On structures and properties of polyethylene during heating and cooling processes based on molecular dynamics simulations

We have investigated the structures and properties of polyethylene (PE) polymers with different chain length during heating and cooling processes in terms of molecular dynamics simulation technique. The conformations and flexibility of PE chains are analyzed using the chain end-to-end distance (Rn)/chain radius of gyration (Rg) with the chain length and temperature. The bond orientation parameter (bop, P2) is applied to reveal the order degree of PE polymer. The approximately linear relationship between the computed and experimental melting temperatures confirms the reliability of simulated results. The relationship between melting temperature (or freezing temperature) and chain length is in good agreement with Fox–Flory equation. The Rg/Rn distribution reveals that the cooling process is accompanied by the conformation transformation from the curled chains to relatively extended ones. For crystalline PE chains, some interesting typical conformations are observed and demonstrated. Interestingly, the PE chains with similar Rg and Rn values have the similar curled/folded structures in melted/crystalline states. The results of the bond orientation parameter (bop) reveal that after freezing, the bop values for short-chain PE polymers are considerably larger than those of long-chain PE polymers, which is attributed to the fact that some chains in the long-chain PE polymers are folded.