Molecular simulation of an initial stage of the ordered-structure formation of linear and ring polymers upon cooling from the melts

• Molecular simulation is used to study the structural formation of linear and ring polymers upon cooling from the melts. • Properties at molecular and bond level are reported to monitor the ordered-structure formation. • Linear polymers seem to form the ordered-structure better than ring chains. Monte Carlo (MC) simulation of the coarse-grained ring and linear polyethylene (PE) models were employed to study the early stage of the ordered-structure formation upon cooling from the melts. The simulations suggest that the ordered structure with the single domain can be formed for both linear and ring PE chains which have the same mean square radius of gyration in the molten state. Molecular and structural properties evaluated from the simulation are the fraction of trans conformation and the sequence length of trans state, the anisotropic change of chain dimension, the overall orientation correlation function, intra- and inter-molecular bond orientation correlation functions and intermolecular pair correlation function. All these properties reveal an ordering process as the sign of crystallization especially for linear chains whereas it is more difficult for ring polymer to form an ordered phase. For ring chains, the ordered structures were formed with multiple folded conformations and smaller magnitude for the fraction of trans conformation, chain orientation, degree of chain packing, intra- and inter-molecular bond ordering. The simulations suggest that PE chains are needed to stretch to some extent before the chain alignment can proceed. Compared to the linear polymers with comparable molecular size, ring PE chains tend to form the less-ordered crystalline structure, have higher melting temperature and exhibit slower crystallization rates.