  Two of the most commonly encountered friction reducing agents used in plastic
sheet production are the amides known as erucamide and behenamide, which
despite being almost identical chemically, lead to markedly different values of
the friction coefficient. To understand the origin of this contrasting
behavior, in this work we model brushes made of these two types of linear chain
molecules using quantum mechanical numerical simulations under the Density
Functional Theory at the B97D/6-31G(d,p)level of theory. Four chains of
erucamide and behenamide were linked to a 2X10 zigzag graphene sheet and
optimized both in vacuum and in continuous solvent using the SMD implicit
solvation model. We find that erucamide chains tend to remain closer together
through {\pi}{\pi} stacking interactions arising from the double bonds located
at C13 C14, a feature behenamide lacks and thus a more spread configuration is
obtained with the latter. It is argued that this arrangement of the erucamide
chains is responsible for the lower friction coefficient of erucamide brushes,
compared with behenamide brushes, which is a macroscopic consequence of
cooperative quantum mechanical interactions. While only quantum level
interactions are modeled here, we show that behenamide chains are more spread
out in the brush than erucamide chains as a consequence of those interactions.
The spread out configuration allows more solvent particles to penetrate the
brush, leading in turn to more friction, in agreement with macroscopic
measurements and mesoscale simulations of the friction coefficient reported in
the literature.
