Photoadaptive Fibers for Textile Materials

Investigators:
G. Mills, L. Slaten, R. Broughton (Auburn)

Students:
K. Malone (graduate), D. Taylor (undergraduate)

Goals

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

Abstract

Fibers of poly(vinylalcohol) that are insoluble in water have been made by crosslinking the macromolecules with dimethylsulfoxide. Incorporation of Ag+ and AuCl4¯ ions into the fibers was achieved after swelling the polymeric materials with methanolic solutions of the metal ions. Illumination of the dry fibers containing metal ions yielded nanometer-sized Ag and Au particles only under high intensity artificial light, or direct sun light, but not under ambient light. Thus, photochemical metallization of the fibers occurs exclusive under high photon fluxes, which is one of the desired properties of the smart fibers that we are attempting to develop. Although a dark oxidation of the metal crystallites was not observed in these simple systems, the metallized fibers are expected to be the basis for flexible materials that act as shields for electromagnetic radiation, where stable metal particles are required. The light-induced changes in the optical properties of these polymeric systems are currently under investigation.

Introduction

Photoadaptive systems are very attractive since they experience reversible changes in their physical and chemical properties in response to illumination. The most interesting photoadaptive (or photoresponsive) systems are those that exhibit desirable and predictable reversible alterations of their properties, these are usually called "smart" systems. An important and complex example is the photoreceptive apparatus that allows three-dimensional (3D) vision in living organisms. Photochromic glasses are simpler examples, where photoreducion of silver halides yields Ag particles that undergo a dark oxidation reaction to reform the metal halides. Novel photoresponsive systems have been recently developed in our laboratories, where AuCl4¯ or PdCl42- ions incorporated into methanol-swollen crosslinked polymers of diallyldimethylammonium chloride (DADMAC) are photoreduced to form nanometer-sized Au or Pd particles. Interestingly, the metal crystallites are oxidized at room temperature in a dark reaction that reforms the starting metal complexes. While oxidation of Ag particles by Cu2+ ions ensures chemical reversibility of the photochromic glasses, the oxidation of metal crystallites in the mild environment of the gels is unusual. These results suggest that, under appropriate conditions, development of photoresponsive fibers is feasible if similar particle generation and decay steps take place inside polymeric fibers. The ultimate objective of this research is to develop photoadaptive fibers that operate in a fashion similar to photochromic glasses where metal particles form by exposure to high intensity visible light, but decay once the photon intensity decreases to ambient light levels. Areas of potential applications of the polymeric fibers include reflection of infrared radiation, 3D storage of optical data and shielding of electromagnetic radiation or resistive heating elements.

Results

Photoreduction of metal complexes occurs mainly in the presence of electron donors that are oxidized by the excited metal ions. While methanol acts as a reducing agent for the photoreduction of AuCl4¯ ions in the photoadaptive poly(DADMAC) gels, incorporation of volatile and flammable liquids into polymer fibers was not considered a logical solution. On the other hand, it is known that AuCl4¯ ions are thermally reduced in solutions containing poly(vinylalcohol), PVA, but formation of Au particles takes place only under illumination when solid NaAuCl4 is mixed with the solid polymer. Thus, photoreduction of the Au complexes seemed to be energetically feasible if incorporation of AuCl4¯ ions into PVA fibers was achievable. Another criteria for selecting these fibers is that simple methods for the their synthesis using dimethylsulfoxide (DMSO) as a crosslinking agent have been reported.

Attempts to load crosslinked PVA fibers with metal ions by exposure to solutions of AgNO3 or NaAuCl4 yielded poor and non-homogeneous incorporation of Ag+ and AuCl4¯ into the polymeric materials. Modifications of the synthetic procedures were therefore needed; the strategy utilized was to include a second crosslinking agent, which was an ionic compound with a charge opposite to that of the selected metal ion. Strong electrostatic binding of the metallic species to the second crosslinking agent was expected to induce high and homogeneous loadings. DADMAC was used for binding AuCl4¯ ions whereas poly(acrylic acid), PAA, was employed in the case of Ag+ ions. Crosslinking of the polymer took place during gelation of solutions containing PVA, DMSO and DADMAC or PAA. After melting the gels, fibers were extruded into cold alcohol, purified for several days followed by drying and drawing at high temperature to increase the strength. The stretched fibers were then exposed to alcoholic solutions of AgNO3 or NaAuCl4 in order to ion-exchange Cl¯ counterions of DADMAC by AuCl4¯, or H+ (from PAA) by Ag+. During this treatment the fibers swell, allowing the metal ions to diffuse into the polymer network and bind.

Photolysis experiments were performed after drying the fibers using either photons of 350 nm from a Rayonet circular illuminator or direct sunlight. A fast generation of metal crystallites was noticed in both cases, for example, under sunlight Ag particles formed in a few minutes whereas for Au particles the process took several tens of minutes. These results are encouraging since fiber preparation has not been optimized to achieve maximum speeds of the photoreduction processes. Generation of crystallites is easy to detect as the colorless fibers turn brown in the case of Ag particles and red for Au particles. Such optical properties are typical of particles exhibiting strong surface plasmon resonances, indicating that the average crystallite diameters are smaller than 100 nm. An important result is that particle generation took place only under high intensity light, that is, no crystallites were formed under ambient light. This is a desirable property for two reasons: a) continuous formation of particles even at low photon intensities would result in an extensive oxidation of the PVA, which would eventually lead to deterioration of the physical properties of the fibers. b) Fast formation of metal crystallites is desired only under the specific conditions of high photons fluxes, for example, when exposed to a burst of radiation.

So far, particle decay not was detected at room temperature in the PVA fibers. This is not an unexpected result because our fibers lack constituents that destabilize the metal crystallites and facilitate their oxidation reactions. Nevertheless, metallized fibers containing stable crystallites are interesting as materials with controllable electromagnetic properties. These fibers may be useful in several military applications where flexible textiles are desired as shielding enclosures for electromagnetic radiation, radar absorbing fabrics, and resistive heating elements for garments. Room temperature stability of the metal particles is also prerequisite for materials capable of performing 3D optical data storage. This concept is based on the idea of storing data along the 3 dimensions of a solid rather than 2D recording as is done in the present. Interestingly, stretching of PVA fibers induces formation of bands perpendicular to the main fiber axis and, if generation of metal particle takes place only along these bands, then data recording will occur in a sequential and 3D fashion. Proper preservation of the stored information is ensured in cases where the particles decay only at temperatures close to the softening point of the polymers (about 60 °C), that is, the metal crystallites will remain stable at temperatures equal to or slightly higher than room temperature. Erasure of the stored data will be achieved by heating the fibers to temperatures close to the softening point of the fibers, where oxidation of the metal particles is initiated by increases in the diffusion rate of chemicals that facilitate this process. Suitable fibers able to sustain high loadings of stable particles and/or formation of particles in preferential orientations are required for these applications and efforts in these directions are currently under way.

Future Plans

The next step is to find appropriate conditions under which formation of small metal particles occurs in a reversible fashion in polymeric matrices. Thin, transparent PVA films will be utilized initially for convenience. The reversibility of the particle formation process will be tested with the films using conventional optical absorption methods to follow the progress of the formation and decay reactions. This procedure is simpler than using fibers, which require more involved optical methods such as reflectance spectroscopy. In addition, determination of other physical properties, such as conductivity, is less complicated in the case of films. Once the reversibility of the formation is demonstrated we will develop similar processes in PVA fibers.