Poly-(4,4′-oxydiphenylene) pyromellitimide or Kapton is the most widely available polyimide with high chemical and thermal stability. It has great prospects for use as a membrane material for filtering organic media due to its complete insolubility. However, the formation of membranes based on it, at the moment, is an unsolved problem. The study corresponds to the rediscovery of poly(4,4′-oxydiphenylene–pyromellitimide)-based soluble copoly(urethane-imides) as membrane polymers of a new generation. It is shown that the physical structure of PUI films prepared by the solution method becomes porous after the removal of urethane blocks from the polymer, and the pore size varies depending on the conditions of thermolysis and subsequent hydrolysis of the membrane polymer. The film annealed at 170 °C with a low destruction degree of polycaprolactam blocks exhibits the properties of a nanofiltration membrane. It is stable in the aprotic solvent DMF and has a Remasol Brilliant Blue R retention coefficient of 95%. After the hydrolysis of thermally treated films in acidic media, ultrafiltration size 66–82 nm pores appear, which leads to an increase in the permeate flow by more than two orders of magnitude. This circumstance provides opportunities for controlling the membrane polymer structure for further optimization of the performance characteristics of filtration membranes based on it. Thus, we proposed a new preparation method of ultra- and nanofiltration membranes based on poly(4,4′-oxydiphenylene–pyromellitimide) that are stable in aprotic solvents.

Polyimides (PIs) are materials that are resistant to high temperatures and crucial for the manufacturing of films, fibers, coatings, and 3D-printed items. PIs are widely used as electrically insulating materials in electronics and electrical engineering. This study investigated how the chemical structure (i.e., choice of initial monomers), the synthesis conditions of the prepolymer (i.e., choice of amide solvent), and the conditions for forming polyimide films (i.e., final curing temperature) affect the thermophysical properties and short-term electrical strength of obtained polyimide films of different chemical structures. In this work, we varied the compositions of the dianhydrides used for synthesizing polyamic acids—pyromellitic acid (PMDA), tetracarboxylic acid diphenyl oxide (ODPA) and 1,3-bis(3′,4-dicarboxyphenoxy)benzene acid (R)—with a constant diamine: 4,4′-oxydianiline (ODA). Additionally, we varied the amide solvents employed: N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP). This study represents the first investigation into how the choice of solvent in the synthesis of thermoplastic polyimide prepolymers affects their short-term electrical strength. The molecular weights of the polyamic acids were determined using gel permeation chromatography (GPC). The deformation and strength characteristics of the investigated films were also assessed. The thermophysical properties of the polyimides were evaluated via dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). X-ray diffraction analysis and infrared spectroscopy (IR) were conducted on the examined film samples. The short-term electrical strength was also evaluated.

Nonwoven materials are fashioned from polyimide (PI) and its copolymer derivative (urethane–imide) (CPUI) through the formation of electrical fibers, referred to as electrospinning (ES). Diaminobiphenyl ether (DABPE) and pyromellitic anhydride (PMA) are chosen as monomers in the synthesis of polyimide. Diamine dianhydride and toluene di-isocyanate (TDI) are used in the synthesis of copoly(urethaneimide), as is polycaprolactone (M_n = 2000). A nonwoven polyimide material is obtained from an aqueous–ethanolic solution of the triethylammonium salt of polyamide acid (SPAA) with subsequent thermal imidization of the nonwoven prepolymer at 250°C. A nonwoven material based on SPAA is obtained from a solution of copoly(urethaneamidoacid) in dimethyformamide (DMF) using a catalytic mixture consisting of triethylamine and acetic anhydride. Optimum conditions are selected for the formation of nonwoven fibers in electrospinning. The chemical structure of nonwovens is confirmed via IR spectroscopy. The morphology of the microstructure of nonwovens is studied by means of scanning electron microscopy (SEM). The thermal and mechanical properties of the obtained nonwovens are determined using TGA, DSC, and DMA, and their deformation and strength characteristics are determined. Membrane filtration properties of the obtained nonwovens are studied. It is concluded that nonwovens can be used as membranes that operate in nonaqueous organic media.

The paper describes changes in the structure, morphology, mechanical and thermal properties of porous film samples of poly(4,4′-oxidiphenylene)pyromellitimide prepared as a result of selective destruction of urethane blocks in copolymers composed of pyromellitimide blocks and polyurethane blocks. The initial samples of the new composition of statistical copoly(urethane-imide)s (CoPUIs) were prepared via polycondensation methods using pyromellitic dianhydride (PMDA), 4,4′-oxidyaniline (ODA), 2,4-toluylenediisocyanate (TDI), as well as polycaprolactone (PCL) and poly(1,6-hexanediol/neopentylglycol-alt-adipic acid) (ALT) as monomers. The molar ratio of imide and polyurethane blocks in CoPUI was 10:1. The initial films were heated up to 170 °C to complete the polycondensation processes, after which they were subjected to thermolysis and hydrolysis. The thermolysis (thermal degradation) of copolymers was carried out by heating the initial samples to temperatures of 300 °C or 350 °C. Then, the thermolized films were subjected to chemical degradation in hydrolytic baths containing an aqueous solution of potassium hydroxide. As a result, urethane blocks were destroyed and removed from the polymer. The resulting products practically did not contain polyurethane links and, in chemical composition, were practically identical to poly(4,4′-oxidiphenylene)pyromellitimide. NMR and IR spectroscopy, atomic force microscopy, X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry and dynamic mechanical analysis and mechanical properties testing were used to determine the differences in the structure and properties of the initial copolymers and targeted products. The effect of the conditions of destructive processes on the structure, morphology and mechanical properties of the obtained porous polyimide films was determined. From a practical point of view, the final porous films are promising as membranes for filtering aggressive amide solvents at high temperatures.