(373j) Force Field Development for Perfluoropolyether with Functional End Groups

Physical properties of nanoscale perfluoropolyether (PFPE) films, which are generally used as a perfluoro linear co-polymer lubricant in hard disk drive, have been experimentally examined. Due to the hydroxyl functional groups at each chain end, PFPE on an amorphous carbon surface shows peculiar characteristics (i.e., layering structure and entanglement on the surface). Via Monte Carlo/molecular dynamics (MD), we qualitatively validated molecular scale phenomena related to nanostructured conformation and dynamic behavior of molecularly nano lubricant film [1, 2]. For the coarse-graining procedure in multi-scale modeling [3], atomic scale phenomena and accurate intra / intermolecular force field parameters for PFPE and amorphous carbon surface are quantitatively required. In this study, we use ab-initio methods to determine force field parameters for the PFPE and its intermolecular interaction with the carbon surface. Calculations based on ab-initio methods as well as density functional theory is used to determine the intramolecular (stretching, bending, torsional) force field parameters by directly using the ab-initio hessian matrix. The interaction among hydroxyl groups in PFPEs and amorphous carbon is also calculated. The PFPE-PFPE dimer potential and PFPE-amorphous carbon surface interactions as a function of the end group structure (e.g., Zdol and Ztetraol) are examined to calculate the intermolecular force field parameters, which will be implemented to the potential energy function of the end group in coarse-grained MD model as well as equilibrium PFPE-PFPE and PFPE-carbon surface geometry as a function of the PFPE structures. The force field is evaluated by comparing vibrational frequencies, interaction energy, and molecular geometry obtained using ab-initio calculations to those obtained using the classical force field.

[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. Mag. 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. (accepted; publish on June 2010).