The Influence of External Electric Field on Local Orientations and Phase Transitions in Polymer Liquid Crystals (PLCs)

Abstract

The starting point is our previous study of influence of the internal molecular mean field of dipole‐dipole interactions on local orientation and phase transitions in polymer liquid crystal (PLC) systems of longitudinal 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 〈P2〉 for LC sequences in the longitudinal PLCs while the first order parameter 〈P1〉 is positive. However, some rapid decreases in 〈P1〉 are observed at points of positive‐to‐negative transitions of 〈P2〉; thus the LC disorientation manifests itself. The limiting case of the monomer liquid crystal (MLC) systems is included also.

Phase Behavior of Lyotropic Liquid-Crystalline Polymers under Varying Solvent Conditions: Effect of External Fields on the Phase Diagram

“…The sensitive response of liquid crystalline materials to external electric and magnetic fields lies at the center of their numerous practical applications. − Accordingly, it is of great importance to develop a detailed understanding of this response and the concomitant effect of external fields on the phase behavior of liquid cystals. As such, the effect of external fields on small-molecule nematogens has been actively studied both experimentally ,− and theoretically. − On the theory side, earlier works − employed continuum-type Landau–de Gennes theory, while more recent studies used a molecular-based Density Functional Theory , (DFT), either of Onsager–Parsons type , for lyotropic liquid crystals − or of Maier–Saupe type for thermotropic ones . In addition to analytical theoretical studies, several molecular simulations of liquid crystals in external fields were also reported. , …”

Thermomechanical processing environment and morphology development of a thermotropic polymer liquid crystal

“…[3][4][5][6] This segmental nature allows easy orientation of the rigid segments under external applied fields, whether electric, magnetic, or mechanical. [7][8][9][10][11][12][13] In fact, their thermotropic character allows them to be transformed by melt processing techniques, such as injection molding, because of their low viscosity resultant from the easy orientation of the rigid segments in the flowing melt. Because of high relaxation times of PLCs, the orientation induced by the flow cannot easily relax during subsequent cooling, leading to highly oriented structures (self-reinforced polymers) with exceptional mechanical behavior and highly anisotropic character.…”