Morphological Development in Alkyl-Substituted Semiflexible Polymers

Abstract

Nonsymmetrically substituted semiflexible polymers were synthesized by reacting (hydroxypropyl)cellulose (HPC) with hexyl, octyl, dodecyl, and octadecyl isocyanates. X-ray diffraction studies indicate that the polymer chains are assembled in regular, layered arrays. Polymers with six and eight carbons in the side chains (C6HPC and C8HPC) showed spherulitic structures typical of common crystalline polymers when examined by a polarized optical microscope (POM), whereas liquid crystalline (LC) textures were observed for C12HPC and C18HPC. When the polymers were slowly cooled from the melt, crystalline (C) textures could be obtained for C12HPC and C18HPC at preselected temperatures slightly higher than the corresponding (isotropic → LC) transition temperatures determined by DSC cooling scans. Upon further cooling, liquid crystalline structures appeared at temperatures 5−6 °C lower. The unusual POM observations are in contrast to conventional thermal transitions reported for liquid crystalline polymers, where the isotropic → LC transition takes place at a temperature higher than that of the LC → C transition, but are consistent with Keller's studies on another liquid crystalline polymer.