I am currently working on the synthesis of reverse osmosis membranes using a polysulfone support and a polyamide active layer. Upon testing membrane performance, I am getting very low salt rejection and high water permeability which is not consistent with literature.
You can use the NMP as a solvent to prepare 18 wt% polysulfone homogeneous solution at 60–70 ◦C under continuous stirring for 7–8 h on the magnetic stirrer. The prepared dope solution is then set to cool down so that the air bubble and moisture present in it get evacuated. Using the homogeneous polymer solution cast the membrane at normal temperature to make the flat sheet membranes. After then the casted membrane needs to immediately immers in the deionized water. The phase inversion/separation process will take place while the immersion. The membrane needs to wash softly with de-ionized water to remove the solvent and other impurities and stored back in deionized water until the next use.
I agree with what the previous colleagues said. However, I think the crux of the matter is that your selection layer has some defects. Just like when the bathtub is filled with water, the water will all slip away from the downpipe. This question has run through the pre-experimental phase of every work I've done since I started doing membranes. Until one day, I suddenly wanted to understand this matter in a dream. Actually every material, every polyamide layer, is different, just like there are a thousand Shakespeares for a thousand people who read Hamlet.So, when you make each membrane, if it is what you said, I think you will get all kinds of strange and irregular results. In fact, for the synthesis process of each polyamide layer, I think that not only following the literature, but finding a method similar to interfacial polymerization, I think that the polyamide layer can be synthesized with peace of mind. As several colleagues said before, if it were me, I would have a high probability of not succeeding if I followed them strictly. Therefore, I think the most important thing for synthesizing a defect-free selection layer is to understand the properties of each component, pay attention to the details that are not mentioned in the literature, and boldly repeat the experiment. Just like some selective layers are not highly cross-linked, if you blow it with nitrogen with too much pressure during the drying process, it is bound to have defects. Therefore, dear colleague, I suggest that you use the slowest speed in history when you make a membrane next time. After each step is completed, think carefully about what contribution this step has made to the synthesis of the selective layer. What are the results. In this way, step by step, even if you are not successful, you will make new discoveries, and then you will be closer and closer to success. Some people attribute this process to "manipulation", but I am more inclined to sum up the experience in a large number of experiments, just like the old saying "practice makes perfect（熟能生巧）".
Hope my seemingly lengthy conversation can solve your problem. In the meantime, any comments from colleagues are welcome. good luck!
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