RATE-DEPENDENT ADHESION BETWEEN OPPOSED PERFLUOROPOLY(ALKYL ETHER) LAYERS - DEPENDENCE ON CHAIN-END FUNCTIONALITY AND CHAIN-LENGTH

Adhesion was measured repeatedly between opposed molecularly thin laye rs of perfluoropoly(alkyl ether)s on mica, with particular attention t o the dependence of the adhesion on chain-end functionality and chain length of the polymer. The polymer layers were kept in contact for spe cified times that differed by 4 orders of magnitude (0.01-500 s) and t hen unloaded (separated) at rates that varied by 5 orders of magnitude (0.003-120 mu m/s of a detachment device with spring constant 1.1 x 1 0(4) N/m) using a modified surface forces apparatus with piezoelectric actuators supporting one surface. Rate dependence resulting in an inc rease of the adhesion by a factor up to 3 was observed when the unload ing rate exceeded a critical value that was found to depend not only o n the polymer sample but also on the contact time between opposed laye rs. At a given unloading rate, the adhesion was larger, the longer the surfaces were left in stationary contact before separation. No histor y dependence was observed when the unloading rate was raised and subse quently lowered. For relatively short contact times (0.01-1 a), the cr itical 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 initia l interdigitation of the chains, since the polar end group had a prefe rential affinity for the underlying surface. This effect vanished for contact times of 100-500 s. For polymer chains without polar end group s, the critical value of the unloading rate was smaller, the larger th e chain length, and the increase in adhesion with unloading rate was m ore pronounced, presumably reflecting slower relaxation of the longer chains. The rate dependence is analyzed quantitatively.