Collapse/expansion dynamics and actuation of pH-responsive nanogels

Abstract

Polyelectrolyte (PE) hydrogels can dynamically respond to external stimuli, such as changes in pH and temperature, which benefits their use for smart materials and nanodevices with tunable properties. We investigate equilibrium conformations and phase transition dynamics of pH-responsive nanogels using hybrid molecular dynamics/Monte Carlo simulations with full consideration of electrostatic and hydrodynamic interactions. We demonstrate that PE nanogels exhibit a closed-loop phase behavior with a discontinuous swelling--collapse transition that occurs only at intermediate pH values. A 50~nm nanogel particle close to a critical point functions as a pH-driven actuator with a microsecond conformational response and work density $\approx 10^5~\mathrm{J/m}^3$, an order of magnitude larger than skeletal muscles. The collapse/expansion time scales as $L^{2}$ and the power density scales as $L^{-2}$ where $L$ is the linear size of the gel. Our work provides fundamental insight into phase behavior and non-equilibrium dynamics of the swelling--collapse transition, and our method enables the investigation of charge--structure--hydrodynamic coupling in soft materials.