Humidity Effects on the Relaxation of Perfluoropolyether Lubricant Films

Humidity effects on the relaxation of molecularly thin lubricant films were studied via examining the bonded fraction and surface energy of fresh dip-coated perfluoropolyether (PFPE) monolayer as functions of elapsed time. The polar surface energy of Zdol films was found to monotonically decrease with the elapsed time, which indicates the reorganization of molecular structure and conformation. Characteristic relaxation time was obtained by analyzing this time-dependent behavior via Kohlrausch-Williams-Watts (KWW) function. Molecular dynamics simulation with a bead-spring model was employed to examine the PFPE monolayer structure in the humid environment. Water molecules were found to couple with functional end groups and wall forming characteristic cluster structure. The normalized autocorrelation function of normal modes for entire PFPE chain was examined to extract the characteristic molecular relaxation processes. The relaxation times become shorter in the higher humidity from both experiment and simulation suggesting that humidity expedites the relaxation process of functional PFPE films