Molecular Dynamics of Polystyrene Films: Comparison Between Atomistic Simulations and beta-NMR Measurements

The enhancement of dynamics near the free surface of glassy polymers continues to attract great interest, as a universally accepted microscopic understanding of this effect remains elusive. To compare to beta-detected nuclear magnetic resonance (beta-NMR) measurements, we have studied the depth dependence of molecular motion in polystyrene thin films with atomistic molecular dynamics simulations. Beta-NMR is a non-destructive technique well suited to study local properties of thin films, including glassy polymers [1]. The technique allows for nanoscale depth-resolved measurements of dynamics occurring at nanosecond timescales, attainable in simulation. The spin-lattice relaxation rate of the ⁸Li⁺ probe is believed to be due to the motion of the phenyl side groups. We compare this to several measures of molecular mobility inferred from the simulated polymer films. Our study is the first side-by-side comparison of molecular relaxation rates in terms of their depth dependence as well as their Arrhenius temperature dependence.

1. McKenzie, I. et al. Enhanced high-frequency molecular dynamics in the near-surface region of polystyrene thin films observed with b-NMR. Soft Matter 11, 1755–1761 (2015).