On the relationships between guest molecular dynamics and free volume in a series of small molecular and polymer glass-formers

► Spin probe TEMPO dynamics in a series of glass-formers has been measured by ESR. ► Ortho-positronium (o-Ps) annihilation studies have been performed by PALS. ► Almost constant o-Ps lifetime at T50G transition. ► Free volume fluctuation as a controlling factor of T50G transition.