To investigate crystallinities based on trans-structures, we determined the differences in the crystallization properties of ring and linear polymers by performing united-atom-model molecular dynamics (MD) simulations of homogeneous polyethylene melts of equal length, N, which refers to the number of monomers per chain. Modified parameters based on the DREIDING force field for the CH2 units were used in order to accelerate the crystallization process. To detect polymer crystallization, we introduced some local-order parameters that relate to trans-segments in addition to common crystallinities using neighboring bond orders. Through quenching MD simulations at 5 K/ns, we roughly determined temperature thresholds, Tth, at which crystallization is observed although it was hard to determine the precise Tth as observed in the laboratory time frame with the present computing resources. When N was relatively small (100 and 200), Tth was determined to be 320 and 350 K for the linear- and ring-polyethylene melts, respectively, while Tth was found to be 330 and 350 K, respectively, when N was 1000. Having confirmed that the crystallization of a ring-polyethylene melt occurs faster than that of the analogous linear melt, we conclude that the trans-segment-based crystallinities are effective for the analysis of local crystal behavior.

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A model is developed for simulating entangled polymers by dissipative particle dynamics (DPD) using the segmental repulsive potential (SRP). In contrast to previous SRP models that define a single-point interaction on each bond, the proposed SRP model applies a dynamically adjustable multipoint on the bond. Previous SRP models could not reproduce the equilibrium properties of Groot and Warren's original DPD model [R. D. Groot and P. B. Warren, J. Chem. Phys. , 4423 (1997)] because the introduction of a single SRP induces a large excluded volume, whereas, the proposed multipoint SRP (MP-SRP) introduces a cylindrical effective excluded bond volume. We demonstrate that our MP-SRP model exhibits equilibrium properties similar to those of the original DPD polymers. The MP-SRP model parameters are determined by monitoring the number of topology violations, thermodynamic properties, and the polymer internal structure. We examine two typical DPD polymers with different bond-length distributions; one of them was used in the modified SRP model by Sirk [J. Chem. Phys. , 134903 (2012)], whereas the other was used in the original DPD model. We demonstrate that for both polymers, the proposed MP-SRP model captures the entangled behaviors of a polymer melt naturally, by calculating the slowest relaxation time of a chain in the melt and the shear relaxation modulus. The results indicate that the proposed MP-SRP model can be applied to a variety of DPD polymers.

Relaxation process of a single dendrimer with excluded volume effects is studied in the free-draining limit for various parameters characterizing the architecture of the dendrimer, which are the functionality of central segment f c , the functionality of branching segments f , the number of generations G and the number of bonds in each spacer between branching segments P. By assuming that the dendrimer relaxes hierarchically from the outermost generation to the central segment after relaxation of the spacers, a scaling law 1 $ f 1À c fð f À 1Þf 1À g GÀ1 P 2 þ1 for the longest relaxation time 1 is derived, where % 0:588 is the Flory exponent. Another scaling law for relaxation times of other antisymmetric relaxation modes is also derived. In order to verify the scaling laws, Brownian dynamics simulations of a single dendrimer are performed and relaxation times are estimated by the relaxation mode analysis method. The scaled relaxation time 1 =ð f 1À c P 2 þ1 Þ is proportional to fð f À 1Þf 1Àx g GÀ1 , where the exponent x corresponding to is estimated as x % 0:66 for f ¼ 3 and 0.63 for f ¼ 4. It is found that 1 =ð f 1À c fð f À 1Þf 1Àx g GÀ1 Þ / P 2yþ1 , where the exponent y that corresponds to is estimated as y % 0:58. The agreement with the predicted scaling law becomes better as G and f becomes larger, where the concentration of segments becomes larger. The other scaling law for other antisymmetric relaxation modes is also verified.