pH Response of Weak Polyampholyte Brushes with Random Charge-Asymmetric Monomer Sequences

Abstract

We perform simulations using the grand reaction method to investigate the pH response of polyampholyte brushes (PABs) with random sequences of weakly acidic and basic monomers. Titration curves, brush height, and chain conformations are shown to depend sensitively on the charge or compositional asymmetry, defined as the difference in numbers of weakly acidic and basic monomers on each chain. For asymmetric systems, the brush height response is a superposition of those for compositionally symmetric PABs and polyelectrolyte brushes. Further, we show that the pH response of PABs with random monomer sequences is distinct from those with ordered blocky sequences, even when parameters such as monomer composition and mean block size are similar. By contrast, we find that the sequence charge decoration parameter collapses structural data characterizing the brush height, lateral fluctuations, and chain conformations for all PABs examined, except for those with ordered sequences where large charge blocks result in strong inter- and intrachain electrostatic interactions. These results support the growing body of work highlighting the importance of charge separation on polyampholyte behavior.