The emerging threats caused by improvised explosive devices (IEDs) have drastically increased concerns about soldier survivability. The ability to identify “friend vs. foe” of any approaching vehicles clearly, quickly, and from a distance, is invaluable to ensure a soldier’s safety, as well as critical to providing protection of facilities at strategic locations. The current state-of-the-art uses externally applied coatings or markings onto vehicles that are not seen in the visible light spectrum, yet are detectable with the use of an ultraviolet (UV) or infrared (IR) interrogation device. However, coatings can wear off or wash off, and are susceptible to being counterfeited or tempered.

The optical clarity of a presoaked Fiber Mat after degassing is shown. (a) PVB mat soaking in PMMA-MMA-AIBN solution; (b) PVB after 5 hours of soaking and vacuum egression.
A need exists for novel transparent polymer composites that contain internally embedded taggant materials that preserve transparency while being capable of patterning for optical tagging. The technical challenge is in the difficulty of fabricating and dispersing the nanofibers in the matrix. Transparency of a material is influenced by both the extent of light transmittance and the percentage of haze. Furthermore, the scattering of light in polymer fiber composites is greatly influenced by the compounding effects from the diameter of fibers, and the difference in refractive index between matrix and fiber. For example, poly(methyl methacrylate) (PMMA) and polyacrylonitrile (PAN) have a small difference between their refractive indexes (1.49 and 1.52, respectively) that would produce significant haze.

The approach for making optically clear, fiber-modified polymer composites is based on the match of refractive index of selected fiber and matrix polymers. PMMA has been used widely in many transparent materials applications. Poly(vinyl butyral) (PVB) has an attractive combination of chemical, mechanical, and optical properties, and it has been used in a wide array of commercial applications, such as structural adhesives for glass laminates, coatings, and matrix polymer for fiber-reinforced thermoset composites. PVB has the same refractive index as PMMA, and is, therefore, selected for the production of electrospun fibers. PVB, however, has extremely high melt viscosity and is generally not melt spun into fibers.

Electrospinning offers a simple and robust method to produce submicron PVB fibers. In addition, electrospinning has the following advantages: (1) it can produce very fine fibers (average diameter ranging from 100 nm to 500 nm) that can minimize the scattering of light if there is a slight mismatch in the refractive indices; (2) it can produce a fibrous mat that has a large surface area to mass ratio for better bonding to the matrix material; (3) it allows the dye to disperse in the spin solution; and (4) it does not compromise the chemical stability of the dye during spinning and composite fabrication.

The scope of this work is to demonstrate the feasibility of making transparent composites that possess chromatic functionalities capable of changing color and/or optical clarity, reversibly, upon exposure to an external irradiation. In-situ polymerization/cast process is used for fabrication of PMMA PVBfiber composites. This study examines the stability of an embedded fluorescent dye within the electrospun fibers and their capability of patterning for optical tagging.

In-situ polymerization/cast was used for preparation of PMMA PVB-fiber composites. The electrospun PVB mat was cut into 4 × 4" pieces. Then several layers of PVB pieces were compacted in a hydraulic press. The stack of PVB mats was soaked in the PMMA-MMA-AIBN solution for 5 hours. The degassed resinimpregnated fiber mat was then transferred to an aluminum mold. The interior dimensions of the mold are 4 × 4 × 0.02". The inner surfaces of the aluminum mold have a mirrorpolished finish. The mold surfaces were sprayed with a release agent (LPS, Dry Film Silicon Lubricant). First, the PVB mat was laid flat into the mold, then an excess amount of PMMA-MMA-AIBN solution was added to the mold. The mold was placed in a hot press for curing. The pressure was set at 1000 kPa and the temperature was set to 55 ºC. The curing process lasted for 12 hours, and then cooled down slowly to room temperature.

PVB has the same refractive index as PMMA, and as a result, the PMMA PVBfiber composites were completely optically clear. The stability of the selected UV-dye within the electrospun fibers appeared to be excellent, even long after fabrication.

This work was done by Jian H. Yu and Alex J. Hsieh of the Army Research Laboratory, and Gregory C. Rutledge of MIT. ARL-0121

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Novel Transparent PMMA Composites for Optical Tagging

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This article first appeared in the December, 2011 issue of Defense Tech Briefs Magazine.

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