Vacuum‐ultraviolet (VUV) Photo‐polymerization of Amine‐rich Thin Films from Ammonia–Hydrocarbon Gas Mixtures

Abstract

Photo‐induced polymerization of hydrocarbon “monomers”, both unsaturated C2H4 and saturated CH4, has been carried out by vacuum‐ultraviolet (VUV) irradiation of the flowing gases at reduced pressure, employing near‐monochromatic radiation from Kr and Xe lamps. Using mixtures with NH3, the source of bound N in the coatings, similar concentrations, [N], can be achieved in both UV‐PE(M):N and low‐pressure plasma polymers, L‐PPE:N, but the former are much richer in primary amines, with selectivity values ([NH2]/[N]) up to 75%. UV‐PE:N and UV‐PM:N films prepared with gas mixture ratios, R, between 0.75 and 1.0 possess the main characteristics required for bio‐technological applications: (i) NH2‐rich, (ii) low loss of [NH2] upon exposure to atmosphere or water, and (iii) very low solubility in aggressive solvents, e.g., water. The VUV photo‐polymerization process, governed by reactions of free radicals from the gas‐phase, appears to result in more stable, more densely cross‐linked deposits than those from plasma‐assisted processes.

Fabrication, Characterization, and Comparison of Oxygen-Rich Organic Films Deposited by Plasma- and Vacuum-Ultraviolet (VUV) Photo-Polymerization

“…Therefore, one would a priori expect quite different outcomes when comparing deposition experiments based on VUV‐ and plasma excitation, under otherwise nominally identical conditions. This has indeed been confirmed in the case of NH 3 /C 2 H 4 or CH 4 reagent gas mixtures …”

“…In Figure , we show plots of deposition rates, r (in nm · min −1 ), versus the gas mixture ratios, R , for the various types of deposits, namely the plasma polymers (PPE:O) and the O‐containing films obtained with the two VUV lamps (UV‐PE:O). The principal features that emerge from Figure are the following: For all of these O‐containing films, plasma‐ and VUV‐activated polymerizations are seen to have led to quite similar r values, a somewhat surprising result; however, we had already noted this earlier, and will discuss it in some detail further below. While r values fall within a broad band (35 ≤ r ≤ 20 nm · min −1 ) and vary little in the range 0 ≤ R ≤ 1 for the case of N 2 O/C 2 H 4 mixtures (except for a slight decrease in the case of PPE:O films, see Figure c), those relating to C 2 H 4 mixtures with CO 2 and O 2 decrease monotonically, the latter rather steeply with rising R ( R ≤ 0.3). This is fully to be expected on account of the rising concentration of O atoms and concomitant reactions producing film‐forming and non‐film‐forming species occurring both at the surface (deposition and etching) and in the plasma phase, with any reaction involving an O atom having a higher probability of forming non‐film‐forming species. …”

“…In spite of the two lamps' different wavelengths and photon fluxes, Φ (the latter being roughly twofold greater for the XeL lamp), corresponding r values did not differ very significantly: Those for the XeL lamp were found to be roughly 20–30% higher, but plots of r / Φ suggest that the KrL lamp was significantly more efficient; the reader is referred to Truica‐Marasescu et al for further details, at least regarding amine‐containing deposits. There and elsewhere, we had also found plots of r with rising R to decrease monotonically for the case of C 2 H 4 NH 3 mixtures ( R ≤ 1) and to nearly overlap for the same three cases (plasma, and the two VUV lamps). In those instances, the steep (fivefold) decrease in r may largely have been attributed to etching by H atoms…”

“…In an earlier article, we had reported co‐polymerizations of binary C 2 H 4 mixtures with NH 3 or N 2 O by comparing the results of plasma‐ and VUV‐initiated reactions. In a later report we compared plasma‐ and VUV‐coatings deposited from mixtures of NH 3 with C 2 H 4 or CH 4 and concluded that VUV photo‐polymerization appears to result in more stable, more densely cross‐linked deposits than those from plasma assisted processes. The purpose of the present research has been to extend those investigations to purely O‐containing organic thin film materials, by comparing photo‐ and plasma‐assisted polymerization of C 2 H 4 with various source molecules of oxygen, namely nitrous oxide, N 2 O, carbon dioxide, CO 2 , and oxygen diluted in argon.…”

Reptation Aggregation of Liquid Silicon Oils Modified by Ar Plasmas

“…The reptation aggregation of silicone oils modified by Ar plasmas is possibly due to the fragmentation of long SiO chains and the subsequent formation of a crosslinked surface layer. Many investigations on polymers exposed to plasmas show that plasmas produced in inert, reactive, or reactive diluted in inert gases can lead to the formation of crosslinked surface layers on polymers, and to the polymerization of monomers; this is due to the energetic ions bombardment along with the vacuum‐ultraviolet (VUV) irradiation. In low temperature plasmas, the energetic ions bombardment can lead to the bonds fragmentation in polymers.…”