Influence of alignment of crystalline confining surfaces on static forces and shear in a liquid crystal, 4 '-n-pentyl-4-cyanobiphenyl

Static force-distance relations as well as linear and nonlinear viscoelasti c responses to oscillatory shear were studied in 4'-n-pentyl-4-cyanobipheny l (5CB) confined between two muscovite mica surfaces at 25 degreesC. The or ientation of the crystallographic axes of the mica sheets was varied from c lose to perfect alignment to a twist angle of theta > 80 degrees, and the s liding direction was kept parallel to the gamma optical axis of one mica sh eet. The layering was unaffected by twist angle when the film thickness exc eeded three molecular layers of 5CB in a planar orientation, but for two an d three layers the adhesive minima in the oscillatory force-distance curve decreased in magnitude with increasing theta. In the linear viscoelastic re sponse (obtained with shear deformations of <20% of the film thickness), an elastic response dominated at shear frequencies of 0.13-130 Hz and small < theta>, whereas a more liquidlike response appeared at low frequency for la rge 8. Apparent discrepancies between the shear responses of alkylcyanobiph enyl films obtained in other investigations were resolved: during shear def ormations of large amplitude, either stick-slip or smooth sliding was obser ved, depending on the number of layers but not on the surface alignment. At the him thickness of two layers, we observed stick-slip above a maximum li miting strain of 0.5 (at the smallest 8), whereas at three layers, the slid ing was continuous with shear thinning at strains larger than 0.7. The effe ctive shear moduli and limiting shear stress decreased with increasing film thickness and misalignment. In contrast to the known friction behavior of muscovite mica in the absence of an intervening fluid layer, no local extre ma were observed at theta = 30 degrees and 60 degrees, indicating that the shear response resulted from the structure of the film of anisotropic molec ules and not directly from the surface crystal lattice.