(468b) Nano-Rheology of Lubricant Films

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], where the tribological
performance due to molecular architecture is one of the critical issues. Using
a pin on disk test, the relationship of PFPE tribology and molecular rheology
was first discussed [2]. Here, we studied the rheological properties as a
complementary tool to estimate the lubricant tribological performance.

Experimentally,
using a rotational rheometer, we measured the bulk rheological properties of
fractionated PFPEs with different molecular weights and endgroup
functionalities (Fomblin Z, Zdol, and ZdolTX) at various temperatures. From the study
of melt viscosity vs. molecular weight (M), we observed a remarkable
crossover at a critical M for ZdolTX while not for Zdol,
which may be due to the peculiar coupling of strongly functional endgroups. The
temperature dependence on viscosity was further examined via the Arrhenius equation, where the
activation energy for PFPE was found to be weakly dependent on M, but increased
drastically with the endgroup functionality strength. The complex shear
modulus, G^*=G'+iG" (G': storage modulus and G": loss
modulus) as well as the modified Cole-Cole plot (G' vs. G" plot)
were also examined. G" is found to be strongly dependent on temperature,
exhibiting different microstructures at various temperatures.

The
measured bulk rheological properties as well as nano/micro-rheological
characteristics were first investigated via equilibrium molecular dynamics
simulations. Clusters of chain-ends were typically observed for functional PFPE
bulk, which is responsible for the peculiar rheological response measured. Further, by
integrating the so-called SOLLD equation of motion and imposing the
Lee-Edwards' boundary condition [3], non-equilibrium
molecular dynamics simulations of static and oscillatory PFPE bulk shearing were realized with a constant
temperature. By
calculating the off-diagonal component of film stress tensor, viscosity and
G^* were
extracted as functions of molecular architecture (e.g., M and endgroup
functionality) and external conditions (e.g., temperature and pressure),
where a critical temperature exists to overcome the activation energy barrier and
modify the PFPE melt structure. Both the discrete and continuous relaxation
time spectra were also estimated, which has shown a broader distribution for strongly
functional PFPEs. Different relaxation models, including Maxwell, Cole-Cole,
Cole-Davidson, and Havriliak-Negami models, were incorporated and compared with
our simulated G^* data. We found that Cole-Cole model fits our
preliminary results best.

Since the confinement of molecules in dimensions comparable to their
size gives rise to a unique behavior, we are currently generalizing our work to
incorporate the nano/micro-scale confinement effect, which provides a full
scale modeling capability for PFPE lubricants.

[Reference]

1. Jhon,
   M.S., "Physicochemical
   Properties of Nano Structured Perfluoropolyether Films", Advances in Chemical Physics, 129
   (2004), 1-79.
2. Karis,
   T.E. and Jhon, M.S., "The Relationship between PFPE Molecular Rheology and
   Tribology", Tribology Letters, 5(1998), 283-286.
3. Allen,
   M.P. and Tildesley, D.J.; Computer Simulation of Liquids; Oxford Science Publications; ISBN 0198556454.