Viscous solvent embrittles long-chain polymer networks

The toughness of polymer networks is commonly attributed to energy dissipation arising from viscoelastic deformation, leading to a design principle: higher viscosity increases toughness. Here, we show that this relationship can be reversed when polymer chains are sufficiently long. To demonstrate, we prepare polyacrylamide hydrogels with identical network structures, fully dry them, and reswell them in glycerol-water mixtures to the same polymer content, thereby varying the solvent viscosity by three orders of magnitude while preserving the network structure. Although the mechanical response under homogeneous deformation does not change significantly, toughness measured by pure shear and trouser tests markedly decreases with solvent viscosity. In particular, toughness collapses onto a master curve when plotted as a function of the product of viscosity and velocity, with a scaling relation consistent with the shear-lag model. Stick-slip occurs under conditions for which the slope of the master curve is negative, corroborating the presence of a master curve. We attribute this embrittlement to limited transmission of tension along polymer chains at the crack tip due to solvent viscosity, which reduces the fraction of load-bearing chain segments at rupture. In contrast, in short-chain networks, the tension can readily be transmitted to the entire polymer chains, resulting in a non-negative slope. At sufficiently high viscosity and velocity, energy dissipation by viscosity dominates the tension transmission, resulting in a positive slope regardless of the chain length. These results advance the understanding of toughness dynamics through tension transmission and viscous dissipation.