Processing
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. 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.
Processed materials are typically in the non-piezoelectric alpha phase. The material must either be stretched or annealed to obtain the piezoelectric beta phase. The exception to this is for PVDF thin films (thickness in the order of micrometres). Residual stresses between thin films and the substrates on which they are processed are great enough to cause the beta phase to form.
In order to obtain a piezoelectric response, the material must first be poled in a large electric field. Poling of the material typically requires an external field of above 30 MV/m. Thick films (typically >100 µm) must be heated during the poling process in order to achieve a large piezoelectric response. Thick films are usually heated to 70–100 °C during the poling process.
A quantitative defluorination process was described by mechanochemistry, for safe eco-friendly PVDF waste processing.
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