Investigators: G. Mills

B.L. Slaten

R. Broughton

Students: G.A. Gaddy (graduate)

K. Malone (graduate)

Goal:

Photoadaptive fibers and films, which experience photo induced reversible optical and heat reflectivity changes, are being developed. Our initial goal of producing fibers and films that are metallized exclusively at high light intensities has been accomplished.

Research:

Adaptive systems that exhibit desirable and predictable reversible alterations of their properties in response to external stimuli are very attractive, and are usually called "smart" systems. Important classes of responsive systems are those that are photoadaptive, that is, systems experiencing reversible changes upon exposure to light. A simple example are photo- chromic glasses, where photoreduction of silver halides yields Ag particles that decay in a dark reaction with Cu²⁺ ions to reform the starting silver halides.

This research is centered on the development of photoadaptive fibers that undergo reversable changes in their optical, heat reflectivity and electrical properties. Potential applications for these fibers include selective reflection of high intensity infrared radiation, shielding of electromagnetic radiation, and as recording media for optical storage of data. Specific textile applications are their use in protective garments for firefighters and clothing against high intensity sunlight. Nanometer-sized metal particles of silver and gold in high concentrations reflect infrared radiation and are, therefore, employed in the photoadaptive fibers as active reflectors. The particles are formed only under high fluxes of photons, that is, under conditions where heat reflection is required. Improved charge transport is expected for fibers containing metal particles, minimizing the charge accumulation effects typical of environments with high light intensity. Manipulation of the electromagnetic properties of flexible textiles can also be achieved by generation of metal particles inside fibers. Applications for such materials include enclosures for electromagnetic radiation and garments containing resistive heating elements. The "metallized" fibers can, in principle, be used as recording media for 3D storage of optical data as well. For this purpose metal crystallites would be generated as layers in sequential fashion within structural anisotropies that are regularly spaced in the fibers.

Preparation of films with uniform thickness is required in order to verify the proposed biphotonic mechanism of metal particle generation via free radical chain reactions. Photonic efficiencies will be determined as a function of photon flux to confirm the biphotonic nature of the particle formation process. The role of PVA free radicals in chain reductions of the metal complexes will be studied by addition of selected free radical scavengers to the fibers.

Preliminary results indicate that Ag particles present in the PVA films are sensitive toward oxidation upon exposure to peroxide solutions. Interestingly, particle formation takes place again when the treated samples are exposed again to light. These results suggest that reversible particle formation can be achieved upon incorporation of mild oxidants in the films. Stable organic peroxides are obvious candidates as oxidizers, but a possible attack of Ag crystallites by molecular iodine will also be investigated, since oxidation of bulk silver with this chemical in non-aqueous media has been demonstrated. Once the oxidation of the metal crystallites is achieved, efforts will be concentrated on optimizing this reaction. The oxidation reaction is expected to proceed much slower than particle generation. Characterization of this process will include determination of the activation energy as well as measurements of oxidation rate constants in dry and wet films in the absence and presence of light.

Figure: Poly(vinyl alcohol)-Poly(acrylic acid) Blend Film Treated with 0.1M AgNO₃ in Methanol

Graduate Students: 2

Undergraduate Students: 4

Presentations: 5

Industry Contacts: 10

Website: www.auburn.edu/~gaddyga

www.auburn.edu/~slatebl