Applicability of mode-coupling theory to polyisobutylene: A molecular dynamics simulation study

Abstract

The applicability of Mode Coupling Theory (MCT) to the glass-forming polymer polyisobutylene (PIB) has been explored by using fully atomistic molecular dynamics simulations. MCT predictions for the so-called asymptotic regime have been successfully tested on the dynamic structure factor and the self-correlation function of PIB main-chain carbons calculated from the simulated cell. The factorization theorem and the time-temperature superposition principle are satisfied. A consistent fitting procedure of the simulation data to the MCT asymptotic power-laws predicted for the α-relaxation regime has delivered the dynamic exponents of the theory-in particular, the exponent parameter λ-the critical non-ergodicity parameters, and the critical temperature T(c). The obtained values of λ and T(c) agree, within the uncertainties involved in both studies, with those deduced from depolarized light scattering experiments [A. Kisliuk et al., J. Polym. Sci. Part B: Polym. Phys. 38, 2785 (2000)]. Both, λ and T(c)/T(g) values found for PIB are unusually large with respect to those commonly obtained in low molecular weight systems. Moreover, the high T(c)/T(g) value is compatible with a certain correlation of this parameter with the fragility in Angell's classification. Conversely, the value of λ is close to that reported for real polymers, simulated "realistic" polymers and simple polymer models with intramolecular barriers. In the framework of the MCT, such finding should be the signature of two different mechanisms for the glass-transition in real polymers: intermolecular packing and intramolecular barriers combined with chain connectivity.

Glassy dynamics of a binary Voronoi fluid: A mode-coupling analysis

“…Therefore, if we number the correlators in the order in which they enter the collapse regime, this numbering is preserved when the correlators leave the regime [57]. This prediction has been observed in many simulations [38,39,61,65,66,67,46]. Figure 3 suggests that it also holds for our Voronoi mixture.…”

“…The factorization theorem, ( 41) and ( 42), provides an additional means to determine whether an analysis of the observed two-step relaxation in terms of MCT is justified or not. A simple test of the theorem works directly with the simulation data without invoking any fit procedure [38,39,46,60,61,62,63,64,65,66,67,68]. To this end, we fix two times t 1 and t 2 (t 2 > t 1 ) in the β regime and calculate the ratio…”

Disentangling Self-Atomic Motions in Polyisobutylene by Molecular Dynamics Simulations

“…The cage effect is the main concept involved also in the mode coupling theory (MCT) [ 73 , 74 ]. The applicability of MCT to the case of PIB was explored in a previous work [ 75 ]. The localization length involved in the MCT was found to be = 0.30 Å for cC and = 0.50 Å for mH.…”

Insight into the Structure and Dynamics of Polymers by Neutron Scattering Combined with Atomistic Molecular Dynamics Simulations

“…The combined consideration of NS and MD-simulations has also been of utmost help in exploring the applicability of theoretical frameworks, like the MCT, in polymers [ 106 , 107 , 108 ]. Consistent results were found when applying MCT to these systems, though with rather high values of the exponent parameter (the key parameter in this theory).…”