Electrostatic-field-induced chain alignment of liquid crystalline copolyether TPP thin films

Abstract

A liquid crystalline (LC) copolyether has been synthesized from 1-(4-hydroxy-4′-biphenyl)-2-(4-hydroxyphenyl)propane with 1,7-dibromoheptane and 1,11-dibromoundecane with a 50/50 (both in %) equal composition of the 7- and 11-methylene monomers [coTPP-7/11(5/5)]. A mono-domain with a homeotropic alignment can be induced by a thin film surface in the LC phase. When an electrostatic field is applied to the surface-induced mono-domains parallel to the thin film surface normal, the molecular alignment undergoes a change from the homeotropic to uniaxial homogeneous arrangement. However, when the field is applied to a direction perpendicular to the thin film surface normal, the molecular alignment is about 10°-tilt with respect to the homeotropic alignment toward the a ∗-axis. This is because the permanent dipole moment of the copolyether is not right vertical to the molecular direction. The calculation of molecular dipoles indicates that the permanent dipole moment of this copolyether is about 70° away from the molecular axis, which leads to a negative dielectric anisotropy. It is speculated that the 10°- rather than 20°-tilt is due to a balance between the alignment induced by the electrostatic field and the surface. In the electrostatic field, molecules are subjected to a torque τ, which is determined by the permanent dipole moment P and the electrostatic field E: τ= P × E . The molecular realignment in both parallel and perpendicular directions to the thin film surface normal is determined by satisfying the condition of τ= P × E =0.