Study of the Shear-Thinning Effect between Polymer Nanoparticle Surfaces during Shear-Induced Aggregation

Abstract

In this research, we studied the impact of material fusion as an adhesion mechanism on the size and structure of fractal aggregates formed during shear aggregation of fully destabilized polymer nanoparticles (NPs). The nanoparticles have a core–shell structure, where the core is composed of poly(methyl methacrylate) (PMMA) and the shell consists of a combination of PMMA and polybutyl acrylate (PBA). Due to significantly different glass transition temperatures (Tg’s) of these polymers, the core acts as a hard sphere, while the presence of PBA in the shell gives the surface a soft character. By varying the system temperature, material fusion is induced between the particles in contact. The strength of the formed physical bond is tested under various shear rate conditions. It was found that the increase in temperature leads to an increase in aggregate size, caused by an increase in adhesion between NP surfaces. This phenomenon occurs due to a material softening of the polymer shell triggered by the increase in temperature, resulting in the formation of a viscous sticky surface. Additionally, it was observed that at temperatures above the Tg of the polymer composing the shell, the increase in the shear rate causes a reduction of the interparticle contact strength suggesting a shear-thinning effect during contact. The interplay between these two contradicting mechanisms determines the final mechanical properties of produced material.