Local dynamics in functionalized polymer grafted nanoparticle systems with weak and strong functional anisotropy.

Abstract

We examine the effect of weak and strong anisotropic interactions of end-functionalized spherical polymer-grafted nanoparticles (PGNs) with a rigid core of radius <i>r</i>₀ and a corona composed of a fixed number of grafted arms, <i>f</i>. Interacting end-functionalized PGNs form bonds when their coronas overlap. In a simple system composed of three PGNs placed on the vertices of a triangular lattice, functional anisotropy is introduced by having adjacent PGN pairs form labile bonds with two different bond energies. We study the local dynamics in such systems by imposing controlled oscillatory shear deformation. Using a probability master equation approach to model the evolution of bonds between PGNs, we study the effect of functional anisotropy on the local dynamics when the oscillation frequency is varied. The resulting shear and normal forces exhibit a broken symmetry with reference to the strain (<i>γ</i>) and strain rate (<i></i>). Furthermore, they display features that are qualitatively different from those of isotropic systems like elastic hysteresis and complex non-Maxwellian response. Fourier analysis of responses indicates that the local rheological features vary strongly with functional anisotropy and the presence of even harmonics plays a critical role in determining the non-linear features of the dynamic response.