A disagreement between effective sizes of elementary free volumes found from the four component PAT-FIT analyses of positronium annihilation lifetime spectra and experiments on low temperature sorption for two membrane materials of internal microporosity poly(tri-methyl-silyl-propyne) PTMSP and spirolinked benzodioxane polymer PIM-1 is described and discussed. This disagreement can be minimized essentially using the fifth lifetime component but not the Gaussian distribution of the fourth component. 1 Introduction Elastic and glassy polymers are normally considered as homogeneous disordered materials with a unimodal (statistical) distribution of elementary free volumes. More complex (bicentral) distributions, obtained from mathematical analyses of experimental positron annihilation lifetime spectra (PALS), were explained recently as "artefacts", i.e. results of incorrect mathematical treatment (de-convolution) of the spectra. Accepting this conclusion as a reasonable formal explanation in some cases, we discussed [1] results of our PAL measurements for a number of elastic and glassy polymers, such as polystyrene (normal and cross-linked) poly-isobutylene poly-butadiene, synthetic rubbers, etc., where observations of the two long-lived ortho-positronium components in PALS, comparison with the results of sorption experiments and irregular (nonlinear) variations of annihilation characteristics in the vicinity of the glass-transition temperature actually revealed structural heterogeneity, confirmed by supplementary techniques: measurements of thermo-stimulated luminescence (TSL), thermo-mechanics, and mobility of penetrants in the systems. The heterogeneity was explained by slow relaxation of the polymer structures. Continuing this line of investigations, we present in this paper annihilation characteristics and sorption data for two membrane materials of internal microporosity: well known poly(tri-methylsilyl-propine) PTMSP [2-4] and a novel material spiro-linked benzodioxane polymer PIM-1 [5,6].A combination of PAL experiments and sorption data for membrane materials is very important for understanding of the mechanisms of Ps annihilation and potential applications of the positron annihilation research. This combination enables testing of the PAL data on the large (1-2nm) pores. Along with

Selected nanoparticles and nanocomposites on the basis of radioactive elements are reviewed. Isotopes of metallic gold, iodine and technetium salts, CeO2 and other lanthanide and actinide compounds, as well as several p- (P, C, F, Te) and d- (Fe, Co, Cu, Cd, Zn) elements form most common radioactive nanoparticles. Methods for their fabrication, including dopation with radionuclides and neutron/proton/deuteron activation, are discussed. These nanocomposites possess a series of useful applications, in particular in biology and medicine, including cancer therapeutics, drug delivery systems and radiotracers, as well as in the studies of several catalytic processes and materials structure.

INTRODUCTIONCalcium carbonate, a material of considerable practical importance [1][2][3], exists in two anhydrous, metastable polymorphs (vaterite and aragonite) and one stable (under ordinary conditions) polymorph (calcite). Each of these polymorphs can be prepared in the form of particles of different morphologies. As a rule, vaterite particles are spheroidal and calcite particles are rhomboidal [4, 5].The vaterite-to-calcite phase transition [6,7] is believed to occur either through spontaneous recrystallization of individual particles or through dissolution of initially precipitating vaterite in an aqueous medium (the solubility of vaterite in water is higher than that of calcite [8]). It seems likely that this transition may also follow a third mechanism, so-called "relay" recrystallization, a process in which metastable particles transform into stable ones through collisions in a stirred suspension. The heat released in the contact region as a result of a collision increases the amplitude of thermal vibrations of atoms (ions, molecules) in the surface layer of the particles, giving rise to structural changes and, eventually, to the transformation of the metastable phase into the stable one. After the collision, the particles may remain in contact (aggregate) or persist as individual particles. In this way, a fast phase transition occurs in all the particles of the metastable phase. The mechanism of such relay processes is schematically illustrated in Fig. 1.The relay mechanism was found earlier to underlie the amorphous-to-crystalline phase transformation of ultrafine hydroxyapatite particles dispersed in an aqueous medium [9] and the crystallization of supercooled melt drops in an inert liquid [10]. Here, we describe a relay vaterite-to-calcite phase transition in an aqueous suspension during vigorous ultrasonic stirring. EXPERIMENTALTo prepare an aqueous vaterite suspension, an aqueous sodium carbonate solution was introduced from above into a fountain of fine droplets of an aqueous calcium nitrate solution, produced by sonication, The starting chemicals were reagent-grade Ca(NO 3 ) 2 and Na 2 CO 3 . We used freshly prepared 0.1 M solutions. The Ca(NO 3 ) 2 : Na 2 CO 3 volume ratio was 1 : 2. Syntheses were carried out in the apparatus shown schematically in Fig. 2. The reaction vessel ( 4 ) was 8 cm in diameter and had a 1.8-cm-diameter acoustic window ( 2 ) fitted with polymer film ( 3 ). The vessel was filled with a Ca(NO 3 ) 2 solution maintained at 20 ± 1 ° C.Abstract -The vaterite-to-calcite phase transition in spheroidal vaterite particles produced by reacting aqueous Ca(NO 3 ) 2 and Na 2 CO 3 solutions at 20 ± 1 ° C under 2.6-MHz sonication was studied by scanning electron microscopy and x-ray diffraction. The results are interpreted as evidence that the transformation of vaterite, a metastable phase under ordinary conditions, to calcite follows a "relay" recrystallization mechanism: vigorous ultrasonic stirring leads to energetic collisions between vaterite and calcite particles, resulting in a t...

Preparation of young people in special chemical groups at the Department of Chemistry of the Moscow State University is described. Actual problems of chemical education in Russia are discussed from the viewpoint of attracting promising young people to the chemical institutions of the country.

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