Thin film metal oxide coatings have been used for commercial electromagnetic filters from the UV to infra red regions
for over half a century. Deposition onto a substrate has typically been accomplished using vapor deposition techniques
and more recently sol-gel methods. These coatings provide very good optical performance under abrasion, thermal
cycles and variable humidity when applied on substrates with similar thermal and mechanical properties. When
conventional metal oxide coatings are applied to flexible, relatively soft substrates such as polymers, mismatches in
mechanical properties can reduce interfacial adhesion or accelerate mechanical failures. The authors recently showed
that a thin film polymer nanocomposite can be applied on a polymer substrate and maintain adhesion even under high
strains. This paper describes the demonstration of an IR mirror using fifteen discrete layers with an IR-reflectance that
exceeds 90 percent at 1064 nm and transparent in the visible spectrum. We will present the results with thin film stacks
containing over 15 discrete layers for IR mirror applications, and our recent work shows that the technology can produce
thin film stacks containing 30 layers or more. Furthermore these coatings have high flexibility and can be applied to
curved polymer substrates. These IR mirrors can withstand thermal cycling and large strains much better than those
made using the state of the art techniques.
Metal oxide nanoparticles can be used in thin film polymer systems to engineer specific material properties while
maintaining visible transparency. High loadings of nanoparticles in a polymer can manipulate refractive index, modulus,
and UV absorption over a wide range. Because the polymer binders can be allowed to dominate the physical properties,
these systems are ideal where materials undergo large strains. While stable dispersions of sub-100nm diameter CeO2,
ZnO, and SiO2 are well understood and commercially available, our group also developed a stable dispersion of TiO2
nanoparticles. These metal oxides are significantly harder than the host polymer, have high UV absorption, and cover a
large refractive index range. Our group has successfully incorporated these materials into PMMA thin films with
loadings up to 60% by volume (approaching the theoretical close packing of spheres). These thin film nanocomposites
have been successfully incorporated into 30 layer, sharp cut optical filters that easily withstand large strains induced by
mechanical loading and thermal cycling. In these films we have adhered to the rule that nanoparticle diameter should be
one-tenth the wavelength of visible light. As the thickness of the overall filter stack increases, light scattering is
intensified, so the dimensions and refractive indices of the nanoparticles become critical for highly transparent systems.
We study here the interactions of particle dimensions, refractive index, loading, thickness, and transparency in
nanocomposites.
Nanocomposites are created by doping host polymers with nanoparticles that typically have higher or lower refractive
indices. The ability to tailor the mechanical and optical performance of these composites has led to their increased use in
transparent materials. Nanocomposites maintain the elastic properties of the binding polymers and exhibit infinite
refractive index tunability between the limits of the system. These unique properties provide distinct benefits for multilayer,
thin-film optical filters. Because the nanoparticles are dispersed in a fluid or bound in a polymer matrix in use,
toxicity risks that may be associated with raw particles are reduced. Using a stable dispersion of titanium dioxide
nanoparticles and a UV curable monomer, we were able to design and produce several quarter-wave filters that
demonstrate control of the height and width of the passband through adjustment of the organic/inorganic ratio and layer
count. The volume loading of the metal oxides can be adjusted from zero to near the theoretical packing density of
spheres, allowing refractive index to be controlled over a large range. Because metal oxide particles exhibit high UV
absorption, these additives provide UV protection to the host polymer and the filter's substrate. Additionally, significant
improvements in abrasion resistance are often observed in films loaded with nanoparticles at the concentrations of interest.
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