A. Yu. Alent’ev

Gas-separating properties of membranes based on star-shaped fullerene (C60)-containing polystyrenes

Based on regular star-shaped PSs differing in the structure of the branching center (one or two covalently bound fullerene C60 molecules) and in the number of branchings (6 and 12), homogeneous gas-separation membranes have been produced. The transport behavior of the membranes with respect to several gases, such as H2, He2, N2, CO, CO2, and CH4, has been studied by mass spectrometry. It has been found that the membranes prepared from six-arm PSs are characterized by a smaller density of macromolecular packing than the membranes obtained from 12-arm PCs and, consequently, they possess higher gas permeability. The starshaped PSs demonstrate a higher selectivity factor for separation of the O2/N2 gas pair compared to the corresponding characteristics of the linear PSs. The analysis of gas-separation characteristics by means of the Reitlinger-Robeson diagrams demonstrates that the transport behavior of star-shaped PSs qualitatively surpasses similar parameters of the known polymers in the separation of the CO/N2 gas pair.

Gas separation characteristics of new membrane materials based on poly(ethylene glycol)-crosslinked polymers and ionic liquids

Gas transport characteristics (permeability and diffusion and solubility coefficients for CO2, O2, N2, H2) of new crosslinked membrane materials synthesized by copolymerization of poly(ethylene glycol) dimethacrylate and poly(ethylene glycol) methyl ether methacrylate in the presence of various ionic liquids have been studied. Comparison of the characteristics of specimens with and without ionic liquids has revealed that the presence of ionic liquids enhances the permeability of the membranes, especially to CO2. It has been shown that the enhancement of the CO2 permeability of films incorporating ionic liquid is due to an increase in CO2 solubility and the increase in selectivity for pairs of gases containing CO2 is determined by thermodynamic selectivity of separation.

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.

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.

Study of the mechanism of the thermochemical reaction of polyimides with hydroxyl groups via vibrational-spectroscopy and quantum-chemistry methods

The thermochemical transformation of polyimides with one hydroxyl group in a diamine fragment that leads to changes in the polymer structure is studied. It is found that in an inert atmosphere in the temperature range 400–450°C, an intramolecular thermochemical reaction accompanied by evolution of carbon dioxide occurs. The FTIR spectra of all studied polyimide films prepared from different solvents and annealed in an inert atmosphere in the range 25–450°C show similar changes that do not agree with the structures of the reaction products (polybenzoxazole and polydehydrocyclobutabenzene) described in the literature. Possible reaction paths are calculated via quantum-chemistry methods. A mechanism of the intramolecular thermochemical transformation and the related structure of the formed polymer (aromatic lactam) are proposed.

Erosion of polyimide modified by amorphous silica sol in the stream of oxygen plasma

Polymeric coatings derived from Kapton H polyimide, thermoplastic polyimide (PI), and a composite with amorphous silica sol (PI-SS) were irradiated in a magnetoplasma dynamic accelerator of oxygen plasma simulating the action of atomic oxygen (AO). The volumetric erosion coefficients of polymers were calculated, and the comparative analysis of the stability of coatings was performed. The changes in morphology of polymer surfaces before and after irradiation were studied by means of scanning electron microscopy. It was shown that the PI-SS composite has increased resistance to atomic oxygen. The composition of the PI-SS composite was found at which the particles of silica sol are uniformly dispersed over the polymer volume that explains a better resistance of PI-SS polymeric-inorganic compositions to the action of atomic oxygen. The lowest stability was registered for the coating based on Kapton H polyimide. The surfaces of all coatings after irradiation were found to possess a carpetlike morphology. Each polymer featured a number of distinctive peculiarities of the surface structure caused by the differences in the chemical structure of the polymides under investigation.

Selection of membrane materials for separation of H2-containing mixtures: Database analysis

Correlation between the chemical structure and transport characteristics of polyimides that are used for separation of hydrogen-containing mixtures is analyzed. The effect of various functional groups (keto-, oxy-, methoxy-, amino-, and others) in polyimides is considered in detail. The group-contribution method is demonstrated to be efficient for prediction of the transport properties of new polyimides and for the search for new promising diamine-dianhydride combinations. The empirical analysis of the database accumulated at the Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, confirms the perspectiveness of further investigations of new membrane materials for separation of H2-CH4 and H2-CO gas pairs.

New membrane materials based on crosslinked poly(ethylene glycols) and ionic liquids for separation of gas mixtures containing CO2

Gas-separation parameters of new network membranes synthesized through the copolymerization of poly(ethylene glycol) dimethacrylate and poly(ethylene glycol) methyl ether methacrylate in the presence of ionic liquids are studied: the permeability, diffusion, and solubility coefficients and the selectivities of gas separation for CO2, O2, N2, and H2. The comparison of filled membranes with an analogous membrane free of ionic liquids shows that an increase in their permeabilities to CO2 is related to an increase in their CO2 solubilities and that improvement of their selectivities for CO2-containing gas pairs is due to the thermodynamic selectivity of separation. The effect of the concentrations of ionic liquids on the gas-transport behavior of network filled membranes is investigated. FTIR studies reveal that ionic liquids in such films occur in two forms: the free form and the form of a “solution” of a network polymer in an ionic liquid. It is found that membranes with the maximum contents of a “solution” of a network copolymer in an ionic liquid possess the maximum permeabilities to CO2 and the maximum selectivities of separation of CO2-containing gas pairs.

Use of hyperbranched polyethoxysiloxane to improve the resistance of thermoplastic polyimide coatings to atomic oxygen environment

Polyimide thin films both unmodified and modified with hyperbranched polyethoxysiloxane were treated with accelerated oxygen plasma flow to imitate a low Earth orbit environment. The resulting changes in the surface morphology were studied using scanning electron microscopy. The erosion yield values were calculated and the relative stability of the tested samples was established. It was shown that the polyimide-polyethoxysiloxane composites have higher atomic oxygen resistance owing to, presumably, the siloxane particles dispersed uniformly in the polyimide matrix. The study found that all plasma-treated samples exhibit the same fibrous, carpet-like surface morphology with some minor variations caused by differences in the chemical composition of polymers.