Yu. P. Yampol’skii

Synthesis and gas separation properties of metathesis polynorbornenes with different positions of one or two SiMe3 groups in a monomer unit

The metathesis polymerization of 5,5-bis(trimethylsilyl)norbornene, 2,3-bis(trimethylsilyl)norbornadiene, and exo,endo-3,4-bis(trimethylsilyl)tricyclo[4.2.1.02,5]non-7-ene with the catalysts WCl6/1,1,3,3-tetramethyl-1,3-disilacyclobutane, RuCl3/EtOH, and the Grubbs Ru-carbene complex Cl2(PCy3)2Ru=CHPh has been studied. New polymers with yields of up to 98% and M w = (2−39) × 105 are prepared. New metathesis copolymers of 5-trimethylsilylnorbonene with 5-(hydroxymethyl)norbornene and 5-(trimethylsiloxymethyl)norbornene are synthesized in the presence of the Cl2(PCy3)2Ru=CHPh catalyst with yields of 78 and 98%. The gas-permeability study of the above series of the metathesis polymers containing one or two Me3Si substituents in each monomer unit shows that the introduction of the second SiMe3 group markedly improves their transport characteristics. A change in the character of the backbone (polynorbornadiene, polytricyclononene) has a small effect on the permeability of the polymers. The metathesis polynorbornene with two vicinal SiMe3 groups exhibits higher gas-permeability coefficients than its isomer with germinal substituents. The homopolymer of 5-trimethylsilylnorbornene is characterized by better transport parameters than its copolymers with -OSiMe3 and -OH substituents.

Chain Structure and Stiffness of Teflon AF Glassy Amorphous Fluoropolymers

The structure of the amorphous perfluorinated polymer Teflon AF 2400 and other structurally close perfluoropolymers was studied by means of a quantum chemistry method. The electronic and structural characteristics of the repeating unit and polymer models with ten and nine monomer units were obtained. It was found that two nonplanar isomers can exist for different models of the perfluorinated dioxole ring with a difference of their energy minimums of 10.8 kJ/mol. The orthogonal-block structure of the polymer chain of the perfluorodioxole homopolymer and its copolymer with tetrafluoroethylene was proposed, the block size was found, and a possible diameter of the void formed by two neighboring polymer chains was evaluated. Potential energy curves for the rotation of certain chain fragments about different bonds of the polymer main chain were constructed, and the polymer stiffness was shown to substantially depend on the molar ratio between perfluorodioxole and tetrafluoroethylene units in the copolymer and on the geometry of the perfluorodioxole ring.

Effect of chloroform on the structure and gas-separation properties of poly(ether imides)

The FTIR spectra of poly(ether imide) films prepared from their chloroform solutions were recorded in a wide temperature interval. The cast films were shown to contain residual solvent. This residual solvent existed in films as unbound chloroform that may be removed by heating to 60–70°C and as bound chloroform that is involved in complex formation with polymers and may be removed by heating at temperatures close to their glass transition temperatures (180°C). Quantum-chemical calculations were performed for structures that model fragments and monomer units of poly(ether imides), as well as their complexes with chloroform. Chloroform was shown to be capable of preferential binding with an oxygen atom in a Ph-O-Ph′ fragment via hydrogen bonds. In this case, the conformational set of poly(ether imide) chains is changed. The above evidence is invoked to explain changes in transport characteristics with time for poly(ether imide) films cast from chloroform solutions.

Effect of structure and conformational composition on the transport behavior of poly(ether imides)

With the example of two isostructural poly(ether imides), the effects of isopropylidene and hexafluoroisopropylidene groups and hydrogen bonding with chloroform on changes in geometry and energy parameters of polymer chains have been analyzed. The relationship between changes in the geometry characteristics of polymer chains and the gas-separation behavior of films cast from these poly(ether imides) has been established. It has been shown that an increase in permeability is related to a rise in rotation barriers and, consequently, to enhancement of chain rigidity, while the improvement of gas-separation selectivity is associated with a reduction in the amount of isoenergy conformers and an increase in the conformational uniformity of polymer chains. As was detected by FTIR spectroscopy and confirmed by quantum-chemical calculations, the conformational nonuniformity and the small-scale mobility of poly(ether imide) chains are possible only in the Ph-O-Ph′ fragment. The data obtained are applicable for estimation of the role of molecular mobility of polymer chains in the mechanism of gas or penetrant transfer through the polymer film and for predicting the transport behavior of polymers from this family. With the example of two other poly(ether imides), the prediction is confirmed by the experimental data on gas separation.

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.

