LAOStrain response of carbon black-polymer hydrogels: Insights from rheo-TRUSAXS and rheo-electric experiments

Colloid–polymer hydrogels are commonly encountered in various applications, from flow batteries to drug delivery. Here, we investigate hydrogels composed of hydrophobic colloidal soot particles—carbon black (CB)—and carboxymethylcellulose (CMC), a food-grade polymer functionalized with hydrophobic groups that bind physically to CB particles. As previously described in Legrand et al. [Macromolecules 56, 2298–2308 (2023)], CB-CMC hydrogels exist in two forms: either electrically conductive when featuring a percolated network of CB particles decorated by CMC or insulating where isolated CB particles act as physical cross-linkers within the CMC matrix. Here, we compare the yielding transition of these two types of CB-CMC hydrogels under large amplitude oscillatory shear, combining rheometry with time-resolved ultra-small-angle x-ray scattering (TRUSAXS) and electrical conductivity measurements. Both types of hydrogels exhibit a “type III” yielding scenario, characterized by an overshoot in G″ and a monotonic decrease in G′, although the underlying microscopic mechanisms differ markedly. Conductive CB-CMC hydrogels display a yield strain γy≃6% concomitant with a drop in DC conductivity, indicative of the macroscopic rupture of the percolated CB network at length scales larger than a few micrometers, beyond the USAXS resolution. At larger strain amplitudes, the conductivity of the fluidized sample increases again, exceeding its initial value, consistent with the shear-induced formation of a transient, dynamically percolated network of CB clusters. In contrast, insulating CB-CMC hydrogels exhibit a significantly larger yield strain, γy≃60%, beyond which the sample flows and the average distance between CB particles decreases, as revealed by rheo-TRUSAXS. This structural reorganization is concomitant with a more than tenfold increase in conductivity, although it remains below that of the conductive hydrogels at rest.