Free volume in poly(propylene glycol) and its relationships to spin probe reorientation

In this work we carried out positron annihilation lifetime spectroscopy (PALS) on pure poly(propylene glycol) (PPG 4000) and an electron spin resonance (ESR) study on PPG 4000 doped with the spin probe TEMPO. The spectral and dynamic features of both the microscopic probes such as the ortho-positronium (o-Ps) lifetime, τ3, the spectral parameter, T50 G, and the correlation time of spin probe, τc, as a function of the temperature were compared. New empirical relationships between the characteristic PALS temperatures ( T b 1 L and T b 2 L ) and the characteristic ESR temperatures ( T X 1 L and T X 2 L ) have been revealed. In addition, these characteristic PALS and ESR temperatures can be connected with the Schönhals and Stickel ( T B SCH , T B ST ) temperatures of the primary α relaxation process of PPG 4000 as obtained by dielectric spectroscopy (DS) studies in the literature. Next, the mean free volume hole sizes of the matrix were related to the spin probe size and to the Arrhenius or the non-Arrhenius motional regimes of TEMPO. Finally, the temperature dependences of the reorientational correlation time have been analysed using the Vogel–Fulcher–Tamman–Hesse (VFTH) equation and its generalized form, i.e., the Bendler–Shlesinger–Fontanella (BSF) one, the latter derived from the defect diffusion model of the dynamics and transport properties. The fitting parameters are then compared with those of the primary α relaxation process.