Internal dynamics of microgels: A mesoscale hydrodynamic simulation study

Abstract

We analyze the dynamics of polymers in a microgel system under different swelling conditions. A microgel particle consists of coarse-grained linear polymers which are tetra-functionally crosslinked and undergoes conformational changes in response to the external stimuli. Here, a broad range of microgel sizes, extending from tightly collapsed to strongly swollen particles, is considered. In order to account for hydrodynamic interactions, the microgel is embedded in a multiparticle collision dynamics fluid while hydrophobic attraction is modelled by an attractive Lennard-Jones potential and swelling of ionic microgels is described through the Debye-Hückel potential. The polymer dynamics is analyzed in terms of the monomer mean square displacement and the intermediate scattering function S(q, t). The scattering function decays in a stretched-exponential manner, with a decay rate exhibiting a crossover from a collective diffusive dynamics at low magnitudes of the wavevector q to a hydrodynamic-dominated dynamics at larger q. There is little difference between the intermediate scattering functions of microgels under good solvent conditions and strongly swollen gels, but strongly collapsed gels exhibit a faster decay at short times and hydrodynamic interactions become screened. In addition, we present results for the dynamics of the crosslinks, which exhibit an unexpected, semiflexible polymer-like dynamics.