| Abstract
| - Surface lamellar decoration (SLD), surface enhanced Raman scattering (SERS) and optical secondharmonic generation (SHG) experiments have been utilized to study the molecular orientation and conformationchanges at a rubbed polyimide alignment-layer surface. This aromatic polyimide containing pendentcyanobiphenyl mesogens was synthesized via a polycondensation of 2,2‘-bis(3,4-dicarboxy-phenyl)hexafluoropropane dianhydride (6FDA) with bis{ω-[4-(4-cyanophenyl)phenoxy]hexyl} 4,4‘-diamino-2,2‘-biphenyldicarboxylate (nCBBP, n = 6), abbreviated as 6FDA−6CBBP. Uniform alignment layers, possessing high pretiltangles ranging from 39° to 43°, have been achieved after mechanical rubbing of the polyimide thin film surfaceat room temperature and subsequent annealing. This is the first time that high pretilt angles have been detectedto possess a negative angle (−θc) with respect to the rubbing direction (i.e., opposite to the rubbing direction),considerably different from the conventional pretilt angle (θc) observed along the rubbing direction. Thisobservation is confirmed using magnetic null and SHG methods. Combined polyethylene (PE) SLD and atomicforce microscopy experiments reveal that the azimuthal orientation distribution of the long axis of the edge-onPE lamellar crystals is oriented normal to the rubbing direction, indicating that the PE chains are alignedparallel to the rubbing direction. This SLD technique probes the anisotropic surface orientation of the outermostmolecules of the rubbed polyimide layer. The SERS results show that prior to rubbing the surface, both thependent cyanobiphenyls in the side chains and backbones possess nearly planar chain conformations at thepolyimide surface. Mechanical rubbing causes not only tilting of the backbone moieties, such as imide-phenylenestructure, but also significant conformational rearrangements of the pendent side chains at the surfaces. Themolecular mechanism of this unusual alignment is due to the fact that the pendent cyanobiphenyls forms auniformly tilted conformation on the rubbed surface, and the polar cyano groups point down toward the layersurface deduced from SHG phase measurements. This conformational rearrangement of the side chains resultsin the formation of fold-like bent structures on the surface, which directly leads to the long axis of cyanobiphenylshaving the −θc pretilt angle with respect to the rubbing direction.
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