I have a rigid polymer which needs to be freeze milled and sieved through 100 mesh. I added the liquid nitrogen inside the stainless steel vial of the polymer containing polymer and milled. After milling, I dried the polymer and made a thin film out of it. The thin film is opaque.The polymer itself was transparent before freeze milling. Does anybody has any idea whats going on?
Thanks
Hello, Sanjiban.
If you are working with a semi-cristalline polymer, your film might crystallize, turning it opaque. It was transparent before either because the polymer was in amorphous state or because the size of the crystals were too small to diffuse the light. Try to do DSC in your pristine material and in the film to verify the differences in crystallinity.
Best regards,
Joao
Hi Joao
Thanks for the explanation. The polymer is amorphous polyurethane. It is very rigid. So crushed the polymer by freeze milling to make it powder. During freeze milling the polymer got into contact with the liquid nitrogen. That's what created the problem. The film made from the polymer before freeze milling was transparent. However the film made from the freeze -milled polymer was non-transparent. It seems the polymer was not melted completely. For both the films the temperature for making the films was same. I dont understand what happened to the polymer after it came into contact with nitrogen. Has some of its physical properties changed?
Thanks
Sanjiban
Nitrogen is inert; it hardly might have reacted with your polymer. Liquid N2 brings the temperature of polymer lower. Tg of polymers get affected with change in ambient temperature. Think about this relation.
Hello, Sanjiban
When you use liquid nitrogen, you go below the Tg of the material. When warming up to the room temperature, you give back mobility to the chains, which can crystallize if your polymer has a semi-crystalline nature. The pristine amorphous state (transparent) can be related to the processing of the film at the first place, allowing it to rapidly cool down avoiding crystallization or limiting it to a size of crystals in which light is not diffused.
If your polymer is 100% amorphous and the crystallinity cannot change, you might have problems with the porosity of the second films. The size and distribution of the pores can be in a range that it is diffusing too much light.
I suggest that, in both samples, you check crystallinity with a DSC and for the porosity an Optical Microscope might be enough to be sure.
Best regards,
Joao
Hi Sanjiban,
I am late to the question but thought I may add some input.
There shouldn't be a reason why the polymer would change color when cooling it. From experience, if you mill using steel- the steel will become brittle under cryogenic temps and you will polish some of that onto your sample. It will darken from the metal.
This is the reason I use Tungsten Carbide for cryogrinding.
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