The effective viscosity and shear behavior of ultrathin films of two structurally different perfluoropolyether fluids have been investigated. The materials used were Fomblin Z, a linear random copolymer of fluoroethylene oxide and fluoromethylene oxide, and Fomblin Y, a branched random copolymer with fluoropropylene oxide and fluoromethylene oxide monomer units. The shearing experiments were conducted with the fluids confined between molecularly smooth surfaces at shear rates ranging from 200 to 4×103 s−1. It was found that when the thickness of the fluid films decreases from 10 to 2 nm, both perfluoropolyethers exhibited a sharp increase in viscosity, from bulk values to ‘‘surface viscosity’’ values that are many orders of magnitude larger. With increasing shear rates, the Z‐type fluid showed a gradual decrease in the shear stress indicating an apparent ordering of the molecules due to the applied shear forces. On the other hand, with the Y‐type copolymer, the shear stresses were significantly lower and were proportional to the shear rate resembling Newtonian fluids. The results are explained in terms of the differences in the molecular architecture of the fluids and suggest a close relation between the molecular structure of the polymer melt near a solid wall and its frictional and lubricating properties.

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Influence of monomer architecture on the shear properties of molecularly thin polymer melts

The development of perfluoropolyethers (PFPEs) represents a major contribution of organofluorine chemistry to advanced technology. These water-white mellifluous fluids, benign toxicologically and environmentally, exhibit an impressive range of physicochemical properties that are already legendary in certain application areas. The aim of this chapter is to substantiate the above statements by dissection and analysis of PFPEs from the viewpoints of their intriguing synthetic chemistry, properties, derivatives, and applications.

Perfluoropolyethers (PFPEs) from Perfluoroolefin Photooxidation

Three copolymeric perfluoroethers with the structure CF3[(OCF2CF2)p(OCF2)q] OCF3, having different p/q ratios, have been fractionated. The fractions obtained have been characterized by Gel Permeation Chromatography and 19F-NMR. The viscosity η the specific volume v and the glass transition temperature, Tg have been measured by standard techniques for all the above samples as well as for some other perfluorinated polyethers.



The temperature dependence of viscosity of the unfractionated samples is described by the W.L.F. equation. The values of fg (fractional free-volume at Tg) and of af (free-volume expansion coefficient) are independent of composition, for p/q ratios from 0.53 to 1.15. The critical molecular weight, Mc, is of the order of 8–9,000. From the molecular weight dependence of specific volume, the contribution to the molar volume of the in-chain CF2 group and the excess molar free volume of the chain ends have been determined. The limiting value of Tg for an infinite molecular weight polymer was found to depend linearly on the compositional ratio O/C and the extrapolated values for polytetrafluoroethylene and for the homopolymer (CF2O)n were found to be respectively 200 K and 120 K.

Physical properties of perfluoropolyethers : Dependence on composition and molecular weight

In liquid‐lubricated bearings used in experimental near‐contact magnetic recording devices, rheological properties of lubricant films at very high shear rates (up to about 109 s−1) and at near‐ambient pressure are required. In this extremely high shear rate regime, shear thinning (non‐Newtonian behavior) becomes extremely important. A rheometer is designed and constructed that makes it possible to study the rheology of lubricants at extremely high shear rates. Since traditional rheometers are limited to moderate shear rates due to viscous heating, the design used in this study is based on a magnetic disk drive configuration in which an ultrathin film of lubricant is sheared between a magnetic disk and a slider at relatively low sliding speeds. By monitoring the film thickness between the slider and the disk using a capacitive method, it is possible to accurately measure the shear rate. Two different types of perfluoropolyether lubricants have been studied up to shear rates of 107 s−1, and these data have been compared to previously published data.

Measurement of rheological properties of ultrathin lubricant films at very high shear rates and near‐ambient pressure

We have developed a parallel-plate rheometer in a magnetic-disk drive configuration constructed of optically transparent materials and operating with a very small gap for measurements at very high shear rates. The friction force at the disk–slider interface has been measured as a function of sliding speed while the film thickness was monitored in situ using a capacitance technique. The shear rate is calculated from the film thickness and the sliding speed. A thin film can be applied on the disk, which allows very high-shear-rate measurements at low sliding speeds with negligible viscous heating. Both disk and slider have been made of optically transparent material to allow optical measurements simultaneously with the rheological measurements. In the present mode, the apparatus is set up for simultaneous rheometery and birefringence measurements on a thin film of polymer lubricant. Rheology and birefringence measurements were made on a perfluoropolyether lubricant over a range of strain rate from 103 s−1 t...

High-shear-rate optical rheometer

The shear dependence of the bulk viscosities of two structurally different types of perfluoropolyether fluids was determined by two different techniques. The first involved direct measurement in a high shear Couette viscometer, the second utilized the time-temperature superposition principle to establish master curves from viscosity determinations at low shear rates and temperature; the results are comparable. Both fluids begin to show non-Newtonian behavior at shear rates above 10,000 s−1.

E. R. George

Papers

Shear dependence of viscosity for perfluoropolyether fluids