The mechanical properties of polymer nanocomposites (PNCs)
depend
sensitively on the structure (e.g., orientation, dispersion, and so
on) of the incorporated nanofillers. Many studies have shown that
the alignment of anisotropic nanofillers can improve the mechanical
properties of PNCs. However, achieving this alignment typically requires
complex preparation processes. To address this challenge, researchers
have introduced dynamic covalent bonds to form reversible cross-linked
polymer systems, which would lead to unique properties, such as self-healing,
recyclability, and reprocessibility. In addition, inspired by the
above ideas, we introduce nanorods as fillers into a linear vitrimer
system to form nanorod vitrimer composites (NVCs). In NVCs, we can
easily manipulate the alignment of the nanorods due to bond exchange
reactions (BERs) in the vitrimer matrices. By using coarse-grained
molecular dynamics (CGMD) simulations, we systematically investigate
the factors affecting the nanorod orientation in NVCs. We find that
the main factor affecting the nanorod orientation is the network rearrangement
caused by BERs. Specifically, the BER potential barrier (ΔE
sw) directly determines the probability of BERs’
occurrence. At the same time, the increase of the interfacial interaction
between polymer chains and nanorods (εnp) confines
the motion of the active beads, which slows down the rate of BERs.
Additionally, the impact of temperature (T) and aspect
ratio (ld) on the orientation of nanorods during uniaxial stretching
or stress relaxation are also discussed. Finally, by combining uniaxial
stretching and stress relaxation processes, we elucidate the orientation
retention mechanism of NVCs and demonstrate the mechanical property
enhancement phenomenon of the pre-oriented NVC systems. This work
provides a simple strategy for manipulating the nanorod alignment
in vitrimer matrices and uncovers guidelines for designing new functional
polymer vitrimer nanocomposites at the molecular level.
Densely grafted polymer chains onto spherical nanoparticles
produce
a diverse range of conformations. At high grafting densities, the
corona region near the nanoparticle surface undergoes intense confinement
due to a high concentration of chains in the concentrated polymer
brush (CPB) region, which results in strong stretching for portions
of the chains located within. In contrast, a semi-dilute polymer brush
(SDPB) forms farther away from the core and offers reduced confinement
for the polymer and more ideal conformations. However, conventional
experimental methods are limited in their ability to provide detailed
information on individual segments of grafted polymers in these regions;
hence, molecular dynamics (MD) simulations are essential for gaining
comprehensive insights into the behavior of the grafted chains. This
study aims to explore the variations in polymer structure and dynamics
that occur along the contour of the grafted chains as influenced by
spatial confinement. We focus on the motions and relative positions
of each bead along grafted polymers. Our results show that only the
initial few grafted beads near the nanoparticle surface exhibit the
strong stretching attributed segments in the CPB region of the brush.
Increased grafting density or decreased chain flexibility leads to
more stretched grafted chains and more aligned bond vectors. As a
result, the relaxation dynamics of local regions of the polymer are
also strongly influenced by these parameters. Although the grafted
beads in the interior of the CPB region are highly sensitive to these
parameters, those farther from the nanoparticle core experience significantly
diminished effects. In comparison to the Daoud–Cotton (DC)
model’s predictions of CPB size, beads near the nanoparticle
surface show slower dynamic decay, especially in high grafting densities,
aligning with the DC model’s estimates. Finally, we compare
our simulations to previous works for additional insight into polymer-grafted
nanoparticles.
A coarse-grained molecular dynamics simulation was employed to examine the relationship between the morphology of carbon black particles and the mechanical properties of elastomer nanocomposites.
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