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Eingeladener Vortrag
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Different surface treatments of polypropylene (PP) were tested to improve the peel strength of aluminium-PP systems. Plasma exposure followed by chemical grafting of spacers with -SiOH endgroups and thin adhesion promoting layers deposited by aerosol-assisted DBD using aqu. solutions of classic polymers were compared. Both methods were effective, however, –SiOH terminated spacers effected no peeling also after ageing.
Functional groups bearing monomers were pulsed plasma polymerised with a degree of retained functional groups of 5591%. The following functional groups could be produced at a maximum of: 30 OH, 18 NH2, and 24 COOH per 100 C atoms. A plasma-initiated radical copolymerisation could also be realised using the functional group bearing monomers as a source of functionalities, olefins as chain-extenders and dienes as chemical cross-linkers. The peel strengths of Al layers on such plasma polymers were correlated to the type and density of functional groups.
The peel strength of aluminium to polypropylene and poly(tetrafluoroethylene) was determined in dependence on the type and the concentration of functional groups on the polymer surface. For this purpose the polymer surface was equipped with monotype functional groups. The first method to produce monotype functionalized surfaces was an introduction of O functional groups using an oxygen plasma treatment and converting these groups to OH groups applying a wet chemical reduction. In result of this two-step treatment the hydroxyl group concentration at the polymer surface could be increased from 34 to 1014 OH groups/100 C atoms. The second method consists in the deposition of a 150 nm adhesion-promoting layer of plasmapolymers or copolymers onto the polymer surface using the pulsed plasma technique. For that purpose functional groups carrying monomers as allyl alcohol, allylamine and acrylic acid were used. Applying the plasma-initiated copolymerization and using neutral monomers like ethylene or butadiene the concentration of the functional groups was varied.
A correlation of peel strength with the ability of forming chemical interactions between Al atoms and functional groups was found: COOH > OH >> NH2 > H(CH2CH2).
This new functionalization method consists of the deposition of very thin plasma polymer layers (20 to 100 nm) of functional group bearing monomers in pulsed plasma. With allylalcohol, a maximum of 30 OH groups per 100 C atoms was measured with a selectivity of about 90% and a significant stability at long-time exposure to air. Allylamine was used to produce primary amino groups, with a maximum of 18 NH2 groups per 100 C atoms. Side reactions were observed during the storage in air, such as oxidation of the amino groups. Carboxylic groups could be produced using acrylic acid with a maximum concentration of 24 COOH groups per 100 C atoms. The most prominent side reaction was the decarbonylation/ decarboxylation of the acid group during plasma deposition.___TAGSTART___BR___TAGEND___ The variation of the density of functional groups using the pulsed-plasma polymerization of functional-group-bearing monomers was possible by the chemically-initiated radical copolymerization with either a chain-extending monomer, such as ethylene, or a cross linker, such as butadiene, in plasma. The density of functional groups could be adjusted continuously (0 to 30 OH, 0 to 18 NH2 and 0 to 24 COOH groups per 100 C atoms).___TAGSTART___BR___TAGEND___ The successful application of these densely functionalized polymer surfaces for producing biocompatible surfaces and for use in metalpolymer composites is proposed.
The production of chemically-defined plasma polymers and the introduction of monotype functional groups onto polymer surfaces are described. One method is to lower the energetic level of low-pressure plasmas. Pressure- and plasma-pulsed plasmas were successfully tested for the production of chemically-defined plasma polymers by increasing the monomer supply during the plasma-off period. Well-defined ultra-thin polymer films with regular structure were deposited from atmospheric plasmas by electrospray techniques. Post-plasma wet-chemical processing was also applied, as were gas/liquid-based aerosols and underwater plasmas.
Polyolefins are chemically inert and do not adhere well to metals, polymers or inorganics. To overcome this problem, polyolefin surfaces were modified thermally, plasmachemically, or by flame treatment with different oxygen-containing groups, however, unfortunately, such treatments were accompanied by undesired, adhesion lowering polymer degradation. To solve this dilemma, solutions of synthetic polymers and copolymers were prepared, sprayed into the barrier discharge or electrosprayed without discharge and deposited as thin adhesion-promoting layers. The deposited polymer layers from poly(vinylamine), poly(ethylene glycol)-poly(vinyl alcohol) copolymers and poly(acrylic acid) were endowed with monotype functional groups. Using the aerosol - dielectric barrier discharge only a fraction of functional groups survived the deposition process in contrast to the electrospray in which all functional groups were retained.
