Critical molecular weight for onset of non‐newtonian flow and upper newtonian viscosity of polydimethylsiloxane

Abstract

AbstractThe flow curves of fractionated polydimethylsiloxanes of different molecular weights were obtained over a wide range of shear rates, from 3 × 10−1 to 4.3 × 106 sec−1, by use of a gas‐driven capillary viscometer designed to decrease the experimental error in high shear rate region. Non‐Newtonian flow can occur at molecular weights below the critical molecular weight Mc for the entanglement of polymer chain. The critical molecular weight M′c for the onset of the non‐Newtonian flow is identical with that of the segment of viscous flow. For the polymer of molecular weights from M′c to Mc, the upper Newtonian viscosity increases with an increase in molecular weight. Above Mc, the upper Newtonian viscosity is almost independent of the molecular weight.