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Fig. 10 (a) Nonbonded phenyl-phenyl (B-B) interaction potential and (b) the coarse-grained
ABAB torsion potential (A
¼
backbone bead; cf. Fig.
8
). The potentials were sampled at two
temperatures [
106
]
Figure
10
shows the CG nonbonded phenyl-phenyl potentials (“B-B”) at two
temperatures obtained by applying the procedure depicted in Figure
9
to PS
sequences which have been coarse grained according to the mapping scheme
shown in Fig.
8
[
106
]. The attractive tail of the nonbonded potential can be fitted
with an
r
6
distance dependency, while the short-range repulsive part is typically
“softer” (
n
12) than the
r
n
12) dependency of the Lennard-Jones potential.
The ABAB torsion potential of the meso diad is also shown in Fig.
10
. The tempera-
ture dependencies of these bonded and nonbonded potentials are noticeable but weak;
therefore the potentials obtained at the higher temperature may potentially also be
used at lower temperatures. Although the parameterization procedure is cheap
enough to be repeated at various temperatures, the potential obtained at 503 K was
used in lower temperature simulations of PS melts in order to test the temperature
transferability of the model.
Figure
11
shows the density of polystyrene 10-mers in the liquid phase (1 atm)
obtained with the detailed-atomistic and CG models. Note that the density is a
prediction of the CG model, since condensed phase data have not entered the
parameterization. The CG model, parameterized at 503 K, is transferable to lower
temperatures in a range as large as 100 K. Another IBI-derived PS model, with one
CG bead per chemical repeat unit, was recently shown not to be transferable to
other temperatures [
107
]. The temperature dependence of the density observed in
that work was opposite to the dependence observed with the atomistic model (i.e.,
the CG model predicted a negative thermal expansion coefficient). The poor
temperature transferability was suggested to be caused by the coarse description
(one bead per repeat unit), which, for example, does not permit one to account for
subtle changes in the liquid state packing of the phenyl groups [
107
].
The coarse-grained PS model in Fig.
8
furthermore describes the global
chain conformations of stereo-regular PS melts in satisfactory agreement with
<
(
n
¼
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