(171c) Structure and Transport Properties of Nohms

Polymer nanocomposites have been a topic of interest in recent
years for their potential in such applications as water desalination, CO₂
capture, photovoltaics and immersion lithiography ^[1-3]. Unlike colloids which tend
to agglomerate irreversibly, polymer grafted colloids are stabilized by
polymer-polymer steric interactions. Nanoscale organic hybrid materials (Nohms)
are a class of such materials which consist of an inorganic nanoparticle
core, functionalized with a corona of organic oligomers.
These differ from common nanocomposites in that the
tethered corona is the suspending medium for the cores. ^[1] The
hybrid nature of the suspension allows the fabrication of materials with tunable
properties by varying parameters such as nanoparticle
chemistry, shape and size, as well as the polymer molecular weight, grafting
density and chemistry.  The range of
properties exhibited by these composites vary from solids, stiff waxes, and
gels for high core content to single component solvent free fluids for low core
content.

 Molecular simulations can help elucidate how
the properties of Nohms are affected (and can be
optimized for specific applications), by changes in molecular design. To this
end, we have used Molecular Dynamics (MD) to get a better understanding of the
structure and transport properties of these systems.

In this work, the translational and rotational
diffusivity for pure Nohms was simulated via
equilibrium MD. Diffusivities are normalized by the corresponding values from
Stokes-Einstein (translation) and Stokes-Einstein-Debye (rotation) relations
for naked nanoparticles suspended in a melt of free
chains. The simulated systems were chosen to mimic experimental systems using silica
cores and PEO chains (having high grafting density and short chains^[2])
and to isolate the contributions of core and corona on Nohms
dynamics by varying the core volume fraction (f=volumeofnanoparticletotalvolume) and polymer length.
Non-equilibrium MD methods were implemented to obtain viscosities (by imposing
a homogeneous steady state shear rate) and yield stresses. A non-Newtonian
shear thinning behavior is observed in all cases along with reduction in yield
stress with decreasing f, a behavior partially consistent with experimental data.^[2] Core
volume fraction affects the
relative viscosity significantly; e.g., for f =0.2, the
viscosities are significantly higher than that of free chains (pentamers) and those of the f=0.1 Nohms. Comparing systems with identical f=0.1, the Nohms with longer chain length have lower viscosities
(closer to that of the melt of free with has the lowest viscosities).
Altogether these observations show that the dynamics of the corona grows in
importance (relative to that of the cores) in Nohms
as f decreases and
chain length increases. The diffusivity results indicate that the system with
the highest f (=0.2) has the smallest translational mobility, and
for the systems with equal
f (=0.1) the Nohms with the
longer chains (20-mers) has higher translational motion and less structured
cores. Changes in the rotational diffusivity are less significant among the all
cases examined, which could be due to a comparable effective friction resisting
rotation.

We also observe a lower shear thinning slope for Nohms+polymer blends than for pure Nohms.
Structural analyses reveal that preferential alignment of grafted polymers in
the direction of shear is one of the dominant phenomena associated with the
shear thinning behavior. Longer chains tend to align more readily causing a
more pronounced reduction in viscosity, consistent with the observed trends in
viscosity. Free polymers are found to align the least, partly explaining the
weaker shear thinning seen for blends (compared with pure Nohms).  Ongoing work aims to provide a more complete
characterization of how f, grafting
density, and polymer length can be tailored to produce Nohms
with a wide range of themophysical properties.

References

1.       
Bourlinos,
A. B., Chowdhury, S. R., Herrera, R., Jiang, D. D., Zhang, Q., Archer, L. A.,
et al. (2005). FUnctionalized Nanostructures with Liquid-Like Behavior :
Expanding the Gallery of Available Nanostructures. Advanced Functional
Materials , 15, 1285-1290.

2.       
Agarwal,
P., Qi, H., & Archer, L. A. (2010). The Ages in a Self-Suspended
Nanoparticle Liquid. Nanoletters , 10, 111-115.

3.       
Rodriguez, R., Herrera, R., Lynden,
A. A., & Giannelis, E. P. (2008). Nanoscale Ionic Materials. Advanced Materials , 20,
4353-4358.