We investigate the chain configuration
and segmental dynamics in
interacting solvent-free polymer brushes using molecular dynamics
simulations. The brush systems are designed to mimic the interstitial
space between a pair of neighboring polymer-grafted nanoparticles
in solvent-free nanoparticle–organic hybrid materials. Each
brush consists of uniformly grafted chains formed by a given number
of monomer beads. In monodisperse systems, two opposing brushes have
the same chain length and grafting density. In mixed conditions, we
consider binary systems with two surfaces being separately grafted
with polymers of distinct chain lengths at different grafting densities
as well as bidisperse systems with polymers of two different lengths
being tethered to the surfaces at a fixed grafting density. We demonstrate
that the brush configuration and interpenetration are both governed
by the need that monomer beads have to uniformly fill the space. For
systems with longer chain lengths and/or higher grafting densities,
the larger interwall separation yields more stretched brush conformations
and reduced extents of interbrush mixing. As a result, the polymer
configurational entropy is generally decreased and the segment-to-segment
relaxation dynamics is slowed down accordingly. The grafting of chains
at a high density not only makes the relaxation dynamics deviate from
the standard Rouse prediction but also leads to distinct relaxation
times for the free and tethered segments. The more slowly relaxing
tethered segments play a more important role in determining the overall
end-to-end fluctuations. Moreover, the two distinct relaxation processes
are consistent with the two-stage decay in the Rouse mode fluctuation
autocorrelation function. In the presence of brush bidispersity, the
collaboration between polymers of different lengths is evidently observed
in the brush profiles. The variations of the chain configuration for
the two polymers are complementary, and the associated relaxation
dynamics of the two species are significantly coupled.
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