Mixed-matrix membranes based on 6FDA-ODA polyimide and silicalite-1 with homogeneous spatial distribution of particles

Abstract

We report an experimental procedure leading to defect-free mixed-matrix membranes with a homogeneous spatial distribution of filler particles, volume fractions of which are within the interval [0.10, 0.35]. A polyimide matrix was synthetized from 4,4′-(hexafluoroisopropylidene) diphtalic anhydride and 4,4′-oxydianiline in the solution with N,N-dimethylformamide. (3-aminopropyl) triethoxysilane was used as a linker to improve the phase contact between silicalite-1 particles and the polyimide matrix. We found that a homogeneous spatial distribution of particles in the resulting membrane was achieved if kinematic viscosity of a polyimide precursor solution was greater than 9.5 cm2 s−1. To do so, an initial mass fraction of the solids in the solution, a reaction temperature and a concentration of water in the solvent had to carefully be selected and controlled. Besides these conditions, we fine-tuned the process of imidization, particularly the rate of evaporation of N,N-dimethylformamide. We arrived at the conclusion that its effect on reproducibility of the entire process was minor rather than major. In addition, we characterised microstructures of the resulting membranes by imaging in a scanning electron microscope. In this context we examined two ways of preparation of membrane specimens and showed that back-scattered electron imaging of polished sections obtained using a metallographic technique clearly outperformed the often-used imaging of fractured surfaces. Finally, we evaluated the homogeneity of spatial distributions of particles by means of morphological descriptors, namely a position-dependent volume fraction of the silicalite-1 phase.