Effect of Br gassing after Ar plasma treatment of polyolefins

Abstract

For simulation and acceleration of artificial polymer ageing, polyolefin foils were exposed to low-pressure Ar plasma. Plasma particle bombardment and irradiation induce C-C and C-H bond scissions by σ → σ ⁄ excitations on the surface and in near-surface layers. Consequently, radicals are generated. They react by recombination, cross-linking, metastable trapping of the radical site or formation of olefinic double bonds. The long-living and metastable trapped C-radicals as well as double bonds in polyolefins were immediately exposed to bromine vapour without breaking the vacuum after switching-off the plasma. These reactive sites rapidly react with the molecular bromine under formation of C-Br bonds. For 5 min of argon plasma exposure, the elemental concentration of bromine was 13% for polyethylene and 22% Br/C for polypropylene as analysed by X-ray photoelectron spectroscopy. Nevertheless, not all C radical sites have reacted with bromine. Later on, when the polyolefins brought in contact with ambient air, an additional post-plasma reaction of the remaining trapped radicals with oxygen was observed. The oxygen concentrations were lower after bromine gassing, thus repressing partially the post-plasma oxidation in the analysed layer (ca. 6 nm) by radical quenching. Such bromination took place either at the surface or in near-surface layers because the Attenuated Total Reflectance (ATR)-FTIR spectra (sampling depth ca. 2500 nm) did not show significant changes for argon plasma-treated PE foils with and without bromine vapour exposure. Further addition of bromine may also occur on C=C double bonds.Keywords: ageing of polyolefins; argon plasma; radical formation; formation of double bonds 1. Introduction Low-temperature plasmas have a wide range of applications like plasma polymerization, surface modification, microbial inactivation, etc. They contain many reactive species such as excited electrons, atoms, ions, etc. Such plasma is well suited to modify chemically inert polyolefin surfaces. The UV irradiation with wavelength shorter than 200 nm can attack the covalent bonds in the polymer. In the case of argon plasma, absorption lines occur at wavelengths of 105 and 107 nm for singly ionized argon and 92 and 93 nm for doubly ionized argon corresponding to energies of 12 and 15 eV.[1] These energies exceed considerably the dissociation energies of all covalent bonds in polyolefins.[2] The scissions of CÀC ($3.5 eV) and CÀH (3.5-4.0 eV) bonds in polyolefins by σ → σ ⁄ transitions are induced by such plasma vacuum UV radiation. Due to the semi-crystalline nature and the stretch-induced orientation of polyolefin foils, the radicals can be formed preferably in the amorphous regions or at the

Free-Radical-Induced Grafting from Plasma Polymer Surfaces

“…These radicals, arising from plasma–surface interactions, can be advantageously exploited for the controlled grafting of a specific chemical group or molecule. ,− Preliminary knowledge of the surface free-radical density, as discussed in the preceding section, can serve as a guide to achieving higher grafting efficiencies. To preserve the maximum number of surface free radicals from postoxidation reactions, the functionalization step, similarly to radical labeling, should preferably take place under a vacuum without air exposure.…”

Green Chemistry Applied to Ground Coffee Volatile Compounds Modification Aiming Coffee Aroma Improvement

“…Photon irradiation and reacting plasma species bombardment can lead to C-C bond scissions that abstract CH 2 from terminal bonds generating metastable C-radicals. This carbon radical can further on react with furfural forming 2-furylmethyl ketone [32]. Part of the reactions observed in the powdered coffee subjected to cold plasma was also observed in the treatment of ready-to-drink coffee subjected to plasma processing [33].…”

Spectroscopic Analysis of Plasma Modified Polymer Surfaces

“…This derivatization is possible via a side reaction by H substitution; however, there should be no nitrogen-containing stabilizers on the plasma-modified surface. A similar reaction between Br 2 and argon plasma treated PP and PE samples was used for quantification of radicals and/or C=C double bonds formed after plasma impact by applying XPS and ATR-FTIR [71]. It was demonstrated that bromine reacts mainly with the radicals which are present in the majority on the plasma-treated top surface layers.…”