A new concept for molecular interface design in metal-polymer systems is presented. The main features of this concept are the replacement of weak physical interactions by strong covalent bonds, the flexibilization of the interface for compensating different thermal expansions of materials by using long-chain flexible and covalently bonded spacers between the metal and the polymer as well as its design as a moisture-repellent structure for hindering diffusion of water molecules into the interface and hydrolysis of chemical bonds.
For this purpose, the main task was to develop plasmachemical and chemical techniques for equipping polymer surfaces with monotype functional groups of adjustable concentration. The establishing of monotype functional groups allows grafting the functional groups by spacer molecules by applying usual wet-chemical reactions. Four processes were favoured for production of monotype functional groups by highly selective reactions: the plasma bromination, the plasma deposition of plasma polymers, the post-plasma chemical reduction of O-functionalities to OH-groups, and the chemical replacement of bromine groups by NH2-groups. The grafting of flexible organic molecules as spacers between the metal layer and polymer improved the peel strength of the metal. To obtain maximal peel strength of aluminium coatings to polypropylene films and occurrence of cohesive failure in the polypropylene substrate, about 27 OH groups per 100 C-atoms or 6 COOH groups per 100 C-atoms were needed. Introducing C6-11-aliphatic spacers 1 OH or COOH group per 100 C-atoms contributed about 60% of the maximal peel strength of the Al-PP system, i.e. 2 or 3 spacer molecules per 100 C-atoms were sufficient for maximal peel strength.
The retention of chemical structure and functional groups during plasma polymerization was investigated. Usually plasma polymer layers, prepared by a continuous wave radio-frequency plasma, are often chemically irregular in their structure and composition. To minimize these irregularities low wattages and the pulsed plasma technique were applied to avoid fragmentations. The first goal was to produce plasma polymers comprising double or triple bonds as precursors for electrically conducting polymers. Acetylene, ethylene, butadiene and polystyrene were used as monomers and deposited as thin polymer films by pulsed plasmas of low wattages. Styrene polymerization was strongly enhanced in the dark phase (plasma off) of a pulsed r.f. plasma caused by the reactivity of the vinyl-type double bond. This could be confirmed by a verification of a rather high chemical regularity of the film sample. The oxygen content of this film measured by X-ray photoelectron spectrometry (XPS) was in situ 0% and after 24 h exposure to air lower than 1%. Additionally, post-plasma oxidations of trapped radicals with air could be suppressed using NO gas as radical quencher. Such quenched plasma polymer layers were completely stable against oxidation for a number of weeks. During the low-wattage pulse plasma polymerization metal atoms were simultaneously or layer by layer evaporated into the growing layer and in situ measured by XPS. These metal atoms are acting as dopants. Here, Li, K, Mg and Cr were used at different concentrations providing electrical conductivity and magnetic properties for the film.
Modelling plasma-induced reactions on polymer surfaces using aliphatic self-assembling and LB layers
(1998)
Several approaches were investigated to produce monosort functionalized polymer surfaces with a high density and homogeneity of functional groups: (i) Plasma oxidation followed by wet-chemical reduction, (ii) formation of radicals and grafting on of functional group carrying molecules, (iii) plasma bromination followed by (iv) Williamson or Gabriel-like synthesis of spacer molecules, and (v) a pulsed plasma polymerization of functional groups bearing monomers or (vi) their copolymerisation with other comonomers. The formation of hydroxyl (OH), primary amino (NH2), and carboxyl (COOH) groups was studied in detail. The oxygen plasma treatment (i) in a low-pressure non-isothermal glow discharge results in the formation of a wide variety of O functional groups, polymer degradation and crosslinking. Low power densities and short exposure times (0.1 to 2 s) are required to functionalize a surface while preserving the original polymer structure. Carbonate, ester, and aromatic groups are rapidly degraded by an oxygen plasma treatment leading to scissions of polymer backbones and loss in molecular weight. Also the formation of macrocycles and C=C bonds was observed in a region of around 4 nm in depth. The investigated polymers could be classified by their degradation behaviour on exposure to the oxygen plasma.___TAGSTART___BR___TAGEND___ In order to maximize the process selectivity for OH groups, the variety of oxygen functionalities formed by the oxygen plasma was wet-chemically reduced by diborane, vitridetrade (Na complex), and LiAlH4. Typical yields were 9 to 14 OH groups per 100 carbon atoms.___TAGSTART___BR___TAGEND___ Plasma bromination (iii) (40 Br per 100 C atoms) of polymers, followed by grafting of spacer molecules (iv), has been proved to be a highly selective reaction.___TAGSTART___BR___TAGEND___ Another way to produce high densities of monosort functionalities was the pulsed plasma polymerization of functional group bearing monomers such as allylamine, allylalcohol or acrylic acid (v). The retention of chemical structure and functional groups during plasma polymerization was achieved by using low power densities and the pulsed plasma technique. The maximum yields were 30 OH, 18 NH2, and 24 COOH groups per 100 C atoms. To vary the density of functional groups a chemical copolymerization with ''chain-extending'' comonomers such as butadiene and ethylene was initiated in the pulsed plasma (vi). Additionally, the often-observed post-plasma oxidations of such layers initiated by reaction of trapped radicals with oxygen from the air were successfully suppressed by using NO gas as radical quencher.
