Bridging Heterogeneity Dictates the Microstructure and Yielding Response of Polymer-Linked Emulsions

Soft materials possessing tunable rheological properties are desirable in applications ranging from 3D printing to biological scaffolds. Here, we use a telechelic, triblock copolymer polystyrene-b-poly(ethylene oxide)-b-polystyrene (SEOS) to form elastic networks of polymer-linked droplets in cyclohexane-in-water emulsions. The SEOS endblocks partition into the dispersed cyclohexane droplets while the midblocks remain in the aqueous continuous phase, resulting in each chain taking on either a looping or bridging conformation. By controlling the fraction of chains that form bridges, we tune the linear elasticity of the emulsions and generate a finite yield stress. Polymers with higher molecular weight (M_w) endblocks form stronger interdroplet connections and display a higher bridging density. Beyond modifying the linear rheology, the telechelic, triblock copolymers also alter the yielding behavior and processability of the linked emulsions. We examine the yield transition of these polymer-linked emulsions through large amplitude oscillatory shear (LAOS) and probe the emulsion structure through confocal microscopy, concluding that polymers that more readily form bridges generate a strongly percolated network, whereas those that are less prone to form bridges tend to produce networks composed of weakly linked clusters of droplets. When yielded, the emulsions consisting of linked clusters break apart into individual clusters that can rearrange upon the application of further shear. By contrast, when the systems containing a more homogeneous bridging density are yielded, the system remains percolated but with reduced elasticity and bridging density. The demonstrated ability of telechelic triblock copolymers to tune not only the linear viscoelasticity of complex fluids but also their nonlinear yield transition enables the use of these polymers as versatile and robust rheological modifiers. We expect our findings to therefore aid the design of the next generation of complex fluids and soft materials.