Electric field‐induced instabilities in nematic and cholesteric polyesters

Electric field‐induced instabilities are investigated in the thermotropic mesophases of polyesters synthesized from 4,4′‐isopropenylenediphenol and adipic acid (polyester 1) or adipic acid and (+)‐3‐methyladipic acid (50:50) (copolyester 2). D. C. and low frequency A. C. voltages are applied. Polyester 1 behaves like a conductive nematogen with negative dielectric anisotropy. The Williams domain pattern and the dynamic scattering mode are observed; at about 5 kHz the existence of a cut‐off frequency is foreseen. According to the behavior of low molecular weight cholesterics, copolyester 2 goes from a predominantly planar texture to a focal conic one when the electric field is applied parallel to the helical axis of the planar texture. Periodic perturbations arise in polyester 1 doped with cholesteric copolyester 2 when the field is applied perpendicular to a cholesteric planar texture with oily streaks. The frequency dependence of the threshold voltage Vt for square grid is reported. As in nematic polyester 1 at about 5–10 kHz Vt appears to diverge. The behavior above Vt is in line with the results reported for low molecular weight nematic‐cholesteric mixtures with εa < 0. The square grid pattern is replaced by the tilted polygonal texture which relaxes when the field is turned‐off. Due to the higher viscosity of the polymer the recovery of the planar texture is slower than in the case of conventional mesogens.