(481e) Optical Nanocomposite Thin Film Filter Undergoing Extreme Strains

Nanocomposites using organic polymers and inorganic nanoparticles will play an important role in applications requiring high visible transparence and mechanical flexibility. A unique class of materials is created when the elastic properties of polymers are combined with the inherent hardness of crystalline nanoparticles. Transparent polymers are generally more flexible than glass materials and exhibit a moderate range of optical, electrical and mechanical properties. The incorporation of inorganic nanoparticles in the polymer system significantly increases the physical parameter range beyond that of the host polymer and allows material properties to be engineered for specific applications. Of particular interest to our group are thin film nanocomposites on polymer substrates in which strain domains are well matched.[1,2]

True nanoparticles are significantly smaller than visible light wavelengths and, therefore, generally do not cause significant optical scattering when incorporated in a polymer. Careful nanoparticle selection can influence refractive index, electrical conductivity, UV absorption, magnetism, and a host of other properties. The composite materials can maintain the elastic properties of the binding polymers while improving abrasion resistance through the inclusion of high hardness nanoparticles. This provides opportunities to improve transparent materials that require clarity and abrasion resistance. A multi-layered, nanocomposite coating having high transparence and flexibility is shown in figure 1.

	Figure 1: A 9-layer, nanocomposite, multi-layered thin-film coating on a polymer substrate demonstrating high flexibility and transparence.

Thin film coatings of less than 100 nm to tens of microns in thickness are important in many polymer substrate applications, but the conditions under which these systems are used must often be limited to maintain film and substrate integrity. Since mainstream coatings are often ceramics applied using vacuum deposition, they normally have strain domains that are significantly different from the polymer substrate. Under large strains these coatings tend to crack and induce objectionable optical scattering that is quantifiable as haze. These cracks allow other types of damage to easily propagate from them as well. The processing temperatures used to apply these coatings also leave behind large intrinsic stresses that further limit their utility due to significant changes in the ultimate strength of the coated substrate.

Here we demonstrate an optical thin film filter composed of a multi-layered nanocomposite undergoing strains in excess of 20 percent. These filters were designed to have a peak reflectance at a chosen wavelength in the visible region. As strain is applied to the system the peak wavelength changes, but the intensity of the reflectance remains constant. This confirms that the films are not failing at large strains and demonstrates a performance unmatched by traditional vacuum deposition techniques.

1- Krogman, K., T. Druffel, and M. Sunkara, Anti-reflective optical coatings incorporating nanoparticles. Nanotechnology, 2005. 16: pp. S338-S343.

2- Druffel, T., Mandzy, N, Sunkara, M. and Grulke, E., Polymer nanocomposite thin film mirror for the infrared region. Small, 2008, 4, pp 459-461