Studies on Polymer Nanocomposites as Shear Thickening Fluids

Abstract

The shear thickening fluids (STF) are dispersions of hard inorganic particles in a liquid medium with a unique combination of properties and are well known for their ability to absorb large impact shear/forces along with reduced weight. When STF is subjected to an impact, at typically high shear rates, it suddenly exhibits a discontinuous increase in viscosity with reversibility. Thus, STF can be utilized in the design of control and damping devices including shock absorbers for sports equipment, clutches, brakes and seismic protectors for buildings etc, due to its ability to self-limit maximum rate of flow. There has been a lot of interest in using STF in liquid body armor by impregnating high performance aramid fabrics such as Kevlar with STF. The impregnated body armor could provide rigidity for resisting piercing by stabbing knife blows, bullets and similar attacks yet allowing the flexibility and reduced weight to the wearer. For shear thickening properties at high shear rates, the attractive forces (depletion and hydrodynamic forces) should be higher than repulsive forces (electrostatic and polymer stabilization forces) in the STF suspensions. The STF reported in literature containing hard inorganic particles dispersed in low viscosity polymers prevent the use of STF for all conditions as they do not provide shear thickening at extreme conditions due to small range of operating shear rates. Modification of conventional STF with improved elasticity and stability is very important and can be used to develop STF for improved or new applications. There are several approaches that can be used to modify the properties of STF. Two typical approaches for this can be distinguished. The first approach involves modifying the hydrodynamic forces by adding high aspect ratio particles and the second approach involves modifying the particle-liquid interaction and depletion forces by refining the liquid medium properties with liquids having specific chemical functionality such as high viscosity polymers. Afo