Liquid Crystalline Properties of Some Monomeric Azo- and Azoxybenzenes and Their Polymers

-We have studied the mesomorphism of p-acryloyland p-methacryloy1-oxy-p'substituted azoand azoxybenzenes. Polymers, obtained by free radical polymerization of these compounds in solution, give anisotropic melts at T,. According tox-ray diffraction data the polymers are characterized by a smectic structure. There is no correlation between the thermal behaviour of monomers and polymers. The optical properties of glassy polymers depend on the nature of solvent. Mesomorphism in these polymers is probably due to the interaction between mesogenic side groups within the macromolecule. The characteristic feature of macromolecules is known to be flexibility which depends on the chemical structure of units and governs the physical properties of polymers. It follows that liquid crystalline behaviour of polymers, in particular the polymers with mesogenic side groups, would depend on chemical structure of monomers, including the nature of the polymerizable group. Naturally, the structure of a monomer is to satisfy the principal requirements for mesomorphic compounds [l]. At present some theoretical conclusions are available, based on extensive experimental data regarding mesomorphism of low molecular weight mesophases, depending on the structure of molecules, particularly on the nature of terminal groups and linkages. They enable to synthesize new compounds with desired liquid crystalline properties. Lack of similar information for polymeric liquid crystals is due to, first of all, the fact that intense investigations at such polymers were begun quite recently. The present paper is devoted to the study of mesomorphism of polymers depending on both chemical structure of the main chain and para-substitutent of the mesogenic side group. It is also of interest to compare the mesomorphic behaviour of monomers and their saturated analogues. For this purpose some p-acryloyland p-methacryloyloxy p'-substituted azoand azoxybenzenes were prepared, shown in table I. 1 . Mesomorphous behaviour of monomers. Of the twelve monomeric azomethines, described by Ringsdorf et al. [2], only five were mesomorphic : four acrylates and one methacrylate. The acrylates proved to have wider mesophase range than their saturated analogues-propionates [3]. In case of azo compounds, acrylates and propionates are inclined to mesomorphism to a greater extent than methacrylates. This tendency increases in replacing the azo group by the azoxy group. The acrylate group as compared with propionate one influences a mesophase thermal stability of azobenzenes in different way depending on the nature of p'-substitutent : the p'-alkoxy substituted acrylates and propionates [4] have practically the same clearing temperature, T,,; whereas the acetyl substituted acrylate has higher T,, than its saturated analogue ; melting temperature (T,) of the ethyl substituted acrylate is lower than T, of the propionate. Azoxy monomers are characterized by higher thermal stability of mesophase (except compound 4) than the corresponding azo monomers, as it is the case with the majority of known azoxy benzenes with saturated p,p'-substitutents [4]. 2. Mesomorphism of polymers. The polymers were obtained by free radical polymerization in dioxane solution at 60 OC using benzoyl peroxide as a initiator. The polymerization products were Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979394