J. Krištiak

Free-volume microstructure of glycerol and its supercooled liquid-state dynamics

A positron annihilation lifetime spectroscopic (PALS) study was carried out on a typical representative of simple hydrogen-bonded glass formers - glycerol - over a wide temperature region from 100 K up to room temperature. Several crossover temperatures, Tbi, were identified in the temperature dependences of the ortho-positronium (o-Ps) lifetime and the relative o-Ps intensity. The onset temperature of free-volume change, 137 K, was compared with the value of 135±3 K obtained from extrapolating thermodynamic data and with the temperatures from relaxation data. In order to determine the influence of the free-volume evolution on the relaxation dynamics above the glass transition temperature Tg, the free-volume fraction from the PALS data was related to the behaviour of the dielectric loss of glycerol. A new relationship between the temperature parameters of the onset of free-volume change and of the high-frequency wing of the dielectric loss is presented. Taking the PALS free-volume data as input for a free-volume model (Doolittle ansatz), it was found that the relaxation times τα can be reproduced above the bend temperature Tb2 = 241 K; the latter lies in the vicinity of crossover temperatures reported from mode coupling theory analyses of dynamic data. In the lower-temperature region Tb2>T>Tb1Tg, a contribution from thermally activated mobility has to be included to reproduce the dielectric relaxation time data.

Free volume factor in supercooled liquid dynamics

The positronium (o-Ps) annihilation lifetime parameters, i.e., the mean o-Ps lifetime, s3, and the relative o-Ps intensity, I3, were measured for a series of molecular and structurally simple polymer glass formers over wide temperature ranges by means of positron annihilation lifetime spectroscopy (PALS). In addition to the most pronounced effect defining the PALS glass temperature, T PALS g , further two groups of the characteristic bend temperatures in the liquid state, one at T lq b1 ffi 1:15–1:4T PALS g and the other at T lq b2 ffi 1:3–1:75T PALS g were found. These characteristic temperatures were compared with certain crossover temperatures from both empirical ðTg; Tm; TB; TA; Tcoll; TMÞ and theoretical ðTcrÞ analyses of the thermodynamic and numerous dynamic data and their coincidences were discussed. Decrease in I3–T plots for glycerol and cis-trans-1,4-polybutadiene can be related to the fast motions on the nanosecond time scale as detected by neutron scattering or dielectric spectroscopy. The corresponding free volume hole characteristics, i.e., the mean free volume hole size, Rh, and the free volume hole fraction, fh, were determined by using a quantum-mechanical model of o-Ps in spherical hole or a model of volumetric and free volume hole properties, respectively. Consequently, the role of the free volume hole fraction from the PALS data in the dynamics of some supercooled liquids was established by analyses of various expressions containing the free volume fraction such as the Doolittle-type equation or the Macedo–Litovitz ansatz. 2002 Elsevier Science B.V. All rights reserved.

Free volume microstructure of tetramethylpolycarbonate at low temperatures studied by positron annihilation lifetime spectroscopy: a comparison with polycarbonate

The annihilation of positrons in amorphous tetramethylpolycarbonate (TMPC) has been investigated in the temperature range from 30 to 300 K. The dependences of the mean lifetime of the ortho-positronium τ3 and its relative intensity I3 on temperature can be described by the empirical relations: τ3 = 2.08(1 + 4.8 × 10−4 T) (ns) and I3 = 20.2(1 + 7.9 × 10−4T) (%). These dependencies are interpreted within the framework of the microstructural free volume concept. The comparison with the polycarbonate (PC) reveals that both values τ3 and I3 are larger for TMPC, indicating larger free volume realized through higher number (concentration) of larger free volume entities. The temperature dependencies of τ3 and I3 for TMPC exhibit a different character and a weaker dependence on the temperature. This different behaviour of the free volume is due to a different packing efficiency of chains and a different dominant mechanism of the free volume generation, as well as a decrease in the flip dynamics of the phenyl rings. The implications of different free volume microstructures on some mechanical properties (e.g. relaxation and fracture) as well as on the transport properties of gases in TMPC and PC are discussed.

Free volume aspects of the strong-fragile classification of polymer liquids

Abstract The strong–fragile classification or, alternatively, the cooperativity concept of polymer liquid dynamics was tested from a free volume point of view by positron annihilation lifetime spectroscopy. For a series of six polymers we found that the expansivity of the mean free volume hole size correlates with the fragility as well as with the non-exponentiality. These tendencies support the original interpretational proposals.

