Effects of chain length on Rouse modes and non-Gaussianity in linear and ring polymer melts

Abstract

The dynamics of ring polymer melts are studied via molecular dynamics simulations of the Kremer-Grest bead-spring model. Rouse mode analysis is performed in comparison with linear polymers by changing the chain length. Rouselike behavior is observed in ring polymers by quantifying the chain length dependence of the Rouse relaxation time, whereas a crossover from Rouse to reptation behavior is observed in linear polymers. Furthermore, the non-Gaussian parameter of the bead displacement is analyzed. It is found that the non-Gaussianity of ring polymers is remarkably suppressed, which is in contrast to the growth in linear polymers, particularly with increasing chain length.

Unraveling the Glass-like Dynamic Heterogeneity in Ring Polymer Melts: From Semiflexible to Stiff Chain

“…This suggests that the distribution of the COM displacement |R m (t) − R m (0)| follows a Gaussian distribution, which was previously reported in our work. 40 The observation of Gaussian behavior in semiflexible ring polymers, even at the dense monomer density of ρ = 0.85, is noteworthy and provides a unique perspective on the dynamics of ring polymers. By contrast, for stiff ring chains, the increase in α 2 (t) was more significant, showing peaks in a long-time regime that approximately corresponded to the onset time scale of the diffusive behavior with ⟨Δr COM 2 (t)⟩ ∼ t, as demonstrated in Figure 1(b) and (d).…”

“…We varied the monomer density ρσ 3 (= NMσ 3 / V ) as 0.1, 0.3, 0.4, 0.5, and 0.55 both for the semiflexible and stiff chains. In addition, we adopted the monomer density ρ = 0.85 for the semiflexible chain with ε θ = 1.5, which was a common choice for MD simulations both for linear , and ring ,− , polymers. It should be noted that a stiff chain system with ε θ = 5 displayed nematic ordering when the monomer densities exceeded ρ = 0.55, which is in agreement with the recent MD simulations reported in ref .…”

“…41 This finding is consistent with the experimental observation of poly(ethylene oxide) ring melts by Braś et al 42 However, our previous study, which also used the same model for MD simulations of ring polymer melts, showed that the NGP remained quite small at all time regimes, even when the chain length was increased up to N = 400. 40 It should be noted that the chain stiffness differed between the two studies. Specifically, the bending potential ε θ (1 + cos θ) (in the unit of energy scale in the Lennard-Jones potential) acts on the bending angle θ formed by three consecutive monomer beads along the polymer chain (eq 3).…”

“…37−39 In addition, we conducted calculations on the NGP for linear polymer melts by MD simulations using the Kremer−Grest (KG) bead-spring model. 40 The chain lengths varied from N = 5 to 400, and the monomer density was set at ρ = 0.85 (in the unit of σ −3 by using the size of the bead σ). Our findings revealed a notable increase in the peak of the NGP as N increases.…”

“…Dynamic heterogeneity (DH) is a key concept used to describe the significant slowing-down of glass-former liquids as they approach the glass transition temperature. − The slowing-down is accompanied by the collective structural relaxation of spatially heterogeneous regions that exceeds the molecular size. − DH is conventionally measured by the non-Gaussian parameter (NGP), i.e., the degree of the deviation from the Gaussian distribution for the molecular displacement within a given time interval. ,− The NGP was utilized to quantify the non-Gaussianity in supercooled linear polymer melts. − In addition, we conducted calculations on the NGP for linear polymer melts by MD simulations using the Kremer–Grest (KG) bead-spring model . The chain lengths varied from N = 5 to 400, and the monomer density was set at ρ = 0.85 (in the unit of σ –3 by using the size of the bead σ).…”