I want to make a thin film from PMMA in CHCl3 solution with a 200 mg/ml concentration. The PMMA dissolved well in CHCl3 and it was optically clear and transparent, but the thin film was opaque. I use spin coating with 3000 rpm for 30 seconds. Moreover, the last droplet, which remains in the tip, becomes milky and not transparent. Does anyone know what is the reason, and how I can get the transparent thin film? I prefer to get the transparent thin film by Chloroform and no other solvent.