High-strength polymer materials are useful components in many multifunctional materials applications. However, because of low reactivity, their use is inhibited in many processing techniques. Polymer films and fibers possess low surface energies and need to be modified through surface activation prior to being used in composite systems where adhesive bonding is an issue. Treatment of polymer materials through plasma processing is one route to greatly increase surface reactivity. Once the surface has been plasma-modified, wettability by liquids can be examined to determine the treatment effects on the polymer material’s surface reactivity.

Water Droplets on the Control Polyethylene Film before plasma exposure (top) and after helium-oxygen plasma treatment (bottom). Prior to plasma exposure, a drop of water was placed on the film and the contact angle was 98°. In this case, the surface can be characterized as hydrophobic. Then, the film was treated under a helium-oxygen plasma for a few seconds and the water contact angle dropped to 25°, revealing a very hydrophilic surface.
In this research, wettability testing of nylon-6, and polyethylene film and ultra-high-molecular-weight polyethylene (UHMWPE) woven fabrics was performed to examine the effects of surface plasma treatment. Each material was plasma-treated under varying treatment times, flow rates, and compositions of helium and oxygen plasmas.

The atmospheric plasma system used for the surface treatments was an industrial scale plasma system from Sigma Technologies. The gases (He and He-O2) were injected into the electrode at atmospheric pressure and allowed to diffuse, forming a filamentary glow discharge. Helium is typically used to initiate and generate the plasma at atmospheric pressure before another gas is introduced into the system. The operation frequency was 90 kHz with an operating power of 1050 W.

UHMWPE fibers (with an average diameter of 100μm), UHMWPE films (Goodfellow, 75 μm thick), and nylon-6 films (Goodfellow, 75 mm thick) were exposed to the plasma created between the just described electrode and a second electrode covered by an alumina layer.

Wettability testing was carried out using one of two methods. For films, a static contact-angle setup using the sessile drop method was used. The setup consists of a micrometer-controlled syringe held in place above a moveable stage. Contact angles were recorded using a goniometer equipped with a charge-coupled-device camera and using an image capture program employing LabVIEW software. Contact angles were measured by defining a circle around the drop, and recording the tangent angle formed at the substrate surface. The LabVIEW program measures this angle on the left and right sides of the snapshot and then averages them for a final contact-angle value.

For static contact-angle measurements on films, small strips (5 × 1 cm) of the sample film are cut and placed onto a glass microscope slide using double-sided tape to ensure a flat viewing surface. The glass slide is placed onto the stage were a 5-μL drop is measured out from the syringe and dropped onto the film surface. The drop is allowed to reach equilibrium before the measurement is recorded and before evaporation occurs. Once the drop has been measured, the slide is moved to allow another drop to be placed on the sample surface. Between three and five drops are used for each sample, with the values being averaged for the final contact-angle value. This exact process was then repeated using methylene iodide, ethanol, and formamide. These liquids were chosen to cover a wide range of polarity.

Results obtained from the control films were compared with those of the plasma-modified surfaces, proving that the surface chemistry is altered and more hydrophilic surfaces are formed through plasma exposure.

This work was done by Daphne Pappas, Craig Copeland, and Robert Jensen of the Army Research Laboratory. ARL-0070

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Wettability Tests of Polymer Films and Fabrics

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This article first appeared in the August, 2009 issue of Defense Tech Briefs Magazine.

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