Combined main chain/side chain polymers. A new class of liquid crystalline polymers with unusual structural, thermodynamic and dynamic properties

Abstract

Combined main chain/side chain polymers carry rigid rod-like mesogenic units both in the side chain and along the chain backbone. The properties of such combined systems are expected to be controlled by the competition of the interactions between the mesogenic units in the side and main chains as well as the tendency of the chain backbones to maximize their entropy. The thermodynamic, structural and dynamic properties of such combined main chain/side chain liquid-crystalline polymers were investigated for various chain architectures. The combined systems were found to display properties corresponding in many cases to a favourable superposition of properties characteristic of both main and side chain systems. These include high transition temperatures (up to 530 K) and broad ranges of stability of liquid-crystalline phases (up to about ΔT = 170 K). The orientational order parameter was found to approach values of 0.9 and it was possible to orient some of the polymers in weak external fields such as 1 T for instance, for magnetic fields. Smectic antiphase formation corresponding to that observed in low molar mass systems carrying strong longitudinal dipoles and resulting from the presence of incommensurable lengths were observed in those cases in which the side groups were attached directly to the rigid part of the chain backbone whereas no such effects occurred if the side groups were fixed to the flexible part of the main chain. The reorientational motion of the mesogens about their short axis, which couples to dielectric and electro-optical properties was found to have a surprisingly low activation energy (as low as 115 kJ/mol) and a strong anisotropy of the dielectric constant (Δε = 7) despite a relatively small concentration of dipoles. It thus seems that the combined systems may be the material of choice for many future applications.