ON THE THEORY OF NEMATIC ORDERING OF COMB -LIKE ELASTOMERS

A thermodynamic model is proposed that describes the rubberlike state of a comblike polymer with mesogenic fragments in the side chains. The model is based on representation of the free energy of the system as a function of the relative elongation and the nematic order parameter, which belong to the class of observable thermodynamic coordinates. Th e nematic ordering of a uniaxially deformed elastomer was studied. The possible types of comblike polymers, differing by the degree of intra molecular cooperativity of mesogenes, are classified with respect to c haracteristic orientational response to the uniaxial deformation. It i s shown that cooling of a weakly deformed elastomer may induce the fir st-order phase transition from a weakly oriented para-nematic phase to a strongly ordered nematic phase. Depending on the type of the effect ive orienting field produced by the deformed network and acting upon m esogenes, the strongly ordered phase may exhibit either a uniaxial nem atic character with mesogenes aligned predominantly in the deformation axis, or a biaxial nematic character, whereby the optical axis of the system (i.e., the direction of preferred orientation of the mesogenes ) is perpendicular to the direction of elastomer deformation. A phase diagram of the system studied is constructed in the coordinates of eff ective orienting field versus temperature.