ИСТИНА

A study was carried out of the rheological and thermophysical properties of individual nematogenic p-alkyl- and p-alkoxybenzoic acids as well as their binary mixtures. Alkoxybenzoic acids are characterized by higher viscosity, activation energy of viscous flow, melting and clearing temperatures compared to alkylbenzoic acids with comparable molecular weight. Mixing the alkyl- and alkoxybenzoic acids leads to the formation of a homogeneous solution, this is characterized by a lower melting point, as compared to the melting points of individual compounds, and a clearing temperature, which is intermediate in value. In mixtures of nematogenic acids, a polymorphism is found, consisting in the formation of a smectic mesophase in addition to the nematic one. The smectic mesophase is characterized not only by a many times higher viscosity compared with the nematic one, but also by viscoplastic behavior, which is expressed in the presence of yield stress and viscoelasticity. Varying the ratio of acids in a mixture affects transition temperatures: an increase in the proportion of the lower-melting component shifts the transition points to the low-temperature region — insignificantly in the case of the melting temperature and greatly in the cases of clearing and polymorphic-transition temperatures. In mixtures of acids, regardless of component ratio, neither rheological nor calorimetric methods reveal transitions characteristic of individual compounds. Alkyl- and alkoxybenzoic acids in mixtures always form unsymmetrical dimers using hydrogen bonds. The existence of such dimers leads to a new complex of properties, and the presence of additionally symmetric dimers in the system does not lead to phase separation and additively affects the properties. Thus, the use of alkyl and alkoxybenzoic acids in mixtures allows one to expand the region of existence of the liquid crystalline state and to obtain a more highly ordered mesophase.