304 The Influence of External Electric Field on Local Orientations and Phase Transitions in Polymer Liquid Crystals (PLCs) Witold Brostow,* 1 Kazimierz Hibner, 2 Janusz Walasek 1, 2 1 Departments of Materials Science and Chemistry, University of North Texas, Denton, TX 76203-5310, USA 2 Department of Physics, Technical University of Radom, 26-600 Radom, Poland 1. Introduction The nature of liquid-crystallinity in polymer liquid crys- tals (PLCs) has been studied using two distinct approaches: lattice and nonlattice. The first method was preferred by Flory who formulated the theory of the athermal PLC systems in his now classic 1956 papers. [3, 4] Flory’s model deals with semiflexible longitudinal PLC chain macromolecules. That is, each chain consists of relatively rigid LC and flexible (F) polymer sequences, with both kinds of sequences occurring in the main chain and the LC sequences oriented along the chain back- bone. [5–7] Longitudinal PLCs are much different from comb or side chain PLCs – such as investigated experi- mentally for instance by Springer, Zugenmaier and co- workers. [8–11] They are also different from double PLCs, with LC sequences both in main and side chains, studied in particular by the University of Halle group [12–16] and also by the Springer group. [17, 18] In the Flory model the macromolecule is represented by a set of lattice cells. He proposed an ingenious proce- dure to calculate the system probability. [3, 4] The model requires a small deviation (no larger than 30 8) of LC “hard rods” from a direction of the so-called system sym- metry axis; it is the axis of the system cylindrical symme- try. The original theory was subsequently amplified by Flory, his collaborators and students. [19–33] However, an extension of the model to larger angles including 90 8 seems to be rather artificial. A nonlattice approach was proposed by de Gennes. [34] A PLC system is represented by three semiflexible chains Full Paper: The starting point is our previous study of influence of the internal molecular mean field of dipole- dipole interactions on local orientation and phase transi- tions in polymer liquid crystal (PLC) systems of longitudi- nal chains. [1, 2] Electric dipoles are created by LC mesogen moieties. The longitudinal PLC is a macromolecule of consecutively copolymerized LC and flexible polymer sequences. We now amplify the model by inclusion of dipole-external electric field interactions. We find that the external fields can seriously modify the local orientational order of the system and affect phase transition parameters dependent on that order. In particular, the external fields induce the formation of disoriented nematic phases with negative values of the second order orientation parameter pP 2 P for LC sequences in the longitudinal PLCs while the first order parameter pP 1 P is positive. However, some rapid decreases in pP 1 P are observed at points of positive- to-negative transitions of pP 2 P; thus the LC disorientation manifests itself. The limiting case of the monomer liquid crystal (MLC) systems is included also. Macromol. Theory Simul. 2001, 10, No. 4 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1344/2001/0404–0304$17.50+.50/0 The PLC phase diagram in terms of parameters u 1 h and j/pnP (after ref. [2] ). Macromol. Theory Simul. 2001, 10, 304–313