Radosław Zaleski

Synthesis and characterization of nanostructural polymer–silica composite: Positron annihilation lifetime spectroscopy study

The swelling of poly(TRIM) spherical particles in TEOS is assessed as a potential way for obtaining polymer-silica nanocomposite materials. Silica deposition was achieved by simply stirring of swollen polymer particles in acidic hydrochloric-water solution. This procedure leads to spherical composite particles with dispersed silica gel within the polymer matrix. The resulting material exhibits the same morphology as the initial polymer. Nanocomposite particles are silica rich (about 17 wt.%). Characterization of the nanocomposites was performed using scanning electron microscopy, FT-IR spectroscopy, (29)Si CP MAS NMR spectroscopy and thermogravimetry. Moreover, the use of positron annihilation lifetime spectroscopy PALS to characterize the structural properties of the nanocomposites is presented. This technique gave more realistic pieces of information about the pore structure of the investigated samples in contrast to nitrogen adsorption studies.

Porosity evolution of VP-DVB/MCM-41 nanocomposite

The porous structure of nanostructured vinylpyrrolidone-divinylbenzene (VP-DVB)/MCM-41 composite was characterized using N(2) adsorption at 77K and positronium annihilation lifetime spectroscopy (PALS), atomic force microscopy (AFM) and Raman spectroscopy. Positron annihilation lifetime spectra were measured during outgassing procedure. The voids of differentiated dimensions were detected in the composite material. The number of free volumes and their dimensions depend on the degree of evacuation of volatile components from the samples.

Vacuum removal of the template in MCM-41 silica studied by the positron annihilation method.

Positron annihilation lifetime spectra were measured for ordered silica before template (octadecyltrimethyl ammonium) removal. An intermediate lifetime component, well pronounced in the spectra, indicates the presence of structural defects in the template being the size of a surfactant molecule. At the temperature near 420 K a radical change of template structure occurs, consisting in appearance of disorder and the degradation of template; evaporation of the fragments begins. It seems that moderate heating (up to about 500 K only) in vacuum for several hours is an effective and simple method of template removal, leaving no pure carbon residue in the pore structure. The results are compared with the data from the liquid nitrogen adsorption/desorption method and elemental analysis, confirming the positron annihilation results.

Positron annihilation and N2 adsorption for nanopore determination in silica-polymer composites

Silica has two different, fragmented and smooth, visual appearances when deposited onto porous Amberlite polymer substrate from TEOS at acidic and basic pH, respectively. The low temperature N2 adsorption isotherms on both materials seem to be a combination of Type 1 and Type 4 isotherms with H2 hysteresis, indicating the presence of both micro (D < 2 nm diameter) and meso (2 nm < D < 50 nm) pores. Their mesopore distributions, computed by both BJH and DFT methods, show maxima near D∼5 nm with a narrower pore size distribution range than the organic support alone. For comparison, pores were also tested by positron annihilation lifetime spectroscopy (PALS) which can scan the full micro to mezo size range in one measurement without cooling and using any adsorbate molecules. These PALS results indicated the presence of D∼0.5, 0.8 and 1.5 nm diameter micropores and an average D∼5 nm diameter mesopores in both the base and the acid set materials. When the organic substrate is burned out from the two differently made composites, the new adsorption isotherms, BET surface areas, and differently measured pore sizes became distinctly different both from each other and from those of the parent materials.

On possible deviations of experimental PALS data from positronium pick-off model estimates

Abstract The sources of possible inconsistency between the models of trapped positronium annihilation and experimental data are discussed. There are two such sources: excessive simplifications in the models of pick-off process and appearance of other processes not accounted in the models. In the case of the smallest traps, the penetration depth of ortho-positronium (o-Ps) wavefunction into the bulk can be larger than usually assumed. In traps of sizes exceeding 1 nm the population of excited levels in the well has to be taken into account. For such large voids the sources of divergence of the experiment and model predictions can be: escape of o-Ps from open pores to outside, chemical interaction of o-Ps with pore surfaces (adsorbate layers), the presence of impurities inside free volumes.

Temperature changes of the template structure in MCM-41 type materials; positron annihilation studies

Abstract The lifetime spectra of positrons annihilating in MCM-41 ordered silica were measured as a function of temperature. It was found that in alkyltrimethylammonium templates degradation in vacuum begins at 380–400 K, that consists in breaking the long micelle cylinders, rise of gaps between the fragments and finally pore evacuation. The latter process is not completed and particularly in C12TAB the spectrum component belonging to fragmented template preserves its high intensity up to 520 K (the highest temperature in our experiment). The pore surface area over 1000 m 2 /g is achieved already at temperatures about 490 K. The lifetime of the longest-lived component indicates that practically all ortho -positronium atoms leave the pores before annihilation.

