Energetics of Molecular Excitation, Fragmentation, and Polymerization in a Dielectric Barrier Discharge with Argon Carrier Gas Sean Watson, † Bernard Nisol, † Sophie Lerouge, ‡ and Michael Robert Wertheimer* ,† † Groupe des Couches Minces (GCM) and Department of Engineering Physics, Polytechnique Montré al, Box 6079, Station Centre-Ville, Montreal, Quebec H3C 3A7, Canada ‡ Research Centre, Centre Hospitalier de l’Universite ́ de Montré al (CRCHUM), and Department of Mechanical Engineering, E ́ cole de technologie supe ́ rieure (E ́ TS), Montre ́ al, Quebec H3C 1K3, Canada ABSTRACT: We report experiments at atmospheric pressure (AP) using a dielectric barrier discharge (DBD) reactor designed for plasma polymerization (PP) with “monomers” at ‰ concentrations in ca.10 standard liters per minute of argon (Ar) carrier gas. We have perfected a method for measuring E g , the energy dissipated per cycle of the applied a.c. high voltage, V a ( f), but the focus here is on ΔE g , the energy difference with and without a flow, F d , of monomer in the Ar flow, with the plasma being sustained at V a ( f) = 2.8 kV rms , f = 20 kHz. From ΔE g and F d , we derive a characteristic energy per molecule, E m (in eV), and investigate plots of E m versus F d and 1/F d for three model “monomers”: formic, acetic, and acrylic acid. These data, along with those for lighter or heavier organic compounds, reveal novel information about energy absorption from the plasma and ensuing polymerization reactions. 1. INTRODUCTION Plasma processing science and technology (PPST) has grown enormously since its humble beginnings in the 1950s and 1960s. A leading subfield of PPST is plasma-enhanced chemical vapor deposition (PECVD), the fabrication of thin film deposits on solid substrates near ambient temperature. 1 In cases where the precursor gas or vapor is an organic compound, one speaks of plasma polymerization; the resulting plasma- polymer (PP) deposits have certain features in common with conventional polymeric solids, but they are fundamentally different in most respects: Their structure is amorphous, highly cross-linked, and their composition depends intimately on the particular set of selected plasma-deposition parameters. 1,2 Major advantages of PP coatings are to enable surface modification of any solid substrate without affecting its bulk properties, and to tailor surface chemistry and -energy by creating polar chemical moieties (e.g., amines, 3 carboxylic or other functional groups 4,5 ) that can govern solid-liquid interfacial interactions. This can help immobilize biomolecules or living cells 5 or, on the other extreme, create antifouling surfaces that inhibit adhesion of proteins and harmful cells like bacteria. 6-9 Of course, there exist numerous other applications for PP deposits, but we shall restrict this discussion to biomedical science and technology. For example, PP-poly- ethylene glycol (PEG-like) coatings are known for their powerful antifouling characteristics, attributed to strong interaction with water molecules. However, retention of the responsible ether groups requires near-perfect control of plasma chemistry, generally achieved by using mild (low-power) activation of the precursor molecules. 6-9 This is merely one example, where strict control of fabrication conditions dramatically affects successful outcome. Like most other PPST procedures, PECVD and PP were almost exclusively carried out under partial vacuum during the first few decades, but this has changed in the past 20 years. Atmospheric-pressure (AP) plasma processing has gained much interest because the absence of a costly vacuum installation promises easier, more economical implementation. 10 Dielectric barrier discharges (DBDs) constitute the main approach that enables scale-up for industrial processing; 11 DBD plasmas may be obtained in gaps between two electrode surfaces, at least one of which is covered by a dielectric. They, too, are nonequilibrium (cold) plasmas, useful in numerous plasma- chemical reactions beside PECVD and PP, such as ozone synthesis, surface modification of polymers, abatement of pollutants, excimer lamps, and others. 11 Even though the primary motivation of this work is related to PP, the emphasis of this current communication will be shown to concentrate on gas phase reactions and energetics. The PP literature has long been interested in correlating deposition kinetics and the physicochemical and structural properties of films with energy absorbed by the organic precursor (or monomer) molecules in the plasma. In the work of Hegemann and co-workers, 12-14 who developed an original approach toward the macroscopic phenomenology of PP, the parameter W/F (W being power input, and F the monomer Received: July 30, 2015 Revised: September 4, 2015 Letter pubs.acs.org/Langmuir © XXXX American Chemical Society A DOI: 10.1021/acs.langmuir.5b02794 Langmuir XXXX, XXX, XXX-XXX Downloaded by ECOLE POLYTECHNIQUE MONTREAL on September 9, 2015 | http://pubs.acs.org Publication Date (Web): September 9, 2015 | doi: 10.1021/acs.langmuir.5b02794