LIQUID-CRYSTAL ALIGNMENT ON POLYTETRAFLUOROETHYLENE AND HIGH-DENSITY POLYETHYLENE THIN-FILMS STUDIED BY OPTICAL 2ND-HARMONIC GENERATION

We have used optical second-harmonic generation to study surface molec ular order in a liquid-crystal (4'-n-octyl-4-cyano-biphenyl, or 8CB) o n shear-deposited polymer films, The films are highly oriented layers of polytetrafluoroethylene (PTFE) or high-density polyethylene (HDPE), with a surface topology of uniaxially aligned nanoscale ridges and gr ooves, which are used as versatile substrates for oriented growth and alignment of other materials. In nematic 8CB cells made with either po lymer, the surface monolayers of 8CB were aligned along the polymer or ientation axis, and showed C-2 nu symmetry. In the isotropic phase, th e surface monolayer alignment in these cells was lost. Monolayers of 8 CB evaporated onto either polymer showed little or no alignment. These data indicate that the PTFE and HDPE films do not produce the strong epitaxylike alignment seen on some cloth-rubbed polymer surfaces. Inst ead, alignment appears to be primarily caused by surface ridges throug h an elastic, bulk-mediated mechanism. (C) 1998 American Institute of Physics.