Determination of solubility coefficients and sorption isotherms of gases in polymers by means of isothermal desorption with a chromatographic detection

A new method is developed for determining the solubility coefficients of gases in polymers that combines the advantages of the static and dynamic approaches to sorption estimation and allows us to determine the equilibrium characteristics of sorption for small quantities of samples (0.1–0.2 g) and low (<0.5 atm) partial pressures of the investigated gas. Sorption isotherms and solubility coefficients of nitrogen, oxygen, carbon dioxide, methane, ethane, and propane in polyvinyltrimethylsilane are obtained, and in poly[3,4-bis(trimethylsilyl)-tricyclononene-7], polyhexafluoropropylene, and OH-containing polyimide for the first time ever. It is shown that the sorption isotherms of gases for all of the gas-polymer systems in the investigated range of pressures are linear. The obtained solubility coefficients are compared to data for other polymers studied earlier.

Relation of gas-transport parameters of amorphous glassy polymers to their free volume: Positron annihilation study

Correlations of transport parameters (diffusion coefficients D and permeabilities P of gases) and thermodynamic parameters (solubility coefficients S and parameters C′ H of sorption isotherms) with the sizes of free-volume elements, v h, as estimated via positron annihilation lifetime spectroscopy are analyzed for the first time on the basis of the data array obtained for glassy polymers. Correlations of logD and logP with 1/v h that agree with the free-volume model under the condition of a weak change in the concentration of free-volume elements in different polymers are ascertained. Certain deviations from linear correlations with 1/v h for polymers with high free volumes are interpreted as evidence that the connectivity (openness) of pores increases with the sizes of free-volume elements. For solubility coefficients and Langmuir parameters of sorption capacity C′ H , good linear correlations with the value of v h are demonstrated.

Membrane separation of gaseous C1-C4 alkanes

The separation of various gaseous hydrocarbons with the aid of polymer membranes is considered; attention is focused on the separation of gaseous C1–C4 alkanes, which are the components of natural gas and associated petroleum gases. It is noted that a practically important property of membrane materials is the thermodynamic selectivity of a membrane, which makes it possible to enrich a permeate with heavier alkanes. Up to this point, polyacetylenes exhibited the best transport parameters for the solution of this problem. The second experimental section of this paper describes a study of the separation of CH4 + C4H10 binary mixtures on films based on additive poly[3-(trimethylsilyl)tricyclononene-7]. It is demonstrated that this highly permeable saturated glassy polymer exhibits thermodynamic selectivity in experiments with both the individual gases (CH4 and C4H10) and their mixtures and provides fivefold butane enrichment of a permeate. The test polymer and others additive Si-containing norbornene polymers are of interest as membrane materials for the separation of hydrocarbon gases.

Transport Properties of Polyimides Containing Phenylquinoxaline Moieties

The gas permeabilities of a number of new structurally related polyimides containing phenylquinoxaline moieties were studied for the first time. The test polymers had different dianhydride units, whereas their diamine components differed in the presence of flexible ether bonds-O-in the main chain, a structure that is reflected in the effective packing of chains and, as a result, in transport parameters. The permeability, diffusion, and solubility coefficients for the gases H2, He, O2, N2, CO, CO2, and CH4, as well as the ideal separation factors for gas pairs, were determined. The transport characteristics of polymers were compared within the given polymer series and with published data for other polymer series.

Transport and physicochemical parameters of polypentenamer

Physicochemical properties of a cis-polypentenamer—a hydrocarbon polymer with a low glass transition temperature (T g = 168.8 K)—have been studied. Measurements of permeability coefficients P in rubbery material for a wide range of gases (He, H2, O2, N2, CO2, CH4, C2H6, C3H8, and n-C4H10) indicate a high permeability of this polymer for which the values of P are only slightly lower than those of the most permeable rubber—poly(dimethylsiloxane). The method of inverse gas chromatography has been employed to estimate solubility coefficients S for n alkanes C3–C10 and cycloalkanes in cis-polypentenamer in the range from 25 to 150°C. It has been shown that the solubility coefficients linearly increase in lnS-T cr 2 coordinates, where T cr is the critical temperature of a solute. In terms of the above correlation, the solubility coefficients of light gases have been estimated and the diffusion coefficients D of gases in the same polymer have been calculated via the formula P=DS. The free volume in cis-polypentenamer has been studied by positron annihilation lifetime spectroscopy. The temperature dependence of the positronium lifetime τ 3 that characterizes the size of the free volume element in a polymer demonstrates saturation at temperatures above 250 K. This effect is probably related to a rapid migration of fluctuation holes in the rubbery polymer at temperatures remote enough from its glass transition temperature.