Diffusion of long ring polymers in a melt is much slower than the reorganization of their internal structures. While direct evidences for entanglements have not been observed in the long ring polymers unlike linear polymer melts, threading between the rings is suspected to be the main reason for slowing down of ring polymer diffusion. It is, however, difficult to define the threading configuration between two rings because the rings have no chain end. In this work, evidences for threading dynamics of ring polymers are presented by using molecular dynamics simulation and applying a novel analysis method. The simulation results are analyzed in terms of the statistics of persistence and exchange times that have proved useful in studying heterogeneous dynamics of glassy systems. We find that the threading time of ring polymer melts increases more rapidly with the degree of polymerization than that of linear polymer melts. This indicates that threaded ring polymers cannot diffuse until unthreading event occurs, which results in the slowing down of ring polymer diffusion.

We study how dynamic heterogeneity in ionic liquids is affected by the length scale of structural relaxation and the ionic charge distribution by the molecular dynamics simulations performed on two differently charged models of ionic liquid and their uncharged counterpart. In one model of ionic liquid, the charge distribution in the cation is asymmetric, and in the other it is symmetric, while their neutral counterpart has no charge with the ions. It is found that all the models display heterogeneous dynamics, exhibiting subdiffusive dynamics and a nonexponential decay of structural relaxation. We investigate the lifetime of dynamic heterogeneity, τ(dh), in these systems by calculating the three-time correlation functions to find that τ(dh) has in general a power-law behavior with respect to the structural relaxation time, τ(α), i.e., τ(dh) ∝ τ(α)(ζ(dh)). Although the dynamics of the asymmetric-charge model is seemingly more heterogeneous than that of the symmetric-charge model, the exponent is found to be similar, ζ(dh) ≈ 1.2, for all the models studied in this work. The same scaling relation is found regardless of interactions, i.e., with or without Coulomb interaction, and it holds even when the length scale of structural relaxation is long enough to become the Fickian diffusion. This fact indicates that τ(dh) is a distinctive time scale from τ(α), and the dynamic heterogeneity is mainly affected by the short-range interaction and the molecular structure.

The experimental extinction spectrum of 38 nm long silver nanorods grown in the 22 nm pores of an anodic aluminum oxide (AAO) template is theoretically analyzed, using the discrete dipole approximation (DDA) method. A single broadband at 485 nm in the spectrum turns out to be the overlap of the two peaks at 430 and 515 nm, corresponding to the transverse and longitudinal modes of the surface plasmon of the nanorods, respectively, at small angles e75°of incidence. Interestingly, the longitudinal mode in the array is blueshifted by about 50 nm from that of an isolated nanorod while the transverse mode does not shift much. Near the glancing incidence, however, the two modes do not contribute much to the spectrum in the visible range due to significant red-shifting and broadening. We show that the incident angle dependence of the optical properties of the silver array may be utilized in controlling local near-field enhancements for surface-enhanced Raman scattering (SERS) and the maximum enhancements are achievable in the incident angles of 30-45°.