I. B. Kevdina

Influence of deformation and chemical structure on elementary free volumes in glassy polymers

Abstract In the scope of the mechanism of positronium formation in polymers before trapping by elementary free volumes (EFV), we demonstrate an approach to quantitative analyses of experimental results. Correspondingly, a relation between the EFV concentration and annihilation characteristics (lifetimes and intensities) is derived. We use PATFIT and CONTIN analyses of positron annihilation lifetime spectra and obtain information on some heterogeneity of the structure of glassy polymers and on the mechanism of the plastic flow of glassy material.

Positronium annihilation data and actual free-volume distribution in polymers

Determination of the size distribution of free-volume holes in solids, in particular, polymers, is an important physicochemical problem. The positron annihilation technique has been proposed for this purpose. The central point in this technique is the quantitative interpretation of data, especially, for substances with a high specific surface area. A developed free-volume system in open-pore membrane materials, such as poly(trimethylsilylpropyne) PTMSP and the spirocyclically bound benzodioxane polymer PIM-1, and polymeric sorbents (hypercrosslinked polystyrenes) makes it possible for the first time to compare the sorption characteristics and positron annihilation data on the character of size distribution of nanopores in these polymers. In combination with the results of mathematical simulation of the structure and radiothermoluminescence measurements, the array of data indicate the structural inhomogeneity of the test amorphous materials. It was shown that this inhomogeneity in relation to the positron annihilation technique is expressed in the insufficiency of the representation of the orthopositronium decay curve by one component that takes into account the Gaussian lifetime distribution (symmetrical pore size distribution) and in the necessity of use of several decay components. The feasibility of revealing a nonrandom character of pore size distribution gives the positron annihilation technique an advantage over other approaches (inverse gas chromatography, 129Xe NMR) to investigation of nanopores in polymers.

When some elementary free volumes in polymers are not seen by positron annihilation experiments

Size distributions of elementary free volumes have been studied in mesoporous micro-heterogeneous polymer sorbents. Positron annihilation lifetime spectroscopy (PALS), low temperature gas adsorption (BET) and thermo-stimulated luminescence (TSL) measurements are employed as complementary instruments for the study. It is shown that small admixtures of rubbers are very effective for variations of the pore size distribution. While BET technique was very informative for measurements of mesopores(2-50 nm), positron annihilation was sensitive to micropores(<2 nm), but not for mesopores. The last specificity is explained by the limited positronium diffusion length in a polymer and also by inhomogeneous distribution of mesoporesin heterogeneous systems. TSL measurements gave information on sizes of rubber inclusions in compositions.

Nanoporosity of Polymer Membrane Materials and Sorbents According to Positron Annihilation and Low-Temperature Gas Sorption Data

Based on the experimental data obtained by the authors in a number of previous studies, the limits of applicability of positron annihilation lifetime spectroscopy (PALS) and low-temperature gas sorption (LTGS) to determination of nanoporosity (size distribution of nanopores in the range from a few fractions of a nanometer to 50 nm) in polymeric membrane materials and sorbents are discussed. It turns out that none of these methods is universal. The possibility of using each of them is determined by different factors, with the cases considered being finely divided polymer materials and the membranes per se cast from powders. It has been shown that the particle size factor is important for the applicability of LTGS. The possibility of using PALS depends on the concentration of nanopores of a given size.

Evaluation of Nonuniformity of Polymeric Membrane Materials by Positron Annihilation Technique

Positron annihilation lifetime distribution was experimentally studied in some polymers, including polymeric membrane materials, in an air and a nitrogen atmosphere. Basic attention was paid to the long-lived distribution component, i.e., to annihilation of orthopositronium (the positron-electron bound system). It was found that ambient oxygen affects the annihilation characteristics of positronium. Newly developed ideas on the mechanisms of formation, localization, and annihilation of positronium in the polymers lead to the conclusion that the distribution of free volumes of different size in the polymeric matrix is not uniform. The number concentration and size of free-volume holes and the size of microirregularities containing these holes were determined