Spin probe reorientation and its connections with free volume and relaxation dynamics: Diglycidyl-ether of bis-phenol A

We report the dynamical behavior of a series of four spin probes of different geometries in amorphous van der Waals-type glass-former diglycidyl-ether of bis-phenol A (DGEBA) as measured by electron spin resonance spectroscopy (ESR). To reveal the relative influence of physical factors influencing spin probe dynamics, the reorientation properties were related to the free volume data of DGEBA from positron annihilation lifetime spectroscopy (PALS) and to the relaxation results of DGEBA from broadband dielectric spectroscopy (BDS). The characteristic ESR temperatures (T50G,TX2Azz′) determined from the temperature dependences of the spectral parameter of mobility, 2Azz′, for all spin probes and those (TX1τ and TX2τ) from the rotational correlation time, τc, as a function of temperatures for the smallest spin probe 2,2,6,6-tetramethylpiperidine-1-oxyl (Tempo) were found to coincide with the characteristic PALS temperatures Tb1, Tb2, and Tσ3 from phenomenological analyses of the temperature dependencies of the mean o-Ps lifetime, τ3, and the width of the o-Ps lifetime distribution, σ3. Moreover, for the smallest spin probe Tempo, the favorable mutual relationship between the spin probe size VPW and hole size distribution g(Vh) and the closeness of T50 G with Tb1 point to the role of free volume in the slow to rapid regime crossover. On the other hand, for the three larger spin probes T50 G lies in the vicinity of Tb2∼Tσ3 and is independent of their size, mass, and shape. The origin of these ESR and PALS coincidences can be traced out to the secondary γ-process for the smallest spin probe Tempo and to the primary α-process for all larger spin probes.