The physics of thermoresponsive polymer networks: Insights from computer simulations.

Microgels are soft particles individually made by cross-linked polymer networks which are nowadays widely used as colloidal model systems because of their responsivity to external control parameters. In particular, thermoresponsive microgels undergo a reversible Volume Phase Transition from a swollen to a collapsed state at a given temperature. Recently, we introduced a numerical protocol to synthesize microgels in silico, providing a realistic description of the particles in comparison to experiments, and we also put forward a way to numerically calculate their elastic moduli. In addition to microgels, we also investigate hydrogels, $i.e.$, bulk polymer networks. By calculating their elastic properties, we find the emergence of auxetic behavior under tension for hydrogels with a low degree of crosslinking. When the limit of auxeticity is reached (Poisson's ratio $=-1$), a condition that we call "hyper-auxeticity", the mechanical instability triggers the onset of a critical point between two networks of different densities.