Dear Nguyen,

It is a high probability of NMP oxidation at 140 C, and this can provide a color change to brown. You can try to dissolve PVDF powder at moderate high temperature, 40-60 deg C during constant mixing. Please use magnetic stirrrer or impeller. Good trick can be also to use hermetically closed heated vessel for avoid an oxidation. Any way a dissolution need pretty long time  also depend from PVDF modification. Please finally try to get a film from your solution into glass surface for checking of it properties.

you can dissolve this polymer in DMF  and evaporate at 80 C

I try to make 3 samples at same amount.

1. Stir at room temperature

2. Stir at high temperature.

3. Not stir at room temperature.

And, 3 samples are different in color.

The first is light yellow, the second is brown yellow, and the third is dark yellow.What's the problem with them?

Interaction between PVDF and NMP is not simple dissolution, it can be also some non-predictable chemical reactions or co-polymere formations. If it is critical point in your research please use IR spectra for checking components. But if not, just made a films formation on the CU or Al foil surface of all 3 samples and choose which have a best adhesion.

Yes I also have similar observation. When PVDF is dissolved in the solvents of DMF, DMF/acetone at elevated temperature, the solution became yellow. In addition, even some solution was transparent after completely dissolved PVDF with carefully controlled temperature, the solution gradually change to yellow after stored for a few weeks or months. Copolymer of P(VDF/TrFE) also have same problem.

One colleague observed solution of P(VDF/TrFE) did not change to yellow when stored in N2 glove box. Does this give some clue? Anybody can give an answer? Thank you very much.

I have Prepared 6% PVDF-DMF solution with avg 85 degree magnetic stirring but I am not able to get clear solution...it is slight yellow ...What is the possible cause? May I use that solution for progress.

I always prepare PVDF/NMP solution in oil/water bath at about 70 C. and most of the time easily get the transparent solution. The one thing you can try is to dry the PVDF overnight before making the solution.

I hope it will give good results.

Definitely you have been observing dehydrofluorination of PVDF units under action of nucleophilic impurities presenting or appearing when heating in your solvent (DMF of NMP). The both solvents facilitate nucleophylic reactions like this one. To avoid this problem you need to carefully clean and what is very important also to dry the solvent and solution of PVDF. One more key thing. Do not be too hurry to accelerate dissolution of PVDF. The point is that typically a polymer needs some time to swell more and more up to complete dissolution. The dissolution rate depends on the polymer molecular mass, state (powder, granules, film, etc.) and the solvent nature, temperature, stirring.

You can find some aspects of the similar dehydrofluorination of PVDF and VDF copolymers in my old paper Article Electrochemical resistance and degradation of Fluorinated Polyolefines

I also do with PVDF and meet the same situation, the reason could be that your material as PVDF/NMP were impure, so when you dissolve the PVDF in NMP, the color of solution will become yellow.