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

Abstract

Adhesion was measured repeatedly between opposed molecularly thin layers of perfluoropoly(alkyl ether)s on mica, with particular attention to the dependence of the adhesion on chain-end functionality and chain length of the polymer. The polymer layers were kept in contact for specified times that differed by 4 orders of magnitude (0.01-500 s) and then unloaded (separated) at rates that varied by 5 orders of magnitude (0.003-120 µm/s of a detachment device with spring constant 1.1 × 10 4 N/m) using a modified surface forces apparatus with piezoelectric actuators supporting one surface. Rate dependence resulting in an increase of the adhesion by a factor up to 3 was observed when the unloading rate exceeded a critical value that was found to depend not only on the polymer sample but also on the contact time between opposed layers. At a given unloading rate, the adhesion was larger, the longer the surfaces were left in stationary contact before separation. No history dependence was observed when the unloading rate was raised and subsequently lowered. For relatively short contact times (0.01-1 s), the critical unloading rate was larger for the polymer that contained polar end groups than for unfunctionalized polymer of similar chain length, indicating a lower mobility in the former case, and thus slower initial interdigitation of the chains, since the polar end group had a preferential affinity for the underlying surface. This effect vanished for contact times of 100-500 s. For polymer chains without polar end groups, the critical value of the unloading rate was smaller, the larger the chain length, and the increase in adhesion with unloading rate was more pronounced, presumably reflecting slower relaxation of the longer chains. The rate dependence is analyzed quantitatively.

Adhesion Between Surfaces Separated by Molecularly Thin Perfluoropolyether Films

“…4). This is consistent with previous results on molecularly thin PFPE films from the literature [19][20][21].…”

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

“…The presence of molecules bridging the two surfaces has been suggested in a study of rate-dependent adhesion, where DOL showed a slightly stronger increase in adhesion force with separation rate than was observed for monofunctionalized samples. It is possible that such bridging would contribute to the adhesion observed in Figure .…”

“…The van der Waals attraction ( F/R = − A /6 D 2 , where the Hamaker constant calculated according to van der Waals−Lifshitz theory 55 is A = 2.47 × 10 -20 J for mica interacting across Fomblin-Z with ε = 2.03 and n = 1.294 70 ) is very small at the separations where adhesion was observed in these systems and cannot account for the values measured. In a previous study on rate-dependent adhesion it has been suggested that possible bridging of difunctionalized chains (hydroxyl-terminated Fomblin-Z) from one surface to the other might occur. We will return to the adhesion results after discussing possible structure of the film.…”

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.…”