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

A liquid crystalline (LC) copolyether has been synthesized from 1-(4-hydrox y-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 ali gnment can be induced by a thin film surface in the LC phase. When an elect rostatic field is applied to the surface-induced mono-domains parallel to t he thin film surface normal, the molecular alignment undergoes a change fro m the homeotropic to uniaxial homogeneous arrangement. However, when the fi eld is applied to a direction perpendicular to the thin film surface normal . the molecular alignment is about 10 degrees -tilt with respect to the hom eotropic 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 degrees away from the molecular axis , which leads to a negative dielectric anisotropy. It is speculated that th e 10 degrees- rather than 20 degrees -tilt is due to a balance between the alignment induced by the electrostatic field and the surface. In the electr ostatic field, molecules are subjected to a torque tau, which is determined by the permanent dipole moment P and the electrostatic field E: tau = P x E. The molecular realignment in both parallel and perpendicular directions to the thin film surface normal is determined by satisfying the condition o f tau = P x E = 0. (C) 2001 Elsevier Science Ltd. All rights reserved.