Thermotropic side-chain liquid crystalline copolymers containing both mono- and bisazobenzene mesogens: Synthesis and properties

The synthesis of a novel series of copolymers containing push-pull bisazobenzene (BAz) and monoazobenzene (MAz) groups with various N-alkyl donor groups and with spacers of two and four methylene units is presented, and the effects of the structural variables on the mesomorphic properties of the polymers are investigated. The copolymer with very short spacer (n = 2) and bulky N-ethyl group exhibits liquid crystalline (LC) properties when the BAz content is at least 30 mol %. Replacing the N-ethyl group with N-methyl increases the tendency toward smectic mesomorphism and expands the mesophases stability. If the flexible spacer is extended to four methylene units, the glass transition temperature (T-g) decreases strongly, whereas the isotropization temperature (T-i) is only slightly affected. X-ray diffraction investigations reveal an orthogonal partial bilayer smectic phase in which the mesogens are interdigitated. Annealing the BAz polymer films above T-g brings about the out-of-plane (homeotropic) orientation of chromophores and the formation of H-aggregates.