Conformational and Structural Relaxations of Poly(ethylene oxide) and Poly(propylene oxide) Melts: Molecular Dynamics Study of Spatial Heterogeneity, Cooperativity, and Correlated Forward–Backward Motion

Abstract

Performing molecular dynamics simulations for all-atom models, we characterize the conformational and structural relaxations of poly(ethylene oxide) and poly(propylene oxide) melts. The temperature dependence of these relaxation processes deviates from an Arrhenius law for both polymers. We demonstrate that mode-coupling theory captures some aspects of the glassy slowdown, but it does not enable a complete explanation of the dynamical behavior. When the temperature is decreased, spatially heterogeneous and cooperative translational dynamics are found to become more important for the structural relaxation. Moreover, the transitions between the conformational states cease to obey Poisson statistics. In particular, we show that, at sufficiently low temperatures, correlated forward-backward motion is an important aspect of the conformational relaxation, leading to strongly nonexponential distributions for the waiting times of the dihedrals in the various conformational states.

Interpretation of 1H and 2H spin–lattice relaxation dispersions: Insights from molecular dynamics simulations of polymer melts

“…In Fig. 9, we see that spatially heterogeneous dynamics associated with the structural relaxation of PPO [25,35] leads to substantial deviations from Gaussian behavior for times between 10 −12 and 10 −9 s. Observing that the long-time limit of this time range well agrees with the short-time limit of the observed range of validity and considering that Gaussian dynamics was assumed to derive Eq. (10), we conclude that the time range, in which insights into translational motion can be obtained from relaxation times, is limited at short times by the non-Gaussian nature of dynamics during structural relaxation.…”

Glass transition in single poly(ethylene oxide) chain: A molecular dynamics simulation study

“…As suggested by the MSD's, the complete relaxation time only at the several higher temperatures is shorter than the simulation duration time (5–10 ns). Similar to the previous works, the α‐relaxation time is estimated as the time (namely τ ) where P 2 ( t ) decays to 1/ e . As shown in Figure (b), these data can be well described by the VFT equation where , , and are the fitting parameters.…”

The influence of internal rotational barriers and temperature on static and dynamic properties of bulk atactic polystyrene

“…First of all, we define a probability, P 2-state (t), that a dihedral has not undergone a conformational transition over a time period, t. 55,56 The conformational transition is defined as a process in which the rotational isomeric state of a dihedral at time t 0 + t differs from that at t 0 . In addition, according to Ref.…”

“…What is more definitively known from simulations is that conformational transitions become increasingly self-correlated with decreasing temperature. 52,56,64 Self-correlated transitions refer to the immediate or near-immediate back jumps between two conformational states due to restricted chain motion imposed by the surrounding matrix. Here, we calculate the distribution of waiting times for 20 conformational transitions, as shown in Fig.…”