National Textile Center

Project Team:

Leader: B. Lewis Slaten, Auburn, Consumer Affairs Department

Expertise: Polymer Chemistry, Fiber Science, Textile Finishing

Email: blslaten@humsci.auburn.edu Phone: 334-844-1330

Members: German Mills, Auburn, Chemistry Department, Physical chemistry

Royall Broughton, Auburn, Textile Engineering, Fiber Science

Objective:

This research is centered on the development of photoresponsive fibers for applications such as reflection of infrared radiation, shielding of electromagnetic radiation, as well as 3D storage of data. Several textile applications are logical for the photoadaptive fibers; the first is as clothing materials for areas of high intensity radiation. One use involves protective garments for firefighters, where IR reflection must be maximized and another is for clothing worn in regions of intense sunlight. This objective can be achieved using high concentrations of small and large metal crystallites. Since the latter particles reflect IR more efficiently than the former, polymers containing high concentrations of both types of crystallites will yield enhanced IR reflection and heat consumption during

the decay of the particles. Secondly, fibers containing metal crystallites are expected to show improved antistatic properties. Enhancement in the capacity of efficient charge transport may prove to be advantageous because

formation of electrostatic charges occur readily in hot environments. Furthermore, formation of large amounts of metal particles inside the fibers is expected to yield materials with controllable electromagnetic properties. These "metallized" fibers may be useful in several military applications where flexible textiles are desired as RFI/EMI enclosures, radar absorbing fabrics, and resistive heating elements for garments. Last, is the development of a 3D optical storage system. The systems that will be evaluated for data storage purposes consist of fibers having regularly spaced structural anisotropies, where metal particles will form as sequential layers.

Relevance to NTC Mission:

Successful completion of this project will provide the U. S. Textile industry with new and innovative products. The production of these products will advance the position of the industry in the global technical textile market place. These products will provide for enhanced performance of protective clothing, unique data storage capability and expand the use of textiles into areas where textiles are not currently being utilized.

State of the Art:

Chromic phenomena are presently being used in many applications. These applications include photoluminescent materials used for safety in clothing, photo responsive dyes in textile fibers that change hue when worn indoors vs outdoors, temperature sensitive dyes used in so-called chameleon fabrics. NTC Project M98-C01 is investigating the use of tunable molecules and oligomeric devices that respond to electrical or magnetic fields which may cause reversible, color changes. These all are different in chemistry and application in respect to the photoadaptive systems being investigated in this project. There is a significant body of work relating to the use of metal particle formation in photochromic glass. The concept is similar but not directly applicable to textile fibers.

Approach:

Adaptive (or responsive) systems exhibit desirable and predictable reversible alterations of their properties, and are usually called "smart" systems. Photoadaptive systems experience reversible changes upon exposure to UV-Vis light. A simple example is photochromic glass, where photo reduction of silver halides yields Ag particles that decay in a concurrent dark reaction with cupric ions to reform the starting silver halides. We developed a new photoresponsive system involving photo reduction of tetrachloro Au(III) complexes incorporated into methanol-swollen crosslinked polymers of diallyldimethylammonium chloride (DADMAC). Nanometer-sized Au particles are formed, these particles are then oxidized at room temperature in a dark reaction to regenerate the starting materials. These results suggest that development of photoresponsive fibers is feasible if similar particle generation and decay steps occur inside polymeric fibers. The photoadaptive fibers envisioned here operate in a fashion similar to photochromic glasses where metal particles form by exposure to high intensity visible light, but decay once the intensity of light decreases to the level of ambient light. Successful development of the photoadaptive fibers will only occur if efficient oxidation of the Ag and Au particles is achieved. From the thermodynamic point of view, the key requirement is to create an environment in the fibers that induces spontaneous decay of Ag and Au crystallites. Conditions that aid the oxidation of the particles are: a) the absence of high concentrations of water (< 0.5% v/v), and b) formation of nanometer-sized particles, since these types of crystallites are relatively unstable towards oxidation even in aqueous solutions. In addition, oxidizing agents will be introduced in the fibers to accelerate the oxidation process. As in the case of photochromic glasses, Cu(II) ions will be the oxidizer in Ag-containing fibers. For fibers containing Au we plan to use high concentrations of chloride ions, which provide higher thermodynamic stability of the starting chloro Au(III) complex with respect to metallic gold. It is known that photoreduction of Au(III) ions to form metal occurs efficiently in solutions of alcohols or films of poly(vinyl alcohol), PVA. 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 their synthesis using dimethylsulfoxide (DMSO) as a crosslinking agent have been reported.

Our ongoing investigation has shown that attempts to load crosslinked PVA fibers with metal ions by exposure to solutions of AgNO3 or NaAuCl4 yielded poor incorporation of Ag+ and AuCl4¯ into the polymeric materials. Therefore generation of particles in the fibers was inefficient and metal crystallites formed only on the fiber surface. 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 ionexchange 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 photon fluxes, for example, when exposed to a burst of radiation.

So far, particle decay was not 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. A further step will be to develop polymeric matrices containing high chloride or Cu²⁺ ion content to insure efficient particle oxidation while retaining desirable fiber properties. Another problem that needs to be resolved is non-uniform distribution of metal crystallites in the fibers. Longer absorption times or increased metal ion concentrations in the doping solutions are expected to yield more uniform particle distribution. 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 a high loading 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.

In summary, we have achieved the generation of small metal particles in PVA fibers. This is a desirable result for applications where flexible materials are needed for shielding against electromagnetic radiation, as radar absorbing materials or for resistive heating elements for gloves and socks. The next task will be to optimize high particle loading and also to develop methods for achieving similar results using copper (lower cost). A key parameter will be uniformity of the particle distribution. It is expected that a significant effort will be required to develop methods that yield particles of uniform size. Since generation of stable copper particles is more difficult, we will explore the possibility of triggering particle formation, using seed particles of Ag or Au; another strategy will be the formation of alloy particles, which are expected to suppress oxidation of copper in the present of ambient moisture.

____________________________________________________________________________________________

This Year’s Goal:

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, DMSO or formaldehyde will be used as crosslinking agents to stabilize the films. 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. Another goal is to optimize loading of stable metal particles in fibers to achieve conductivity values that are appropriate for resistance heating and shielding applications.

Outreach to Industry:

Fiber and technical textile producers will be contacted and consulted where technical input is needed. Potential users of the new materials will be informed of their possible applications. The fundamental studies will conducted at Auburn. Successful completion of this project may lead to the use of pilot scale equipment at a selected producer in a subsequent tech transfer project.

New Resources Required:

Laboratories in the three participating Auburn departments are able to provide most of the equipment needed to complete this project. A double beam UV/Visible spectrophotometer for reflectance measurements has been purchased. A spin-coater for production of uniform films is needed.