The rheological properties and molecular weight distribution of polydimethylsiloxane

The rheological properties of a polymer melt (polydimethylsiloxane) have been measured for fractions and unfractionated polymer in steady shear flow and in relaxation and recovery after steady shear flow. The limiting values of the viscosity and normal stress coefficient at low shear rates have been found to depend on the first and second moments respectively of the weight fraction mol. wt. distribution. The steady shear compliance of a monodisperse polymer has been correlated with the entanglement mol. wt. using rubber elasticity theory. Lodge's elastic liquid model appears to be the best basis for a theory for the mol. wt. dependence of the behaviour of polymer melts at low stress levels. However, the stress relaxation results could not be predicted adequately by the Lodge theory. A model for the non-Newtonian viscous behaviour of polymer melts has been proposed; it is based on the mol. wt. distribution and an approximation to the behaviour of a monodisperse polymer.