Samira M. Matson

This paper studies the chemical modification of a polymer promising for the development of gas separation membranes to improve its CO₂ selectivity. The possibility of the introduction of butylimidazolium bromide (BIm⁺Br^–) into the structure of poly(1-trimethylsilyl-1-propyne) (PTMSP) using a two-stage process—bromination of the initial polymer with N-bromosuccinimide and subsequent interaction with a tertiary amine, N-butylimidazole—is demonstrated. Supercritical CO₂ and CHF₃, which have a number of advantages over organic solvents, such as nontoxicity, nonflammability, and environmental safety, are used as the reaction media. Depending on the conditions of the process, the proposed method makes it possible to regulate the BIm⁺Br^– content in the polymer structure. The resulting modified polymers are characterized by good film-forming properties, thermal stability, and increased resistance to aliphatic, alicyclic, halogenated, and aromatic hydrocarbons. It is found that with an increase in the BIm⁺Br^– content in the polymer, the selectivity of CO₂/N₂ and CO₂/CH₄ separation increases while maintaining high gas permeability. These results open up new possibilities for the development of efficient membrane materials for gas separation in industrial conditions.

In this work cross-linked polymer membranes were obtained by heat treatment of films prepared from a solution containing a mixture of brominated poly(1-trimethylsilyl-1-propyne) [PTMSP] and polyfunctional amine polyethylenimine [PEI] as a cross-linking agent. The cross-linked products were identified on the basis of IR spectra, elemental analysis data, and the stability of the reaction products to the solvent (CCl4) in which the original brominated PTMSP is soluble. According to the IR spectra, the crosslinking reaction occurs via the reactive C-Br bond in bromine-containing PTMSP with the participation of PEI amino groups at temperatures above 90°C. Cross-linking of bromine-containing PTMSP makes it resistant to organic solvents. An increase of PEI-content in the mixture correlates with an increase in the proportion of bromine atoms that have reacted. The cross-linked polymer transport parameters were studied for individual gases and in a methane/n-butane mixture (98.4 mol.% methane and 1.6 mol.% n-butane). In the series PTMSP – brominated PTMSP-Br – PTMSP-Br/PEI (before cross-linking) – PTMSP-Br/PEI (after cross-linking) the permeability to individual gases decreases. Cross-linked PTMSP in a methane/n-butane mixture demonstrates high permeability coefficients of n-butane (P n-C4H10 = 12000 Barrer) and selectivity for the release of n-butane from a mixture with methane (α n-C4H10/CH4 = 13).

The thermo-oxidative stability and gas transport properties of poly(4-methyl-2-pentyne) (PMP) samples of the mixed configurational composition (50% cis-units) in the presence of phenolic antioxidants Irganox 1076, Irganox 1010, Vulkanox BHT, and Rianox 1520 differing in the nature and structure of substituents on the benzene ring is studied. The annealing of PMP films at 140°С demonstrates that the films with the added Irganox 1076 and Irganox 1010 maintain the strength and stability of gas transport properties for at least 150 h of total heating after moderate decrease in permeability (∼30%) for the first 48 h of heating. PMP films with Vulkanox BHT and Rianox 1520 upon 48 h of heating at 140°С show an increase in gas permeability coefficients, indicating the onset of polymer degradation, and after 100 h of heating they are destroyed. The wide-angle X-ray diffraction of the films with the most effective stabilizers Irganox 1076 and Irganox 1010 reveals a decrease in interchain distances upon annealing which provides evidence for an increase in the packing density of macromolecules as a result of thermally activated relaxation.

The permeability of individual lower hydrocarbons and in a С₁/С₄ mixture is studied for the first time for films based on PTMSP of the new cis-enrich configurational composition (the content of cis-units in the samples is 80 and 90%). The methane permeability of freshly prepared cis-regular PTMSP films (90% cis-units) and exposed to air for a month is higher than the corresponding values of PTMSP films containing 80% cis-units. The X-ray diffraction data indicate a looser packing of cis-regular PTMSP. The introduction of antioxidant Irganox 1076 into the PTMSP films leads to a decrease in the initial level of methane flux. At the same time, the permeability of the films with the added Irganox decreases much more slowly over time (for a month) compared with the films without the added antioxidant. The permeability of individual lower hydrocarbons through PTMSP films without and with the added Irganox 1076 increases in the sequence С₁ < C₂ < C₃ < C₄. The value of the separation factor in the n-butane/methane mixture attains 33, which is almost 7 times higher than selectivity for individual components. A high level of n-butane permeability for cis-enrich PTMSP is maintained at least for a month of film storage in air.

The effect of the stereoregularity of poly(1-trimethylsilyl-1-propyne) [PTMSP] (cis-content from 50 to 90%) on physical aging was investigated by measurement of the gas permeability. Films from pure PTMSP as well as those with the addition of the antioxidant Irganox 1076 were exposed to the air. The permeability of pure PTMSP films increases with an increase in cis-stereoregularity and correlates with an increase in interchain distances (according to X-ray analysis). For pure PTMSP films, the most significant aging (up to 50% of permeability drop) was observed for polymers with mixed microstructure, and the slowest aging (10–30% of permeability drop) was observed for polymers with cis-regular structure. For PTMSP films with added Irganox 1076, some decrease in permeability with time is also observed. The addition of Irganox 1076 to PTMSP in mixed as well as cis-enriched configurations visibly slows down aging. In the case of cis-regular PTMSP with a slow aging rate, the introduction of an antioxidant does not provide any advantages. The high stability of cis-regular PTMSP demonstrates the possibility of obtaining more stable membrane materials with the highest equilibrium state of the polymer selective layer prepared by casting solution.

The Effect of Introduction of Phenolic Antioxidants on the Thermal-Oxidative Stability and Gas Permeability of Poly(4-methyl-2-pentyne) Влияние введения фенольных антиоксидантов на термоокислительную стабильность и газопроницаемость поли(4-метил-2-пентина)

Poly(1-trimethylsilyl-1-propyne) of a cis -Rich Configuration — a Membrane Material for the Separation of Butane/Methane Mixtures as Natural Gas Components Поли(1-триметилсилил-1-пропин) цис -обогащенной конфигурации – мембранный материал для разделения смеси бутан/метан как компонентов природного газа

The thermally activated relaxation of poly(1-trimethylsilyl-1-propyne) (PTMSP) samples of various cis-/trans-compositions (50–80% units of cis-configuration) in the presence of phenolic antioxidants of various structures was investigated. It was pointed out that polymers with a high content of cis-units exhibited greater thermal-oxidative stability due to the greater flexibility of the cis-enriched macrochains. The use of hindered phenols as antioxidants made it possible to prevent the process of thermally initiated oxidative degradation. At the same time, the most effective stabilizing agents were antioxidants with larger molecules such as Vulkanox BKF, Irganox 1010, and Irganox 1076. It was shown that the permeability coefficients of stabilized PTMSP during thermal treatment initially slightly decreased (by 20–30%), which, according to the X-ray diffraction data, was associated with an increase in the density of the macrochain packing, and during further heating remained practically unchanged. Note that for the cis-enriched samples, no signs of oxidation or decrease in the transport characteristics were observed during polymer heating for 240 h at 140 °C.