The chemical and morphological stabilities of polymer segments in the near-surface layer were investigated by spectroscopic methods such as X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. Model studies were undertaken with LangmuirBlodgett films, self-assembled monolayers and oligomer films. For thin polymer layers (30 to 500 nm), the changes in molecular-weight distributions of some polymers were investigated systematically by size exclusion chromatography, matrix-assisted laser desorption/ionizationtime-of-flight mass spectrometry and thermal-field flow fractionation for oxygen- and helium-plasma exposures. The polymer surfaces were found to be relatively stable at exposure to an oxygen low-pressure plasma up to ca. 2 s. This is important information to get maximum adhesion to metals in composites. In correlation to their redox potentials, potassium, aluminium and chromium react with oxygen functional groups at the polymer/metal interface. In a dedicated study, chromium was found to attack aromatic rings and form different reaction products.
Plasma-chemical bromination has been shown to be well-suited for highly selective and efficient C–Br-functionalization of polyolefin surfaces and has now been applied to graphitic materials, including highly oriented pyrolytic graphite (HOPG), natural graphite, carbon nanotubes, and graphitized carbon fibres. In contrast to the radical substitution of hydrogen by bromine at hydrogen-terminated sp3-hybridized carbon, bromination of all-carbon bound sp2-hybridized carbon in graphene requires addition to aromatic double bonds. The related change of sp2 to sp3 hybridization of the carbon atoms involved helps to explain experimental results of a loss in surface planarity on brominated HOPG. Bromine concentrations of up to 50 bromine atoms per 100 C-atoms were achieved in elemental bromine vapour under low-pressure plasma conditions using low-energetic inductively coupled radio-frequency plasma excitation. The time scale of the bromination progress was found to increase with the specific surface area of the material. The results of plasma-chemical bromination are compared to that of wet-chemical electrophilic addition reactions, which achieved 28 Br per 100 C for MWCNT. The resulting C–Br bonds from plasma-chemical processing of graphitic materials were shown to be well-suited for hydroxyl functionalization and for grafting of organic molecules by nucleophilic substitution with 1,6-diaminohexane and (3-aminopropyl)triethoxysilane.
Polyolefin surfaces (polyethylene and polypropylene) were exposed to haloform (CHX3) plasmas for introduction of monosort halogen groups. Bromoform and chloroform plasmas produced selectively C-Br (100 Br/100 C) and C-Cl (200 Cl/100 C) groups in high yields. The bromoform plasma showed 1-3 and the chloroform plasma 2-7 post-plasma introduced O-functionalities per 100 C. The polyolefin C-Br groups were grafted wet-chemically or by exposure to the vapour of amines, diols and glycols. Thus, spacer molecules could be covalently bonded to maximal 15 spacer molecules per 100 C for the smallest grafted molecules and 1.2 molecules/100 C for larger molecules as octaaminophenylene-POSS. After metal evaporation the end groups of these polymer-bonded spacer molecules formed also covalent bonds to the metal. Thus, flexible, hydrophobic and barrier elements were introduced into the polymer-metal interfaces for high-impact, high-durable and water-resistant metal-polymer composites as measured by means of peel strengths.
Barrier properties of plasma and chemically fluorinated polypropylene and polyethyleneterephthalate
(1995)
The functionalization, perfluorination or an coating of polypropylene (PP) and polyethyleneterephthalate (PET) surfaces in different plasma atmospheres and in an F[2] + N[2] gas mixture were investigated. The aim of this work was to form a barrier layer against solvent and fuel permeation. Typical test solvents were n-pentane, toluene, tetrachloroethylene and mixtures of n-pentane and methanol. The permeation was gravimetrically measured. The permeation of the solvent mixtures was analyzed by mass spectrometry and Fourier transform IR spectroscopy. The permeation of the solvent mixtures through the plasma-modified polymer samples was associated with increased absorption of polar solvent components in the polymer film. The thickness of the plasma-fluorinated PP surface layer was 10-100 nm and that of chemically fluorinated PP about 5000 nm. Nearly the same barrier efficiency was measured for both plasma- and chemically fluorinated samples.
