After adding polyethylene terephthalene (PET) to TFA and DCM in volume ratio of 80:20 and let the PET to dissolve, the colour of the mixture solution based on journal, it should be colourless after 4 hours of stirring, but what if I get a thin tranparent top layer, and the layer below is white (original PET granule colour),does it means the time for dissolving it still not enough?

And seem like the solution mixture is easily evaporated, can i cover the solution mixture to prevent serious evaporation?