Construction of polydisperse polymer model and investigation of heat conduction: A molecular dynamics study of linear and branched polyethylenimine

Molecular dynamics (MD) simulation is a powerful tool for investigating the molecular mechanism of heat conduction in polymers. However, existing MD studies are mostly on monodisperse polymers and the effect of polydispersity, which typically occurs in commercial polymers, remains to be clarified. In this work, various types of polydisperse polyethylenimine (PEI) composed of molecules having different molecular weights and branching structures were constructed by in silico step-growth polymerization. The effect of the polydispersity and molecular structure on heat conduction in PEI was investigated using all-atom MD simulations. The number and weight fraction distributions of the polydisperse purely-linear PEI agreed well with the Flory-Schulz distributions, and therefore it can be concluded that the in silico polymerization used in this work reasonably mimics the step-growth-like polymerization observed in the actual synthesis of PEI. Thermal conductivity increased with increase in the radius of gyration dependent on the degree of branching of the molecules. In addition, thermal conductivity of a polydisperse PEI exhibited a similar value to that of a monodisperse PEI of a representative polymer chain in the polydisperse system. By analyzing in detail the thermal energy transfer among and inside molecules, their microscopic mechanisms could be understood and it was discovered that the average molecular weight is a critical factor in determining heat conduction.