Different types of plasma, irradiative and chemical activation were compared in terms of surface functionalization. Corona and spark jet plasmas are characterized by low gas temperatures and high rates in surface modification. UV irradiation in the presence of ozone does not involve any particle bombardment and acts only by enhanced photooxidative processes. Although ion implantation can be avoided, this method is not free of radiative damage in both the surface-near region and the bulk of polymers. Furthermore, its functionalization rate is low. In relation to low-pressure O2 plasma modification, all treatments mentioned here have a low efficiency in adhesion promotion due to oxidative degradation of macromolecules and formation of molecular debris known as the ''weak boundary layer''.
Polymers synthesized with plasma techniques are very interesting materials for electronic, optic, and bio compatible applications. Thin films of plasma polymers shows a good adhesion to metals, glass, or other polymers. But the supramolecular structure, the durability, and the chemical and mechanical behavior of these polymers is poorly understood. Therefore dielectric investigations are carried out to study the dynamic behavior of the plasma polymers. As polymer system allyl alcohol/alkene is chosen to get polymers with a defined concentration of hydroxyl groups. The dielectric investigations shows several relaxation processes and a dependency of the dielectric parameters from the ratio of allyl alcohol in the polymer is observed. This results indicated that the alkene monomers were assembled continuous into the polymer matrix.
Single walled carbon nanotubes (SWNT) have been produced in an electric arc discharge by using a graphite rod as anode which is filled with a powder mixture of graphite and metallic catalysts like nickel and yttrium. The soot material containing approximately a third of SWNT was collected from the cathode region as a soft, voluminous material. The main by-products in this process are amorphous carbon and graphitic nanoparticles which have to be removed by an appropriate purification process. The as-produced soot material was purified by gas phase oxidation in air at 355 °C. The oxidation of amorphous carbon in air is advantageous against wet chemical methods because it is better controllable. Thermogravimetric analysis and electron microscopy were used to investigate the oxidation behavior of the different kinds of carbon. Oxidation of SWNT and amorphous carbon occurs simultaneously even at moderate temperatures, whereas amorphous carbon is more rapidly oxidized than SWNT. For optimizing the purification procedure by gas phase oxidation, kinetic studies were used to determine the oxidation time for each component in the soot at a given temperature. So it is possible to remove the amorphous carbon quantitatively with minimal losses of SWNT. But it was found that graphitic nanoparticles have the highest stability against oxidation and could therefore not be quantitatively removed by this method without the complete destruction of SWNT. Therefore, the electric arc discharge process has to be a controlled process for minimum production of graphitic nanoparticles to obtain a material with a high content of SWNT.
Surface-Enhanced Infrared Absorption (SEIRA) and Its Use in analysis of Plasma-Modified Surface
(1999)
Organic thin films with defined chemical structures and physical properties are required for various applications. Plasma polymerization is of technological interest, since the deposition of plasma polymers is possible on any material of any shape in the desired thickness. We report on the TOF-SIMS (TOF: time-of-flight) and XPS investigations of pulse plasma poly(acetylene) and alkali metal doped pulse plasma poly(acetylene) films. The combination of TOF-SIMS and XPS supplies detailed information on the surface chemistry of these films. Application of both methods provides the possibility for cross-checking certain results. Moreover the different sensitivities of the methods can be utilized. According to the TOF-SIMS and XPS data it can be concluded that the interaction of alkali metals with plasma poly(acetylene) results in electrovalent bonds of the type Me?+C?- between carbon and metal. Furthermore carbonoxygenmetal interactions were observed. These interactions are of the type C=O?-cdots, three dots, centeredMe?+ and/or CO?-Me?+.
Thin metal-filled polyterafluoroethylene films with various metal concentration were produced by co-deposition in vacuum. Metal nanocluster size increased with metal concentration. Films were heated up to 300 degrees C, their optical spectra were recorded during heating. The changes in plasmon band shape and wavelength of the nanocluster ensemble during heating are not linearly related with metal concentration and heating temperature. This is caused by different thermal behavior of the complex processes, which are taking place in each of the two materials present in the film. The metal cluster size and optical properties of the whole ensemble can be purposefully formed by varying metal nature, its concentration and annealing temperature of the film. Nano- and micro-domains with properties different from original film were generated by focused excimer laser or electron beam. Gold-filled PTFE nano-structured films were used as substrate for surface enhanced Raman scattering measurements of ultrathin film of Rhodamine 6G dye.
