Surface and bulk dynamics of compressed polystyrene films: A β-NMR study

Glasses gradually densify as they relax towards equilibrium, and in doing so, their spectral density of molecular fluctuations is modified. In polystyrene (PS), only a few percent increase in density is equivalent to the passage of millions of years. To access this regime, we have plastically deformed a PS thin film with nanoimprint lithography, resulting in permanent compactification without rejuventation¹. We present depth-resolved ⁸Li⁺ β-NMR measurements in PS films and discuss the dynamical effects of the mechanical processing. β-NMR is a technique sensitive to nanosecond molecular dynamics through the spin-lattice relaxation of a short-lived radioisotope. Because the isotope is implanted as a low-energy ion beam, the depth is controllable with nanometer-scale resolution. This has allowed β-NMR to directly probe the dynamics in glassy polymer thin films near both the free and buried interfaces².

¹ H. D. Rowland et al., Science 322, 720–724 (2008).
² I. McKenzie et al., Soft Matter 14, 7291–7544 (2018).