Nanocomposites dichroic

Figure 9.1. Microscope image of a dichroic nanocomposite consisting of gold particles embedded in wood (probably fir), published in 1905 [36], The polarization plane of the light is parallel to the orientation axis of the wood in the left image and perpendicular in the right image. See color insert.

Dichroic nanocomposites with pronounced color changes are potentially useful in liquid crystal display (LCD) applications. The setup of a traditional... 

In 1910, Kolbe was the first to prove that dichroic nanocomposite samples based on gold contained the metal indeed in its zero-valence state. Such affirmation was confirmed a few years later by X-ray scattering. In particular, it was shown that zero-valence silver and gold were present in the respective nanocomposites made with oriented ramie fibers, and the ring-like interference patterns of the metal crystallites showed that the individual primary crystallites were not oriented (32). Based on Scherrer s equation, which was developed just in this period, the average particle diameter of silver and gold crystallites was determined in fibers of ramie, hemp, bamboo, silk, wool, viscose, and cellulose acetate to be between 5 and 14 nm (33). 

There is a general agreement that nanocomposites imply nanoscale fillers with sizes less than 100 nm in at least one dimension. The fillers can be classified into three groups depending on their shape. One-dimensional nanofillers are nanorods, fibers, or tubes with varying aspect ratios. Rodlike nanopartides can introduce anisotropic properties to composite materials, for example, silver or gold nanorods are used for preparation of dichroic nanocomposites as described later (Figure 1). 

Further applications of metal-polymer nanocomposites are in the fields ofultrahigh and ultralow refractive index materials [Weibel et al., 1991 Zimmerman et al., 1992, 1993], dichroic color filters [Dirix et al., 1999a,b], nonlinear optical filters [Qu, S. et al., 2002], and catalytic polymer membranes [FritschandPeinemann, 1995 Troger etal., 1997]. 

Around 1900, widely forgotten reports indicated the preparation of polymer nanocomposites with uniaxially oriented inorganic particles, and their remarkable optical properties (14,15). Dichroic plants and animal fibrils (eg, linen, cotton, spruce, or chitin, amongst others) were prepared by impregnation with solutions of silver nitrate, silver acetate, or gold chloride, followed by reduction of the corresponding metal ions under the action of light (16). 

Other examples that focus more on the optical aspects of nanocomposites include UV absorption stability enhancement, formation of translucent nanocomposites with tunable color, dichroic materials, low emissivity systems controlling energy loss, near-infrared absorbing additives for laser engraving and welding, luminescent materials, and refractive index adjrrst-ment. Such materials can be expected to show additional property improvements with additional research. 

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