2018 (1) 3

Rheological characteristic of hydrophobic aerosol dispersions in the hydrocarbon medium. Rheopexy and a percolation threshold

V.F. Shumsky, L.F. Kosyanchuk, V.V. Davidenko, I.P. Getmanchuk, O.I. Antonenko, A.P. Sirovets

Institute of Macromolecular Chemistry NAS of Ukraine

48, Kharkivske shose, Kyiv, 02160, Ukraine

Polym. J., 2018, 40, no. 1: 23-30

Section: Physics of polymers.

Language: Russian.

Abstract:

The rheopectic effect was studied in the flow of dispersions at low stresses. At use the shear rate sweep experiments showed that the upward and downward branches of the flow curves coincide above some shear rate value. At the increase of a shear rate it was observed the existence of the Newtonian part on the flow curve in the low-shear-rate domain, while in the downward curve it was shown the effect of yielding . Transient processes proceed in the range of shear deformation of the order of several units. The yield stresses of dispersions with different concentration of a filler were determined in three ways. In one case, steady shear experiments were performed over a range of incrementally decreasing shear rates. The resulting flow curves, plotted as shear stress against shear rate, clearly showed the existence of a yield stress for dispersions at φ ≥ 1,39 vol. %, the Herschel-Bulkley model being fitted to obtain values. In the second case, oscillatory amplitude sweeps were performed at three frequencies, and the “dynamic yield stress” was defined as the stress at which deviation from linearity occurred. It was found that the dynamic yield stress is frequency dependent, and cannot therefore be thought of as physically meaningful material property. At no frequency did the dynamic yield stress correlate with the yield stress obtained from the flow curves. Dependences of the loss modulus on frequency were evidence of existence of the yield stress as well (the third case). It was determined the percolation threshold for dispersions under the study: φ* = 1, 35 ± 0,03 vol. %.

Key words: dispersion, hydrofobic aerosil, flow curves, yield stress, amplitude oscillation, percolation threshold.

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