Thermotropic Polymorphism in Liquid-Crystalline Lead(II) Alkanoates

Abstract

Lead(II) carboxylates with even chain lengths from octanoate to octadecanoate have been investigated by differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction, and dilatometry, together with Raman, infrared, and ²⁰⁷Pb N.M.R. spectroscopy. The reassignment of the high temperature mesophase as smectic A (L_x) for the octanoate to dodecanoate is confirmed. This phase is most stable for the decanoate. The intermediate temperature mesophase is a highly ordered three dimensional lamellar structure based on ionic layers. The alkyl chains in this phase are partially disordered (melted) through the introduction of gauche conformations and show decreased lateral interactions; however, they remain largely in an extended conformation due to the constraints imposed by the ionic network. Melting of the alkyl chains occurs therefore in more than one step. Re-cooling to the crystalline phase appears to result in a different polymorphic crystalline form (albeit still lamellar) from the original.