Polyelectrolytes are polymers containing ionic groups on their backbones or on branches. They are mainly used because of their ion exchange properties.
There are different methods to produce thin layers of polyelectrolytes on surfaces
of almost any bulk material. A lot of work was done on graft copolymerisation of
olefinic ionic monomers onto polymers after introducing reactive sites as e.g.
radicals or peroxides.
Another way to obtain polyelectrolyte layers is a direct plasma polymerisation
process. It is possible to deposit ultrathin layers containing e.g. a high content
of anionic functional groups from electron.cyclotron-resonance-microwave-plasmas.
These layers are very homogeneous, stable and unsoluble in water.
Plasma Systems
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Plasma reactors
The fundamentals of the polyelectrolyte layer deposition were investigated in a research plasma reactor.
The results were used to scale the process to a semitechnical plasma reactor.
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Monomers
As monomers for the plasma process, we use olefinic molecules with a high vapour pressure, e.g. acrylic acid. The deposition kinetics of these monomers as well as the properties of the resulting layers are well characterized (see list of publications).
Layer Characterization
The characterization of the ionic plasma layers includes the detection of the volume
concentration of ionic groups within the layer via labeling of these functional groups
with a cationic fluorescent dye as well as the investigation of the overall chemical
constitution by infrared spectroscopy and electron spectroscopy for chemical analysis.
Furthermore, an investigation of surface properties can be performed by contact angle
goniometry and zeta potential measurements. Last but not least, .the determination of
the physical appearance is possible via atomic force microscopy and electron microscopy.
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Dye Thionine Acetate, CI52000 (C12H9N3S.C2H4O2). This dye is also known as Lauth´s Violett or Hoechst 33258. |
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Fluorecence labeling
The deposited layers are suspended in a solution of the cationic dye Thionine to stain all anionic groups with this cationic label. The dye can be removed from the layer by exchanging it in acid solution; the fluorescence intensity of this solution is a direct measure for the volume concentration of anionic groups in the plasma layer.
It is possible to obtain polyelectrolyte layers containing up to 3e-4 mol/ccm ionic groups.
Further information:
Fluorecence labeling
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X-Ray photoelectron spectrum of a Plasma Layer (Courtesy of W. Unger, Bundesanstalt für Materialforschung) |
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X-ray photoelectron spectrometry
The high content of carboxylic ionic groups is confirmed by XPS.
The total oxygen content of the plasma layers is 38%.
Further information:
X-ray photoelectron spectrometry (XPS)
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IR spectrum of a Plasma Layer (Courtesy of M. Nitschke, Technische Universität Chemnitz-Zwickau) |
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Infrared spectrometry
An Infrared spectrum of a carboxylic groups containing plasma layer reveals the expected peak abov 1700 cm-1 as well as the hydrogen bond signal between 3400 and 3000 cm-1. Obviously, the layer is chemically quite heterogeneous, as can be seen from the fingerprintregion between 1400 and 900 cm-1.
Further information:
Infrared spectrometry
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Surface of a Plasma Polymer on Polyethylene. The total surface area is 130µm x 130µm. (Courtesy of H. Kamusewitz, GKSS) |
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Atomic force microscopy
A medium resolution AFM-investigation of the plasma layer reveals a pinhole-free, smooth surface. Experiments show that the surface flattens with increasing deposition time.
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Contact angle goniometry
The carboxylic groups containing plasma layers have a relatively high surface energy of about 57 mJ/sqm. The water contact angle is 41°.
Further information:
Contact angle goniometry
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Plasma Polymer on Polyethylene. The yellow arrow points to the interface between Polyethylene and Plasma Layer. (Courtesy of A. Purz, IAP) |
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Electron Microscopy
SEM images of cryobroken plasma layers on polyethylene give an idea of the density and extent of crosslinking in comparison of the familiar structure of polyethylene.
Further information:
SEM on polymer-analysis.com
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Zeta potential of a Plasma Layer compared to Polyethylene. (Courtesy of K. Richau, GKSS) |
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Zeta Potential
Compared to ethylene, the zeta potential of the plasma layers in 1e-3 mol KCl is - due to the ionic groups on the layer surface - higher. The isoelectrical point can be found at lower pH's.