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- Plasma (2)
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- Bromination (1)
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Response of linear, branched or crosslinked polyethylene structures on the attack of oxygen plasma
(2014)
Linear, branched and crosslinked polyethylenes (PE) were exposed to the low-pressure oxygen plasma for 2–120 s. In the following the samples were washed with solvents to remove low-molecular weight oxidized material and to excavate the subjacent polymer structure for microscopic characterization. X-ray photoelectron spectroscopy (XPS) measurements provided information about changes in elemental composition and chemical structure of PE after plasma exposure and washing. The calculation of the concentration of tertiary C atoms using XPS data was a measure of branches and crosslinking in the polymer before and after exposure to oxygen plasma. Linear PE was most sensitive towards oxygen plasma and showed the highest concentration in tertiary C atoms after plasma exposure. On the other hand branched PE types, which possess originally more tertiary carbon atoms, have lost two-third of them after 2 s oxygen plasma exposure. Branched PE show also topological changes at their surface as detected by atomic force microscopy. Differential scanning calorimetry measurements confirmed strong changes in crystallinity and molecular orientation of linear PE already after 120 s exposure to the oxygen plasma interpreted as amorphization. These effects should be interpreted as result of crosslinking caused by the recombination of dangling bond sites.
Melamine and melamine resins are widely used as fire retardants for polymer materials used in pharmaceutical, plastic, textile, rubber, and construction industry. Melamine-based flame retardants act by blowing off intumescent layers, char formation, and emission of quenching ammonia gas and diluent molecular nitrogen. Special advantages are: low cost, low smoke density and toxicity, low corrosive activity, safe handling, and environmental friendliness. Methylated poly(melamine-co-formaldehyde) (mPMF) was used as thick (≥40 µm) fire-retardant coating for plasma pretreated polymers. A combined low-pressure plasma pretreatment consisting of oxygen plasma exposure followed by deposition of thin poly(allylamine) (ppAAm) and poly(allyl alcohol) (ppAAl) plasma polymers as adhesion promoters have improved the adhesion of thick mPMF coatings strongly. Chemical structure and composition of deposited polymer films were characterized by infrared-attenuated total reflectance and X-ray photoelectron spectroscopy (XPS). After peeling, the peeled layer surfaces were also investigated for identification of the locus of failure and their topography using optical microscopy and XPS. Often the adhesion promotion was so efficient that the peeling of coating was not possible. Thermal properties of plasma polymers and dip-coating films were analyzed by thermogravimetric analysis. Significant improvement of fire-retardant properties of coated polymers was confirmed by flame tests.
Plasma-chemical bromination applied to graphitic materials, in particular to highly ordered pyrolytic graphite is reviewed. The resulting bromination efficiency of the plasma-chemical treatment was subject of systematic process optimization. The plasma of elemental bromine vapour produced bromine concentrations on graphene surfaces of more than 30 % Br/C using either inductively or capacitively coupled low-pressure radio-frequency plasmas. Plasma brominated graphite surfaces have been studied by Near Edge X-ray Absorption Fine Structure, X-ray Photoelectron Spectroscopy, Atomic Force Microscopy and Scanning Electron Microscopy. The introduction of bromine into graphene-like graphite layers and its binding situation were investigated. To study the physical effect of the plasma bromination process, Kr plasma was also used because of its chemical inertness but similar atomic mass. Covering the samples with a Faraday cage or with a LiF window should help to differentiate between physical, chemical and radiation effects of the plasma. Bromination was assigned to radical or electrophilic addition of bromine onto fully substituted aromatic double bonds (sp² C) with exothermal reaction enthalpy. Low bromination shows a strong decay of aromatic double bonds, higher bromination percentage let disappear all aromatic rings. The formed C–Br bonds were well suited for efficient grafting of organic molecules by post-plasma wet-chemical nucleophilic substitution. This grafting onto the graphene surface was demonstrated using aminosilane and different diamines. The bromination of double bonds changes the hybridization of carbon atoms from plane sp² to tetrahedral sp³ hybridization. Thus, the plane topography of graphene is destroyed and the conductivity is lost.
Activators regenerated by electron transfer–atom radical transfer polymerization (ATRP) as a controlled living polymerization are distinguished by their acceptance of small amounts of transition-metal complexes and oxygen and by their tolerance of reducing agents at a high concentration. The precondition of all ATRP applications is the use of homolytic or heterolytic cleavable halides as a dormant species; this allows the propagation of monomer chains. Hence, alkyl bromides are slightly cleavable and are the preferred initiators for ATRP. The bromination of polymer slides used as macroinitiators was carried out under gentle bromoform plasma conditions. This led to an oxidation-resistant stable bromine layer. More than 20 bromines per 100 carbons on the polymer scaffold were permanently bound to the substrate after plasma treatment. The resulting amounts of secondary and tertiary bromines on the polymer scaffold exhibited a suitable macroinitiator concentration for the surface-initiated polymerization of methyl methacrylate and glycidyl methacrylate.
Polymer deposition morphology by electrospray deposition - Modifications through distance variation
(2014)
Electrospray deposition (ESD) of highly diluted polymers was examined with regard to the deposited surface structure. Only the flight distance (flight time) onto the resulting deposited surface was varied from 20 to 200 mm.
An apparatus without any additional heating or gas flows was used. Polyacrylic acid (PAA) and polyallylamine (PAAm) in methanol were deposited on Si wafers. The polymer layers were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, derivatization reactions and Fourier transform infrared spectroscopy using a grazing incidence unit.
SEM images illustrated the changing structures of PAA and PAAm. For PAA the deposited structure changed from a smooth film (20 mm) to a film with individual droplets on the coated surface (100 mm and 200 mm), while for PAAm individual droplets can be seen at all distances. The ESD process with cascades of splitting droplets slows down for PAA after distances greater than 40 mm. In contrast, the ESD process for PAAm is nearly stopped within the first flight distance of 20 mm. Residual solvent analysis showed that most of the solvent evaporated within the first 20 mm capillary-sample distance.