Positron annihilation and relaxation dynamics from dielectric spectroscopy: poly(vinylmethylether)

We report on the temperature dependence of the lifetime of the ortho-positronium (o-Ps), tau(3), annihilation in amorphous polymer poly(vinylmethylether) (PVME) from positron annihilation lifetime spectroscopy (PALS). We show that the behavior of tau(3)(T) can be divided into five regions, each of them having a linear temperature dependence, and that the crossover PALS temperatures situated at T-b1(G) = 0.76T(g)(PALS), T-g(PALS), T-b1(L) = 1.14T(g)(PALS) and T-b2(L) = 1.37T(g)(PALS), and marking the discontinuity in the free volume microstructure are related to various dynamic features from neutron scattering (NS) and broadband dielectric spectroscopy (BDS). First, a slight change in the PALS response in the glassy PVME at T-b1(G) is related to the onset of the excess wing in an apparent correspondence with the fast secondary beta motion from NS. A further slight bend in the liquid state at T-b1(L) is related to a high-frequency tail of the primary alpha process as well as to the slow secondary beta relaxation from BDS. In addition, it lies also in the vicinity of the crossover temperature, T-B(beta KWW), in the spectral dispersion of the primary alpha process, indicating a connection of the change in the o-Ps lifetime with the variation in the width of the primary alpha relaxation times distribution. Finally, the tau(3) value at T-b2(L) is close to the mean relaxation time of the primary alpha process, tau(alpha), in coincidence with the crossover in the secondary effective beta process between two regimes in the liquid PVME. All these relationships point to very close connections between the PALS response and the dynamic behavior of PVME, which can be explained in terms of the temperature dependence of the probability function of the liquid-like and the solid-like domains, as obtained from the two-order parameter (TOP) model description of the liquid to glass transition in glass-formers.