Polytetrafluoroethylene (PTFE) films have been deposited onto polycarbonate (PC) substrates from the products of PTFE evaporation, activated by a cloud of accelerated electrons. A 40.68 MHz glow discharge was used during the deposition process. The polymer films have been characterised by XPS, FTIR and AFM. The use of the low power plasma during film growth led to the formation of PTFE films with modified structure. Films are amorphous and contain more cross-links, but in general, the structure of their macromolecules is still linear. An increase of RF-power leads to the formation of films with large amount of double bonds and enhanced internal stresses.
Deposition of PTFE on PC without plasma treatment led to the formation of PTFE clusters up to 50 nm in diameter. The RMS roughness of the films, deposited without plasma, was about 4 nm, while the films deposited with plasma treatment had a roughness of 1.5 nm. The use of plasma has an additional effect if a PTFE coating is deposited on the PC substrate with submicrometer-sized steps. Without plasma the steps retain a rectangular shape. Deposited with the RF-discharge the PTFE layers resemble plasma-polymerised films. Under certain conditions the deposited films can fill trenches in the substrate like a wetting liquid, while under other conditions they avoid trenches and grow in between them.
Indirect UV-absorption spectrometry was shown to be a valuable tool for chemical characterization of functionalized carbon nanotubes (CNTs). It complements data from X-ray photoelectron spectroscopy (XPS) or FTIR analysis since it helps to clarify the type and concentration of functional groups. The principles of indirect application of UV-spectrometry and its mathematical interpretation are discussed. Their facile application, together with their adequate sensitivity and high flexibility, make UV-absorption-based approaches a valuable alternative to fluorescence spectrometry. Here, the approach was applied to the chemical analysis of oxidizing substances on CNTs. For this, pristine CNTs of low but finite oxygen content as well as brominated CNTs were analyzed by reaction in suspension with UV-active thiol reagents and a styrene derivative. It was shown that carefully selected reagents allow differentiation and quantification of bromine and generally oxidizing entities like oxygen. For brominated CNTs, it was shown that physisorbed bromine may dominate the overall bromine content.
Chemical functionalization of carbon nanotubes (CNT) with nucleophiles requires introduction of electrophilic reactive sites on the CNTs. This can, for instance, be accomplished by the chemical bromination procedure with elemental bromine and a set of Lewis acids (BBr3, BF3 × Et2O, AlBr3, FeBr3, ZnBr2, TiBr4, SiBr4, SnBr4, VBr3) or a radical starter like dibenzoylperoxide (DBPO) in appropriate solvents at varied temperature. The present approach to electrophilic sites relies on the well-known electrophilic aromatic substitution or addition of bromine with aromatic structural units. In addition to the use of bromine, the introduction of haloalkyl groups was also investigated here using bis-electrophiles or haloalcohols and Brønsted acids. The advantages and drawbacks of the studied reaction conditions, the obtained degree of bromination as analyzed by X-ray photoelectron spectroscopy (XPS) and the amount of introduced bromine that can be substituted by a nucleophile are presented and discussed.
Wet-chemically oxidized carbon nanotubes (CNTs) generally exhibit both covalently-bound acidic functional groups on the surface and surface-adsorbed acidic substances, i.e. carbonaceous CNT fragments from the oxidation procedure. Direct potentiometric titration of oxidizable high surface area materials with dynamically desorbing acidic fragments is slow and inaccurate. Adsorbed acidic fragments are deprotonated by sodium hydroxide and form anions in solution which is not the case for covalently bound acidic groups on the CNTs, so the following filtration after NaOH treatment separates desorbable acidic substances from non-desorbable or covalently bound groups. For a known concentration of NaOH, titration of the eluate with hydrochloric acid (HCl) allows determination of the concentrations of both types of acidities. However, contrary to reports in the literature, the NaOH consumed by non-desorbed acidic groups has to be accounted for and impedes distinction of desorbed acidic groups via their pKa values. Results are presented of a study on the information content and the reliability of indirect potentiometric Boehm titration for different oxidized multi-walled CNTs.
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.
Polypropylene supports were functionalized by plasma-deposition of polymeric allylamine layers. The surface amino groups generated were wet-chemically reacted with xanthene dyes resulting in fluorescent polymer films. The effect of polymer-attachment of the dyes on their emission features was studied fluorometrically and different methods were tested to improve the fluorescence properties of the films. Modification with cucurbit[6]uril (CB6) yields a moderately enhanced fluorescence as well as an improved photostability. The observed effect is most likely due to CB6-induced rigidization of the linker molecules which seems to reduce fluorescence quenching dyedye and fluorophoresurface interactions.