A Fast Stimuli-Responsive Brush with Regulated Exposed–Adsorbed Transition of Minority Polyelectrolyte Chains

Multicomponent polymer brushes with minority responsive charged chains serve as a promising model system for achieving fast stimuli-responsive switches. Understanding the conformational transitions of these responsive chains under external fields is critical for optimizing functional material performance in ionic environments and uncovering fundamental mechanisms of biological response processes. This work systematically investigates the conformational behavior of sparsely grafted minority polyelectrolyte chains (responsive chains) under the stimulation of an external electric field, surrounded by densely grafted neutral polymer chains. Through self-consistent field (SCF) calculations, a significant first-order exposed–adsorbed transition of the polyelectrolyte chains is observed. This transition becomes barrierless when the responsive chains are weakly charged and slightly longer than the neutral chains. Moreover, when the degree of ionization of the charge chains exceeds some critical values, two crucial transition characteristics, namely transition width and barrier, which typically constrain each other in traditional phase transitions, become decoupled and decrease simultaneously with appropriate environmental parameter adjustments. These phenomena of barrier-free transition and decoupling of the transition characteristics indicate fast and sensitive stimulus responses. Through systematic investigations of the influences of environmental parameters on the transition characteristics and the critical ionization, optimized parameters are identified. To comprehensively characterize parameter interdependencies, approximate relationships between transition characteristics and environmental parameter combinations are derived in the limit of low ion concentration and short responsive chains. We hope that our findings provide theoretical guidance for the design of fast and sensitive externally stimulated-responsive materials in ionic environments, such as biosensors, targeted drug carriers, and nanoswitches.