Chemistry Reference
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equilibrate the chain conformations and melt packing of high molecular weight
polymers and to investigate polymer dynamics [
100
-
102
] as well as the dynamics
of slowly diffusing additives [
99
]. Today, these models are used to describe static
and dynamics properties of soft matter systems at length scales ranging between
0.3 nm and 100 nm and time scales up to 10
3
s. Since the degree of coarse graining
is moderate, atomistic chain conformations can easily be mapped onto the CG chain
conformations, as illustrated in Fig.
8
.
Switching resolution, from atomistic to coarse grained and back [
56
] offers
new opportunities for scale-bridging approaches in multiscale modeling of complex
soft matter systems. For example, recently developed hierarchical PS models have
significantly contributed to computationally efficient, quantitative modeling of
chain diffusion in entangled melts [
102
], liquid and vapor permeation data [
99
],
and detailed-atomistic structures of free-standing polystyrene surfaces [
103
]. Most
of these properties are not amenable to modeling with a detailed-atomistic model
alone.
A coarse-grained force field for a selected macromolecule should ideally be
developed such that it is transferable to conditions where the chemical environment
or thermodynamic conditions are different. Different methodologies exist to obtain
the CG potentials for polymer models. The IBI method [
104
] optimizes a set of CG
potentials (bonded and nonbonded interactions) against selected target quantities.
These target quantities are obtained from atomistic simulations of oligomer melts
(or polymer solutions) and include pair correlation functions between the CG bead
centers and probability distribution functions of the bonded degrees of freedom
corresponding to the coarse-grained mapping points. The iterative procedure is
usually performed under constant NVT conditions. The virial pressure of the
atomistic system is often included as an addition target in the iterations. Because
IBI is an automated procedure [
104
,
105
], CG models can readily be obtained once
an atomistic trajectory is available. However, it usually remains unclear whether the
resulting CG models can be applied outside the thermodynamic state point where
the IBI parameterization was carried through. Since IBI-derived models are fitted to
the structure and pressure at a selected density and temperature, the models may fail
to describe condensed phase properties at different temperatures and densities. In a
recently developed, alternative coarse graining method [
98
], coarse-grained bonded
and nonbonded potentials have been derived from sampling single oligomers and
oligomer pairs in vacuum, respectively, with a detailed-atomistic model. Hence,
fitting of the potentials on condensed phase structures or conformations is avoided
in this method and the resulting potentials are not biased to represent any environ-
ment-dependent (melt, solution) property, such as chain conformations or bead
packing characteristics. In order to obtain CG bonded potentials, single oligomers
were sampled in vacuum, taking into account all atomistic interactions along the
backbone up to a cutoff distance which is determined by the local interactions that
influence the bond stretching, angle bending, and torsional degrees of freedom of
the CG model. Longer-ranged nonbonded interactions along the backbone, that in
the final CG model are accounted for with nonbonded interactions, were all
switched off during the single-oligomer sampling stage in order to avoid “mixing”
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