LAOStrain response of carbon black-polymer hydrogels: insights from rheo-TRUSAXS and rheo-electric experiment

Abstract

Colloid-polymer hydrogels are 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 binding physically to CB. As described in [Legrand et al., Macromolecules 56, 2298-2308 (2023)], CB-CMC hydrogels exist in two flavors: 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. We compare these two types of CB-CMC hydrogels under Large Amplitude Oscillatory Shear (LAOS), 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 (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 microns, beyond USAXS resolution. At larger strain amplitudes, the conductivity of the fluidized sample increases again, exceeding its initial value, consistent with shear-induced formation of a transient, dynamically percolated network of CB clusters. In contrast, insulating CB-CMC hydrogels exhibit a larger yield strain (60%), beyond which the sample flows and the average distance between CB particles decreases. This reorganization is concomitant with a more than tenfold increase in conductivity, although it remains below that of conductive hydrogels at rest.