CRYSTALLIZATION OF MONOTROPIC LIQUID-CRYSTALLINE POLYCARBONATES BASEDON A METHYL-STILBENE MESOGEN AND A METHYLENE-CONTAINING FLEXIBLE SPACER

Crystallization behavior has been studied for a series of recently syn thesized liquid crystalline polycarbonates1 based on a methyl-substitu ted stilbene mesogen and a methylene-containing flexible spacer. The s tability of the nematic phase and its relationship to three-dimensiona l crystallinity were studied. Development of crystallinity was followe d by isothermal and nonisothermal crystallization using differential s canning calorimetry (DSC) and polarizing optical microscopy. A monotro pic liquid crystal phase is identified for these polycarbonates. The m ultiple melting endotherms seen in the heating scan are assigned to th e melting of crystals forming either from the mesophase (melting at hi gher temperature) or from the isotropic phase (melting at lower temper ature). The number, n, of -(CH2)- units in the flexible spacer affects the transition temperatures seen in the cooling and heating scans. On ly a weak odd-even effect is seen in the upper melting of the heating scan from n = 5 to 8. In this range of spacer length, the region of st ability of the liquid crystal phase seen in the cooling scan becomes s maller. When the length of the spacer becomes greater than that of the mesogen (n greater-than-or-equal-to 9), all the transition temperatur es drop.