Tools to Probe Nanoscale Surface Phenomena in Cellulose Thin Films: Applications in the Area of Adsorption and Friction - The Nanoscience and Technology of Renewable Biomaterials - page 101

the shear viscosity η eff and moduli G and G became quite different from those of
the bulk. On entering the tribology regime (film thickness < 30 nm) PBD exhibited
highly nonlinear behavior and yield points indicative of phase transitions to 'glassy'
or 'solid-like' states. Klein et al . (Klein and Kumacheva 1998) discovered that the
transition between liquid-like behavior and a solid-like phase of the liquids under pro-
gressive confinement take place abruptly at a distance of few molecular layers. The
films that are thinner behaved in a solid-like fashion and they required a critical stress to
shear them.

4.6.2 Orientation of Lubricant Films

Why can lubricants reduce friction? How do lubricant molecules work and behave under
shear? These questions are currently being investigated by several groups. Lubricant
molecules organize themselves under shear as illustrated in Figure 4.7 by Yoshizawa et al .
(Yoshizawa, Chen et al . 1993). A critical velocity V c * exists; if the sliding velocity of
two surfaces are below V c * a polymeric lubricant film exhibits amorphous structure and
the polymer chains interplay and entangle with each other. In this case high friction is
produced (static-kinetic sliding). This phenomenon supports experimental observations
in which chain interdigitation was found to be an important molecular mechanism giving
rise to 'boundary' friction and adhesion hysteresis of monolayer-coated surfaces. If the
sliding velocity of two surfaces is above the critical velocity, polymer chains will be
aligned or 'combed' by shear into an ordered conformation and therefore will result in
very low friction (superkinetic sliding).
The phenomenon of shear-induced alignment of lubricant molecules has been vali-
dated by a number of experiments. For example, Frantz and co-workers (Frantz, Perry
et al . 1994) adsorbed polyisoprene onto a single solid surface and found that the back-
bone of the polymer oriented in the direction of flow. They also found that the extent
of orientation increased with increasing molecular weight. The structure of the lubri-
cant, such as chain length (Frantz, Perry et al . 1994), packing densities (Ruths 2003;
Ruths, Alcantar et al . 2003), and nature of the polymer (brush-like (Zappone, Ruths
et al . 2007) or grafted polymer (Urbakh, Klafter et al . 2004) and chain ends (Chen,
Maeda et al . 2005)) have been found to influence molecular alignment of the lubricant
under shear.
Within these investigations, the work of Urbakh et al . (Urbakh, Klafter et al . 2004)
is very significant. They used grafted polyelectrolytes, hyaluronan and hylan, to mimic

V < V c* V > V c*

D

Static-kinetic sliding Superkinetic sliding

Figure4.7 Lubricantmoleculesorganizedbyshear.ReprintedfromYoshizawa,Chenetal.
Copyright(1993)withpermissionfromElsevier.