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The aim of the present work was to elucidate the impact of the structural changes of polymeric excipients during the course of storage on the drug release stability of tablets containing different polymers. Matrix tablets were formulated with famotidine as a model drug, using polyvinylpyrrolidone and carbopol matrix. Dissolution tests were carried out before and after storing the tablets under stress conditions for different time intervals. Parameters characterizing the release kinetics of matrix tablets, just as difference and similarity factors, were calculated to compare the release profiles as a function of storage time. Positron annihilation lifetime measurements were carried out to track the structural changes of the physical mixtures containing polymers during the course of storage. The changes in the positron lifetime distribution curves of the famotidine-polymer mixtures were in good correlation with the significant changes of release parameters of tablets. Thus the method would be a valuable tool for the screening of possible destabilizing interactions in the preformulation phase.

Temperature dependence of positron lifetime parameters have been studied in pure cyclodextrins and their supramolecular complexes with C 60 and C 70 molecules. It was found that while C 60 molecules are localized in molecular cavities in gamma-cyclodextrin, they are too small to be trapped by the larger cavities of delta-cyclodextrin. On the other hand, the larger C 70 molecules fill these larger voids completely. In cases where fullerene molecules are localized in molecular cavities, a low-temperature phase transition was observed. The activation energies for the phase transitions were calculated assuming a simple two-state annihilation model.

The behaviour of vacancies during isothermal ageing following quenching of an Al-1.7at.% Mg-1.1at.% Cu alloy has been studied by positron annihilation lifetime spectroscopy. The positron lifetime parameters vary in parallel with the Vickers hardness of the alloy, suggesting simultaneous migration of vacancies and changes in the size and concentration of vacancies containing clusters and dislocation loops. The results also explain the long hardness plateau observed previously in ageing experiments carried out between 100 and 240°C in terms of a continuous growth of Mg-Cu-vacancy clusters during ageing. The vacancy concentration of the clusters increases gradually until the setting in of the cluster Guinier-Preston-Bagaryatsky zone transformation.

The CO2 absorption of several ABO3 type perovskites was studied by positron lifetime spectroscopy. The longer positron lifetime was associated with positrons trapped by A site vacancies. The evaluated positron lifetime data indicated the relative stability of the crystal structure of Sr(Co0.5Fe0.5)O3−δ against Ca doping at low Ca concentrations. Oxygen desorption and CO2 absorption/desorption could also be followed by positron lifetime spectroscopy. It was shown that the concentration of oxygen vacancies has a large effect on positron lifetime data through the electron density of A site vacancies.

Positron annihilation spectroscopy was applied to study the effects of pre-treatment and composition of substrates on the quality and defect structure of electrodeposited thick chromium coatings. The results show that both parameters are important, and a scenario is proposed why the mechanically polished substrate gives more defective film than the electropolished one.

Random polymer structures were simulated in the non-continuous space of a diamond lattice. Free volume distributions were calculated for different polymer chain lengths and densities. The calculations provided free volume distributions dependent on the number average molecular weight and the density of the polymer. Using the results of the simulations, it was shown that positron lifetime data might provide distorted free volume distributions under certain conditions.

The paper offers three applications of nuclear methods in the research of chemical structure. First, progress in positron annihilation spectroscopy is illustrated by a positron beamline study, which obtained results that are not available through conventional experiments. The positron beam was used for the study of Langmuir-Blodgett (LB) films containing 4-58 layers of arachidic acid and its salts. These measurements have shown that this emerging technique is capable of characterizing even such elusive systems. Second, the potential of Mossbauer spectroscopy to answer current challenges of solid state chemistry are shown in a study on perovskites of recent interest. 151Eu Mossbauer spectroscopy was used to study the effect of Pr substitution in EuBa2Cu3O7-δ. It was shown that the introduction of Pr into the rare earth site as well as into the Ba site results in the appearance of extra electrons both in the copper oxide planes and at the 4f shell of Eu cations. The observed effects were explained by the hole filling effect of Pr. Finally, a survey is presented on the recently developed techniques for nuclear resonant scattering of synchrotron radiation, an exciting and very rapidly developing extension to conventional Mossbauer spectroscopy. An interesting new result is that nuclear inelastic scattering experiments performed on solutions of 57Fe complexes show contribution from vibrations rather than from diffusion to the inelastic spectra.

