Correlated Segmental Dynamics in Amorphous Atactic Polystyrene:  A Molecular Dynamics Simulation Study

Abstract

Molecular dynamics simulations of bulk atactic polystyrene have been performed in a temperature range from 100 to 650 K at atmospheric pressure. Local translational mobility has been investigated by measuring the mean square translational displacements of monomers. The long-time asymptotic slope of these dependencies is 0.54 at T > Tg, showing Rouse behavior. Crossover from motion in the cage to Rouse-like dynamics has been studied at T > Tg with a characteristic crossover time follows a power law behavior as a function of T, as predicted by mode-coupling theory (MCT). We studied the coherent intermediate scattering function and show the agreement with scaling predictions of MCT. Local orientational mobility has been studied via the orientational autocorrelation functions, ACFs (Legendre polynomials of the first and second order), of both the main-chain and side-group bonds. The ACF relaxation times analysis shows strong coupling between the phenyl groups and the backbone relaxation. The relaxation times of the orientational R-relaxation follow the same power law (γ ∼ 2.9) as the characteristic translational diffusion time and the intermediate scattering function. Below Tg, both types of dynamics are described by the same activated law. The ACFs time-distribution functions reveal the existence of activated local rearrangements already above T g.

Simulation of polymer glasses: From segmental dynamics to bulk mechanics

“…This change in the character of the relaxational behavior from the VFT-like behavior above T g to the activated behavior below T g is qualitatively similar to that observed for PS in [1][2][3]. The MCT fit [4], s $ (T À T c ) Àc , is also used to fit the T-dependence of the a-relaxation times above T g (the MCT-like fit is very close to the fit produced by Eq.…”

“…United-atom models are used in the present study; the models, the force fields and details of the cooling procedure are described in detail by Lyulin et al [1][2][3]. In short, the NPT MD simulation has been performed for a model PS melt consisting of 1-8 polymer chains of N p = 80-160 monomers each.…”

“…Our recent molecular dynamics (MD) simulation studies [1][2][3] on the atomistic model of an atactic-polystyrene (PS) melt reveal the details of segmental polymer dynamics in the glassy state. The simulations show that above T g the characteristic translational diffusion time for this polymer follows the same universal power law as the characteristic time of dynamic scattering, in good agreement with the predictions of the mode-coupling theory (MCT) [4].…”

Time Scales and Mechanisms of Relaxation in the Energy Landscape of Polymer Glass under Deformation: Direct Atomistic Modeling

“…Models and simulation procedure.-United-atom models for nonentangled polymer melts are used; they have been described in detail by Lyulin et al [13,14]. Briefly, constant temperature-pressure NPT MD simulations are carried out for a system of 4 -8 atactic PS chains of N 80-160 monomers each, and 64 PC chains of N 10 monomers.…”

Local polymer dynamics under strong connectivity constraints: The dendrimer case

“…[3][4][5][6] At such subnanometer scale and in the dynamic range of the ␤-relaxation where the glass-forming units are moving within a "cage" formed by their immediate neighbors, [7][8][9][10] polymer specific effects are seemingly not directly involved. Notwithstanding, even at these very short spatial dimensions indirect effects such as the modification of local packing characteristics due to the applied intramolecular potentials in polymeric models, 4 or even correlation of motion within the cage 11,12 should properly be taken into account.…”