Molecular Design, Synthesis, and Characterization of Liquid Crystal−Coil Diblock Copolymers with Azobenzene Side Groups

Abstract

The synthesis and characterization of a family of well-defined liquid crystal−coil (LC−coil) diblock copolymers have been carried out. The block copolymers in this study have been designed to have nearly identical molecular weight azobenzene-containing LC blocks in order to eliminate possible variations in LC behavior caused by the differences in the LC block molecular weight. Quantitative hydroboration chemistry was used to convert the pendent double bonds of an isoprene block to hydroxyl groups to which the mesogenic groups were attached via acid chloride coupling. The LC homopolymer and the block copolymers (LC volume fraction from fLC = 0.82 to fLC = 0.20) all exhibited smectic mesophases with similar clearing transition temperatures. The clearing transition enthalpies strongly depend on the block composition ratio and decrease as the LC block volume fraction decreases. Solvent-casting of a lamellar LC−coil copolymer (SICN5-66/60) resulted in an oriented bulk film in which both the axes of the mesogenic groups and the lamellar interfaces lie parallel to the film surfaces. A LC cylinder morphology was observed with a fLC = 0.22 LC-containing block (SICN5-176/55) using TEM and confirmed by SAXS measurements. This is the first observation with the LC block in a cylinder microdomain. Other morphologies (bicontinuous, LC sphere) were observed by TEM while retaining the smectic order in the LC microphase.