All atom molecular dynamics simulation study of polyethylene polymer in supercritical water, supercritical ethanol and supercritical methanol

In spite of valuable applications of plastic materials, they should be recycled. Polymers like polyethylene are usually depolymerized to their monomers in supercritical water or alcohols. In this manuscript structural changes of polyethylene in supercritical water, ethanol and methanol are investigated by molecular dynamics simulation. Temperature and pressure of each solvent was chosen 50 °C and 50 bar upper their critical temperature and critical pressure, respectively. OPLS all atom force filed was used for molecular dynamics simulation. Root mean square fluctuation of monomers of polyethylene, radius of gyration, diffusion constant and hydrophobic solvent accessible surface area of polymer molecule is calculated in investigating supercritical solvents. The results showed that flexibility of both ends of molecule is more than flexibility of internal monomers in three supercritical solvents. Also, flexibility of polyethylene oligomer in supercritical water is greater than flexibility of polyethylene in supercritical alcohols. Compactness of polyethylene was increased during molecular dynamics simulation in supercritical water. Among three solvents, supercritical ethanol has greater diffusion constant than the other ones. Although single aggregate structure was not observed, ethanol comes closer to polyethylene molecule during simulation than the other solvents. It seems that supercritical water has two roles in depolymerization of polyethylene; solvent and reactant, but supercritical alcohols acts only as solvent.