Direct evidence of free-volume relaxation and the crossover effect in Ni25Zr55Al20 metallic glass

Abstract The changes in positron annihilation lifetime in a thermally treated Ni 25 Zr 55 Al 20 alloy were observed. The glass transition temperature T g lies around 700 K. Isothermal annealing was used to treat samples sequentially in the amorphous (at 640 K) and the undercooled liquid (at 740 K) states. This provides insight into the microstructural processes taking place via determination of the evolution of the positron capture site density and distribution. The observed shifts in the positron lifetime distribution are analyzed in terms of relaxation of the free volume V f , i.e. chemical and topological short-range ordering (CSRO, TSRO) which are known to be influenced in an important manner by the relaxation processes. The upper limits of the activation energies of the two relaxation processes (2.2 and 2.6 eV) are determined. For the first time, the “crossover” effect in the free-volume distribution has been observed.

Free Volume Seen by Positronium in Bulk and Confined Molecular Liquid

Results of positron annihilation experiments on bulk hexadecane and hexadecane filled into silica gel nano pores will be reported. The lifetime and Doppler broadening measurements were performed at the temperature range (14-300) K. The annihilation data were combined with output of the differential scanning calorimetry at range from 270K up to 300K.

Short range ordering in the melt and its manifestation in glassy Fe-Co-B. Investigation by positron annihilation lifetime

Abstract Six sets of ribbons of Fe 64 Co 21 B 15 metallic glass obtained by rapid quenching of the melt having different temperature and time history, selected with respect to the temperature-time evolution of melt viscosity, have been investigated by positron annihilation spectroscopy. The interest has been focused on the correlation between the expected melt precursor structure and the presence of ordered units (clusters, associations) in the amorphous matrix. Ribbons have been annealed for suitable times at temperatures below and above the crystallization temperature. Such selective thermal treatment of the samples has enabled to accentuate or anneal-out the effects of the presence of small ordered structural units as reflected by the positron annihilation lifetimes and positron affinities and by changes of electron structure obtained from electrical resistivity measurements. Fine structure of the amorphous matrix has been determined by analyzing the positron annihilation lifetime spectra with the use of a numerical Laplace transform method which provides continuous distributions of the position annihilation lifetimes. Differences in local ordering have been found for samples from master alloys with different melt histories.

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), τ(3), annihilation in amorphous polymer poly(vinylmethylether) (PVME) from positron annihilation lifetime spectroscopy (PALS). We show that the behavior of τ(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(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 β 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 α process as well as to the slow secondary β relaxation from BDS. In addition, it lies also in the vicinity of the crossover temperature, T(B)(βKWW), in the spectral dispersion of the primary α process, indicating a connection of the change in the o-Ps lifetime with the variation in the width of the primary α relaxation times distribution. Finally, the τ(3) value at T(b2)(L) is close to the mean relaxation time of the primary α process, τ(α), in coincidence with the crossover in the secondary effective β 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.

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, τ3(T), and the various spectral and relaxation features have been observed. Thus, the phenomenological evaluation of the τ3(T) dependence of the PALS response of the amorphous sample reveals three characteristic PALS temperatures: TgPALS, Tb1L = 1.15TgPALS and Tb2L = 1.25TgPALS, 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 Tb1L 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 Tb1L. The high-temperature plateau effect in the τ3(T) plot occurs at Tb2L and agrees with the characteristic Stickel temperature, TBST, marking a qualitative change of the primary α process, but it does not follow the relation Tb2L < Tα [τ3(Tb2) < τα]. Both effects at Tb1L and Tb2L correlate with two crossovers in the spectral shape and related non-exponentiality parameter of the structural relaxation, βKWW. 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, Tmc, close to Tb1 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.

New Organic-Inorganic Hybrid Ureasil-Based Polymer Materials Studied by PALS and SEM Techniques

The new organic-inorganic materials, based on polyether chains covalently linked to a silica framework through urea bridges, referred as ureasilicates or ureasils, and semiconducting As2S3-ureasil composites are investigated using positron annihilation lifetime spectroscopy (PALS) and scanning electron microscopy (SEM) techniques. The results obtained show that incorporation of the As2S3 clusters into ureasil affects on the ortho-positronium (o-Ps) intensity or positronium formation probability and micro-/nanoscopic structure compared to the pure polymer, the effect is more essential as the loading fraction of As2S3 increases.