Disentangling Self-Atomic Motions in Polyisobutylene by Molecular Dynamics Simulations

Area of Science:

• Polymer Physics
• Materials Science
• Computational Chemistry

Background:

• Understanding polymer dynamics is crucial for material properties.
• Polyisobutylene (PIB) exhibits complex dynamics above its glass transition.
• Previous studies on PIB dynamics relied on neutron scattering, with some interpretations needing revision.

Purpose of the Study:

• To characterize the atomic-level dynamics of polyisobutylene (PIB) using atomistic molecular dynamics simulations.
• To investigate the relationship between main-chain and methyl-group motions in PIB.
• To re-evaluate previous findings on the dielectric beta-process in PIB.

Main Methods:

• Fully atomistic molecular dynamics simulations of PIB across a wide temperature range.
• Validation of simulation cell against experimental neutron scattering data.
• Analysis of atomic trajectories using anomalous jump diffusion and rotational rate distribution models.

Main Results:

• Simulations accurately reproduced experimental neutron scattering data.
• All atoms in PIB, including main-chain carbons, exhibit a crossover to non-Gaussian dynamics.
• Methyl group rotations are found to be coupled with main-chain segmental motions, contradicting assumptions of statistical independence.

Conclusions:

• Methyl group rotations in PIB are dynamically coupled to the polymer's main-chain motion.
• The anomalous jump diffusion model effectively describes the observed dynamics.
• Simulation results provide a revised perspective on the origin of the dielectric beta-process in PIB.