β-NMR Studies of the Depth Dependence of Secondary Relaxations in Polystyrene Thin Films

β-detected nuclear magnetic resonance (β-NMR) of implanted ⁸Li⁺ has been used to probe the depth dependence of secondary relaxation processes in thin films of atactic polystyrene (PS). β-NMR measurements of the average ⁸Li spin-lattice relaxation rate were used to determine the depth dependence of the fluctuation rate of the fast γ-relaxation process in PS films at 317 K, which is well below its glass transition temperature [I. McKenzie et al. Soft Matter, 14, 7324 (2018)]. A novel hole-burning (selective saturation) β-NMR technique has been used to measure the depth dependence of the fluctuation rate of the much slower β-relaxation process at the same temperature. Both the fluctuation rates of the β- and γ-relaxation processes are larger near the free surface than in the bulk and both return to the bulk value over a length scale of several nm.