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

Abstract

Strong dependence of the tribological response upon the nature of the polar end groups of perfluorinated linear chains was observed for Fomblin-Z perfluoropolyalkyl ethers of similar length and composition but terminated by a different polar group at both chain ends. The number-average molecular weight of the polymers was M n ≈ 2000 g mol -1 and the chain-end functionality was either carboxylic acid, hydroxyl, piperonyl, or the p-phenoxyanilinium salt of a carboxylic acid. The method of investigation was a surface forces apparatus modified for dynamic oscillatory shear at variable frequency and effective shear rate. Differences were observed as concerns not only the shear forces but also the minimum thickness to which the films could be compressed under a given normal load and the adhesion measured on separation of the surfaces after prior compression. The shear forces were studied at normal pressures of 1 and 3 MPa, both in the linear viscoelastic regime and at high shear amplitudes corresponding to shear rates of 10 -2 -10 5 s -1 . The carboxylic acid terminated polymer displayed solidlike responses to shear, possibly reflecting dimerization owing to hydrogen bonding. This contrasted with the more fluidlike shear rheology of the hydroxyl-and piperonyl-terminated polymers, in which the association from hydrogen bonding and polar interactions is believed to be weaker and result in a different structure. The sample comprised of the p-phenoxyanilinium salt of a carboxylic acid could not be compressed to less than an exceptionally large film thickness, around 100 Å, and did not appear to solidify at the pressures studied. This study suggests that not only the affinity of the functionalized chain ends to a solid surface but also the self-association of polar end groups in the nonpolar environment of fluorinated chains influences the lubricating performance of these films.

Thickness and drainage of perfluoropolyethers under compression

“…Granick et al 10,11 have also studied the tribology of confined Zdol and Z03 with the SFA. These authors find values of D 0 that are substantially larger, by a factor of 2 in some FIG.…”

Addition of Perfluoroalkylethane Thiols to Bis(Propargyl) Polyoxyethylene Ethers: Synthesis of Tetrakis-(F-Alkylethanethio) Polyoxyethylene Ethers

“…The separation of the two asymmetric centers of the molecule by the polyoxyethylene ether chain renders the interaction between these two sites negligible in a manner that they can be considered as two independent asymmetric centres and NMR spectroscopy is therefore expected to be unable to distinguish between the two diastereoisomers. 17 The coexistence of four F-alkylated chains and four sulfur atoms linked to a modulable polyoxyethylene chain may be the origin of the interesting applications associating the well-known proprieties of highly fluorinated compounds 18,19 to those related to the existence of sulfur atoms.…”

Interaction between two solid surfaces across PDMS: influence of chain length and end group

“…In all cases, an immobilized, rubber-like layer of thickness typically 1-3 times the radius of gyration R g was observed. Beyond this layer the two interacting surfaces experience a longer-ranged repulsion across PPMS [12], PB [10], and perfluorinated polyethers [5,8], typically reaching 1-3 R g . With PDMS, an oscillatory, exponentially decaying interaction was observed [14,15].…”