As a curious non-expert in this area, I do have to ask: Do you modify the PE or PP surfaces first in order to enable adhesion of the highly polar polydopamine? Presumably you need either hydrogen bonding or pi-pi interactions or both. Otherwise, molecules of polydopamine are more likely to stick to one another and form floating particles than to stick to the polyolefi surface.

Polymerization of dopamine can be obtained via chemical oxidation of dopamine with dissolved O2 in an alkaline solution or in the presence of APS, ammonia, etc. So, I suggest controlling the "rate of deposition" by tuning the dopamine/oxidant ratio and changing the oxidant type.

Howbeit, the color change on the membrane surface means that the reaction happens properly. To increase the coating thickness you need to increase the oxidant (e.g. O2) amount in the polymerization medium.

If you are taking advantage of the dissolved oxygen, leave open the reaction vessel and by shaking the vessel let more O2 enter into the solution. For this, chose a vessel with a large opening.

You may refer to the following paper and the supporting information"

Article Nano-scale modification of polysulfone membrane matrix and t...

Dear Emanuel Cooper , thank you for your reply. At them moment I'm not modifying the separator's surface previous to the coating, I've found a couple of articles that performed the same procedure and yield good results.

Dear Ali Nematollahzadeh ,

thank you so much for your helpful hint. That might indeed be the cause, at the beginning I was using a beaker as reaction vessel and then I switched to glass vials. After the switch I observed a poorer deposition rate.

Thank you very much for attaching the reference, it is highly appreciated.

Best

The polymerization of dopamine into polydopamine is highly dependent of the pH of the solution. Why do you put methanol 1:1 tris? I think you should control better the pH of the reaction (at least in the first 4 hours).

The aggregates you osberved in dissolution are usual, since once that the surface to be coated is "saturated" (let's say), the rest of polydopamine oligomer tend to aggegrate themselves.

If you want to increase the polydopamine coating thickness, try a second polymerization in the same conditions (however, you would obtain a highly rough surface).

Thank you Rocio Tejido-Rastrilla , what I meant is that I prepared a buffer solution of Trizma base at 10mm and then mix it with methanol in a 1:1 ratio.

However, I noticed that the pH decreases significantly when I mix the buffer with methanol and also when I add dopamine. Therfore what I do now is first mixing water and methanol, then I add dopamine and tris, and finally I correct pH to 8.5 ( even if usually at this point pH is 8.7).

I still have troubles getting an homogeneous deposition on both sides of the separator, if I leave the membranes floating on the surface, there will be big difference in contact angle between the submerged surface and the other one.

I reccomend you to immobilize the separator (somehow) inside the solution, in vertical position, in order to avoid the deposition of big aggregates in one of the sides.

On the other way, to obtain a better film homogeneity, try to use a light agitation while deposition process. Probably you'll get a thiner layer, but more homogeneous.