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 115

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RP10E13
RP13E17
E26P40E26
E133P50E133
Air
Water

Cellulose

0.1

0.01

0 0 0 0 0 0 0 0 0

Normal force (nN)

Figure4.17 Therelationshipoffrictioncoefficient(COF)andnormalforce(F n )oncellulose
films in air, water and in the presence of four types of nonionic polymers. E: polyethylene
oxide;P:polypropyleneoxide;R:alkylgroups.

because the shared hydrophilicity nature of the E blocks and silica surface (affinity
between the E blocks and the silica surface). Consequently the P blocks are repelled
from the surface. A competition between solvency of E segments and the enthalpic
E-to-surface attraction is likely to be present in the case of cellulose substrates. Therefore,
an anchor-buoy-anchor configuration may exist on the hydrophilic cellulose surface.
Molecular self-assembled structures are formed on the interfaces between sliding surfaces
as a result of morphology changes at a nanoscale level. These changes mainly depend on
the chemical nature of the surface and the liquid. In boundary lubrication, it is believed
that surface coatings of organized, molecular liquid films will control friction and reduce
wear in fiber processing.
Figure 4.17 shows an LFM curve for coefficient of friction at different applied
loads. Under low normal forces the coefficient of friction decreases as the normal
force increases. However, at high normal forces, the value of friction coefficient does
increase. The threshold for this transition was around 30-40 nN. This behavior can
be explained by the fact that lubricant molecules self assemble onto the surface and
form a layer under shear and normal forces. At higher shear rates or normal forces,
the polymer aligns better and forms a more compact structure with a low coefficient
of friction. However, at higher pressures and loadings, the polymer film might be
distorted (molecules can be driven out from the interface) and the tip can make direct
contact with the unlubricated surface thus measuring a higher coefficient of friction.
This phenomenon is especially relevant in the case of sharp LFM tips where even a
normal force of only 30-40 nN can produce a substantially high pressure.

4.12 Summary

In this chapter we discussed the use of QCM and SPR as tools to monitor the adsorp-
tion of molecules on solid surfaces. Some examples were provided with regards to the