M. Walker

Proton-conducting polymers with reduced methanol permeation

The permeability of Nafion® 117 and some types of acid-base and covalently crosslinked blend membranes to methanol was investigated. The methanol crossover was measured as a function of time using a gas chromatograph with a flame ionization detector. In comparison to Nafion, the investigated acid-base and covalently crosslinked blend membranes show a significant lower permeation rate to methanol. Additionally, another method to reduce the methanol permeability is presented. In this concept a thin barrier layer is plasma polymerized on Nafion 117 membranes. It is shown that a plasma polymer layer with a thickness of 0.3 μm reduces the permeability to methanol by an order of magnitude.

Sterilisation with low-pressure microwave plasmas

Abstract The aim of this article was to show that low-pressure microwave plasmas are suitable for the sterilisation of materials for all kinds of applications. Therefore, several microwave plasma sources were used to treat test substrates with a defined initial contamination of 10 6 test spores. Spore reduction kinetics are presented for four different test spores obtained with different plasma sources and different working gases. A fast spore reduction of four orders of magnitude after less than 1 s of plasma treatment could be achieved. Furthermore, the results prove that the main mechanism responsible for this fast inactivation is the effect of UV and VUV light that is produced in the plasma.

Improvement of the adhesion of low-energy polymers by a short-time plasma treatment

Abstract Foils of polyethylene, polypropylene and polyethylene terephthalate are exposed to low-pressure plasmas. The plasma is produced by microwaves ( f =2.45 GHz) at the electron cyclotron resonance, with electron densities of about 10 11 cm -3 and electron temperatures between 2 and 4 eV. Oxygen and hydrogen are used here for plasma treatment at a pressure of typically 3 Pa. Quartz-like and polyethylene-like thin films are produced on the polymers as adhesion agents. The adhesion power between the polymers and the adhesive is measured by pull-off tests. The adhesion strongly increases by a plasma treatment, for polyethylene by a factor of 20 after treatment of a few seconds. The surface tension of the polymer foils is measured using the Cahn surface tensiometer. It is shown that the polar part of the surface tension of the tested polymers increases with plasma treatment, whereas the dispersion part is only weakly influenced. The adhesion power and the polar part of the surface tension of the polymers are correlated. The polymer surfaces are investigated by scanning electron microscopy and by infrared spectroscopy.

Barrier properties of plasma-polymerized thin films

Abstract Thin hydrocarbon films are deposited on Nafion™ membranes in a low-pressure plasma excited by microwaves. Gas mixtures of hexane (C 6 H 14 ) with hydrogen (H 2 ) were used as monomers. The permeability of methanol through the Nafion membranes modified by plasma polymer films is investigated as a function of the C 6 H 14 /H 2 ratio of the gas mixture. The methanol permeability was measured as a function of time using a gas chromatograph with a flame ionisation detector. It is shown that a plasma polymer film reduces the permeability of methanol by a factor of about 15.

Plasma polymerized barrier films on membranes for direct methanol fuel cells

The methanol permeation through fuel cell relevant Nafion® membranes is investigated at different concentrations of methanol in aqueous solutions. Thin plasma polymerized barrier films are deposited on Nafion® membranes in a low pressure microwave generated plasma to reduce their methanol permeability. The methanol permeability was measured as a function of time using a gas chromatograph with a flame ionisation detector. It is shown that a plasma polymer layer with a thickness of approximately 0.27 μm on Nafion® membranes reduces the permeability to methanol by a factor of approximately 20.

