The effect of surface grafting polymer chains on the shear thickening of hard microsphere suspensions

Abstract

Shear thickening of suspensions was generally researched as a macro-perspective, the neglect of microstructure and micro-interactions may deviate the truth. Here, four suspensions were prepared by matching SiO2 microspheres, SiO2-g-PMMA microspheres with oligomeric fluids PEG, PPG, respectively, the rheological behavior of them under the controlled temperature was investigated to figure out the influence of grafted chains on shear thickening. For all suspensions, the reversible shear thickening with significant temperature dependency was observed, the critical shear rate (γ˙) decreases rapidly and reaches to a balance with the decreased temperature. Under the same media viscosity, the γ˙ of SiO2/PEG suspensions is higher than that of other suspensions at the lower temperature due to the stronger hydrogen bond interaction between the hydroxyl of microsphere and media. Comparing PEG, using low polar PPG as media raises the γ˙of SiO2 suspensions and decreases the γ˙ of SiO2-g-PMMA suspensions by reason of the changed interaction between microsphere and media, which is also affected by temperature. Moreover, the critical shear stress (τc) of SiO2/PEG suspensions is higher than that of other suspensions in all test temperature region, revealing that the grafted chains weaken the jamming effect. Two models were used to predict τc, the Wagner’s hard-sphere model predicts the SiO2/PEG suspensions well, Kaldasch’s model based on polymer stabilized colloidal system could fit the SiO2-PMMA/PEG suspensions better.