Structure and dynamics of a polymer- nanoparticle solution
Résumé
Understanding the motion of nanoparticles in polymer solutions
and melts is a problem of broad importance, with applications to
many different fields, such as material science, biophysics, and
medicine. If the nanoparticles are larger than the polymer's radius
of gyration, their structure and dynamics can be well described in
terms of effective pair potentials. However, much remains to be
understood in the so-called “protein limit”, where the size of the
nanoparticles becomes comparable to or smaller than that of
the polymers. Moreover, most of the previous study considering
this size range have only focused on the dilute nanoparticle
regime, which is easier to handle since inter-nanoparticle
interaction can be neglected and the properties of the polymer
solution/melt are expected to be unchanged.
Using molecular dynamics simulations, we study the dynamic and
structural properties of a semidilute polymer solution containing
well dispersed spherical nanoparticles of size smaller than the
polymer's radius of gyration. We consider various nanoparticle
diameters and a broad range of nanoparticle volume fractions,
up to values for which the inter-nanoparticle interaction
becomes important.
We find that the polymers slow down when the nanoparticle
concentration is increased, in qualitative agreement with the
confinement parameter theory (Choi et al.,ACS Macro Lett.
2013,2,485−490), according to which polymers slow down
because they have to squeeze through “bottlenecks” created by
the presence of the nanoparticles. Also the nanoparticles slow
down when their concentration is increased, with the magnitude
of the slowing down depending in a non-trivial way on their size.
Surprisingly, if the concentration of the nanoparticles is increased
past the range in which the nanoparticle dispersion is good, the
diffusivities of polymers and nanoparticles reach a minimum and
then start to increase.