Desorption or evaporation is one of the mechanisms for loss of perfluoropolyalkylether (PFPE) lubricants from them surfaces of data storage media. One approach to minimizing PFPE loss to desorption is the use of lubricants with increasing molecular weight or increasing average chain length. In order to understand the effects of chain length on the lubricant evaporation kinetics we have studied the desorption kinetics of monolayer films of oligomeric ethers with varying chain length adsorbed on the surface of graphite. The desorption preexponents, nu, and desorption barriers, DeltaE(des)double dagger, have been measured for poly(ethylene glycol) dimethyl ethers, CH3O(CF2CH2O)(m)CH3, with m = 1, 2, 3, 4, 8 and 10. These are models for the PFPE known as Fomblin Z, which has a structure CF3O(CF2CF2O)(x)(CF2O)(y)CF3. The results show that the desorption pre-exponents are independent of chain length and have an average value of nu = 10(18.7+/-0.3) s(-1). The DeltaE(des)double dagger for the poly(ethylene glycol) dimethyl ethers vary non-linearly with chain length and can be fit with a power law expression of the form DeltaE(des)double dagger =a +b . N-gamma, where N is the total number of atoms in the oligomer backbone (N = 3m + 3) and the scaling exponent has a value of gamma approximate to 1/2. This non-linear dependence of DeltaE(des)double dagger on chain length has also been observed in recent studies of the desorption kinetics of straight chain alkanes from graphite. A desorption mechanism is described that explains the non-linearity of DeltaE(des)double dagger for the poly(ethylene glycol) dimethyl ethers. The implication for the lifetime of lubricants on data storage media is that the long chain PFPE lubricants desorb more rapidly than one might expect based on simple linear scaling of the DeltaE(des)double dagger of lower molecular weight PFPEs.
