Computer simulations of thermo-shrinking polyelectrolyte gels

Abstract

In this work, thermo-responsive polyelectrolyte gels have been simulated using polymer networks of diamond-like topology in the framework of the primitive model. Monte Carlo simulations were performed in the canonical ensemble and a wide collection of situations has been systematically analysed. Unlike previous studies, our model includes an effective solvent-mediated potential for the hydrophobic interaction between non-bonded polymer beads. This model predicts that the strength of the attractive hydrophobic forces increases with temperature, which plays a key role in the explanation of the thermo-shrinking behaviour of many real gels. Although this hydrophobic model is simple (and it could overestimate the interactions at high temperature), our simulation results qualitatively reproduce several features of the swelling behaviour of real gels and microgels reported by experimentalists. This agreement suggests that the effective solvent-mediated polymer-polymer interaction used here is a good candidate for hydrophobic interaction. In addition, our work shows that the functional form of the hydrophobic interaction has a profound influence on the swelling behaviour of polyelectrolyte gels. In particular, systems with weak hydrophobic forces exhibit discontinuous volume changes, whereas gels with strong hydrophobic forces do not show hallmarks of phase transitions, even for highly charged polyelectrolyte chains.