Electrorheological Fluids Based on Porous Carboxyl-Functionalized Polytriphenylamines

Electrorheological fluids (ER) are a class of smart materials that are distinguished by their capacity to alter their rheological characteristics in a controlled and reversible manner in response to an externally applied electric field (E). As an inherent polarizable anhydrous ER-active material, polyaniline (PAni)-based materials are among the most frequently utilized ER-active materials. However, PAni can only be used as an ER-active material after carefully adjusting its conductivity to an appropriate range. Three-dimensional (3D) conjugated microporous polymer (CMP) analogs of PAni have a nitrogen-rich porous and hierarchical structure, low density, and appropriate conductivity that can be used to explore ER performance and dispersion stability. In this study, a carboxylic acid functionalized version with a greater polar porous surface was designed to obtain higher polarizability and appropriate conductivity without the need for any dedoping process and thus enhanced ER performance. For this purpose, polytriphenylamine (PTPA) and PTPA-COOH were synthesized as ER-active materials by Buchwald-Hartwig cross-coupling. The structural, morphological, electrical, microstructural, and surface properties were investigated in detail before the ER performance was determined. Dispersions of the CMPs were prepared in silicon oil and examined under different E by measuring shear stress, shear viscosity, and moduli values. PTPA-COOH dispersion with 10 wt% concentration exhibited excellent dispersion stability of 99% and enhanced ER performance, including high shear stress (static yield stress of 370 Pa at 3.5 kV/mm), repeatable and reversible electric field response, and obvious dielectric loss peak (relaxation time of 0.01 s). This class of functionalized 3D analogs of PAni shows significant promise ER-active materials for applications in smart fluids.