PVDF may be synthesized from the gaseous VDF monomer via a free radical (or controlled radical) polymerization process. This may be followed by processes such as melt casting, or processing from a solution (e.g. solution casting, spin coating, and film casting). Langmuir-Blodgett films have also been made. In the case of solution-based processing, typical solvents used include dimethylformamide as well as the more volatile butanone. In aqueous emulsion polymerization, the fluorosurfactant perfluorononanoic acid is used in anion form as a processing aid by solubilizing monomers.[4] For characterization of the molecular weight via gel permeation chromatography (also called size exclusion chromatography), solvents such as dimethyl sulfoxide or tetrahydrofuran may be used.