3.2. Methods

PVDF and PVC were vacuum-dried at 60 °C for 12 h before use. PVDF-PVC mixture was dissolved in DMF, then stirred at 60 °C for 6 h, and the membrane solution of PVDF-PVC at 12 wt% (85:15) (M-12) was prepared and degassed for more than 30 min. The composite membrane was prepared by NIPS, and about 10 mL of the cast membrane was scraped on MEMCAST^TM (Porometer Ltd., Nazareth, Belgium) with a knife gap of 200 µm. After scraping, the cast membrane was exposed to the air for 10 s and then immersed into a 25 °C cold pure water coagulation bath for 5 min. After the flat membrane was solidified, it was soaked by ultrasonic cleaning with anhydrous ethanol for 30 min, washed with deionized water, and then soaked in deionized water for 12 h. The residual solvent was removed and dried at air temperature. Finally, the membrane is dried in a vacuum oven of 60 °C for 6 h before use.

Through a literature search, it was found that polyethylene glycol with a low degree of polymerization can alleviate the solubility of PVDF and PVC in DMF. Therefore, polyethylene glycol 200 (PG) antidissolver [8,9] was selected. However, the concentration of PG has yet to be investigated. It is necessary to meet the requirements of dissolving the coating material, and at the same time, the PVDF-PVC base membrane should not be dissolved as much as possible.

PVDF and PVC were vacuum-dried at 60 °C for 12 h before use. The casting agent is DMF, the anti-solving agent is polyethylene glycol 200 (PG), and the casting solution concentration of the coating layer is 4 wt% PVDF-PVC (85/15). Seven groups of solutions with PG: DMF of 10:90, 15:85, 20:80, 25:75, 30:70, 35:65, and 50:50 (recorded as PG-10, PG-15, PG-20, PG-25, PG-30, PG-35, and PG-50, respectively) were prepared and placed in the bottle. Place the powder with a mass concentration of 4 wt% PVDF-PVC (85/15) in seven groups of solutions and place it in a water bath clock. Raise the temperature of the water bath kettle from 30 °C to 40 °C, 50 °C, and 60 °C. Observe the dissolution of the powder.

In order to prevent the dissolution of the base membrane before scraping, DMF is used as the membrane casting agent, PG is the anti-solvent agent, and M-12 is used as the base membrane (the base membrane is cut to 15 mm × 15 mm for standby). Seven groups (bottles) of solutions (PG-10, PG-15, PG-20, PG-25, PG-30, PG-35, and PG-50) are prepared for the experiment, respectively. Put the cut base membrane into seven groups of solutions, put it into the water bath, and change the temperature of the water bath from 30 °C to 40 °C, 50 °C, and 60 °C. Observe the dissolution of the basement membrane within 30 min.

In order to prevent the deformation of the base membrane in the solvent, and at the same time, it is hoped that the polymer PVDF and PVC on the surface of the base membrane will have a certain extension in the developing agent, so as to increase the adhesion between the two layers, the base membrane expansion experiment is designed. According to Section 3.2.2., the M-12 base membrane (60 × 60 mm) was soaked in different temperatures and solvent ratios for 3 min, then immersed in purified water, then immersed the membrane in ethanol solution for 1 h, and then ultrasonic at 60 °C for 1 h in the ultrasonic cleaner. Take out the membrane and wash the ethanol with purified water, dry it naturally, and dry it in a vacuum at 60 °C for 8 h. Measure the contact angle and surface morphology of the base membrane after expansion.

Dry PVDF and PVC in a vacuum at 60 °C for 12 h for standby. PVDF and PVC are used as membrane materials, and polyethylene glycol (PE200) and DMF in different proportions are used as solvents. The total mass of the membrane material is 4%. PVDF-PVC (w/w = 85/15) is dissolved in different proportions of polyethylene glycol (PE200) and DMF as the solvent, stirred at a constant speed for about 2 h at 60 °C, and then ultrasonically defoaming at 60 °C for more than 30 min to obtain the casting solution.

Immerse the M-12 base membrane in the optimum temperature and solvent ratio for 3 min, and then immerse it in pure water. Then, immerse the membrane in ethanol solution for 1 h, and then an ultrasound at 60 °C for 1 h in the ultrasonic cleaner. Take it out and wash the ethanol with pure water, dry it naturally, and dry it in a vacuum at 60 °C for 8 h.

As shown in Figure 10, the 4 wt% PVDF-PVC (w/w = 85:15) coating material membrane casting solution with the membrane casting agent was used, and the membrane casting solution was coated on the substrate at 60 °C, 50 °C, 40 °C, and 30 °C, respectively. After the completion of scraping, immerse the membrane in pure water. After the membrane is completely separated from the scraping template, immerse the membrane in ethanol solution for 30 min, and then ultrasound for 30 min at a constant temperature of 60 °C in the ultrasonic cleaner. Take it out, wash the ethanol with pure water, and then soak it in deionized water for 12 h, remove the residual solvent, and let it dry naturally. Dry it for 6 h under the temperature control of 60 °C in the vacuum drying box, and wait for standby.

Preparation process diagram of D-PP/PP super-hydrophobic composite membrane.

In order to prevent the aggregation of nano-graphite in solution and further reduce the number and size of nano-graphite layers, a high-power ultrasound instrument was selected to disperse the nano-graphite through ultrasound. Add 2.5 g nano-graphite into 500 mL DMF solution (initial concentration of nano-graphite: 5 mg/mL), and ultrasonic peel it at 50 °C for 30 min.

Dry PVDF and PVC, and wait for 12 h in a vacuum at 60 °C. A casting solution containing 1% nano-graphite and 4% PVDF-PVC (85%/15%) was prepared using PVDF, PVC, and nano-graphite as membrane materials, polyethylene glycol (PE200), DMF, and nano-graphite dispersion as solvents.

Preparation of base membrane according to the method outlined in Section 3.2.3.

As shown in Figure 11, a casting solution containing 1% nano-graphite and 4% PVDF-PVC (85%/15%) with the membrane casting agent was used, and the membrane casting solution on the substrate was coated. After the completion of scraping, immerse the membrane in pure water. After the membrane is completely separated from the scraping template, immerse the membrane in ethanol solution for 30 min, and then ultrasound for 30 min at a constant temperature of 60 °C in the ultrasonic cleaner. Take it out, wash the ethanol with pure water, and then soak it in deionized water for 12 h, remove the residual solvent, and let it dry naturally. Dry it for 6 h under the temperature control of 60 °C in the vacuum drying box, and wait for standby.

Preparation process diagram of D-PPG/PP super-hydrophobic composite membrane.