Linearly extended plasma source for large-scale applications

Abstract A linearly extended plasma source — the Duo-Plasmaline — is characterised. The plasma is excited by microwaves of 2.45 GHz in a pressure range between 5 and 500 Pa. The device is similar to a coaxial wave guide. A quartz tube conveys through a vacuum chamber. A copper rod centered in the quartz tube is the inner conductor, and the plasma produced in the low-pressure regime outside the quartz tube represents the outer conductor. The microwaves are fed from both ends of the quartz tube into the wave guide and propagate mainly along the inner rod and the tube, filled with air at atmospheric pressure. The device generates a linearly extended plasma up to several meters, mainly controlled by the pressure and microwave power. The plasma source used here is expanded by a second parallel quartz tube both mounted and supplied parallel as a double line system. A sample stage movable perpendicular to the plasma source was mounted. The plasma was characterised for different plasma conditions by measurements of the electron density in relation to the axial and radial distances from the plasma source. The excellent axial homogeneity of the electron density is reflected in the homogeneous axial distribution of the etch rate of polymethylmetacrylate in an oxygen plasma. Also, the axial homogeneity of the deposition rate of quartz-like films polymerised in a plasma from hexamethyldisiloxane (HMDSO) and oxygen, is demonstrated. The newly designed linear plasma source is well suited for large-area plasma treatment and coating.

Short-time plasma pre-treatment of polytetrafluoroethylene for improved adhesion

Abstract Fluoropolymers with their unique properties (chemical inertness, thermal stability, low surface tension, mechanical stability) are used in many industrial applications. One disadvantage of these fluoropolymers is their poor adhesion to other materials. In this work a short-time plasma pre-treatment of polytetrafluoroethylene (PTFE) with a low pressure microwave plasma was investigated. The Plasmodul ® source allows the pre-treatment of PTFE substrates with an ammonia plasma (NH 3 ). The newly developed plasma source Planartron ® , which is derived from the Duo-Plasmaline ® , was used for improving the adhesion properties of PTFE by generating oxygen and nitrogen plasmas. PTFE foils were modified on both sides by plasma treatment for typically 30 s. After bonding the foils to aluminum dollies using a 2-K-epoxy resin the bonding strength of the adhesive joint was measured directly by pull off tests. Some modified foils were additionally investigated by Fourier-transform infrared spectroscopy.

XPS and IR analysis of thin barrier films polymerized from C2H4/CHF3 ECR‐plasmas

Thin fluorocarbon polymer films are prepared on PE-foils in low-pressure electron cyclotron resonance plasmas using ethylene (C2H4) and trifluoromethane (CHF3) as monomers. The thin fluorinated hydrocarbon layers strongly reduces the permeability of polyethylene to alkanes. For example, the permeation of toluene was decreased by a factor of about 100 by a single, thin fluorocarbon layer. A further reduction of the permeation down to a factor of 1600 can be obtained by a multilayer coating. X-ray photoelectron spectroscopy and Fourier transform IR spectroscopy are used to characterize the plasma polymerized films. It is shown that the addition of CHF3 to a C2H4 plasma leads to an increase of CF3—, CF2—, and CF— groups and to a decrease of CH3— and CH2— groups in the film. The chemical composition of the polymer layers and their toluene permeabilities are discussed.

Influence of plasma surface treatment on the adhesion of thin films on metals

Abstract Thin quartz-like polymer films are prepared in a low pressure electron cyclotron resonance plasma operated at 2.45 GHz. Hexamethyldisiloxane is used as monomer in a mixture with oxygen or hydrogen. The adhesion of the polymer films on metal surfaces is studied by measuring the adhesion power with an adhesion test instrument. It is shown that a plasma surface pre-treatment is necessary to obtain good film adhesion. The deposited films are characterized by their IR spectrum measured by the Fourier transform IR technique. The adhesion power depends on the chemical nature of the film; in some cases the surface energy determined by the test ink method correlates with the adhesion power.

Development and Characterization of an Atmospheric-Pressure Microwave Plasma Torch

The presented microwave plasma torch finds application in many areas of treatment and conversion of different gases as well as in surface treatment. The torch is based on an axially symmetric resonator. Microwaves of 2.45 GHz are fed into the cavity, resulting in a sufficient high electric field for ignition and maintaining stable plasma operation. The characterization of the plasma is performed by optical-emission spectroscopy, which revealed a rotational temperature of about 3600 K in the center of the plasma.