystallize exclusively in isochiral (uncompensated) helical conformations. In such cases, handedness of twisting appears to be rigidly controlled by chirality of molecular conformation, and the twisting itself appears to derive from causes other than those believed to apply in optically inactive polymers. We discuss and evaluate methods for determining handedness of twisting, particularly in tightly banded spherulites, and illustrate them by application to poly(3-hydroxybutyrate), poly(3-hydroxyvalerate), and to enantiomeric polymer pairs in the cases of poly(epichlorohydrin) and poly(propylene oxide). Although rigidly controlled twisting in such polymers commonly shares the handedness of the underlying molecular conformation we have clear evidence in poly(3-hydroxyvalerate) that such a correlation is not universal.