Rubbing-induced molecular reorientation on an alignment surface of an aromatic polyimide containing cyanobiphenyl side chains

Surface lamellar decoration (SLD), surface enhanced Raman scattering (SERS) and optical second harmonic generation (SHG) experiments have been utilize d to study the molecular orientation and conformation changes at a rubbed p olyimide alignment-layer surface. This aromatic polyimide containing penden t cyanobiphenyl mesogens was synthesized via a polycondensation of 2,2'-bis (3,4-dicarboxy-phenyl)hexafluoropropane dianhydride (6FDA) with bis{omega-[ 4-(4-cyanophenyl)phenoxy]hexyl} 4,4'-diamino-2,2'-biphenyldicarboxylate (nC BBP, n = 6), abbreviated as 6FDA-6CBBP. Uniform alignment layers, possessin g high pretilt angles ranging from 39 degrees to 43 degrees, have been achi eved after mechanical rubbing of the polyimide thin film surface at room te mperature and subsequent annealing, This is the first time that high pretil t angles have been detected to possess a negative angle (-theta (c)) with r espect to the rubbing direction (i.e., opposite to the rubbing direction), considerably different from the conventional pretilt angle (theta (c)) obse rved along the rubbing direction. This observation is confirmed using magne tic null and SHG methods. Combined polyethylene (PE) SLD and atomic force m icroscopy experiments reveal that the azimuthal orientation distribution of the long axis of the edge-on PE lamellar crystals is oriented normal to th e rubbing direction, indicating that the PE chains are aligned parallel to the rubbing direction. This SLD technique probes the anisotropic surface or ientation of the outermost molecules of the rubbed polyimide layer. The SER S results show that prior to rubbing the surface, both the pendent cyanobip henyls in the side chains and backbones possess nearly planar chain conform ations at the polyimide surface. Mechanical rubbing causes not only tilting of the backbone moieties, such as imide-phenylene structure. but also sign ificant conformational rearrangements of the pendent side chains at the sur faces. The molecular mechanism of this unusual alignment is due to the fact that the pendent cyanobiphenyls forms a uniformly tilted conformation on t he rubbed surface, and the polar cyano groups point down toward the layer s urface deduced from SHG phase measurements. This conformational rearrangeme nt of the side chains results in the formation of fold-like bent structures on the surface, which directly leads to the long axis of cyanobiphenyls ha ving the -theta (c) pretilt angle with respect to the rubbing direction.