The viscoelastic behavior of perfluoropolyether lubricants

Molecular rheology of perfluoropolyether (PFPE) systems can be particularly important in designing effective lubricants that control the friction and wear in their tribological applications. Using equilibrium/nonequilibrium molecular dynamics simulation, we examined the viscoelastic properties of PFPE melts under oscillatory shear, such as dynamic moduli and viscosity, by monitoring the time-dependent strain-stress curve. Strong dependences of PFPE melt viscoelasticity on molecular architecture (e.g., endgroup functionality) and external conditions (e.g., temperature and oscillation frequency) were observed. Nonfunctional PFPEs exhibit liquid-like behavior, while "pseudoreptation-like" behavior is captured for functional PFPEs, where endgroup couplings are found to be dissociated at high temperatures. The empirical Cox-Merz rule was also discussed.