(188t) Rheology and Tribology of the Nano-Scale Confined Polymeric Thin Films Using Molecular Dynamics

Authors: 
Chung, P. S., Carnegie Mellon University
Jhon, M. S., Carnegie Mellon University


We examined the rheological and tribological properties of perfluoropolyether (PFPE) nanofilms to investigate the rheological and tribological properties of the polymeric thin films [1, 2] using molecular dynamics (MD) simulation [3].

The PFPE molecules were modeled via a coarse-grained, bead-spring model, which neglects the detailed atomistic structural information while keeping the essence of the molecular structure. Lennard Jones and finitely extensible nonlinear elastic models were used to describe potential energies among the beads in the system. Solid surfaces that confine the PFPE film are assumed perfectly flat. With the total potential energy constructed, MD simulations were performed using the Langevin equation.

We examined the shear deformation of confined, nano-scale PFPE films. By analyzing the viscosities and the relaxation processes as a function of wall interaction/separation and endgroup functionality, the nano-rheological response of PFPE films with and without functional endgroups was examined. We further examined the system with ultra-thin films to demonstrate solid-like deformation. The dynamic performance of nano-films by tuning the film thickness and the endgroup functionality was discussed in conjunction with the relaxation processes.

To examine the tribological fundamentals, we simulated the nano-mechanics (i.e., ?compression? and ?tension?) of molecularly thin PFPE films. For ?compression? process, functional PFPEs give higher clearance than nonfunctional PFPEs; while for ?tension? process, a fluid bridge was more apparent for functional PFPEs. The normal stress profiles were calculated for both ?compression? and ?tension? processes, where the observed hysteresis phenomena indicate the irreversible nature of functional PFPE nano-mechanics. N-modes Maxwell model was then used to analyze the relaxation processes and we obtained a second relaxation process for N=2 case, suggesting the role of functional endgroups.

Reference:

1. T.E. Karis and M.S. Jhon, ?The Relationship between PFPE Molecular Rheology and Tribology,? Tribology Letters, 5, 283-286 (1998).

2. R.N. Kono, S. Izumisawa, M.S. Jhon, C.A. Kim, and H.J. Choi, ?Rheology of perfluoropolyether lubricants,? IEEE Trans. Mag. 37, 1827 (2001).

3. Q. Guo, P.S. Chung, H.G. Chen, and M.S. Jhon, ?Molecular rheology of perfluoropolyether lubricant via nonequilibrium molecular dynamics simulation?, Journal of Applied Physics, 99, No. 08N105 (2006).

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