Ultrastable glasses (USGs) exhibit strikingly different properties from liquid-cooled normal glasses (NGs), yet their molecular dynamics remain poorly understood. Using $\beta$-detected NMR of implanted spin-polarized ${}^8{\mathrm{Li}}^{+}$, we directly track the depth and temperature dependence of the $\gamma$ relaxation—phenyl ring twisting—in highly monodisperse atactic styrene oligomers. Near the bulk glass transition temperature $T_{\mathrm{g}}$, USGs show much faster surface dynamics but slower bulk dynamics than an NG. Cooling below $T_{\mathrm{g}}$ reveals a sharp crossover at $T^{*}$, where surface dynamics vitrify on tens to hundreds of nanosecond timescales and become slower than in the bulk. The NG further displays two distinct $T^{*}$ values near the surface, pointing to heterogeneous local structure. Despite these differences, all dynamics obey entropy-enthalpy compensation, suggesting a common molecular mechanism. These results show that preparation and thermal history lock styrene oligomeric glasses into distinct regions of the potential energy landscape, giving rise to fundamentally different glassy dynamics.

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• Polymer glasses
• Polymer thin films
• Beta-detected nuclear magnetic resonance