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. Aforesaid STF would combine the material properties of the additives. Current trends towards nanoparticles and viscoelastic polymers make performance of STF increasingly relevant. Thus, improvement of existing STF by additives provides an elegant route towards STF having new and advanced properties. vii In the present study the primary raw materials and processing conditions for STF have been extensively studied. Fumed silica (11nm) in Polyethylene glycol (PEG) having molecular weight 200g/mol, gives high shear thickening properties at very low concentrations as compared to dispersion of simple silica particles (400nm). Furthermore, considerable improvement in stability is seen for fumed silica samples with respect to sedimentation and shear thickening properties over time as compared to simple silica. The probe sonication of 10 minutes for 20ml sample of 20% fumed silica PEG-STF is an effective method to attain uniform and homogeneous dispersion and prior sonication for 2 min is required for dispersing clay particles in PEG. The 20% STF of fumed silica PEG demonstrated a better and stable dispersion as well as shear thickening properties suitable for studying the effect of different particles and liquids as additives. The rheological behaviors of STF have been investigated as these new STF exhibited better shear thickening behavior with respect to different parameters. Significantlly, the additives provide an opportunity to vary the shear thickening range of STF. The findings suggest that these STF could be useful for varying engineering applications where a range of critical viscosity and critical shear rate are required. Building on the base STF material of 20% of fumed silica in PEG200, the present study reports desirable viscoelastic behavior for the STF with different additives. The concentrations of these additives are much lower than fumed silica concentration. Among the nanoclays, MMT, Kaolin, Halloysite, NanomerI.28E and Closite®15A along with alumina particles were used as additives, Closite®15A and NanomerI.28E demonstrated significant enhancement in shear thickening properties in the steady as well as dynamic state and appreciable stability at low and high temperatures. Among different high molecular weight PEGs i.e. PEG4600, PEG6000 and PEG10000, as additives to PEG200, PEG with molecular weight of 6000 showed promising properties. Silicone oil of different viscosity, namely, 100, 500 and 1000cst in different concentrations were added to the primary liquid medium (PEG200) for processing of STF. Silicone oil being hydrophobic in nature required modification with PEG to interact with hydrophilic fumed silica therefore, silicone oil has only been used as additive. As viscosity of viii silicone oil increases, STF properties (critical viscosity) increase but at the cost of dispersion of fumed silica particles and stability with respect to temperature due to an increase in gel like behavior. Additionally, suspension of hydrophobic fumed silica as primary particles instead of hydrophilic fumed silica with PEG200 and PPG425 (polypropylene glycol; molecular weight 425g/mol) have been studied. Hydrophilic fumed silica dispersed in semi-polar PPG causes an intense increase in value of critical viscosity at very low shear rates that confirm slight gel like behavior and later transforms to a gel after a week of storage whereas hydrophobic fumed silica in PPG gives smooth shear thickening properties with high stability. In general, it was found from steady-state rheology results that with an increase in temperature and time of storage, the shear tickening behavior decreases i.e critical values of viscosity decreases a lot. This decrease in shear thickening is more pronounced in high molecular weight PEGs and high viscosity silicone oil added as additives as compared to nano-clays and alumina particles added as additives. Shear thickening fluids have been studied under steady-shear deformations as well as oscillatory or dynamic shear deformations under different amplitudes and frequencies. As shear thickening phenomena is dependent on the velocity of impact (time rate of deformation), thus both strain and frequency sweep studies are also very important. Dynamic shear results showed that as concentration of additives increase, dynamic state parameters i.e. storage and loss modulus also increase and this increase is very significant for high molecular weight polymers used as additives compared to clay used as additives. It can be concluded that the distinct particles and liquids for STF may leads to diverse effects and sensitivity towards temperature as well as stability. Thus, cost effective and material utilization improvements can be made by using combination of fumed silica/silica and clays as dispersed particles and high molecular weight and high viscosity liquids as dispersion medium. Also significantly, the different compositions of STF show a vast range of critical viscosity and shear rates, and present work will serve as a valuable resource to select the tunable compositon as per the requirements for different existing and new applications.