Control of Gas Phase and Surface Processes in Plasma Polymerization

Plasma polymerization covers several interesting fields from highly functional plasma polymers to hard coatings. While the film growth for hard coatings are mainly determined by ion bombardment yielding densification, rather mild plasma conditions are typically used for the retention of functional groups within less crosslinked plasma polymers. Certain applications, however, require functional plasma polymers showing a high stability. Hence, both the gas phase and the surface processes have to be controlled during the plasma polymerization process. We could demonstrate that the energy invested per particle within the plasma (activation of the chemical reaction pathway) as well as the energy dissipated during film growth (crosslinking) can be used as reaction parameters. While the former scales with power input per unit of gas flow W/F, the latter is governed by the energy density, i.e. ion flux times ion energy per deposition rate, enabling the control by macroscopic parameters. Siloxane (HMDSO) as well as hydrocarbon-based plasmas (CO$_{2}$/C$_{2}$H$_{4})$ were used to optimize the functionality of hydrophobic and hydrophilic plasma polymers revealing that plasma chemical gas phase processes should prevail over surface processes, since strong ion bombardment yields a loss in functionality.