Methyl Dynamics and β-Relaxation in Polyisobutylene: Comparison between Experiment and Molecular Dynamics Simulations

Molecular dynamics simulations were employed in bulk polyisobutylene, to provide a detailed characterization of the methyl dynamics and to examine its relation to the β-relaxation as described by different experimental techniques. The simulation results were compared to new 13 C NMR measurements probing methyl motion at temperatures above the glass transition, as well as to lower temperature literature NMR data. It was found that the 13 C NMR results, and those for the β-relaxation reported by dielectric relaxation spectroscopy and neutron scattering experiments, can be interpreted on a common basis through a coupling mechanism between methyl rotation and a recently described fast relaxation motion associated with the chain backbone. In addition, a molecular mechanism for the description of the dielectric β-process observed in polyisobutylene is proposed, and investigated by simulations.