Dual Origin of Viscoelasticity in Polymer-Carbon Black Hydrogels: A Rheometry and Electrical Spectroscopy Study

• Published in: Macromolecules 2023-03, Vol.56 (6), p.2298-2308
• Main Authors: Legrand, Gauthier, Manneville, Sébastien, McKinley, Gareth H., Divoux, Thibaut
• Format: Article
• Language: English
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/acs.macromol.2c02068

Nanocomposite gels formed by mixing nanoparticles and polymers offer a limitless creative space for the design of functional advanced materials with a broad range of applications in materials and biological sciences. Here, we focus on aqueous dispersions of hydrophobic colloidal soot particles, namely, carbon black (CB) dispersed with a sodium salt of carboxymethylcellulose (CMC), a food additive known as cellulose gum that bears hydrophobic groups, which are liable to bind physically to CB particles. Varying the relative content of CB particles and cellulose gum allows us to explore a rich phase diagram that includes a gel phase observed for a large enough CB content. We investigate this hydrogel using rheometry and electrochemical impedance spectroscopy. CB–CMC hydrogels display two radically different types of mechanical behaviors that are separated by a critical CMC-to-CB mass ratio r c. For r < r c, i.e., for low CMC concentration, the gel is electrically conductive and shows a glassy-like viscoelastic spectrum, pointing to a microstructure composed of a percolated network of CB particles decorated by CMC. In contrast, gels with a CMC concentration larger than r c are nonconductive, indicating that the CB particles are dispersed in the cellulose gum matrix as isolated clusters, and act as physical cross-linkers of the CMC network, hence providing mechanical rigidity but limited conductivity enhancement to the composite. Moreover, in the r > r c concentration range, CB–CMC gels display a power-law viscoelastic spectrum that depends strongly on the CMC concentration. These relaxation spectra can be rescaled onto a master curve, which exhibits a power-law scaling in the high-frequency limit, with an exponent that follows Zimm theory, showing that CMC plays a key role in the gel viscoelastic properties for r > r c. Our results offer an extensive experimental characterization of CB–CMC dispersions that will be useful for designing soft nanocomposite gels based on hydrophobic interactions.