Further studies on the molecular dynamics of the glass transition and the glass state using EPR probes

Studies of an organic glass-forming liquid, dibutyl phthalate, with electron-spin-resonance probe molecules indicate a change in the primary relaxation mechanism about the glass transition. This is interpreted in terms of two different relaxation mechanisms of liquidlike and solidlike cells in the glass phase, as postulated in the work of Grest and Cohen. The observed gradual change of $\tau$, the rotational diffusion time, above the glass-transition temperature is derived from the Grest-Cohen model. The sudden changes in line shape and signal intensity observed at the glass transition are interpreted in terms of EPR signal saturation effects and a possible abrupt change of $T_1$, the spinlattice relaxation time, due to the change of a substantial amount of the sample from the supercooled liquid state to the solidlike glass state. Spectral line simulations are used to verify phenomenological slow-tumbling $\tau$ formulas. The best fits to the experimental spectra at temperatures in the region of the glass transition are sums of simulated liquid and rigid EPR spectra. This provides evidence for the coexistence of two different types of regions from approximately 220 K and below.