Secondary Dynamics in Ultrastable Polystyrene Thin Films Studied by β-NMR*

Ultrastable, highly mono-disperse polystyrene (PS) thin films can be produced by physical vapor deposition. These films can exhibit properties similar to those of a normal glass that has been aged for thousands of years. β-detected nuclear magnetic resonance (β-NMR) of implanted ⁸Li⁺ probes can be used to study dynamics in thin polymer films, which is not possible with conventional magnetic resonance techniques [I. McKenzie et al. Soft Matter 14, 7324 (2018)]. We have used β-NMR to study the temperature dependence of the secondary γ-relaxation process in an ultrastable PS film and a normal PS film that was produced by rejuvenating an ultrastable film by heating to T_g+25 K for 2 minutes. The γ-relaxation is ~38% slower at 295 K, and the activation energy is ~20% larger in the ultrastable glass compared with the normal glass. Our interpretation of the results is that the denser packing in the ultrastable glass hinders motion of the phenyl rings.