Steady‐shear viscosity of polydimethylsiloxanes over the range from the lower to the upper Newtonian region

The mechanism of non‐Newtonian behavior for flow from the lower to the upper Newtonian region is explained by a modification of Graessley's theory. In the theory proposed here, a viscosity ηfric, which is based on friction between polymer segments and is almost shear‐independent, is introduced in addition to Graessley's entanglement viscosity ηent, which decreases with increasing shear rate. The theory is applied to previously obtained data on steady flow of polydimethylsiloxanes of different molecular weights. The agreement between calculated and experimental results is good. In polymers with the molecular weight above the critical molecular weight for entanglement Mc, the major contribution to viscosity near zero shear rate is ηent. As the shear rate increases, the flow curve has an inflection where ηfric cannot be disregarded in comparison with ηent. In the upper Newtonian region, ηfric has more influence on the viscosity than ηent. The theory can also explain the experimental results on flow of polymers with molecular weight below Mc, which were shown to be slightly non‐Newtonian in the previous paper.