Positron-annihilation-lifetime response and broadband dielectric relaxation spectroscopy: Diethyl phthalate

We report the results of a combined phenomenological analysis of the data from positron-annihilation-lifetime spectroscopy (PALS) and the relaxation data from broadband dielectric spectroscopy (BDS) on diethyl phthalate (DEP). The ortho-positronium (o-Ps) lifetime, τ3, as a function of temperature over a temperature range from 67K up to 300K is compared with the spectral features and the relaxation parameters of the BDS spectra decomposed into the primary α and the secondary β processes in the temperature range from 140K up to 380K by using the Williams-Watts scheme. Phenomenological model-free analysis of the τ3−T plot provides the three characteristic PALS temperatures, where the two most pronounced ones at TPALSg=185K and Tb2=245K=1.32TPALSg are related to the glass-liquid transition and the onset of a quasiplateau region, respectively. In the case of a weaker bend effect at Tb1=210K=1.14TPALSg, a number of new coincidences with changes in the dielectric β process have been found. They concern the changes in width parameter of the distribution function for the β relaxation time and the activation energy of the βeff process, a crossover from the Arrhenius to the non-Arrhenius type of temperature dependence as well as with the onset of a short-time tail of the β relaxation time distribution and finally, with changes in the relaxation strength of the α and βeff processes. All these findings indicate a close connection of the o-Ps annihilation parameters and relaxation characteristics of BDS response for the DEP matrix.This work was supported by the VEGA Agency, Slovakia under Grants No. 2/6035/26. to one of the authors (J.B.) and No. 2/7121/27 to one of the authors (O.S.). Two of the authors (A.A., J.C.) acknowledge the support by the Spanish Ministry of Science and Technology (MICyT -Contract No. MAT 2004/01617) and by the Government of the Basque Country -Contract No. 9/UPV 00206.215-13568/2001.Peer reviewe

Nanopores in Silica Aerogels

The size of nanopores in thermally treated samples of silica aerogels was determined by ortho-positronium lifetime measurements at room temperature. The annealing of samples in the temperature region of 300-800°C does not make substantial changes in pore size felt by positronium. The annealing at higher temperatures from an interval of 800-1050°C causes shrinkage of the samples reducing the pore size to 1-15 nm. The macropores vanish at temperature treatment at 1000°C and the positron annihilation is not affected by oxygen from air

Positron annihilation response and broadband dielectric spectroscopy: Salol

A phenomenological analysis of the ortho-positronium (o-Ps) annihilation from positron annihilation lifetime spectroscopy (PALS) and the dynamics from broadband dielectric spectroscopy (BDS) are reported on a small molecular glass former of intermediate H-bonding and fragility: salol. The dielectric spectra extend over a very broad frequency range of about 2 × 10−2−3.5 × 1011 Hz, providing information on the α-relaxation, the secondary relaxation giving rise to the excess wing, and the shallow high-frequency minimum in the micro- to milli-meter wave range. A number of empirical correlations between the o-Ps lifetime, τ 3(T), and the various spectral and relaxation features have been observed. Thus, the phenomenological evaluation of the τ 3(T) dependence of the PALS response of the amorphous sample reveals three characteristic PALS temperatures: T g PALS , T b1 L = 1.15T g PALS and T b2 L = 1.25T g PALS , which are discussed in relation to similar findings for some typical small molecular vdW- and H-bonded glass formers. A slighter change of the slope at T b1 L appears to be related to the transition from excess wing to the primary α-process-dominated behavior, with the secondary process dominating in the deeply supercooled liquid state below T b1 L . The high-temperature plateau effect in the τ 3(T) plot occurs at T b2 L and agrees with the characteristic Stickel temperature, T B ST , marking a qualitative change of the primary α process, but it does not follow the relation T b2 L < T α [τ 3(T b2) < τ α ]. Both effects at T b1 L and T b2 L correlate with two crossovers in the spectral shape and related non-exponentiality parameter of the structural relaxation, β KWW. Finally, the application of the two-order parameter (TOP) model to the structural relaxation as represented by the primary α relaxation times from BDS leads to the characteristic TOP temperature, T m c , close to T b1 from PALS. Within this model the phenomenological interpretation is offered based on changes in the probability of occurrence of solid-like and liquid-like domains to explain the dynamic as well as PALS responses. In summary, all the empirical correlations support further very close connections between the PALS response and the dielectric relaxation behavior in small molecule glass formers