Diminished Splay Stiffening in Weak Gels of Calamitic-Bent-Core Nematic Composites

Composites of calamitic and bent-core nematic molecules exhibiting a nematic to nematic-gel transformation have been investigated using thermal, electrical, X-ray, and mechanical probes. The studies focusing on the Frank elastic behavior show a surprising result that the thermal behavior of the threshold voltage and the dependent splay elastic constant (K11) are remarkably different in temperature regions identified as weak and strong gels. In the former gel, the parameters exhibit values significantly smaller than the higher-temperature fluid nematic, effectively canceling out the underlying thermal variation due to the order parameter. This is especially attractive from the viewpoint of display devices. The X-ray diffraction data suggest that the fibers have a plastic nature in the weak gel and 3D-crystalline ordering in the strong gel. We argue that the different elastic behavior in the two gel phases is caused by the nature of the fibers; they are stiff in both gels but the interfiber interaction is weaker in the weak gel allowing the splay elastic constant to be lowered. The X-ray and rheological data lend support to the characterization of the fibers.