V. P. Shantarovich

High transport parameters and free volume of perfluorodioxole copolymers

Abstract Permeability (P) and diffusion (D) coefficients of perfluoro-2,2-dimethyl-1,3-dioxole-tetrafluoroethylene copolymers were determined with respect to different gases (He, H2, O2, N2, CO2, hydrocarbons C1C3). The copolymers with large content of perfluorodioxole comonomer exhibit high permeability with respect to lighter gases comparable to that of poly(trimethylsilyl propyne). However, the copolymers studied are much more permselective than the latter polymer. Free volume as estimated via Bondis method and free volume size distribution parameters which were determined by means of positron annihilation lifetimes (PAL) method are also unusually high if compared with other glassy polymers. A novel correlation of the P and D values, as well as of the solubility coefficients S with the PAL parameter t 4 I 4 are reported.

Effects of cis-trans-configurations of the main chains of poly(trimethylsilyl norbornene) on its transport and sorption properties as well as free volume

Samples of poly(trimetylsilyl norbornene) having different content of cis configurations of double bonds in the main chains were prepared by ring opening metathesis polymerization in the presence of the catalysts characterized by different stereoselectivity (Re2O7, RuCI3, WCI6). It was shown that the polymer obtained in the presence of the WCI6 catalysts having the closest content of the cis-and trans-configurations is distinguished by the largest gas permeability, diffusion, and solubility coefficients. Determination of the parameters of gas sorption isotherms and estimation of free volume using group contribution and positron annihilation methods allowed to conclude that microstructure of this polymer (cis/trans ratio) affects its free volume and, therfore, its transport and thermodynamic properties.

Synthesis, transport, and sorption properties and free volume of polystyrene derivatives containing Si and F

Gas permeation, sorption, and free volume was studied in a series of glassy vinylic type Si-containing polymers. They included p-substituted derivatives of polystyrene, which were compared with polystyrene, and poly(vinylphenyldimethyl silane). The strongest increase in permeability and diffusion coefficients were observed for the polymer containing the Si(CH3)3 group directly attached to phenylene ring. An introduction of spacers between phenylene ring and Si(CH3)3 group results in much lower gas permeability. Positron annihilation study of free volume in these polymers showed that lifetime spectra are composed of four components. It implies that size distribution of free volume in these polymers, which are characterized by a modest level of gas permeability, is bimodal, a feature previously observed only for extra large free volume and permeability materials such as poly(trimethylsilyl propyne).

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.

Physical and Mechanical Behavior of Polymer Glasses. VI. the Role of Free Volume

Abstract For various polymer glasses, the temperature-induced recovery of residual deformation was studied. The ratio between the low-temperature and high-temperature recovery components is controlled by the difference between deformation temperature and glass transition temperature T g of polymer samples independently of their chemical structure. This ratio correlates with polymer macroscopic mechanical characteristics such as elastic modulus and yield stress. Experimental results were treated in terms of the dynamics of segmental mobility within different structural sublevels with different packing densities. To correlate this mechanical response with the structural state of glassy polymers, positron annihilation lifetime spectroscopy (PALS) was used. For different polymer glasses, the microscopic segmental mobility and resultant macroscopic mechanical properties were shown to be controlled only by the development of the adequate free volume content which depends on the difference between testing temperature and T g . These results allowed us to propose the general correlation between microstructure, microscopic molecular mobility, and Macroscopic mechanical behavior of polymer glasses.

Positron annihilation study of the laminated system structure

Some of the ideas and experimental results discussed in this contribution seem to demonstrate valuable possibilities for traditional angular correlation measurements of annihilation radiation in studies of laminar structures and in solving some problems of solid-state physics and technology.

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