On the thread-like morphology of LC/I block copolymers in nematic solvents

AB and ABA block copolymers are synthesized by direct living anionic polymerization where the A-block is a liquid crystalline side chain polymer (LC-block) and the B-block is an isotropic polymer (I-block). Similar to amphiphilic block copolymers with a "hydrophilic" water soluble segment and a "hydrophobic" water insoluble part, the LC/I block copolymers consist of the "nematophilic" LC-block that is soluble in a nematic solvent and the "nematophobic" segment that is insoluble in nematic solvents. The behaviour of these LC/I block copolymers and corresponding homopolymers is analyzed in dilute nematic solutions of low molar mass liquid crystals. The effects of varying molecular weight, chemical constitution of blocks and block length ratio are investigated. Copolymers with a high fraction of the I-block are insoluble in the nematic solvent as indicated by phase separation. Copolymers with a high fraction of the LC-block are completely soluble. If the weight fraction of the LC-block is in the range of 0.46 to 0.85, thermoreversible thread-like aggregates are formed in the nematic solvent. These thread-like aggregates have a uniform diameter of 2-3 mu m and their long axis perfectly follows the director field of the nematic matrix. The formation of these structures is discussed in terms of copolymer concentration, temperature and influence of aligning surfaces. From simple geometrical packing arguments it can be assumed that these thread-like aggregates consist of bilayered vesicles. In contrast to the spherical shape of amphiphilic block copolymer vesicles in aqueous solution, the rod-like shape of vesicles in nematic solution is caused by the minimization of free elastic energy density of the nematic matrix.