The overall goal of this procedure is to prepare and characterize freestanding films and impregnated membranes based on zwitterionic hydrogels for membrane water purification. This method provide guidance for repairing hydrogel base freestanding film in impregnated membrane and as well as for characterizing their physical and transport properties. The main advantage of this technique is that it's simple, fast, and easy to be scaled up in industrial production.
Visual demonstration of this method is critical as the integrated hydrogel preparation and characterization steps are difficult to learn because custom built apparatus is used, and interdisciplinary skills are required. To prepare the freestanding films, first place two spacers of known thickness on a clean quartz disc. The thickness of the spacers controls the thickness of the obtained polymer films.
Transfer a small amount of the pre-polymer solution to the quartz disc using a disposable pipette. Place another quartz disc on top of the liquid and ensure that there are no bubbles in the liquid film. Then, place the sample in an ultraviolet or UV crosslinker and irradiate it for five minutes using UV light with a wavelength of 254 nanometers.
When the polymer film is removed from the quartz disc, it is critical to work slowly to avoid tearing the film. Separate the polymer film from the quartz discs using a sharp blade. Use tweezers to transfer the film to a deionized water bath.
Change the water twice during the first 24 hours to remove the solvent, unreacted monymer and crosslinker, and solve from the film. To prepare dried films for analysis, remove the film from the water bath and allow it to air dry for 24 hours. Place the film in a vacuum oven at 80 degrees Celsius to dry overnight under vacuum.
Place a sheet of porous support onto a quartz disc. Using a foam brush, coat each side of the support twice with a prepolymer solution that is based on the water ethanol mixture. Place another quartz disc on top of the support.
Then place the sample in a UV crosslinker and irradiate it for five minutes using UV light with a wavelength of 254 nanometers. To remove the impregnated membrane from the quartz discs, immerse the whole assembly in a deionized water bath for five minutes and carefully remove the membrane using a sharp blade and tweezers. Keeping the membrane in deionized water, change the water twice to remove the solvent, the unreacted monymer and crosslinker, and the sol from the membrane.
To prepare dried, impregnated membranes for analysis, remove the membrane from the water bath, allow the membrane to dry at ambient conditions for 24 hours. Then, dry the membrane in a vacuum oven overnight at 80 degrees Celsius under vacuum. To measure the contact angles using the pendant drop method, first cut a rectangular strip of the membrane sample.
Soak this strip in deionized water for 10 minutes. And then dry it for five minutes. Place the dried sample on the sample holder.
Then, submerge the sample holder in a transparent environmental chamber containing the deionized water. In the next step, it is critical to work slowly and dispensing the n-decane drops onto the membranes. Using a microliter syringe with a stainless steel needle, dispense drops of n-decane onto the membrane sample.
Leave the setup undisturbed for two minutes to ensure the stabilization of the droplets. Next, use appropriate image analysis software to determine the contact angle of the samples by measuring the angles of the dispensed droplets on the membrane surface. To characterize the water permeability using a dead end filtration system, first cut coupons of freestanding films and impregnated membranes with a hammer driven hole punch of an appropriate diameter.
Place a prepared coupon on the porous support inside a dead end filtration cell. Then, place the O ring on top of the sample. Screw the two halves of the permeation cell together.
Next, add approximately 50 milliliters of deonized water to the permeation cell. Screw on the cap and place the permeation cell on a magnetic stirrer. Set the stirring rate to between 300 and 900 RPM.
Then, place a covered beaker on a balance to collect the permeate water and tear the balance. Open the valve on the gas cylinder. Turn the pressure regulator valve clockwise until the desired pressure is reached.
Finally, open the release valve to deliver the pressure to the permeation cell. Monitor and record the weight of the beaker with time. Shown here are photos of the freestanding film, porous support, and impregnated membranes.
It is evidence that after the porous support is filled with the hydrogel, it changes from opaque to transparent. The infrared spectra indicate the almost complete conversion of the acryllate groups in the hydrogels. Due to the absence of acryllate characteristic peaks in the spectra.
The glass transition temperature of hydrogels is determined by differential scan calorimetry as shown here. Lower contact angles in the freestanding films and impregnated membranes indicate the greater hydrophilicity than the porous support. After watching this video you should have a good understanding of how to repair hydrogel base materials and characterize their properties.
This work describes a faster method to fabricate hydrogel based material via photopolymerizations as well as how to characterize them for water purification. The method and material may also be used to repair membrane for gas operations such as CO2 capture. Don't forget that working with chemical and UV radiation can be extremely hazardous and precautions such as personal protective equipment should always be taken while performing this procedure.
