Boundary viscosity of polydimethylsiloxane liquids and their binary mixtures

A systematic study is made of the boundary viscosity of a number of polydimethylsiloxane liquids (PMS) of molecular weights ranging from 1,000 to 40,000 by the blow-off method, film thicknesses being measured by a modulation-polarimetric (elliposmetric) precedure. It is established experimentally that the viscosity of the liquids studied is not the same throughout the thickness of the boundary later.

PMS applied to glass and steel substrates retain their bulk viscosity values down to a layer thickness of ca. 150–200 Å, below which the viscosity increases slightly. On further decrease of the distance to 10–15 Å from the substrate, the viscosity becomes anomalously low, amounting to about 10–20 % of the bulk value. The existence of a layer of anomalously low viscosity may be attributed to orientation of the PMS molecules in the plane of the substrate, and hence, to the ease with which these oriented layers slip relative to one another.

When studying the boundary viscosity of binary mixtures of PMS of different molecular weights, boundary phases of elevated viscosity were found to exist at a distance of 15–30 Å from the substrate. This phenomenon may be attributed to a change in concentration of the mixture at the solid surface i.e., to an increase in content of the component with the higher molecular weight in the wall-adjacent layer.