Grafting of carboxyl groups using CO2/C2H4/Ar pulsed plasma: Theoretical modeling and XPS derivatization

The grafting of carboxyl groups enhances cell adhesion and can be used for immobilization of different biomolecules onto plasma-treated materials. The process, however, was not well optimized due to lack of clear understanding of the mechanisms of carboxylic group incorporation into plasma and their grafting to polymer surface. In this work the deposition of COOH plasma polymers from CO2/C2H4/Ar pulsed discharge has been studied depending on the gas mixture and duty cycle. We have demonstrated that the CO2/C2H4/Ar plasma with adjustable thickness of COOH functionalized layer and high stability of the grafted functions in water is a better solution for the COOH surface functionalization compared to the thoroughly analyzed CO2 plasma. The concentration of different carbon environments and the density of COOH groups have been measured by using chemical derivatization combined with X-ray photoelectron spectroscopy. It has been found that the CO2/C2H4/Ar plasma mainly contains ester groups (COOC), the COOH/COOC ratio being between 0.03 and 0.08. The water stability of the COOH groups was significantly higher compared to ester environment, so immersing in water for 24 h allowed to increase the COOH/COOC ratio by a factor of 3. The mechanisms of the CO2 molecule attachment to hydrocarbon chains on the polymer surface and those located inside the plasma were modeled using ab initio calculations. (C) 2017 Elsevier B.V. All rights reserved.