I think you should visit the following web:

https://www.ika.com/en/Products-Lab-Eq/Overhead-Stirrers-Agitator-Blender-Lab-mixer-Accessories-cspacc-187/

if you use the condition but you change the impellers , then you should see the characteristics of each impeller, to determine its effectiveness in mixing.

before answering in more detail, I would need to know at least 4 things:

- what polymer

- what solvent

- which concentration

- how do you stirr - i.e., which rpm, what are the dimensions of the vessel you are working with

Dear Abdul Al Lafi,

Thank you so much for your answer. Yes, I have to compare the impellers. But I need to know what are the most important comparable features.

Regards.

the important question is which shear rate will be applied, and that is where my questions are leading to

Dear Foroogh Khodadadi

as Bernhard Wessling pointed out, assuming that you are using the same concentration, solvent, rpm and size of the vessel, then the type and value of shear rate is the important. in other simple words, the effect of stirrer on the dispersion of the polymer particle in the solvent and the collision of these particle with the wall of the vessel as well as the speed of that collision, all would have effects. ofcaurce, I presume that the dissolution or stirring was carried out for the same time at the same temperature and the solvent did not evaporate and you cover your vessel.

hope that would help

kind regards

Dear Dr. Bernhard Wessling ,

Thank you for your attention.

Polymer= PVDF (15-20 %)

Solvent= DMAc & DMF (72-78%)

Mixing speed= 90- 600 rpm

Mixing speed at the first seconds is around 550-600 and then decreases slowly to mix for 24 h in 90 rpm.

Regards

Dear Abdul Al Lafi,

What is the effect of collision of the polymer powder's particles with the wall of the vessel? Could you please clarify this more? Thank you so much for your help and attention. I greatly appreciate it.

Best Regards.

the process start with swelling of the solvent by the polymer particles, stirring will destroy this soft layer of swollen polymer and so the solvent will penetrate more in the particle until eventually dissolution occurs. with the swollen particle hits the wall of the vessel similar thing will happen and so the solvent penetrate more and so on.

Foroogh, what is the viscosity of the various liquids? (you can see, I did not call them "solution", see below next answer)

2 important aspects need to be considered:

a) 15 - 20% is well above the solubility limit of PVDF in DMAc or DMF (did you make a micture or used both separately?) - and because this is the case, you are preparing a dispersion (not a solution)

b) in dispersions, your just swollen polymer particles, surrounded by layers of solvent, will achieve a certain particle size; with different shear rate / shear stress, you will get a different particle size; the smaller the particle size, the higher the viscosity; so I would assume that where you see a higher viscosity, will will have had a higher shear rate.

you should check the particle size in a suitable Laser Doppler or ligh scattering apparatus.

Dear Abdul Al Lafi,

Thank you so much! I did not see the relation between the walls and dissolving process in any place. That was wonderful !

Kind Regards

Abdul, the particles will not "hit the wall", because they are surrounded by adsorbed solvent molecules which are surrounded by free solvent molecules (and those form an adsorbed layer, a film on the wall surface).

But what happens is that between the stirrer and the wall, there is a tight space, and in this space the shear rate is the highest, one has high turbulence, and that's where the dispersion takes place.

Dear Bernhard Wessling,

Your knowledge and guidance completely impressed me!

I use the solvents separately.

Why did not you call them solution? If the mentioned process is for dispersion, so what is the definition of dissolving process in your field?

I know that Anchor impeller has a high shear rate, the Radial flow turbine offers a medium shear rate and 3-bladed propeller have low shear rate. But I do not know what is the exact shear rate which every one offers.

Foroogh, you have 2 question, here answer to no 1:

A solution is a system, in which the material you want to dissolve is *completely* surrounded by solvent molecules, if a polymer is to be dissolved, every monomer unit must be "solvated" = surrounded by solvent molecules.

For high molecular weight polymers, this is usually the case only at concentrations around 1%. (for PVDF, I do not know the concentration for a "saturated solution", nor at which concentration it starts to become "over-saturated" which then is a dispersion).

Anything above that concentration is not a true solution any more, but a colloidal system, a dispersion which behave completely different compared to true solutions. *dispersions* are thermodynamically spoken non-equilibrium systems.

(2)

Many thanks Bernhard Wessling for your clear explanation. yes, I do agree with you.

(2) to your 2nd question:

The stirrer shape is one aspect, the even more important aspect is how large and how tight is the gap between stirrer surface and vessel wall, and then the rpm. That will determine which shear rate you get.

Foroogh, what I said does not mean one should not prepare 20% PVDF in DMF or so, but what I mean is, that *because* such systems are not true solutions, their behaviour is critically dependant upon their preparation *history*, here: which shear rate, time, temperature etc.

For true solutions, all that doesn't matter, whatever way you produce a true solution, they will always behave the same, will always have the same properties, if you compare the same concentration.

Dear Dr. Bernhard Wessling,

That is a deep wisdom! I think I got the difference between true solutions and pseudo-true solutions!

I did not know how much the solution quality can be affected by these factors! Could you please clarify more this sentence: " how large and how tight is the gap between stirrer surface and vessel wall, and then the rpm. That will determine which shear rate you get"? Or introduce a reference which offers: HOW these factors (the gap between stirrer surface and vessel wall & the rpm) can determine the shear rate?

I do not need to know mathematically and so much deep, but I need to explain a little about them in my report.

I am far curious about it.

Regards

Dear Dr. Bernhard Wessling,

The molecular weight of PVDF (SOLEF 1015) is 537000 Da and in the most solutions the PVDF concentration is 18% and the solvent is DMAc (Mw= 87.12 g/mol). The temperature is 60-70 C. The mixing time is 24 h to be completely sure about homogeneity. During adding polymer powder the mixing speed is high around 450-600 to avoid gel formation and then it decreases.

Can I introduce "dispersion" in this system because some of the features make me sure about their non-equilibrium property.

Is it important to consider Reynolds number during mixing the polymer solution?

for the calculation of shear rate, you can look into textbooks (you need to check the geometry of your devices); for more basic literature, I would like you to look at this (apart from other papers in my RG pages):

Article Critical Shear Rate - the Instability Reason for the Creatio...

I also would like to refer to this paper:

Article On turbulence in dilatant dispersions