Optically Active Polyethers. 3. On the Relationship between Main-Chain Chirality and the Lamellar Morphology of Solution-Grown Single Crystals

Abstract

The morphology of chiral (R)- and (S)-poly(epichlorohydrin) (PECH) single crystals grown from solution was studied to obtain insight into the mechanism by which the chiral character of the main chain can be transmitted to higher organizational levels, such as the lamellar twisting orientation in banded spherulites. Poor solvents were selected so that the crystallization from solution occurred at high temperatures, where the resulting lamellar habits simulate most closely those obtained in crystallization from the melt. Interestingly, the TEM examination of folded-chain single-crystal lamellae showed no morphological evidence that could be related to the chiral character, R or S, of the polyenantiomer. In addition, it was established that the handedness (or sense) of screw dislocations in these lamellae is not controlled by the main-chain chirality. This result supports previous evidence which shows that, although the screw dislocations may contribute to the banding through lamellar branching, they appear not to be primary determinants of either the handedness or magnitude of the twist of lamellae in banded spherulites. Similar morphology patterns were obtained for lamellar single crystals of chiral (R)- and (S)-poly(propylene oxide) and of the optically active biopolyester, (R)-poly(3-hydroxyvalerate), supporting the generality of these features. In searching further for chiral effects, the examination of lamellar morphology in large PECH aggregates, e.g., highly dendritic crystals and hedrites, grown from solution indicated that some of the lamellae are nonplanar while in solution. Some considerations pertaining to the factors that may contribute to the nonplanar habits observed in the morphology of these large aggregates are discussed as well.