(104f) Multiscale Modeling of Perfluoropolyether Lubricants with Functional Endgroups

            Physical properties of nanoscale perfluoropolyether (PFPE) films, which are generally used as a perfluoro linear co-polymer lubricant in hard disk drives (HDDs) has been previously studied in disparate molecular/mesoscale simulations.  The PFPE molecules must exhibit strong adhesion to the carbon overcoat surface as well as rapid self-healing properties; therefore, a fundamental understanding of nanoscale characteristics of lubricant structure which contribute to desired macroscale performance must be established for successful nanotechnology design.  

            Due to the hydroxyl functional groups at each chain end of well-known PFPE (e.g., Zdol and Ztetraol), PFPE on an amorphous carbon surface shows peculiar characteristics (i.e., layering structure and entanglement on the surface). Via coarse-grained (CG) molecular dynamics (MD), we qualitatively validated molecular scale phenomena related to nanostructured conformation and dynamic behavior of molecularly thin lubricant film [1, 2]. To validate this coarse-grained model, a procedure in multi-scale modeling [3] which links atomic scale phenomena to CG MD is established thereby providing a form of synergy between the atomistic and molecular scales.  Accurate intra / intermolecular force field parameters for PFPE and carbon surfaces are implemented in CG MD to enable improvements in our molecular scale model.

             In this study, we use ab initio methods to determine intramolecular force field parameters for PFPE molecules and their intermolecular interaction with other PFPEs as well as with various carbon overcoat surfaces. Calculations based on ab initio methods are used to determine the intramolecular (stretching, bending, and torsional) force field parameters via solving the ab initio Hessian matrix eigenvalue problem. The intermolecular interaction between endgroups in pure and blended PFPEs is quantified using the supermolecular approach to molecular interaction.  In addition, we explore novel carbon overcoat technology in HDD by calculating PFPE interaction with graphene.  The implementation of atomically thin graphene as a carbon overcoat has the potential for a factor of two improvement in areal recording density.  We compare the PFPE interaction with the traditional carbon overcoat versus graphene.  We also evaluate PFPE-PFPE and PFPE-carbon surface geometry as a function of the PFPE structures particularly for new-generation lubricants including DDPA. The force field is evaluated by comparing interaction energies and molecular geometries of equilibrium configurations obtained using ab initio calculations to those obtained via the classical force field analysis.

[1] Q. Guo, L. Li, Y.-T. Hsia, and M.S. Jhon, “A Spreading Study of Lubricant Film via Optical Surface Analyzer and Molecular Dynamics”, IEEE Trans. Magn. 42(10), 2528-2530 (2006).

[2] P.S. Chung, H. Chen, and M.S. Jhon, “Molecular dynamics simulation of binary mixture lubricant films”, J. Appl. Phys., 103, 07F526 (2008).

[3] D. Kim, P.S. Chung, P. Jain, S.H. Vemuri, and M.S. Jhon, “Multiscale modeling of head disk interface”, IEEE Trans. Magn., 46, 2401-2404 (2010).