Membrane fabrication

with and subsequent stirring for varying times. This was approximately 20 minutes in case of interchanging Na+ with H+. Here the titanosilicate has to be exposed to an acidic solution. However it is known that in too strong acidic media JDF-L1 loses its structural stability resulting in the formation of amorphous material. In order to keep this effect minimal a weak acid (acetic acid) diluted to a pH value of not lower than 3.5 was used and the pH value was monitored during the reaction. As soon as the solution reached a pH between 6 and 7 (around 10 minutes) it was carefully acidified once more to pH 4 and stirred for 10 more minutes until the pH remained unchanged and the reaction was considered finished. In case of the other cations the titanosilicate was added to a solution of the respective chloride salt and the stirring time varied between 4 and 24 hours. The used alkaline and alkaline earth chloride salts allowed a much higher excess of the cation to be interchanged with due to their high solubility. In this work a 80-fold excess of the new cation in relation to interlamellar sodium was used. As salts lithium chloride (≥99 %; Sigma Aldrich), potassium chloride (Panreac, 99.5 %), calcium chloride dihydrate (≥99 %; Sigma Aldrich) and magnesium chloride anhydrous (≥99 %; Sigma Aldrich) respectively were used. In both cases (weak acid, chloride salt) the extraction of the exchanged material was carried out via three subsequent steps of centrifugation for 10 minutes at 10,000 rpm and washing with demineralised water. The detailed preparation methods of the individually synthesised batches can be found in the appendix (see chapter 6.2).

4.3.4 Simultaneous disaggregation and cation exchange

Based on the finding of Rubio[123] that as-synthesized JDF-L1 can also be disaggregated using sodium hydroxide a reaction was tested to perform disaggregation and ion exchange in one step. Here the following procedure was applied. In a 50 mL one necked flask a one molar solution of different bases is prepared. Then 500 mg of as-synthesized JDF-L1 are added and the mixture is stirred for 16 hours at 80 °C. After letting the flask cool down to room temperature the sediment is extracted and washed at least three times with demineralised water via centrifugation at 10,000 rpm for 10 minutes. The wet product is finally dried at 100 °C for 24 hours. As bases NaOH (pellets, 98 %, Alfa Aesar), NH3 (solution, 25 %, Panreac), LiOH*H2O (powder, 98 %, Alfa Aesar) and KOH (pellets, 85 %,

Alfa Aesar) are used.

4.4 Membrane fabrication

Though all membranes comprise the same copolyimide the fabrication methods differ significantly depending on whether a pure polymer or e.g. a mixed matrix membrane was to be prepared. Also different kinds of filler materials required different dispersion methods. The membrane fabrication methods used in this work are given below. In all cases the polymer 6FDA-4MPD/6FDA-DABA 4:1 was used. To simplify matters however only the much shorter term “copolyimide” is used in the following. At this point some basic information applying to all membrane preparation methods is given. The polymer solution had to be filtered using a syringe filter (PTFE, 0.45 µm pore size, Carl Roth). This step is necessary, because during the synthesis and subsequent processing of the polymer powder dust particles are usually attracted. These hamper defect free film formation if not removed. Subsequently the filter was washed out using small additional amounts of solvent in order to reduce the loss of polymer solution due to the dead volume of the filter (100 µL according to manufacture

specifications). As the mould for membrane casting different diameter metal rings were used. They were either placed on a PET sheet (Hostaphan® RN, colourless transparent, thickness 190 µm) or on a commercial mirror tile (20 x 20 cm). In order to keep away dust particles from contaminating the casting solution the rings were covered with a funnel whose open end was lightly plugged with tissue paper after casting of the solution.

4.4.1 Preparation of copolyimide membranes

In a 20 mL snap-cap vial 640 mg copolyimide (0.225 mmol) were dissolved in 6.5 mL THF and the solution was filtered through a syringe filter. Subsequently the filter was rinsed with additional 1.5 mL of THF. Now 4 mL each of the filtered solution were cast into metal rings (diameter 10 cm) lying on a piece of PET sheet, which then were covered with a funnel. After evaporation of the solvent the membranes were dried for 24 hours at 150 °C and 80 mbar in a vacuum oven.

4.4.2 Preparation of mixed matrix membranes

Not all investigated MMMs contained the same amount of inorganic material. However after development of the following successful general preparation method, all conditions apart from the filler load were retained for every MMM prepared within this work. Below a method is given exemplarily for the preparation of a membrane containing 8 wt.-% of inorganic material.

The copolyimide powder (1.6 g, 0.526 mmol) was dissolved in a 20 mL snap-cap vial using 16 mL THF. Subsequently the solution was filtered through a syringe filter (PTFE, 0.45 µm pore size) and the filter rinsed with additional 2 mL of THF. After portioning the solution into two snap-cap vials (9 mL each), 64 mg JDF L1 were added to both parts and the mixture was stirred over night. To ensure good dispersion the casting solution was now treated in the ultrasound bath (ELMA Transsonic 310, 35 Hz) for 30 minutes and afterwards stirred for another 30 min. These steps were repeated three times in total. Now 4 mL each of the mixture were cast into metal rings (diameter 7.5 cm) on a PET sheet and covered with a funnel. After evaporation of the solvent the membranes were dried for 24 hours at 150 °C and 80 mbar.

4.4.3 Preparation of MWCNT/copolyimide membranes

As for MMMs also in this case membranes containing different filler loads were prepared. After a successful preparation method was found no parameters except of the filler amounts added were altered. In the following the method is given for the preparation of a MWCNT/copolyimide membrane containing 1 wt.-% of MWCNT.

In a 20 mL snap-cap vial 0.7 g of the copolyimide (0.246 mmol) were dissolved using 10 mL THF and filtered through a syringe filter (PTFE, 0.45 µm pore size) which was subsequently rinsed with 5 mL THF. Now 7 mg of MWCNT-OH were added and the mixture was dispersed using ultra sound treatment (Sonifier 450 W) for 30 minutes at 30 % amplitude (results in 10 % power, ca. 20 W), while cooling the vial with an ice bath. After this the mixtures was stirred for 30 minutes at room temperature. These steps were repeated three times in this order. Finally two membranes were