124,140 research outputs found

### "Wet-to-Dry" Conformational Transition of Polymer Layers Grafted to Nanoparticles in Nanocomposite

The present communication reports the first direct measurement of the
conformation of a polymer corona grafted around silica nano-particles dispersed
inside a nanocomposite, a matrix of the same polymer. This measurement
constitutes an experimental breakthrough based on a refined combination of
chemical synthesis, which permits to match the contribution of the neutron
silica signal inside the composite, and the use of complementary scattering
methods SANS and SAXS to extract the grafted polymer layer form factor from the
inter-particles silica structure factor. The modelization of the signal of the
grafted polymer on nanoparticles inside the matrix and the direct comparison
with the form factor of the same particles in solution show a clear-cut change
of the polymer conformation from bulk to the nanocomposite: a transition from a
stretched and swollen form in solution to a Gaussian conformation in the matrix
followed with a compression of a factor two of the grafted corona. In the
probed range, increasing the interactions between the grafted particles (by
increasing the particle volume fraction) or between the grafted and the free
matrix chains (decreasing the grafted-free chain length ratio) does not
influence the amplitude of the grafted brush compression. This is the first
direct observation of the wet-to-dry conformational transition theoretically
expected to minimize the free energy of swelling of grafted chains in
interaction with free matrix chains, illustrating the competition between the
mixing entropy of grafted and free chains, and the elastic deformation of the
grafted chains. In addition to the experimental validation of the theoretical
prediction, this result constitutes a new insight for the nderstanding of the
general problem of dispersion of nanoparticles inside a polymer matrix for the
design of new nanocomposites materials

### Diffusion dynamics of star-shaped macromolecules in dilute solutions

Polymer chains dissolved in a solvent take random conformations due to large
internal degrees of freedom and are characterized geometrically by their
average shape and size. The diffusive dynamics of such large macromolecules
play an indispensable role in a plethora of engineering applications. The
influence of the size of the polymer chain on its diffusion is well studied,
whereas the same cannot be said for the shape of the polymer chain. In the
present work, the influence of shape on the center-of-mass diffusion of the
star-shaped chains in solution is investigated using Multi-particle Collision
Dynamics. Star-shaped chains of varying degrees of functionality are modeled in
a good solvent at infinite dilution. The radius of gyration($R_g$) of the
star-shaped chains follows a functionality-independent scaling law with the
chain length($N$), $R_g \sim N^{\nu}$, where $\nu \sim 0.627$. The shape of the
polymer chains is calibrated by relative shape anisotropy. Highly anisotropic
star-shaped polymer chains are found to have a faster rate of diffusion along
the translational direction due to a slower rate of rotational diffusion when
the radius of gyration of the polymer chains is maintained constant.Comment: 24 pages, 11 figure

### The Hydrodynamic Interaction in Polymer Solutions Simulated with Dissipative Particle Dynamics

We analyzed extensively the dynamics of polymer chains in solutions simulated
with dissipative particle dynamics (DPD), with a special focus on the potential
influence of a low Schmidt number of a typical DPD fluid on the simulated
polymer dynamics. It has been argued that a low Schmidt number in a DPD fluid
can lead to underdevelopment of the hydrodynamic interaction in polymer
solutions. Our analyses reveal that equilibrium polymer dynamics in dilute
solution, under a typical DPD simulation conditions, obey the Zimm model very
well. With a further reduction in the Schmidt number, a deviation from the Zimm
model to the Rouse model is observed. This implies that the hydrodynamic
interaction between monomers is reasonably developed under typical conditions
of a DPD simulation. Only when the Schmidt number is further reduced, the
hydrodynamic interaction within the chains becomes underdeveloped. The
screening of the hydrodynamic interaction and the excluded volume interaction
as the polymer volume fraction is increased are well reproduced by the DPD
simulations. The use of soft interaction between polymer beads and a low
Schmidt number do not produce noticeable problems for the simulated dynamics at
high concentrations, except that the entanglement effect which is not captured
in the simulations.Comment: 27 pages, 13 page

### A finite excluded volume bond-fluctuation model: Static properties of dense polymer melts revisited

The classical bond-fluctuation model (BFM) is an efficient lattice Monte
Carlo algorithm for coarse-grained polymer chains where each monomer occupies
exclusively a certain number of lattice sites. In this paper we propose a
generalization of the BFM where we relax this constraint and allow the overlap
of monomers subject to a finite energy penalty \overlap. This is done to vary
systematically the dimensionless compressibility $g$ of the solution in order
to investigate the influence of density fluctuations in dense polymer melts on
various s tatic properties at constant overall monomer density. The
compressibility is obtained directly from the low-wavevector limit of the
static structure fa ctor. We consider, e.g., the intrachain bond-bond
correlation function, $P(s)$, of two bonds separated by $s$ monomers along the
chain. It is shown that the excluded volume interactions are never fully
screened for very long chains. If distances smaller than the thermal blob size
are probed ($s \ll g$) the chains are swollen acc ording to the classical
Fixman expansion where, e.g., $P(s) \sim g^{-1}s^{-1/2}$. More importantly, the
polymers behave on larger distances ($s \gg g$) like swollen chains of
incompressible blobs with P(s) \si m g^0s^{-3/2}.Comment: 46 pages, 12 figure

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