Understanding dynamics of pure ring polymer in the melt systems: Molecular Dynamics (MD) Simulations in both 2-Dimension and 3-Dimension

Abstract

The internally closed-loop molecular architecture without chain ends of ring polymers overall makes a significant influence on structural and dynamical properties of ring systems. In particular, ring polymer melts is known to exhibit a collapsed chain structure in the limit of long chain length. This is ascribed to an effective pressure acting on individual ring molecules via intermolecular topological interactions under the unknotted and uncrossability (or nonconcatenation) conditions. In addition, there has been a supposition in the community on the effect of interchain penetration on the rheological behaviors of ring melts. These two factors may play a key role in determining ring polymer rheology. In this study, we investigated a direct correlation between the topological influence leading to a compact structure and dynamics of ring polymers, separately from the interchain penetration effect. To this, we first analyzed the structural and dynamical properties of ring polymer melts in 3-dimensional (3D) space by using molecular dynamics (MD) simulations implemented with an accurate united-atom model and a well-known coarse-grained Kremer-Grest model. These results show the general rheological behaviors of ring polymers in the melt. Second, we conducted 2-dimensional (2D) MD simulations of ring melts to exclude the interchain penetration effect, with all other conditions being preserved similar to the 3D melt systems. The topological effect resulting in a collapsed structure was found to become much more prominent for 2D systems, as compared to 3D systems: Flory’s scaling exponent approaches to 1/2 and 1/3 for 2D and 3D systems, respectively. 2D ring polyethylene melts ranging from C50H100 to C500H1000 of molecular weight have been subjected to extensive atomistic MD simulations while the chain length in the Kremer-Grest 2D systems ranges from 30 beads up to 400 beads. We found that the 2D ring melts display distinctive scaling behaviors in dynamical properties; e.g., the longest relaxation time with and the chain center-of-mass diffusion coefficient with , as compared with the corresponding results and for 3D ring melts which have intermediate length. On the basis of the results of 2D ring systems possessing considerably collapsed structures even at moderate chain lengths, we may further predict the rheological behaviors of 3D ring melts of large chain lengths.