Tribology of confined Fomblin-Z perfluoropolyalkyl ethers: Role of chain-end chemical functionality

Strong dependence of the tribological response upon the nature of the polar end groups of perfluorinated linear chains was observed for Fomblin-Z perf luoropolyalkyl ethers of similar length and composition but terminated by a different polar group at both chain ends. The number-average molecular wei ght of the polymers was M-n approximate to 2000 g mol(-1) and the chain-end functionality was either carboxylic acid, hydroxyl, piperonyl, or the p-ph enoxyanilinium salt of a carboxylic acid. The method of investigation was a surface forces apparatus modified for dynamic oscillatory shear at variabl e frequency and effective shear rate. Differences were observed as concerns not only the shear forces but also the minimum thickness to which the film s could be compressed under a given normal load and the adhesion measured o n separation of the surfaces after prior compression. The shear forces were studied at normal pressures of 1 and 3 MPa, both in the linear viscoelasti c regime and at high shear amplitudes corresponding to shear rates of 10(-2 )-10(5) s(-1). The carboxylic acid terminated polymer displayed solidlike r esponses to shear, possibly reflecting dimerization owing to hydrogen bondi ng. 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 dif ferent 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 Angstrom, and did not appear to solidify at the pressures studied. This study suggests that not only the affinity of the f unctionalized chain ends to a solid surface but also the self-association o f polar end groups in the nonpolar environment of fluorinated chains influe nces the lubricating performance of these films.