HYDRODYNAMIC AND NONHYDRODYNAMIC BEHAVIOR OF LAYER-COMPRESSION MODULUS-B AT THE NEMATIC-SMECTIC-A PHASE-TRANSITION IN 8OCB

We present dynamic compression experiments which show that layer-compr ession modulus B does not exhibit dispersion in the 100 Hz-l kHz domai n and varies with temperature according to a simple power law over fou r decades. These results indicate that the saturation effect predicted by Nelson and Toner does not exist for this compound, and that the th ermal variation of B determined at higher frequencies by a second-soun d technique is not hydrodynamic, contrary to what is claimed. We show that it is affected by two relaxation mechanisms.