Positron probing of the micellar template interior in MCM-41

The properties of positronium annihilating in the micellar template in MCM-41 materials were investigated. For alkyltrimethylammonium bromide surfactants (Cn=14,16,18) the template interior is liquid-like; the bubble model of positronium, elaborated for bulk liquids, can be applied. The equivalent surface tension is linearly dependent on temperature, like in classic liquids. A very long-lived component in o-Ps lifetime spectrum (≈20ns) can be ascribed to o-Ps in cracks in the elongated template structure.

Influence of Atmospheric Gases Present in the Pores of MCM-41 on Lifetime of Ortho-Positronium

The modification of the extended Tao-Eldrup model accounting ortho-positronium quenching in air is presented. Taking into account quenching by oxygen molecules adsorbed on the surface of porous material gives reasonable agreement between results of the model and the experimental positron annihilation lifetime spectroscopy data. Pore size distributions calculated using this model from the spectra for MCM 41 mesoporous sieve obtained in air, oxygen or vacuum are compared and discussed taking into account effect of ortho-positronium migration from small open pores to the larger ones. The rates of ortho-positronium quenching by air (47.2 µs-1 MPa 1), oxygen (220 µs-1 MPa-1) or nitrogen (1.7 µs-1 MPa-1) obtained from pressure dependences of the lifetimes observed in MCM 41 agree reasonably with the experimental results of other authors, if the correction for oxygen adsorbed on the surface is applied.

Odd-Even Differences in o-Ps Properties in Some Solid Long-Chain Alkanes

Positron lifetime spectra in solid long-chain alkanes were measured as a function of the storage time with positron source between the samples, and as a function of temperature and pressure. Large differences of the time constant of I3 increase were observed for odd-numbered and even-numbered molecules. High pressures also induced the I3 instability, but in this case the I3 intensity decreased. In the rigid solid phase the o-Ps lifetime in even-numbered alkanes was shorter than in odd ones, which was due to different thickness of the empty layer between the lamellas in the crystal structure. Introduction Variation of ortho-Ps intensity I3 at low temperatures was observed in polymers by many authors and explained as the result of positron combination with trapped electrons produced during previous ionization acts [1,2]. The same effect can be observed also in some molecular crystals, particularly in long-chain n-alkanes. Well defined crystal structure and simple molecular geometry make them a more convenient object for studies of the electron trapping effect. Moreover, the length of molecular chain can be easily changed by selecting appropriate hydrocarbon. Results and discussion The object of the study were n-alkanes with the chain composed of 17 24 carbon atoms (SigmaAldrich, 99% purity). Positron source activity placed between the samples was 0.9 MBq. The samples were initially kept near the melting point and then cooled to a predetermined storage temperature. The PAL spectra were decomposed into three discrete exponentials. A rise of I3 was observed at several temperatures as a function of time. The result is shown in Fig.1 for eicosane C20H42, tetracosane C24H50 and pentacontane C50H102 stored at 103 K. The time dependence of I3 increase resembles the exponential function and one can introduce the time constant of that process. Several interesting features can be observed: a) contrary to polymers, the saturation value of I3 need not to diminish at higher temperatures, e.g. in octadecane the maximal I3 value is reached at 250 K and amounts 60%, comparing to 28% without e trapping; b) the range of I3 variations extends approximately up to the room temperature; c) the time constant of I3 rise, , increases with temperature for evennumbered alkanes, while it decreases for odd-numbered ones. At 200 K the odd-even difference in can reach almost two orders of magnitude. In odd Fig.1 Intensity of long-lived component I3 as a function of storage time at 103 K for three even-numbered alkanes. 0 30 60 90 TIME, h 5 10 15 20 25 30 35 40 45

Principles of positron porosimetry

Abstract The paper deals with positron porosimetry (PP), which is based on positron annihilation lifetime spectroscopy (PALS). The numerical analysis of positron lifetime spectra for PP is more demanding than in most of other applications of PALS. The resulting intensity distributions of ortho-positronium (o-Ps) lifetimes are interpreted in terms of the extended Tao-Eldrup (ETE) model, which provides the dependence between the o-Ps lifetime and pore size. Additionally, the relation between the intensity of an o-Ps component and the pore volume allows obtaining pore size distribution (PSD). The value of the empirical parameter Δ, which is dependent on material, can be estimated from the temperature dependence of an o-Ps lifetime. The most unique feature of PP among other techniques that allow determination of PSDs is its ability to perform measurements in almost any conditions. This makes this method suitable for various in situ studies. In this review article, both the capabilities and the limitations of PP are discussed. The methods to overcome some of the limitations are presented.