Method for producing a polyelectrolyte complex membrane
The present invention relates to a method for producing a polyelectrolyte complex (PEC) membrane having a predetermined porosity via salt dilution induced phase separation, in which a liquid polymer solution (P) containing polyanions (A) and polycations (C) dissolved in an aqueous medium at an overcritical salt concentration is exposed to an aqueous medium.
1 . A method for producing a polyelectrolyte complex (PEC) membrane having a predetermined porosity via salt dilution induced phase separation, the method comprising the following steps:
(i) providing a liquid polymer solution having a homogeneous single phase at an overcritical salt concentration, where the overcritical salt concentration is a salt concentration higher than the salt concentration at the critical point, by mixing polyanions and polycations in an aqueous medium, the liquid polymer solution containing the polyanions and polycations dissolved in the aqueous medium in a state where the polyanions and polycations do not form a polyelectrolyte complex (PEC), wherein the complexation of the polyelectrolyte is suppressed by the overcritical salt concentration of the liquid polymer solution, which is adjusted either by the addition of a salt before the mixing step and/or by the addition of the salt after the mixing step; and
(ii) thereafter, without preparing the solid polyelectrolyte complex (PEC) or a coacervate phase, exposing the overcritical salt rich liquid polymer solution having a homogeneous single phase to an aqueous medium so as to decrease the salt concentration, which triggers the complexation of the polyelectrolyte and precipitation of the polyelectrolyte complex (PEC), thereby forming the polyelectrolyte complex (PEC) membrane,
wherein the overcritical salt rich liquid polymer solution has a viscosity, measured as dynamic viscosity at room temperature using a rheometer, of 10 to 100000 mPa·s before exposing it to the aqueous medium in step (ii),
wherein the salt concentration of the overcritical salt rich liquid polymer solution in step (i) is sufficiently high so that the formation of the solid polyelectrolyte complex (PEC) and of the coacervate phase can be suppressed, and
wherein the method does not employ any organic solvent.
2 . The method according to claim 1 , wherein the polyanions are polymers having at least one anionic functional group selected from the group consisting of sulfonates, carboxylates, and phosphates.
3 . The method according to claim 1 , wherein the polycations are polymers having at least one cationic functional group selected from the group consisting of primary amines, secondary amines, tertiary amines, quaternary amines, amides, and pyridinium moieties.
4 . The method according to claim 1 , wherein the salt is selected from the group consisting of potassium bromide (KBr), sodium chloride (NaCl), potassium chloride (KCl), trisodium phosphate (Na 3 PO 4 ), sodium carbonate (NaCO 3 ), sodium sulfate (Na 2 SO 4 ), potassium sulfate (K 2 SO 4 ), magnesium sulfate (MgSO 4 ), magnesium chloride (MgCl 2 ), calcium chloride (CaCl 2 )), calcium carbonate (CaCO 3 ), disodium hydrogen phosphate (Na 2 HPO 4 ), sodium dihydrogen phosphate (NaH 2 PO 4 ), sodium hydrogen carbonate (NaHCO 3 ), potassium hydrogen sulfate (KHSO 4 ), sodium hydrogen sulfate (NaHSO 4 ), sodium sulfite (Na 2 SO 3 ), and sodium hydrogen sulfite (NaHSO 3 ).
5 . The method according to claim 1 , wherein the salt concentration of the overcritical salt rich liquid polymer solution in step (i) is in the range of 0.01 mol/L to 7.0 mol/L.
6 . The method according to claim 1 , wherein the polyanions and polycations are strong polyelectrolytes, the functional groups of which are selected from the group consisting of sulfonates and quaternary amines, respectively.
7 . The method according to claim 1 , wherein the polyanions include poly(styrene sulfonate) and the polycations include poly(diallyldimethylammonium chloride).
8 . The method according to claim 1 , wherein the total concentration of the polyanions and polycations in the overcritical salt rich liquid polymer solution is in the range of 1 to 80 wt.-%.
9 . The method according to claim 1 , wherein the molar monomer ratio of the polyanions to the polycations in the overcritical salt rich liquid polymer solution is 10:90 to 90:10.
10 . The method according to claim 1 , wherein step (ii) comprises applying the overcritical salt rich liquid polymer solution onto a substrate by casting, printing, coating or spinning, and thereafter, immersing the same in the aqueous medium, thereby forming a freestanding flat-sheet polyelectrolyte complex (PEC) membrane in case of using a flat substrate, or forming a tubular polyelectrolyte complex (PEC) membrane or a freestanding hollow fiber polyelectrolyte complex (PEC) membrane in case of using a tubular substrate.
11 . The method according to claim 10 , wherein the overcritical salt rich liquid polymer solution is applied onto a tubular non-woven substrate thereby forming a tubular composite polyelectrolyte complex (PEC) membrane.
12 . The method according to claim 10 , wherein two or more overcritical salt rich liquid polymer solutions are applied onto the substrate, which differ from one another in at least one of the salt concentration, the concentration of the polyanions and polycations, and the molar monomer ratio of the polyanions to the polycations.
13 . The method according to claim 1 , wherein step (ii) comprises forming a hollow fiber by extruding the overcritical salt rich liquid polymer solution with a bore solution through a spinneret, and optionally immersing the extruded hollow fiber into a coagulation bath comprising the aqueous medium, thereby forming a freestanding hollow fiber polyelectrolyte complex (PEC) membrane.
14 . The method according to claim 1 , wherein the polyelectrolyte complex (PEC) membrane is a microfiltration membrane, an ultrafiltration membrane, a nanofiltration membrane, a reverse osmosis membrane, a diffusion dialysis membrane, a contactor membrane, an ion-exchange membrane, or a gas separation membrane.
15 . The method according to claim 1 , wherein the polyelectrolyte complex (PEC) membrane is one of the following:
a freestanding flat-sheet polyelectrolyte complex (PEC) membrane having a symmetrical or an asymmetrical porosity,
a tubular composite polyelectrolyte complex (PEC) membrane including a non-woven substrate, or
a freestanding hollow fiber polyelectrolyte complex (PEC) membrane having a symmetrical or an asymmetrical porosity.