I'm trying to make smooth PVDF film using solvent casting dried under vacuum from 20 mL of 15% PVDF in DMF solution. The vacuum dryer temperature was at 55 deg C. After some time, this is what I got (refer to the image attached):
The film appears to have shrunk and turned out to be opaque. I was expecting a smooth transparent film since I had it dried under vacuum at 55 deg C (and because DMF is hygroscopic). I've checked out one of the working conditions for spin coating done by Li et al. (http://pubs.rsc.org/en/content/articlepdf/2013/TC/C3TC31774A) and tried it in my case.
Where have I gone wrong? Thanks in advance for any insight.
This happens when the PVDF forms regions of different refractive index - either crystallites or voids - that then scatter light. For a transparent film it must be all amorphous or with very small crystallites that won't scatter visible light, and no voids.
Try lowering the temperature - this may prevent initiation of crystallites or boil off of the solvent (making voids). We have had success with non-vacuum drying at similar temperatures - the vacuum may be causing voids to form as DMF bubbles in the film, while drying the film withou vacuum will evaporate without forming bubbles.
The right conditions will make a transparent film from the PVDF.
I prepared PVDF with thicknesses from 100 to 600 microns. I dissolved PVDF in DMSO and dried it at lower temperatures (30-40 C) in non-vacuum conditions. Moreover, Keeping the films in coagulation bath for a longer time may allow the solvent to properly leave the films.
Hello Syed, did you obtain transparent films in your case? Also, what was your solvent for the coagulation bath? Thanks!
Even very thin films of PVDF may be opaque if prepared in humid conditions. DMSO (if not dry) contains water which causes crystallization of PVDF. We have obtained clear films in glovebox, i.e. in humidity free atmosphere.
Look at this:
Article Controlling the microstructure of poly(vinylidene-fluoride) ...
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