Controlling Equilibrium Swelling of Polyampholyte Gels Using Charge Sequence

The swelling behavior of polymer gels is vital for many applications that depend on their volume change during swelling and their ability to retain solvents. One promising approach to tailoring this behavior involves controlling electrostatic interactions in polyampholyte gels by arranging opposite charges. This study investigates the effects of three distinct charge sequences─alternating, random, and diblock─on the equilibrium swelling ratio Q of polyampholyte gels. Both molecular simulations and theoretical analysis illustrate that the electrostatic tension σe, together with the elastic stress σ× of the gel network, balances the osmotic pressure Π to establish the equilibrium state of the polyampholyte gel. As the Bjerrum length lB increases, stronger σe causes Q to decrease from a value that is minimally affected by electrostatic interactions to a value of 1 for the dry state with ionic bindings. The two asymptotic regimes of the decrease are independent of the charge sequence. However, in between, the slope for the decrease of Q with lB is steepest for the alternating sequence, moderate for the random sequence, and shallowest for the diblock sequence. Resolving σe, σ×, and Π directly in the simulations reveals the sequence dependence of Q stems from the variation of σe with the sequence, while σ× and Π are independent of the electrostatic interactions. Moreover, σe scales with the volume fraction ϕ of overlapping polyampholyte network strands in an apparent power-law manner. The theoretical analysis also demonstrates a sequence-dependent power-law scaling of σe with ϕ derived from the Random Phase Approximation theory for single polyampholyte chains. These findings suggest a charge-sequence-based pathway for designing the swelling behavior of polyampholyte gels.