Study of nanopulsed discharges for plasma-polymerization: experimental characterization and theoretical understanding of growth mechanisms in the deposition of functional polymer thin films

Plasma processes are highly versatile methods able to promote the formation of thin films from a vast variety of compounds thanks to the numerous energetic species they generate. Notably, plasma-enhanced chemical vapor deposition (PECVD) processes have already led to the simultaneous synthesis and deposition of a wide variety of organic functional materials. Reciprocally, the many reactive species composing the plasma induce a non-negligible number of side reactions, yielding altered chemistry compared to conventional polymerization processes. Therefore, there is a strong need for controlled methods promoting conventional polymerization pathway in plasma-based processes to increase their range of application.
To this end, the atmospheric-pressure plasma-initiated chemical vapor deposition (AP-PiCVD) process was developed, differing from classical PECVD by its initiation source. AP-PiCVD relies on square-wave nanopulsed discharges with single ultra-short plasma on times (t_on ≈ 100 ns) and long plasma off-times (t_off = 0.1 – 100 ms) rather than alternative current sources yielding long and repeated discharges (t_on = 10 µs).
Methacrylate monomers initiated by nanopulsed discharges highlighted a shift of growth mechanisms from classical PECVD pathways (plasma-polymerization) to conventional free-radical polymerization. Notably, the growth of polymer layers with an extremely high retention of the monomer’s chemistry and unprecedented molecular weights for an atmospheric-pressure plasma process is demonstrated at long plasma off-times. Moreover, a transition from gas phase to surface growth mechanisms is observed, allowing the deposition of conformal coatings similarly to low-pressure alternative chemical vapor deposition processes.
A thorough investigation of the thin films’ chemistry confirms the conventional nature of the layers grown by AP-PiCVD and shed light upon the growth mechanisms in low-frequency nanosecond pulsed plasmas. While the on-time induces the formation of free radicals from the monomers’ fragmentation which are able to initiate and terminate the chain addition process, conventional polymerization mechanisms are strongly promoted during the off-time yielding linear polymer core. Interestingly, new insights on selective initiation mechanisms based on sacrificial functions for an enhanced control of the molecular fragmentation are put forward and discussed.
The development of a model describing the kinetic of pulsed plasma was carried out and correlated with experimental observations, providing deeper understanding of the interaction between the gas phase and the surface. The extraction of important physical parameters for the description of the growth kinetics in AP-PiCVD is demonstrated and their significance discussed.
Using the fundamental knowledge developed on nanosecond pulsed plasma-initiated polymerization mechanisms, thermoresponsive copolymer layers grown by AP-PiCVD are reported for the first time. The properties of the layers are evaluated and related to their chemistry, allowing the determination of an optimal co-monomers ratio to integrate both of their individual properties as a unique functional thin film.