Energetics of Molecular Excitation, Fragmentation, and Polymerization in a Dielectric Barrier Discharge with Argon Carrier Gas

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 Eg, the energy dissipated per cycle of the applied a.c. high voltage, Va(f), but the focus here is on ΔEg, the energy difference with and without a flow, Fd, of monomer in the Ar flow, with the plasma being sustained at Va(f) = 2.8 kVrms, f = 20 kHz. From ΔEg and Fd, we derive a characteristic energy per molecule, Em (in eV), and investigate plots of Em versus Fd and 1/Fd 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.

Energetics of Reactions in a Dielectric Barrier Discharge with Argon Carrier Gas: III Esters

“…Inspecting Figure (b), we note two key characteristics: (a) sharply rising E m at low F d , culminating in a peak value ( E m ) max at ( F d ) crit , characteristic for each precursor (see Table ); and (b) a decreasing portion for greater F d , which proves to have a 1/ F d functional dependence. The leading and trailing portions before and after the peak, respectively, correspond to region I (monomer‐lean) and region II (monomer‐rich) . Now, yet a third feature is encountered, particularly noteworthy in the case of the present monomers: (c) a transition region, characterized by a sharp drop at the beginning of region II , identified by “B” on the curve representing AMA (see Figure b).…”

“…For AMA and PgMA, ( E m ) max is particularly high, roughly twice (105.5 eV) or threefold (181.2 eV) greater than the MMA value. We believe that this results from the presence of even heavier reaction products in the gas phase, which also become subject to fragmentation . Interestingly, AMA and PgMA are chemically quite different from (i) to (iii): AMA contains a second CC double bond (allyl), while PgMA has an additional CC triple bond (propargyl).…”

“…We showed that this can lead to a multifold overestimate of W / FM , hence to erroneous conclusions . Accurate determination of energy absorbed by reagent (monomer) molecules was not accomplished until recent work in this laboratory, but the method for determining Δ E g and E m that we have developed now avoids pitfalls (i) and (ii) above . Clearly, E m (energy absorbed from the AP plasma per monomer molecule, in units of eV) is broadly equivalent to the Yasuda/Hegemann parameters; numerical results that we have presented give good reason for confidence, and the method appears to constitute a breakthrough …”

“…In Table , one notes similar ( E m ) max values for MMA, PIB, and n‐PMA (57.3, 59.9, and 67.1 eV, respectively); of course, ( E m ) max refers to the condition in which species traversing the discharge absorb the most energy possible. For these three compounds [(i)–(iii)], this is believed to correspond to near‐total breakage of all constituent covalent bonds . For AMA and PgMA, ( E m ) max is particularly high, roughly twice (105.5 eV) or threefold (181.2 eV) greater than the MMA value.…”

“…Figure (a) shows another plot of E m , this time versus 1/ F d , which reverses regions I and II ; (b) shows a zoom on the near‐linear region closest to the origin (lowest 1/ F d , or highest monomer flow rates). From a simple dimensional analysis, the slope of this near‐linear region has distinct physical meaning, namely power provided by the DBD plasma (in W); an alternative interpretation is the facility of the particular monomer to absorb energy from the Ar plasma's energy “reservoir,” deep into region II . Table lists values for compounds (i) to (v), ca.…”