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

Le résumé fourni par la source

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.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.