The shear viscosity dependence on concentration, molecular weight, and shear rate of polystyrene solutions

Abstract

The solution viscosity of narrow molecular weight distribution polystyrene samples dissolved in toluene and trans-decalin was investigated. The effect of polymer concentration, molecular weight and shear rate on viscosity was determined. The molecular weights lay between 5 ⋅ 10⁴ and 24 ⋅ 10⁶ and the concentrations covered a range of values below and above the critical valuec ^*, at which the macromolecular coils begin to overlap. Flow curves were generated for the solutions studied by plotting logη versus log$\dot \gamma$. Different molecular weights were found to have the same viscosity in the non-Newtonian region of the flow curves and follow a straight line with a slope of − 0.83. A plot of logη ₀ versus logM _w for 3 wt-% polystyrene in toluene showed a slope of approximately 3.4 in the high molecular weight regime. Increasing the shear rate resulted in a viscosity that was independent of molecular weight. The sloped (logη)/d (logM _w ) was found to be zero for molecular weights at which the corresponding viscosities lay on the straight line in the power-law region.

On the basis of a relation betweenη _sp and the dimensionless productc · [η], simple three-term equations were developed for polystyrene in toluene andt-decalin to correlate the zero-shear viscosity with the concentration and molecular weight. These are valid over a wide concentration range, but they are restricted to molar masses greater than approximately 20000. In the limit of high molecular weights the exponent ofM _w in the dominant term in the equations for both solvents is close to the value 3.4. That is, the correlation betweenη _sp andc · [η] results in a sloped(logη _sp)/d(logc · [η]) of approximately 3.4/a at high values ofc · [η] wherea is the Mark-Houwink constant. This slope of 3.4/a is also the power ofc in the plot ofη ₀ versusc at high concentrations.