Dynamical signature of two "ideal glass transitions" in nematic liquid crystals

A temperature scaling analysis using the same mode coupling theory (MCT) scaling relationships employed for supercooled liquids is applied to optical heterodyne detected optical Kerr effect data for four liquid crystals. The data cover a range of times from similar to1 ps to 100 ns and a range of temperatures from similar to50 K above the isotropic to nematic phase transition temperature T-NI down to similar toT(NI). The slowest exponential component of the data obeys the Landau-de Gennes (LdG) theory for the isotropic phase of liquid crystals. However, it is also found that the liquid crystal data obey MCT scaling relationships, but, instead of a single scaling temperature T-C as found for supercooled liquids, in the liquid crystals there are two scaling temperatures T-CL (L for low temperature) and T-CH (H for high temperature). T-CH is very close to T*, which results from LdG scaling, just below the isotropic to nematic phase transition temperature, T-NI, but is 30-50 K higher than T-CL. The liquid crystal time dependent data have the identical functional form as supercooled liquid data, that is, a fast power law decay with temperature independent exponent, followed by a slower power law decay with temperature independent exponent, and on the longest time scales, an exponential decay with highly temperature dependent decay constant. For each liquid crystal, the amplitudes of the two power laws scale with expressions that involve T-CL, but the exponential decay time constant (long time dynamics) scales with an expression that involves T-CH. The existence of two scaling temperatures can be interpreted as a signature of two "glass transitions" in liquid crystals. In ideal MCT developed for spheres, T-C is the "ideal glass transition temperature," although it is found experimentally to be similar to20%-30% above the experimental glass transition temperature, T-g. The transition in nematic liquid crystals at T-CL corresponds to the conventional ideal MCT glass transition, while the transition at T-CH can occur for nonspherical molecules, and may correspond to the freezing in of local nematic order. (C) 2003 American Institute of Physics.