Pigments Based on Liquid Crystal Polymers

Interference and angle-dependent color effects can also be achieved by layers or particles based on liquid crystal polymers (LCP) (8, 35]. Such effects can, for example, be produced by small plate-hke substances which consist of an LCP material itself, or by small platelets which are uniformly coated with a cross-linked Hquid crystalline polymer in a chiral-nematic arrangement (36-38). 

The thickness of a 360° turn of the director represents the pitch length p of the helix [35]. The color design of LCP pigments in paint fihns or of pure LCP films has its origin in an interference phenomenon. In this case, only incident Ught with a wavelength equal to the LCP lattice separation interferes and is reflected. 

On the other hand, a structure which has a helical superstructure with no change in the refractive index can also reflect Hght just like cholesteric phases [38]. In this case, it is not so much a change in the refractive index that gives rise to the optical effect but rather the superstructure. 

The reflected light waves from the layers increase the intensity of the total reflection. The maximum reflectivity of one polarization state requires at least 6 helices or a thickness of about 3 pm. The most efficient reflection is given by layers with a thickness of up to 10 pm. 

Cholesteric materials are temperature sensitive and show a thermochromic effect The reason for this is that the pitch length of the helix and the refractive index are temperature dependent [35, 36]. 

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Pigments Based on Liquid Crystal Polymers 99 Figure 7.17 Scheme of a liquid crystal film. 

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