Imaging of the microstructure of Carbopol dispersions and correlation with their macroelasticity: A micro- and macrorheological study

Abstract

We developed a new data analysis strategy, the so-called micro-rheo-mapping technique, based on multiparticle tracking experiments to obtain an accurate and direct visualization of the microstructure of commercial acrylate thickeners of Carbopol-type with high (Ultrez 10), intermediate (ETD 2020), and low (ETD 2050) degree of crosslinking. At low polymer concentration, aggregates made of several primary Carbopol particles are formed with an average diameter of 43 ± 11, 56 ± 14, and 10 ± 2.5 μm for Ultrez 10, ETD 2020, and ETD 2050, respectively. For ETD 2050, the least crosslinked thickener, the shell of dangling polymer chains covering the aggregate surface is thicker than for ETD 2020 and Ultrez 10. At technically relevant polymer concentrations, our results indicate, for all three thickeners, that the microstructure is highly heterogeneous with regions of different crosslink densities. One region inaccessible for tracer particles corresponding to a mixture of polydisperse aggregates and individual primary particles with a core mesh size less than 200 nm and a second, diluted enough to be accessible and which exhibits both elastic and viscous characteristics. The study of the impact of pH, polymer concentration, and crosslink density on these local structural and viscoelastic heterogeneities as well as macrorheological properties allowed us to establish a correlation between microstructure and macroelasticity. In particular, we found that the bulk shear modulus strongly depends on the fraction of inaccessible areas, making this microscopic parameter most relevant for describing the macroelasticity of Carbopol gels, whereas the local elasticity of the interstitial regions is of minor importance.