Dynamics from the Surface to the Bulk in Ultrastable and Liquid-Cooled Oligomeric Glasses

Ultrastable glasses (USGs) exhibit strikingly different properties from liquid-cooled normal glasses (NGs), yet their molecular dynamics remain poorly understood. Using β-detected NMR of implanted spin-polarized ^{8}Li^{+}, we directly track the depth and temperature dependence of the γ relaxation-phenyl ring twisting-in highly monodisperse atactic styrene oligomers. Near the bulk glass transition temperature T_{g}, USGs show much faster surface dynamics but slower bulk dynamics than an NG. Cooling below T_{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.