Combining control of branching and sulfonation in one-pot synthesis of random sulfonated polyarylethersulfones: effects on thermal stability and water retention

In functional polymers with tunable hydrophilic behaviour like sulfonated polyarylethersulfone (SPES) high ionic conductance can be obtained directly by increasing the concentration of sulfonic moieties along the macromolecular chain. This, however, often comes to the cost of excessive water sorption, which can lead to membrane rupture and consequent device failure. To overcome this drawback, and to reconcile high hydrophilicity with the high mechanical properties of aromatic polymers, we propose the use of SPES copolymers with a low degree of branching. A series of branched SPES was synthesized using homogeneous (one-pot) copolymerization by replacing an amount up to 0.9% mol of the difunctional monomer 4,4'-dihydroxydiphenyl with the trifunctional 1,3,5-trihydroxybenzene (THB). The polymers were characterized by H-1-NMR spectroscopy, intrinsic viscosity, water sorption measurements, thermogravimetric analysis and differential scanning calorimetry. Like linear SPES, branched SPES is totally amorphous and soluble in polar aprotic solvents, lending itself to easy fabrication of membranes and coatings. Besides reducing the water sorption and in-plane swelling of SPES membranes upon equilibration in liquid, branching also improves the retention of water in swollen membranes.