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Title: Microscopic nature of the Johari-Goldstein secondary relaxation
Plain: Below the glass point most molecules are frozen, yet glasses of even rigid molecules still show a faster, smaller relaxation, the Johari-Goldstein $\beta$ process. Which molecular motions produce it, and whether it is the precursor of the main relaxation, is disputed.
Precise: In molecular glass formers, including glasses of rigid molecules, dielectric and mechanical spectra show a secondary $\beta$ relaxation that persists below $T_g$ with Arrhenius $\tau_\beta(T)$ and an empirical activation energy of roughly $24\ k_B T_g$ per molecule (approximate). Determine whether it is carried by localized islands of mobility or by small-angle motion of all molecules, and whether it is the precursor of the alpha relaxation as assumed in coupling-type models. An answer identifies the motions in a model glass equilibrated near or below T_g and reproduces the measured $\beta$ peak and its link to $\tau_\alpha$.
Settled by: Simulation of a swap-equilibrated model glass that resolves which particles and motions produce the $\beta$ peak and reproduces its activation energy and its relation to $\tau_{\alpha}$, consistent with NMR and dielectric data.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: contested
Status note: (empty)
Posed since: 1970