DYNAMIC-MECHANICAL RELAXATION BEHAVIOR OF LOW-MOLECULAR-WEIGHT SIDE-CHAIN CYCLIC LIQUID-CRYSTALLINE COMPOUNDS NEAR THE GLASS-TRANSITION TEMPERATURE

In this paper, we report the mechanical relaxation behavior of low mol ecular weight glass-forming side-chain liquid crystalline compounds wi th cyclic cores and their linear polymeric analogues near the glass tr ansition temperature. We examined two systems: one based on cyclic and linear siloxane backbones and one based on cyclic (cyclohexane) and l inear aliphatic backbones. Dynamic mechanical spectroscopy is used to measure the dynamic shear moduli and the complex viscosity near but ab ove T-g. The temperature dependence of the zero-shear viscosity of the cyclic compounds is well described by the Vogel-Tammann-Fulcher (VTF) equation, The strong temperature dependence of the viscosity along wi th the values of the fitted parameters of the VTF equation shows that the cyclic LC compounds are ''fragile'' liquids. All cyclic LC compoun ds, regardless of chemical structure, show identical relaxation behavi or when viscosity is plotted versus normalized temperature (T-g/T), wh ere T-g is the temperature obtained calorimetrically at a heating rate of 10 K/min. All cyclic compounds show lower viscosity than their lin ear analogues when plotted versus T-g/T. The difference in viscosity b etween the cyclic and linear siloxane compounds is much more pronounce d than the difference observed in the aliphatic compounds. For the cyc lic compounds, master curves of G' and G '' can be described by a sing le Maxwell mode. The linear compounds exhibit much broader mechanical spectra, suggesting a more complex relaxation phenomenon is taking pla ce. Our results show that, while there is little difference in relaxat ion behavior among low molecular weight cyclic liquid crystalline comp ounds, the behavior of the linear polymeric systems is quite different .