In situ lateral force microscopy (LFM) and X-ray photoelectron
spectroscopy (XPS) were used to probe the lubrication behavior of
an aqueous solution of poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene)
(PEO-PPO-PEO) symmetric triblock copolymer on thin films of polypropylene
(PP), polyethylene (PE), and cellulose. LFM experiments were carried
out while the substrates were immersed in water and in solutions of
the copolymer. The friction coefficient on PP and PE was reduced after
adsorption from the PEO-PPO-PEO aqueous solution while the opposite
effect was observed for cellulose surfaces. A critical normal loading
force, at which the friction coefficient of the lubricated and unlubricated
surfaces is equal, was identified and related to the affinity of the
polymer with the substrate. Further experiments were performed to
mimic practical operations involving lubricant addition during manufacturing
and postprocessing removal. XPS was used to verify the presence of
the lubricant on the polymeric substrates and to evaluate its removal
by water washing. The lubricant layer was easily removed by water
from the PP and cellulose surfaces while a durable layer was found
on PE. The XPS results were in agreement with the highest critical
normal loading force measured for PE (52 nN for PE in contrast to
a minimum of 10 nN for cellulose). While several reports exist on
lubrication on hard surfaces, friction behavior on soft surfaces is
still not well documented as the substrates usually deform under loading
pressure. Therefore, we also propose a simple lubrication model for
PP, PE, and cellulose and the use of critical normal loading force
as a parameter to predict lubricity and durability of adsorbed nonionic
block copolymers.
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