Surface diffusion of thin perfluoropolyalkylether films

Abstract

The spreading characteristics of thin polymeric liquid films of perfluoropolyalkylethers ͑PFPEs͒ Fomblin Z15 and Fomblin Zdol ͑hydroxyl terminated PFPE͒ on silica surfaces have been measured by scanning microellipsometry ͑SME͒. We estimated the surface diffusion coefficients and propose a modified diffusion equation to interpret the spreading phenomenon from film thickness profiles measured with SME. We investigated the spreading of Z15 as a function of binary blend ratio of monodisperse Z15 fractions and found that the surface diffusion coefficients of the blends do not obey a simple linear mixing rule. A summary of thin PFPE film spreading characteristics as a function of molecular weight, film thickness, chain-end functionality, temperature, and humidity is presented.

Tribology of Confined Fomblin-Z Perfluoropolyalkyl Ethers: Role of Chain-End Chemical Functionality

“…It is, however, unlikely that a polar group like carboxylic acid 21,42 would fail to physisorb to a polar surface as, for example, the hydroxyl groups in DOL, [19][20][21][22][23][24][25][26]28,29,[32][33][34][35][38][39][40]73 the piperonyl groups in AM2001, 30,31 or the p-phenoxyanilinium group 41 are expected to do.…”

“…[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][38][39][40] It has been found experimentally that the introduction of carboxyl, hydroxyl, or piperonyl end groups increases the ability of the Fomblin-Z polymer to protect the underlying substrate against wear. [19][20][21][22][23][24][25][26][27][28][29] It has been demonstrated that these polar end groups are preferentially located near the polar surface of the hydrogenated carbon overcoat that forms the top layer of the computer hard disk 22, [30][31][32]39,40 and also that they are located near other polar surfaces such as silica 21,23,33,34,41 and ZrO 2 . 42 Heating ("thermal bonding") introduces chemical bonds between functionalized chain ends and polar groups on the surface to further improve the stability of the lubricant film.…”

“…Several end-functionalized Fomblin-Z polymers are widely used in commercial magnetic storage devices. [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][38][39][40] This family of chain polymers therefore provides a useful system in which to examine the influence of chain-end functionality.…”

Rate-Dependent Adhesion between Opposed Perfluoropoly(alkyl ether) Layers: Dependence on Chain-End Functionality and Chain Length

“…Since perfluoroheptane is a good solvent for Fomblin and Demnum perfluoropoly(alkyl ether)s at room temperature, 36 the driving force for the physisorption of these nonpolar molecules to the interface between the polar mica and the nonpolar solvent is likely the difference in their dielectric properties: 37 The refractive indexes of mica, Demnum S-20, and perfluoroheptane at 25°C are 1.58-1.60, 37 1.290, 38 and 1.259, 39 respectively. The polymers containing polar functional groups are expected to have an additional affinity for the polar surface, [27][28][29][30][31] but no difference in adsorbed layer thickness was observed for these samples. The layer thickness of 12 Å corresponds to not more than 2-3 segmental diameters of the perfluoropoly(alkyl ether), since the cross-sectional area of a -CF 2 -chain 40 is 28.3 Å 2 and the chain diameter of the Fomblin backbone has been estimated to be around 7 Å.…”

“…The surfaces studied were thin, physisorbed layers of four different linear perfluoropoly(alkyl ether)s. Perfluoropoly(alkyl ether)s are very stable against oxidation and therefore useful not only for hard-disk lubrication 11,[26][27][28][29][30][31] but also as hightemperature gas turbine oils. 32 Furthermore, since they are liquid in a wide range of temperatures and have very low volatility, they are used for lubrication in aerospace applications and in laboratory ultrahigh-vacuum equipment.…”