(547e) Temperature-Controlled Nanomechanical Shear-Thinning between Polymer Nanoparticle Surfaces during Shear-Induced Aggregation.

José Francisco Wilson^{a, [1]}(wilsonj@vscht.cz), Miroslav Soos^a (soosm@vscht.cz).

^a University of Chemistry and Technology Prague, Technicka 5, 16628 Prague 6, CZECH REPUBLIC

Abstract

In this research, we studied the impact of material fusion as adhesion mechanism on the size and structure of fractal aggregates formed during shear-induced aggregation of fully destabilized NPs. To precisely control particle adhesion, we used nanoparticles (NPs) with a Core-Shell (C-S) structure, where the core is composed of polymethyl methacrylate and the shell is composed of a combination of polymethyl methacrylate and polybutylacrylate. The core acts as a hard-sphere with a high glass transition temperature (T_g) to maintain the structure of the NPs during changes in temperature. On the other hand, the composition of the shell was designed to target a T_g of 50°C , giving the surface of the NPs the capability to soften upon increasing temperature. During aggregation experiments we observed that the size of aggregates grow significantly at temperatures above T_g of the shell, indicating an increase of adhesive force between the NPs. Additionally, at temperatures higher than T_g, it was observed that the increase in adhesion is countered by a reduction of aggregate size when higher shear rates are applied. This suggest that the bond between nanoparticles weakens upon increase of shear rate when the material on the surface experiences a transition from soft to a viscous polymer. This finding indicates that polymer NPs surfaces at temperatures above T_g might experience a local shear thinning effect. To prove this hypothesis, we used DEM simulations and study the interplay between the increase of adhesion due to material softness and the decrease of bonding strength upon increase of shear rate. Obtained results support the hypothesis that these two contradicting mechanisms determine the final mechanical properties or produced material.

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