Theory of nematic comb-like polymers

Abstract

Comb polymers with semi-flexible main chains and rod-like nematogenic side chains form interesting nematic phases, largely as a consequence of competition between polymer entropy and rod order. The authors model this competition taking main chains of various stiffness and side groups of various length. Depending on volume fractions, temperature, nematic coupling and stiffness they get one of three nematic phases which they call N_I, N_II or N_III. They find that at least one component, main or side chain, must be ordered toward a direction, i.e. have a positive order parameter, and the order parameter can be positive or negative. They identify the molecular trends leading to each possibility. The theory, in addition to complex phase diagrams, also predicts unusual properties such as anomalous temperature variation of optical anisotropy and molecular conformational changes. The latter, amenable to small angle neutron scattering, shows a prolate (rod-like) form for the main chain dimension in the N_II and N_III phases where the main chain order parameter is positive. The oblate (disk-like) form, the N_I phase, is examined in some detail.