Sequential surface derivatization of PET films

The present study describes the surface tailoring of a polymeric substrate by a multistep process using two consecutive plasma treatments, followed by derivatization reactions. The chemical concept of this approach is presented in this paper, using a poly(ethylene terephthalate) (PET) substrate. In the first step, tetrafluoroethylene (TFE) was plasma polymerized, generating a highly cross-linked perfluoric surface layer. The next step introduced amine groups into the plasma polymer, by exposing the surface to plasma of ammonia. The reactive amine moieties were then used as anchoring sites for further derivatization. Finally, poly(ethylene glycol) (PEG) chains were grafted onto the surface via a hexamethylene diisocyanate (HDI) spacer. The ESCA spectrum of treated PET revealed that the surface chemistry obtained after plasma polymerizing TFE was one clearly dominated by CF₂ and CF moieties, as demonstrated by the large peaks appearing at 291.1 and 289.5 eV, respectively. As expected, substantial amounts of nitrogen could be seen after exposing the surface to a plasma of ammonia, as revealed by the large N 1s peak at 402.0 eV. ESCA also demonstrated the presence of PEG chains bound to the surface. These findings were confirmed by FTIR spectroscopy and supported by water contact angle measurements. Special attention was given to the absorption bands of the CF groups and ether bonds belonging to the fluorinated plasma polymer and the PEG chains. While the water contact angle of untreated PET was 76°, it increased sharply after the fluorinated layer was created (93°), decreasing drastically (to less than 20°) once the highly hydrophilic PEG chains were grafted on the surface.