Structure and Transport Properties of Nohms

Structure and transport properties of Nanoparticle organic hybrid materials

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=volume of nanoparticletotal volume ) 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.