(692f) The Viscoelastic Behavior of Perfluoropolyether Lubricant Via Molecular Dynamics

In current information storage devices, molecularly-thin lubricant films, e.g., perfluoropolyether (PFPE) series are dip-coated onto various carbon overcoats for hard disk drives' reliability and lubrication [1]. As a matter of further increasing the areal density of hard disk drives, the head-disk spacing has been steadily reduced, which promotes more frequent intermittent contacts between the head and disk. Therefore, PFPE rheology becomes a relevant study since PFPE shear can occur in the head disk interface either due to the air shear stress or due to the direct contact between the slider and disk. Here, we examined the rheological properties of PFPEs as a complementary tool to estimate the lubricant tribological performance. Our previous work has investigated steady shear of PFPE melts [2], which elucidates the importance of PFPE molecular architecture and external conditions in determining the viscous behavior of PFPE melts. However, we are still lacking in the understanding of the linear viscoelastic responses of PFPE melts under oscillatory shear.

In this study, we first conducted the equilibrium molecular dynamics simulations to investigate the nanostructure of PFPE lubricant bulk system using a bead-spring model. The clusters of functional chain-ends are observed, which is responsible for the peculiar rheological response measured. By integrating the so-called SOLLD equation of motion and imposing the Lee-Edwards' boundary condition, non-equilibrium molecular dynamics simulations were carried out to examine the viscoelastic properties of PFPE bulk system under oscillatory shear. A sinusoidal form of shear strain was first applied to the PFPE melt, and the resultant shear stress was calculated. Then, in terms of the simulated strain-stress curve, one can calculate the complex modulus G* = G'+iG". Here, G' is the elastic, storage, or in-phase shear modulus related to the elastic energy stored by the PFPE melt, which measures the solid-like behavior of the melt; and G″ is the viscous, loss, or out-of-phase shear modulus related to the energy dissipated by the viscous flow, which measures the liquid-like behavior of the melt. Our study has suggested that the viscoelastic behavior of PFPE lubricants strongly depends on the molecular architecture (e.g., endgroup functionality) and external conditions (e.g., temperature and oscillation frequency). Nonfunctional PFPEs exhibit liquid-like behavior; while ?pseudo-reptation-like? behavior is captured for functional PFPEs, where endgroup couplings are found to be dissociated at high temperature. The dynamic viscosity was further calculated and its comparison with our calculated shear viscosity is also provided to validate the Cox-Merz rule.

Since the confinement of molecules in dimensions comparable to their size gives rise to a unique behavior, we also incorporated the nano/micro-scale confinement effect into the PFPE system. To study this, the nano-mechanics of PFPE films, including ?compression? and ?tension? were examined. In our ?thought experiment?, the PFPE films are ?coated? onto two nanoscale separated solid surfaces facing each other which can be verified by atomic force microscope. As the top surface moves downward, a complete contact of nanofilms occurs. As moving the top surface up afterward at a constant speed, molecules are elongated to form the fluid bridge between the two surfaces. The film normal stress was calculated as a function of wall separation for both nonfunctional and functional PFPEs, where a characteristic ?hysteresis? nature was observed and can be used to illustrate the viscoelastic properties of PFPE films via the N-mode Maxwell model. We have found that functional PFPE exhibits an additional mode with a smaller elastic constant but a much longer relaxation time, due to the strong endgroup coupling.

[Reference]

1. Jhon, M.S., ?Physicochemical Properties of Nano Structured Perfluoropolyether Films?, Advances in Chemical Physics, vol. 129, pp.1-79, 2004.

2. Q. Guo, P. S. Chung, H. G. Chen, and M. S. Jhon, "Molecular rheology of perfluoropolyether lubricant via non-equilibrium molecular dynamics simulation," J. Appl. Phys., vol. 99, pp. 08N105, 2006.