Insights in the initiation and termination of poly(alkyl acrylates) synthesized by atmospheric pressure plasma‐initiated chemical vapor deposition (AP‐PiCVD)

Abstract

A comprehensive mass-spectrometry study of a set of poly(alkyl acrylate) layers synthesized by atmospheric pressure plasma-initiated chemical vapor deposition (AP-PiCVD) is provided. High-resolution mass spectrometry investigations demonstrate that exposure of the alkyl acrylate monomers to ultra-short and lowfrequency plasma pulses produces a defined number of radical and neutral fragments, which can play both the roles of polymerization initiation or termination groups. Further inquiries illustrate the competition between a conventional free-radical polymerization pathway and plasma-polymerization. On the basis of the massspectrometry observations and the bond dissociation energies calculated by density functional theory, guidelines are made to select appropriate AP-PiCVD monomers. K E Y W O R D Satmospheric-plasma CVD, conventional polymerization, high-resolution mass-spectrometry, nanopulsed discharge, polymerization mechanisms 1 | INTRODUCTION In many synthesis reactions, plasma provides a convenient alternative to thermal heating [1] or chemical reactants. [2] Noticeably, plasma-enhanced chemical vapor deposition (PECVD) processes can lead to the simultaneous lowtemperature synthesis and deposition of crystalline metal oxide thin films on polymer substrates [3] or ensure the "polymerization" of chemically non-polymerizable precursors. [4,5] The PECVD of many inorganic and organic materials has already been reported and the fine-tuning of the PECVD parameters, e.g., the composition of the plasma gas and the plasma excitation mode, can trigger a wide range of reactions, including the reduction of metal salts, [6] the oxidative polymerization of aromatic compounds, [2] and the free-radical polymerization of vinylene monomers. [7] Nevertheless, due to the intrinsic nature of plasmas, consisting of many reactive species with a wide energy range, a nonnegligible number of side reactions occurs. [2] As a consequence to the non-specificity of plasma-induced reactions and the sensitivity of the organic bonds, the chemical structure of monomers is only partially retained and the resulting PECVD This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

Pulsed plasma initiated chemical vapor deposition (PiCVD) of polymer layers − A kinetic model for the description of gas phase to surface interactions in pulsed plasma discharges

“…The present kinetic study relies on previously reported data highlighting the different behaviors of two series of methacrylate‐based thin films deposited by AP‐PiCVD . Notably, the deposition rate as a function of the plasma discharge frequency displays maxima that are dependent on the investigated monomer (i.e., MMA or GMA) (Figure a).…”

“…More particularly, in spite of a similar deposition rates, the GMA‐based thin films elaborated at 100 and 10 000 Hz exhibit smooth and highly granular morphologies, respectively . High‐resolution mass spectrometry (HRMS) analyses allowed for a global description of the growth mechanisms while employing ultra‐short plasma pulses, elucidating the free‐radical polymerization initiation and the numerous side‐mechanims that eventually become predominant for very high frequencies (Figure d) . The competition between these growth mechanisms also correlated with the strong disparity in average molecular weights between MMA and GMA‐based thin film (Figure b) .…”

“…Summary of previous experimental data for MMA and GMA‐based thin films grown by AP‐PiCVD on which the present kinetic study relies on, namely (a) the deposition rate according to the discharge frequency, (b) the size distribution by size exclusion chromatography (SEC) of thin films grown at 100 Hz, (c) the surface topography by scanning electron microscopy (SEM) of the thin films grown at 100 and 10 000 Hz, and (d) the matrix‐assisted laser desorption/ionization high‐resolution mass spectra (MALDI‐HRMS) of the thin films grown at 100 Hz…”

“…The combination of long plasma off‐time (i.e., up to hundreds of milliseconds) together with ultra‐short (i.e., tens to hundreds of nanoseconds) plasma pulses are known to efficiently initiate the free‐radical polymerization of vinyl monomers while greatly reducing the detrimental plasma fragmentation . High‐resolution mass spectrometry (HRMS) evidenced that the ultra‐short plasma pulses cleave a small number of the vinyl monomer units into defined radical and neutral fragments that are able to either initiate or terminate the polymerization . Subsequently, part of the formed radicals and the vinyl monomers diffuse and adsorb from the vapor phase onto the surface where the free‐radical polymerization occurs .…”

“…The chemical and physical properties of the thin films grown using ultra‐short plasma pulses result from a competition between conventional polymerization reactions (initiation, propagation, and termination) and plasma‐polymerization reactions (fragmentation and recombination) . Such competition is notably expressed by the switch from oxygen integration at low plasma discharges frequencies to the oxygen fragmentation and carbon integration when the frequency of plasma pulses increases . This transition is accompanied by variations in the morphology of the produced thin films, as a result of a shift from predominant gas phase mechanisms to predominant surface mechanisms .…”