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Exceeding the limits of polyimide membranes for less-energy intensive CO2 separation and capture by employing the PIM concept and blending technique
Project status: Ongoing
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A sumary of the main results obtained during project implementation in 2022 can be found below:

Stage I (year 2022) of memPIM-PIs project aimed at developing a technology for preparation of polymer membranes for CO2 separation/capture based on polyimides containing voluminous DOPO units and mixtures of polyimides based on them. For this purpose, an aromatic diamine-type monomer containing DOPO group grafted on a triazine ring was synthesized. Based on it, three polyimides with DOPO units and different chain flexibilities modulated by the variable structure of the diimide segment were prepared. Polymer synthesis reactions were accomplished by direct polycondensation, in a single step, at high temperature, in a polar aprotic solvent. The polymers with DOPO units were structurally characterized by spectral methods (FTIR, 1H and 31P-NMR), which confirmed their correct structure. Upon excitation with UV light, the synthesized polyimides with DOPO units exhibited weak fluorescence in the blue-green spectral range, due to the presence of the charge transfer complex developed between the diamine segment as an electron donor and the diimide segment as an electron acceptor. All polymers showed good solubility in polar aprotic solvents such as DMF, NMP, DMAc, which allowed their processing from solution in form of films. Although an extensive work was carried out to optimize the processing conditions of these polymers in form of self-standing membranes, only brittle films were obtained, which cannot be detached from the support. Therefore, the DOPO-containing polyimides were mixed with a polyimide based on 6FDA with very good film-forming properties, whose synthesis was carried out starting from commercial monomers. Thus, 7 polymer blends were prepared in different mass ratios, which were processed in form of membranes with good mechanical integrity. In order to obtain uniform membranes, the application of a controlled thermal regime to the blend solutions was applied after establishing the optimal processing conditions and the optimal ratios between the two components of the blend. The resulting self-standing membranes were characterized from the morphological and miscibility point of view, the experimental results demonstrating their uniformity and the homogeneous mixing at the molecular level of the two polymeric components. In the next stage of the project, these membranes will be characterized from the physico-chemical point of view and explored with regard to the gas permeation capacity, with an emphasis on the separation/capture of CO2. Part of the outcome results of this project stage were disseminated in the form of 2 oral communications at international scientific events and a review paper was published in the ISI journal "Chemistry". 

A sumary of the main results obtained during project implementation in 2023 can be found below: 

Stage 2 (year 2023) of memPIM-PIs project has as main objective to investigate from the physico-chemical point of view all membranes obtained from the polymer blends reported in Stage 1, in which one of the components is a polyimide with a bulky DOPO unit (DOPO-PI), as well as to test them for permeation of different gases, with emphasis on CO2 separation/capture. Another objective of this stage consisted in the development of a technology for membranes preparation  suitable for CO2 separation/capture based on polyimides containing triphenylamine substituted with trityl units (TTA) and their mixtures with a fluorinated polyimide. Thus, the blends based on DOPO-PI were analyzed from thermal, mechanical, dielectric, and chemical resistance point of view. They showed high thermal stability, up to 404–509°C, function on the composition of the blend, while the glass transition temperature was between 222 and 250°C. The membranes had good mechanical integrity and flexibility, with tensile strength in the range of 47–91 MPa. In addition, they did not degrade in acidic or basic corrosive environments, demonstrating a very good chemical resistance, necessary in gas separation processes. Dielectric spectroscopy studies have shown that segmental and chain mobility is allowed in these blended membranes, despite the strong interactions in which the two constitutive components are involved. The membranes were tested in permeation processes of several gases, such as O2, N2, He and CO2, and based on the obtained results, the ability to separate CO2 from gas mixtures was evaluated. Also, the results were highlighted in Robeson diagrams, and solutions to improve their performances were proposed. A second structural strategy was further approached to obtain membranes based on polymer blends. For this purpose, we resorted to the synthesis of a new monomer containing triphenylamine substituted with trityl units (TTA), which has a central nitrogen atom with affinity towards CO2. After structural identification, this diamine-type monomer was used in polycondensation reactions with different aromatic dianhydrides to obtain 3 polyimides with bulky TTA units. These polymers were characterized from structural and physico-chemical point of view, in order to evaluate the optical, thermal and electrochemical properties. Since these polyimides based on TTA did not give free-standing films with good mechanical integrity, they were subjected to blending in different proportions with a polyimide based on 6FDA, with excellent film-forming ability. Following an extensive optimization process regarding the processing conditions and the ratios between the two components of the blends, 7 membranes with good mechanical integrity were obtained, whose investigation sugested the miscibility at the molecular level of the two components. These membranes will be studied in detailed, and afterwards tested in the separation process of CO2 from gas mixtures in the Stage 3 of the project. Part of the results obtained in Stage 2 of memPIM-PIs project were disseminated in international scientific events in the form of 2 invited conferences, 2 communications (including a short one) and 2 posters. Also, based on the obtained results, 2 ISI papers were published in Molecules and Progress in Organic Coatings journals, as well as 2 chapters in the book Polyimides: Advances in Blends and Nanocomposites, edited by Elsevier. 

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