Vacuum-ultraviolet induced oxidation of the polymers polyethylene and polypropylene

The emission from low-pressure microwave plasmas in the vacuum-ultraviolet (VUV) region (λ < 200 nm) was investigated in order to use these plasmas as light sources for the study of the VUV photochemistry of polyethylene (PE) and polypropylene (PP) as part of the study of plasma-polymer interaction. These polymers, immersed in low-presure oxygen, were exposed to radiation with wavelengths down to 112 nm, the cut off of magnesium fluoride used as a window to separate the polymer specimen from the plasma light source. Total oxygen incorporation in the surface [O], and the formation of hydroxyl, carbonyl, and carboxyl groups were measured using XPS in combination with chemical derivatizations, particularly their dependence upon the radiation spectrum and the oxygen pressure around the sample. In most experiments the surface oxygen concentration [O] attained a constant value that appears to be related to the initial oxidation rate; this suggests a competition between oxygen incorporation and chain scission reactions, followed by the removal of volatile oxidation products. PE is usually oxidized to a higher level than PP, the latter appearing to be more susceptible to reaction with atomic oxygen than PE. A general initiation mechanism for the VUV experiments is proposed that allows us to explain the observed differences in behavior between PE and PP, and the results obtained under different irradiation conditions. The nature of oxidation products is in both cases very similar to what is observed after direct plasma treatment of the polymers. We conclude that short wavelength radiation contributes very appreciably to the observed surface modification effects during plasma treatment of PE and PP. © 1995 John Wiley & Sons, Inc.     

Vacuum Ultraviolet Irradiation of Polymers

The interest in incoherent sources for wavelength-selective photochemistry has increased lately, but little is still known about the behavior of polymers when exposed to far UV and vacuum UV (VUV) radiation. The same dearth of information exists regarding UV (VUV) radiation emitted by low-pressure plasmas during polymer treatment. In order to study VUV-UV effects on several polymers (polyethylene - PE, polystyrene - PS, hexatriacontane - HTC, and poly(methyl methacrylate) - PMMA), we have used the well-characterized emissions from hydrogen (broad-band emission) and hydrogen/argon mixture (near-monochromatic radiation) plasmas as light sources. During irradiation, samples were kept under vacuum or in a flow of pure oxygen at low pressure; in both cases the radiation fluxes at the sample position have been precisely determined by careful spectroscopic calibration experiments. We have employed a quartz crystal microbalance (QCM) to measure in-situ any possible mass change of the various polymers. Following irradiation, samples were analysed by ellipsometry (for thickness and refractive index), X-ray photoelectron spectroscopy (XPS, to evaluate the near-surface composition and content of various functional groups), and atomic force microscopy (AFM, for surface topography and roughness measurements).     

Mechanism of adhesion between protein-based hydrogels and plasma treated polypropylene backing

We studied the mechanism of adhesion between N₂ plasma treated polypropylene (PP/N₂) backing and a hybrid hydrogel (HG) produced by chemical crosslinking between poly(ethylene glycol) and soy albumin. The work of adhesion, measured by peel testing, was found to be 25 times higher for PP/N₂ compared to untreated PP (≈5.0 J/m² versus ≈0.2 J/m²). In order to understand the adhesion mechanism, we performed a detailed analysis of the surface chemical composition of PP and PP/N₂ using X-ray photoelectron spectroscopy (XPS), chemical derivatization and attenuated total reflectance infra-red (ATR-IR) measurements. The results confirm incorporation of different nitrogen- (amine, amide,…) and oxygen- (hydroxyl, carboxyl,…) containing chemical groups on the PP/N₂ surface. The derivatized functions were primary amine, hydroxyl, carboxyl and carbonyl groups. Chemical derivatization reactions validated the XPS results (except for carbonyl groups), and they clearly underlined the essential role of primary amine groups in the adhesion process. In fact, after derivatization of the amine functions, the work of adhesion was found to be 0.41 ± 0.12 J/m². Participation of amine groups in the formation of covalent bonds at the interface between PP/N₂ and HG was directly confirmed by ATR-IR measurements.     

The influence of vacuum-ultraviolet radiation on poly(ethylene terephthalate)

Poly(ethylene terephthalate) was exposed to radiation from different kinds of low-pressure plasmas in an oxygen atmosphere. The lower wavelength limit of the spectrum investigated, λ = 112 nm, is the cut-off of magnesium fluoride used for separating the specimen chamber from the plasma light source. The total surface oxygen concentration, and the formation of hydroxyl, carbonyl, and carboxyl groups were evaluated from XPS measurements in combination with chemical derivatizations, and their dependences on the radiation spectrum and the oxygen pressure in the sample chamber have been investigated. © 1996 John Wiley & Sons, Inc.     

Modification of Active Carbon by Hydrophobic Plasma Polymers: II Fabric Substrates

Active carbon, used in various types of chemical protection devices, can absorb an extremely wide range of hazardous compounds. Unfortunately, this universal absorption includes ubiquitous water molecules, which can result in saturation and premature loss of efficacy. The purpose of this work has been to treat active carbon-impregnated fabric with appropriate low-pressure plasmas, so as to render the carbon surface hydrophobic, while minimally affecting its ability to absorb toxic gases or vapors (simultated here by CCl₄). The best results have been achieved using plasma polymerization of organosilicon (PP-HMDSO) thin films onto the fabric surface: This has been done in a pilot-scale microwave plasma reactor system, designed for treating continuously-moving flexible web materials up to 30 cm in width. Under optimal treatment conditions, a plasma exposure duration of 10 s (PP-HMDSO film thickness, d 40 nm) is found to be sufficient to reduce water absorption by 85%, while the corresponding reduction in CCl₄absorption is small (<20%). The treatment demonstrates long-term stability, and it holds promise for commercial implementation on existing roll coaters.