Pervaporation of organic-water liquid mixtures using polyimide membranes

Abstract

A series of polyimides were prepared to investigate the pervaporaton performances of the fluorinated and non-fluorinated polyimide membranes. Four polyimide membranes with fluorinated side groups were prepared by polycondensation of 2-(perfluorohexyl)ethyl-3, 5-diamino benzoate (PFDAB) with four aromatic dianhydrides(6FDA, ODPA, BTDA, PMDA) in N-methyl-2-pyrrolidone. The fluorinated diamine, PFDAB was synthesized through esterification of dinitrobenzoyl chloride(DNBC) with 2-(perfluorohexyl)ethyl ethanol (PFOH), followed by reduction of the nitro compounds. As reference materials for comparison, two polyimide membranes without fluorinated side groups were prepared by polycondensation of m-phenylene diamine(m-PDA) with the same aromatic dianhydrides. The fluorinated polyimides show lower Tg, lower surface free energy, lower solubility parameter, higher density and higher fractional free volume(FFV) than the non-fluorinated polyimides. X-ray diffractograms showed that all the polyimides prepared were amorphous. Their pervaporation performances for dilute aqueous organic solutions were investigated with two fluorinated(6FDA-PFDAB, ODPA-PFDAB) and two non-fluorinated(6FDA-m-PDA, ODPA-m-PDA) aromatic polyimide membranes. The pervaporation experiments were carried out as functions of nature of permeating solvents, feed concentration, permeate pressure and feed temperature. The polyimide membranes containing fluorinated side groups showed high pervaporation selectivity and high permeating flux toward hydrophobic solvents which were explained in terms of enhanced sorption/sorption selectivity and reduced diffusion resistance toward organic solvent over water due to enhanced hydrophobicity and fractional free volume. These results demonstrated that the introduction of fluorinated side groups into polyimide membranes improves significantly their pervaporation performances for the separation of hydrophobic organic solvents from water. Polyimide composite membranes were pr...