Samples of TiN hard coatings prepared by physical vapour deposition (PVD) were investigated by means of depth-sensitive positron annihilation spectroscopy. The results indicate that the samples are at the limits of the applicability of this method presumably due to the high defect concentration. Our findings also show that, though the samples are thoroughly characterized by other independent methods, they might not be sufficient to explain all aspects of positron-solid interactions in these cases.

Arachidate Langmuir−Blodgett (LB) films of different chemical composition and number of monomolecular layers deposited on silylated silica glass substrates were studied by means of positron annihilation spectroscopy. The applied methods included the measurement of the Doppler broadening of the annihilation photopeak with variable energy positron beams and bulk positron lifetime measurements. The studied samples were 58 monomolecular layers (MML) thick Mg- and Cd-arachidate, arachidic acid (50 MML) and a series of Pb-arachidate samples with 4, 10, 20, 40, and 58 MML. The investigation showed that the variable energy positron beam technique is capable of measuring the thickness of the deposited LB films. The measured positron annihilation parameters are sensitive to the chemical composition of the films and the behavior of the films in a vacuum. The results confirmed the stability of salt base LB films in high vacuum conditions and showed the desorption of pure acid films. These investigations have also sho...

Positron lifetime spectra were obtained from 1,1-bis(paramethoxy-meta- methyl- phenyl)-cyclohexane (BMMPC) vs. temperature T between 23 and 348 K. At low temperatures, almost up to the glass transition at Tg, a fast pick-off annihilation mechanism is proposed for explaining the variations in the annihilation parameters of positronium (Ps). Above the Vogel temperature T0 the lifetime of o-Ps is coherently interpreted in terms of a thermodynamic model which assumes the molecules to occupy solid- and liquid-like states. The transition temperature of the model calculated from o-Ps lifetimes is significantly less by ~ 20 − 30 K than that obtained from all other physical quantities available, including free positron lifetimes.

Our work proves that positron annihilation spectroscopy is an excellent tool to follow the structural changes in chemical species. We present four examples to support the above statement. The sizes of defects in electrodeposited chromium layers were studied and estimated on the basis of positron lifetime spectra decomposed into two components. Vacancies, di-vacancies and vacancy-clusters could be identified in the electrodeposites. The changes of the size distribution of the free volume units in poly(methylmetacrylate) on the dependence of molecular weight and dispersity were described by the correlation between the lifetime ofortho-Ps and the free volume units in polymers. It was found that the free volume is affected by both the molecular weight and dispersity. The effect of dispersity was explained by the sample preparation technique, namely by the application of high pressure. The ortho-para conversion ofortho-Ps was used to follow the partial spin-crossover in [Fe(1-ethyl-1H-tetrazole)6](BF4)2. The spin-crossover temperature was identified to be 105 K. A conformal structural transformation was found in [Zn(1-propyl-1H-tetrazole)6](BF4)2 between 170 and 90 K by positronium lifetime measurement and the role of (BF4)2− anion, in this transformation, was proved by19F NMR spectroscopy.

BaTiO3, was studied by positron lifetime spectroscopy in static electric fioeld. The strength of the field was varied from 0 up to 104 V/cm. Two characteristic points were found at 4500 V/cm and at 8500 V/cm. These field strengths were related to subsequent ordering processes of the crystal. Additional temperaturee dependent measurements were perforrmed in the 70–310 range to confirn the interopretaion of the two positron lifetimes. The two expected phase transitions were observed and these measurements showd that positron are trapped by Ba vacancies.

