Per-Fluorinated Azo- and Azomethane-Based Organic Polymers Developed Via Nucleophilic Aromatic Substitution Mechanism: Synthesis, Thermal, Optical Properties, Porosity and Environmental Applications

Abstract

Abstract A new series of ether-linked, per-fluorinated organic polymers bearing azo- (-N = N-) and azomethane (-C = N-) organic linkers was reported. The synthetic methodology relied on applying the nucleophilic aromatic substitution reaction (NAS) to fluorinated linkers (e.g. decafluorobiphenyl and hexaflourobenzene) and diols of azo- and azomethane linkers. The successful formation of the new structures revealing ether-linkage substitution of selected fluorine sites was confirmed by 1H-, 13C-, 19F-NMR and FTIR. All polymers were thermally stable in the range of 350–500 °C due to the variation of fluorine and nitrogen contents. The extended conjugation of the polymers was confirmed by the changes in the UV-Vis spectra of the organic linkers and their corresponding polymers. A notable hypsochromic shift was observed in all cases, which was more pronounced with azo-based fluorinated chains due to the H-bonding on the nitrogen sites, chain conformations and planarity. The optical band gap (Eg) of the polymers was determined from the UV-Vis. The Eg values of azo-based fluorinated polymers were higher by 1eV compared to their corresponding linkers. The 19F-NMR analysis confirmed two types of NAS on both the ortho- and para- positions of the fluorinated linkers. These connections created the possibility of developing cross-linked frameworks beside the open-chain confirmation with tailored hydrophobic nature. The cross-linking formation was characterized by porosity measurements, including surface area (SA), pore size and pore volume. The highest measured values were recorded for the azo-based polymer (DAB-Z-1h), which reached 438 m2/g and a pore volume of 0.35 cm3/g. A surface area of 105 m2/g was the lowest for the open-chain azomethane-based polymer (DAB-A-1O) with a pore volume of 0.0872 cm3/g. The beneficial formation of porous structures with varied hydrophobic nature was investigated as adsorbents for separating water/benzene, water/phenol and the selective binding of methane/carbon dioxide gases from the air. The most hydrophobic polymers that contain the decafluorbiphenyl linker were suitable for benzene separation, and the superior methane uptake values were 6.14 and 3.46 mg/g, for DAB-Z-1O and DAB-A-1O, respectively. On the other hand, DAB-Z-1h, with the highest surface area (438 m2/g) and rich with nitrogen sites, has the highest CO2 uptake at 298 K (17.25 mg/g).