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Numerical simulations can help to shed light on some of these issues. However,
they also face some technical problems, namely, if the dynamics is slow in experi-
ment, it will also be slow in the simulation, provided the
correct
interaction
potentials are used. Therefore speeding up the dynamics, for example via a reduc-
tion of the number of degrees of freedom, is essential. Therefore, to obtain a
realistic description of PEMs, current models and techniques need to be further
improved.
Recently we have revisited the usage of the generic coarse-grained model to
study PEM formation out of solution [
186
,
187
]. There are several issues during the
build-up of a PEM. First, one has to develop a suitable simulation protocol which
allows one to mimic the dipping process closely in order to obtain realistic adsorbed
chain conformations. Since in experiments the surface can be exposed to the
solution in the order of minutes, one has to speed up the adsorption process in the
simulation. In recent work on the adsorption of one layer of PEM, Carrillo et al.
[
188
] have introduced a so-called “stirring” step where, after a certain time, chains
that remain non-adsorbed are randomly repositioned in order to improve adsorption
rates. They showed that, without stirring steps, the adsorption might not be in the
real saturation regime within the time intervals used in their previous works [
189
].
We followed a somewhat different route and introduced several dipping-rinsing
subcycles in which we “refilled” the bulk solution of the simulation box in order to
keep the bulk polymer concentration constant. This is necessary since in a finite box
under NVT conditions the adsorption process depletes the “bulk” from polymers.
With this we were able to achieve excellent saturation conditions, and we could
simulate larger systems with substrate areas
A
up to
A
¼
40
40 (in units of the
square of the particle diameter).
Fig. 15 Simulations showing the thermodynamic instability of a bilayer (
upper plot
). However,
a fast deposition step of a third layer stabilizes the then tri-layer (
bottom figure
). The interaction
parameters are
e
¼
1.0,
e
¼
10. Figure adapted from [
187
]
mm
ms
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