Counterion-Mediated Hydrogen Bonding Making Strong Polyelectrolytes pH-Responsive: Current Understanding and Perspectives

The counterion-mediated hydrogen bonding (CMHB) is related to the hydrogen bonding between bound counterions and polyelectrolyte chains in polyelectrolyte systems, where the counterions can both electrostatically bind to the charged groups of polyelectrolyte chains and act as hydrogen bond donors or acceptors to form hydrogen bonds with the hydrogen bond sites associated with polyelectrolyte chains simultaneously. A large number of literatures illustrate that strong polyelectrolytes (SPs) are insensitive to pH, which severely limmits the applications of SPs as smart materials. However, our studies have demonstrated that the CMHB makes SPs pH-responsive. This perspective discusses the mechanism of pH responsiveness of SPs and the pH-tunable properties of SPs, based on the pH-controlled CMHB effect. The future research directions on the pH responsiveness of SPs are also discussed here. It is anticipated that the study of the pH responsiveness of SPs not only will provide a new understanding of the fundamental properties of SPs, but also will greatly expand the applications of SPs in the field of smart materials.