NEMATIC WETTING AT THE FREE-SURFACE OF 4-CYANO-4'-N-ALKYL-BIPHENYLS

The free surface of a homologous series of low molecular weight liquid crystals 3-cyano-4'-n-alkyl-biphenyls (nCB, n=5,6.7,8) is studied by reflection ellipsometry. On approaching the bulk isotropic-nematic tra nsition temperature T-NI from the isotropic phase, the growth of a nem atic layer at the surface is observed. The director orientation in the layer is identified as homeotropic. The ellipsometric data are analyz ed in terms of a perturbation calculation. It is shown that for 7CB an d 8CB very close to T-NI, where the layer has a thickness d greater th an or equal to 50 Angstrom, both d and the order parameter S-0 at the surface can be determined. In all other cases only one parameter, whic h corresponds to the nematic coverage, can be extracted from the data. Results are discussed in terms of a wetting process, Partial wetting by the nematic phase is observed for n =6,7.8. On approaching T-NI fro m lower temperatures where the bulk is in the nematic phase a small in crease of excess surface order is measured which is interpreted as a h omeotropic nematic wetting layer possessing higher order than the nema tic bulk. (C) 1995 American Institute of Physics.