Gas separation membranes made through thermal rearrangement of ortho methoxypolyimides

Abstract

Ortho-methoxypolyimides were prepared by the classical chemical imidization method and also by azeotropic imidization, using 3,3’-dimethoxybenzidine (DMAB) and hexafluoroisopropylidene diphthalic anhydride (6FDA) as monomers. High molecular weights were achieved by both methods, and the physical properties of the materials were investigated by spectroscopic and thermal analytical techniques. Polymers exhibited excellent thermal properties and film-forming ability, which allowed them to be tested as dense membranes for gas separation after a convenient treatment at high temperature (450 °C) to promote thermal rearrangement (TR). Spectroscopic evidences indicated that the final composition of the TR materials seemed not to correspond to TR-polibenzoxazoles, as it is the case when ortho-hydroxypolyimides are exposed to similar thermal treatments. A detailed study was carried out with the purpose of elucidating the actual mechanism of rearrangement, comparing ortho-hydroxypolyimides and ortho-acetylpolyimides with the ortho-methoxypolyimides obtained in this work. Results led to the conclusion that the chemical nature of the final TR materials attained from ortho-methoxypolyimides is rather complex because imide, cyclolactam and benzoxazole groups seems to be formed under thermal treatment, and the proportion of the different moieties greatly depends on the ortho-substituent, on the synthesis route used to prepare the polyimide and on the time-temperature schedule applied in the thermal rearrangement. The gas permeation properties exhibited by thermally treated ortho-methoxypolyimides compared very well with those of other TR-PBOs reported so far, showing O2 permeability of 94 barrers and CO2 permeability of 540 Barrers.