Thickness and drainage of perfluoropolyethers under compression

Abstract

The Surface Forces Apparatus was used to study the compression and drainage of perfluoropolyethers (PFPEs) between two flat parallel mica surfaces. In the case of Zdols and Demnum-SA, the PFPEs can be squeezed out during slow compression to a final residual film one gyration diameter in thickness. This thickness remained constant up to the highest applied pressure of ∼10 MPa. The residual thickness for Demnum-SA, with one active end group, was found to be approximately 40% larger than that for Zdol of the same molecular weight, with two active end groups. In contrast, Z03, with no active end groups, could be displaced completely from the contact. The dynamics of expulsion were studied by monitoring the variation of the gap width as a function of time after fast (a few milliseconds) step increase in the compressive load. It was found that Zdol behaves as the bulk liquid down to gap widths of four equivalent gyration diameters. A viscosity increase of more than ten times was observed when the gap width was between four and two gyration diameters. Finally, slow compression to the maximum achievable pressure (∼10 MPa) led to a residual layer one gyration diameter in thickness trapped between the mica surfaces.