Glass Transition Dynamics and Crystallization Kinetics in the Smectic Liquid Crystal 4-n-Butyloxybenzylidene-4'-n'-octylaniline (BBOA).

Abstract

The molecular dynamics of 4-n-butyloxybenzylidene-4'-n'-octylaniline (BBOA, abbreviated also as 4O.8) was studied by broadband dielectric spectroscopy (BDS) for samples that were exposed to various thermal treatments. Phase transitions between liquid crystalline phases (N, SmA, SmBhex, and SmBCr) were evidenced by abrupt changes in the temperature dependence of the dielectric permittivity spectra and dielectric relaxation times. A particularly complex dynamic behavior was revealed for the highly ordered SmBCr phase that showed clear evidence for cooperative dynamics of a glass-forming liquid as manifested by a Vogel-Fulcher-Tammann (VFT)-type temperature dependence of its structural relaxation time τ(T). At low temperatures, dependence τ(T) again changes from VFT to Arrhenius behavior, a phenomenon commonly observed for supercooled liquids confined to nanometer length scales and occasionally discussed as strong-fragile transition on heating. In this context, our observation supports the idea of partial orientation disorder in a quenched SmBCr phase, where the length scale of cooperative motions is restricted to the nanometer size of glassy domains causing the deviation from "bulk" VFT-type behavior. Finally, the isothermal crystallization kinetics of the metastable SmBCr phase was studied in detail. On the basis of structural information about the SmBhex and SmBCr phases, determined by X-ray diffraction, the subtle relation between molecular order and relaxation behavior is discussed.