Coincident Correlation between Vibrational Dynamics and Primary Relaxation of Polymers with Strong or Weak Johari-Goldstein Relaxation

Abstract

The correlation between the vibrational dynamics, as sensed by the Debye-Waller factor, and the primary relaxation in the presence of secondary Johari-Goldstein (JG) relaxation, has been investigated through molecular dynamics simulations. Two melts of polymer chains with different bond length, resulting in rather different strength of the JG relaxation are studied. We focus on the bond-orientation correlation function, exhibiting higher JG sensitivity with respect to alternatives provided by torsional autocorrelation function and intermediate scattering function. We find that, even if changing the bond length alters both the strength and the relaxation time of the JG relaxation, it leaves unaffected the correlation between the vibrational dynamics and the primary relaxation. The finding is in harmony with previous studies reporting that numerical models not showing secondary relaxations exhibit striking agreement with experimental data of polymers also where the presence of JG relaxation is known.

Mutual Information in Molecular and Macromolecular Systems

“…If these are not known, their estimation is a delicate (and sometimes uncertain) procedure leading to higher computational cost [13,15]. Building on previous studies carried out by Molecular Dynamics (MD) simulations, the present paper reports novel insight provided by MI-based approaches on two distinct aspects of transport and relaxation close to GT, namely dynamical heterogeneity [23,24] and its influence on secondary relaxations [25,26]. Details about the model systems and numerical methods are found elsewhere [23][24][25][26].…”

“…In agreement with previous studies [53,54], we found that the chains having shorter bond length exhibits a characteristic two-step decay after a first fast decay at t ∼ 0.1, which is a signature of the presence of two distinct relaxation processes. We ascribed the faster one to the JG β relaxation and the slower one to the structural α relaxation [25,26]. Effective torsional potential resulting from the Lennard-Jones interaction between the first and the last monomers of the fragment.…”

“…We considered N c = 512 linear chains made of M = 25 monomers each, resulting in a total number of monomers N = 12, 800. Further details are given elsewhere [25,26]. An interesting aspect of the model is that, even if the force field does not include a torsional interaction, an effective torsional barrier may hinder the dihedral rotations depending on the bond length l 0 due to the LJ repulsion between two non-adjacent monomers.…”

“…For simplicity we set t 0 = 0. We ascribed the faster one to the JG β relaxation and the slower one to the structural α relaxation [25,26]. The data were taken from Reference [25].…”

State-of-the-Art Polymer Science and Technology in Italy

“…Leporini and co-workers, by the means of molecular dynamics simulations, studied two melts of polymer chains with different bond length, resulting in rather different strength of the Johari-Goldstein (JG) relaxation. They found that, even if changing the bond length alters both the strength and the relaxation time of the JG relaxation, it leaves unaffected the correlation between the vibrational dynamics and the primary relaxation [8].…”