Controlling equilibrium swelling of polyampholyte gels using charge sequence

The swelling behavior of polymer gels is crucial for many applications, such as flexible electronics, energy storage devices, and biomedical implants, which rely on their volume changes during swelling and their ability to retain solvents. One promising approach to tailoring the swelling behavior involves controlling electrostatic interactions in polyampholyte gels by arranging opposite charges. This study investigates the effect of three distinct charge sequences - alternating, random, and diblock on the equilibrium swelling ratio Q using molecular simulations and theoretical analysis. Both molecular simulations and theoretical results demonstrate that electrostatic tension σ_e and elastic stress σ_x balance osmotic pressure Π to establish equilibrium swelling. As the Bjerrum length l_B increases, enhanced σ_e drives Q from a weakly interacting regime toward complete collapse (Q = 1) with ionic binding. While the asymptotic limits remain sequence independent, the transition behavior exhibits sequence dependence: the alternating sequence shows the steepest decline in Q with l_B, random sequences display intermediate behavior, and diblock sequences exhibit the most gradual response. Direct resolution of stress components reveals that sequence-dependent swelling originates from variations in σ_e, while σ_x and Π remain unaffected by electrostatic interactions. Furthermore, σ_e scales with the volume fraction Φ of overlapping network strands in an apparent power-law manner. Theoretical analysis based on Random Phase Approximation confirms sequence-dependent power-law scaling of σ_e with Φ. These findings provide a rational framework for designing polyampholyte gels with programmable swelling responses through charge sequence engineering.