Triborheology and orientational dynamics of ionic liquid crystals

We present studies of triborheological properties and orientational dynamics of three imidazolium-based ionic liquids (ILs) [C14mim][BF4], [C14mim][PF6] and [C14mim][Tf2N], by utilizing a variety of physical methods including DSC, SAXS/WAXS, rheology, flow dichroism and tribology. Results revealed that two ILs ([C14mim][BF4] and [C14mim][PF6]), which possess a liquid crystalline (LC) phase show ultralow friction and lower wear as poly-alpha-olefin (PAO) standard lubricant oil in the isotropic phase at 120 °C. Moreover, the [C14mim][BF4] IL exhibits also ultralow coefficient of friction (COF) in its LC phase. Such unique tribological behavior originates from bilayered domain structure, which is maintained by the specific molecular interactions and orientation in the shear flow. SAXS data revealed that cation tails in LC domains are interdigitated in a bilayer smectic phase. In strong shear flows LC domains tend to be oriented along the flow direction which is confirmed by sigmoidal growth of flow dichroism values. The dichroism signal does not relax even when the flow is stopped due to long-range orientational order in LC domains. The negative sign of the dichroism indicates orientation of anisotropic mobile cation tails perpendicular to the flow directions. This specific bilayered domain structure is well pronounced in LC phase and the "precursors" of these domains exist also in the isotropic phase, giving rise to ultralow friction regime in sliding contacts.