Positron lifetime measurements were performed to study the swelling dynamics of an amphiphilic polymer network (APN). Lifetime spectroscopy proved to be suitable to investigate APNs. Very quick structural changes were indicated by lifetime parameters at very low swelling ratios. On the basis of the results a possible mechanism is proposed for the hydration of the investigated materials.

The most obvious symmetry breaking in Nature is the left-right asymmetry of living beings: sugars and amino acids are almost exclusively represented by one of their stereoisomers (D-isomer for sugars and L-isomer for amino acids) at the expense of other possible isomers. In our experiment we studied the aqueous solutions of tartaric acid, alanine, and cysteine in the function of temperature by positron lifetime spectroscopy. The results were compared with those obtained in pure water under similar conditions. We always observed a sharp minimum of the positron lifetime at around 41 °C in water and in the solutions of the natural stereo-isomers. On the other hand, the same minimum occurred at considerably lower temperatures in the solutions of the “unnatural” isomers.

Positron lifetime spectroscopic and Dopplerbroadening measurements have been carried out on trans-stilbene single crystals. The results (including the complementary DSC measurement) give new evidence for the existence of phase transitions in trans-stilbene at temperatures between 20 and 300 K.

Positron lifetime measurements were carried out in normal hexane vs. temperature in the range of 12.5–62.5°C. The evaluation of the lifetime spectra indicates the presence of four lifetime components, as published first by Jacobsen et al. An attempt is made to interprete the properties of the longest-living two components in terms of the microscopic structure of the liquid.

Positron lifetime measurements were carried out on [Fe x Zn1−x (ptz)6](BF4)2 single crystals in the temperature range of 72–300 K. Complementary Doppler broadening spectra were also recorded at room temperature. The probability of the formation of o-Ps (lifetime 500–1400 ps) was between 10–30%. Both the spin density and the lattice parameters of the sample affected the lifetime of o-Ps through the ortho-para conversion and the pick-off annihilation, respectively. The positron annihilation technique proved to be a sensitive detector of the spin-crossover5T2 ↔1A1 and of the concomitant phase transformation.

Positron lifetime measurements were carried out on [Fe x Zn 1− x (Ptz) 6 ](BF 4 ) 2 single crystals in the temperature range of 72–300 K. Complementary Doppler broadening spectra were also recorded at room temperature. The probability of the formation of ortho-positronium (o-Ps) (lifetime 500–1400 ps) was found to be between 10–30%. It was established that the electron spin state transformation can be monitored by positron lifetime spectroscopy also in the solid state.

Positron lifetime measurements have been performed on differently manufactured PEEK samples of different molecular weights. Our measurements confirmed an old assumption: the o-Ps lifetime changes linearly with the molecular weight of the polymer. We have found that both the size of the free volumes and their number increase with increasing molecular weight of the investigated polymers.

Calculations were performed on a model of multicomponent, fine-grained materials to simulate positron distributions immediately after the thermalization process. A real three-component system (β-Ni(OH)2/β-NiOOH/graphite) was fitted by the parameters of the model and positron lifetime parameters were determined for the interfaces of the system.

Two bisphenol-A type epoxy resins were irradiated by electron beam and studied afterwards by positron lifetime spectroscopy. An interesting result is that despite the considerable amount of free-radicals no inhibition of positronium formation was observed in the two epoxies. Nevertheless several serious differences were detected between the studied polymers. The results suggest that the radiation-resistant properties of epoxies depend strongly on the amount of the curing agent.

The implantation of positrons from a22Na source has been studied in two polymer coatings. Transmitted intensities, effective mass absorption coefficients and implantation profiles have been determined.

Tin-vacancy interaction was studied in a dilute Al(Sn) alloy, quenched from 613°C to −70°C and isochronally heat treated up to 250°C, using parallel positron lifetime and Mossbauer spectroscopy.

