Preparation And Properties Of Shear Thickening Fluid And Its’ Composites

Shear thickening fluid（STF）is a concentrated suspensions composed of the solid dispersed phase and the liquid dispersed medium.The viscosity of it will increase rapidly with the increase of shear rate as the critical shear rate reached,even changed from a liquid-like to a solid-like,and the shear thickening behavior is reversible.Since the unique non-Newtonian rheological behavior of STF was found,related researches have been made on its composition,rheological properties,mechanism and application,and some achievements have been obtained.However,most of the existing researches had focused on the STF with SiO₂ paticles in polyethylene glycol（PEG）.Researches on the novel STF are very few.Meanwile,reported mechanisms about STF also have different viewpoints and the application about it also remains to be further developed.Based on the above background,this thesis mainly carried out the following research content.Firstly,the traditional STF was selected as the starting point,and the rheological and viscoelastic properties of STFs with nano-silica or submicron silica in PEG were investigated systematically through the steady shear and the oscillatory shear by the high precision rotional rheometer,respectively.The influence of different factors on the shear thickening behavior was determined and its internal mechanism was analyzed comprehensively.The theoretical model of the shear thickening was established,which provided a theoretical guidance for further researches.Secondly,different monodisperse polystyrene（PS）microspheres were prepared,and STFs with them were developed.Then,a series of different kinds of ionic liquids（ILs）were synthesized,and novel STFs were made tentatively by using ILs as the dispersed medium.Influencing rules of different factors on the shear thickening behavior of these STFs were studied,and the mechanism of them was discussed.Finally,the STF/Kevlar composites were prepared,and the mechanical properties and stabbing resistance of them were investigated.This provides an experimental and theoretical evidence for the development and application of shear thickening fluid.This thesis mainly drew the following conclusions through the above studies.The STF with nano-silica or submicron silica had a good reversibility and transient response property.STFs with different concentrations of nano-silica behaved as a continuous shear thickening（CST）,while those with submicron silica would change from the CST to the discontinuous shear thickening（DST）with increasing concentration.The critical shear rate of STF would decrease and the shear thickening behavior of it would enhance when the size of submicron silica decreased,or the concentration increased,or the temperature decreased,or the average molecular weight of the dispersed medium increased.Meanwhile,the critical shear strain would decrease,the complex viscosity,loss modulus and storage modulus would increase,and the strain shear thickening behavior would be enhanced.The critical shear stress was basically constant for different conditions.The effect of temperature on the shear thickening behavior was observably,and the shear thickening behavior almost disappeared at much high temperature.The shear thickening behavior of STF with submicron silica was superior to that of STF with nano-silica,and both of them can be explained by the clustering mechanism.The STF with PS microspheres also had a good reversibility and transient response property.The critical shear rate of it would decrease and the shear thickening behavior of it would enhance when the concentration increased,or the average molecular weight of the dispersed medium increased,or the temperature decreased.Meanwhile,the critical shear strain would decrease,the complex viscosity,loss modulus and storage modulus would increase,and the strain shear thickening behavior would be enhanced.The critical shear stress was basically constant at different conditions.The effect of temperature and the size of PS microsphere on the shear thickening behavior was observably,and it almost disappeared for much high temperature or large microsphere size.It would transform CST to DST with increasing concentration.The sehar thickening behavior was inferior to that of STF based on submicron silica,and it conformed with the clustering and jamming mechanism.The STF could be prepared by the nano-silica and the hydrophilic ILs,while the nano-silica and the hydrophobic ILs could not be used to make STF.The shear thickening behavior declined when the length of alkyl chain on the cationic of the[BMIm]BF₄ was increased.It also would be enhanced when the hydroxyl was introduced on the cationic of the imidazolium tetrafluoroborate.The ILs-prepared STF had a good reversibility,transient response property and high conductivity.The shear thickening bahevior was presented under steady shear and oscillatory shear for the STF with different concentrations in ILs,respectively.The critical shear rate of the ILs-prepared STF was less than that of other STF and the shear thickeing of it was superior to that of STF with nano-silica in PEG-400.With increasing concentration or decreasing temperature,the critical shear rate or the critical shear strain of them declined,the critical shear stress was constant,and the shear thickening and the viscoelastic properties of it enhanced.The sehar thickening behavior of it also conformed with the clustering and jamming mechanism.The preliminary investigates found that only the[EOHMIm]BF₄ could be used to make STF with submicron silica.The shear thickening behavior and the viscoelastic properties of this STF were enhanced with the increase of concentration.The continuous shear thickening behavior appeared in the whole range of shear rates for the STF with high concentrations.It would change from STF into a gel at high temperature.Effects of the strain sweeping frequeny under the oscillatory shear on the ILs-prepared STF was not significant.However,with increasing the strain sweeping frequency,the critical shear strain decreased,and the storage modulus and loss modulus increased for other STFs.Compared with the neat Kevlar fabrics,both the knife stabbing resistance and mechanical properties of the STF/Kevlar composites were enhanced.Both the knife stabbing resistance and the mechanical properties of the STF/Kevlar composites,prepared by the STF with submicron silica in PEG-400 or[EOHMIm]BF₄,were superior to those of the STF/Kevlar composites prepared by the STF with nano-silica in PEG-400 or[EOHMIm]BF₄ or the STF with PS microsphere in PEG-400,respectively.The knife stabbing resistance became bad when the absorbing capacity of STF in the STF/Kevlar composites was too low or too high.The better knife stabbing resistance for the STF/Kevlar composites would be got for the absorbing capacity between 30 wt%and 70 wt%.Especially for the absorbing capacity was about 30 wt%,the knife stabbing resistance of the STF/Kevlar composites was enhanced and the area weight of them was lightened.Effects of the absorbing capacity of STF in composites on the mechanical properties were not observably.The tensile strength of Kevlar fabric was doubled after treated by the STFs which were composed of submicron silica or PS microspheres,and the tearing strength of the STF/Kevlar composites increased by as high as eight times than that of the neat Kevlar fabric.While,the tensile strength of the STF/Kevlar composites increased by fifty percent after treated by the STF which was composed of nano-silica.The mobility of fibers or yarns was confined for the shear thickening of STF and the mechanical hinge formed by the particles between fibers,which was the intrinsic factors leading to both the knife stabbing resistance and the mechanical properties of the STF/Kevlar composites enhanced.Both the conductivity and the electromagnetic shielding of the STF/Kevlar composites,prepared by the STF which was composed of ILs,were superior to other STF/Kevlar composites.This provided a new direction for developing the functional composites which was based on STF.