Modeling the Liquid-Liquid Phase Transition Induced by Polymerization

We suggest a simple model to describe phase separation induced by polymerization, as was recently observed in liquid sulfur. The model contains three types of interactions: i) atoms attract each other by van der Waals forces which produce the liquid-gas phase transition at low pressures, ii) atoms may form covalent bonds which induce polymerization, and iii) polymerized atoms attract each other stronger than non-polymerized atoms. We show that the latter couples polymerization with phase segregation and produces the liquid-liquid phase transition that was observed in sulfur. We implement (i)-(iii) using a discrete molecular dynamics approach, and obtain a qualitative agreement with the behavior of sulfur.