Positron lifetime spectroscopy was used to characterise the beta -Ni(OH)2/ beta -NiOOH electrode system. The lifetime parameters for both chemically and electrochemically treated samples are presented. Three different stages in the oxidation-reduction process were observed and identified. For intermediate-oxidation states the samples were assumed to be heterogeneous and the interface between the differently oxidised phases proved to be an effective positron trap. The positron lifetime parameters provided valuable information about the oxidation processes occurring in different ways in the chemically and the electrochemically treated samples.

Positron implantation and annihilation and the influence of water exposure on the annihilation properties were studied in a number of organic coatings deposited on metal substrates. Change of the lifetime spectra upon water exposure was related to the corrosion protective properties of the coatings.

Vacancy–tin interaction is studied in dilute AlSn alloys quenched from 613 to −70°C and annealed up to room temperature using parallel positron lifetime and Mossbauer effect measurements. Evidence is found on the formation of small vacancy clusters around tin atoms during the fast quench. The Mossbauer line with an isomer shift of (2.86 ± 0.01) mm/s is identified with these vacancy clusters. Further clustering of isolated vacancies is observed upon isochronal annealing below room temperature. The results also suggest that single vacancies have minor effect on the isomer shift of the line corresponding to tin in solid solution. Mit Positronenlebenszeitstudien und Mossbauereffektmessungen wird in verdunnten AlSn-Legierungen die Wechselwirkung von Leerstellen und Zinnatomen nach Abschrecken von 613°C auf −70°C untersucht. Die Ergebnisse zeigen die Bildung von kleinen Leerstellenclustern an Zinnatomen wahrend des Abschreckens. Die Isomerverschiebung der Mossbauerlinie betragt dabei (2,86 ± 0,01) mm/s. Bei isochroner Erholung oberhalb der Raumtemperatur wird weitere Clusterbildung isolierter Leerstellen beobachtet. Einfachleerstellen haben nur einen geringfugigen Einflus auf die Isomerverschiebung des gelosten Zinns.

Positron lifetime parameters were measured for the spin-crossover complexes [Fe(R-1H-tetrazole)6](BF4 ) 2 (R = 1-ethyl, 1-n-propyl) and for the diamagnetic [Zn(1-n-propyl-1H-tetrazole)6](BF 4 )2. Positronium forms with significant intensity in the studied compounds. The ortho-para conversion of ortho-positronium was used to follow the spin-crossover. Changes of the dynamic structure were found in the propyltetrazole complex between 150 K and 90 K. PACS numbers: 78.70.Bj, 36.10.Dr, 75.20.–g, 33.25.+k

Positron lifetime parameters were measured for the spin-crossover complexes [Fe(1-alkyl-1 H -tetrazole) 6 ](BF 4 ) 2 and [Fe(1,10-phenathroline) 2 (NCS) 2 ]. If there are structural free volumes large enough (as for [Fe(1-ethyl-1 H -tetrazole) 6 ](BF 4 ) 2 and Fe(1-propyl-1 H -tetrazole) 6 ](BF 4 ) 2 ), there is a significant intensity of positronium (Ps) and the lifetime of o -Ps is a detector of spin-crossover. The intensity of the shortest lifetime component also signals the spin-crossover.

The poly[(2-dimethylamino)ethyl methacrylate]-l-polyisobutylene (PDMAEMA-l-PIB) amphiphilic network (APN), a new class of cross-linked systems, was synthesized by the use of methacrylate−telechelic PIB obtained via quasiliving carbocationic polymerization. The swelling dynamics and the free volume changes of this APN was followed by simultaneous swelling and positron annihilation measurements. It was found that the lifetime of ortho positrons (o-Ps) increases with increasing swelling ratio (R), reaches a maximum at relatively low R, and decreases to a constant value at equilibrium swelling. These findings indicate the collapse of the hydrophobic PIB domains and expansion of the hydrophilic PDMAEMA phase in the network upon contacting with water. After reaching the maximum, the decrease of the lifetime parameter is caused by filling the free volume with water in the network. A striking observation was obtained for the o-Ps formation intensity as a function of time (or swelling ratio): the intensity rapidl...