β-NMR studies of the depth and temperature dependence of dynamics in normal and ultrastable polystyrene glasses

We have used β-detected nuclear magnetic resonance (β-NMR) with implanted spin-polarized ⁸Li⁺ to study the depth and temperature dependence of the γ-relaxation of polystyrene (PS), which involves motion of the phenyl side groups.^1,2 Information about the dynamics is obtained by measuring the average spin-lattice relaxation rate, 1/T₁^avg of ⁸Li. Ultrastable glass (USG) films of highly monodisperse, low-molecular weight PS were prepared by physical vapor deposition. USG films exhibit properties similar to those of normal glass (NG) films that have been aged for several years. β-NMR measurements of USG and NG films indicate the bulk γ-relaxation is slower in the USG compared with the NG while the opposite is true near the free surface. We observe a change in the temperature dependence of 1/T₁^avg for ⁸Li implanted near the free surface that we associate with the glass transition. The near-surface T_g is lower in the USG film than in the NG film. These trends are more significant for samples with a larger apparent age, which is related to the fictive temperature. There is also a difference between the USG and NG films in the thickness of the near-surface region with enhanced dynamics.

1. I. McKenzie et al., Soft Matter 14, 7324 (2018)

2. I. McKenzie et al., J. Chem. Phys. 156, 084903 (2022)