Plasma-grafted PVDF polymers as anion exchange membranes for the electrotransport of Cr(VI)

Grafting of aminosiloxane groups on PVDF supports was performed in a low-frequency plasma deposition process using a tetramethylsilane/ammoniac mixture. The morphology and thickness of plasma-grafted PVDF membranes were characterized by SEM observations; their chemical structure was determined by FTIR analysis, XPS analysis, and contact angle measurements. The membranes anion-exchange capacity for the transport of Cr(VI) was investigated by electro-electrodialysis using a three compartments Kel’F cell. We are able to correlate the plasma grafting parameters with the structure and the electrotransport performances of the synthesized membranes. Starting from 0.10 for the virgin PVDF films, the best Cr(VI) transport number (0.155) is obtained for membranes both rich in substituted amine functions and hydrophobic enough synthesized from an equimolar tetramethylsilane/ammoniac plasma monomers mixture.

We have shown that the plasma grafting process using a TMS/NH3 monomers mixture is an efficient technique to synthesize anionexchange membranes for the Cr(VI) electrotransport. Chemical characterizations reveal that, depending on the proportion of NH3 in the precursors mixture, it is possible to adjust the quantity of amine functions in plasma films and their hydrophobic character in order to optimize the transport ability of Cr(VI) species. The postreaction of plasma membranes with CH3Br induces a partial substitution of amine functions leading to a further improvement of the electrotransport performance. Nevertheless, the enhancement provided by the synthesis conditions used in this study remains moderated due to the very low quantity of amines incorporated in membranes and consequently the limited impact of the substitution reaction. Some further experiments are planned to be performed in order to get around these limitations with the aim of reaching a Cr(VI) transport number suitable for the treatment of industrial chromium plating baths.