Cholesteric liquid crystals molecular alignment

To form cholesteric liquid crystalline polymers, one either polymerizes cholesteric monomers or mixes low molecular mass cholesteric liquid crystals with polymers. In the latter case, two components may be mixed homogeneously or in such a way that the polymers act as a matrix while the small molecular mass cholesteric liquid crystals are in droplets, known as the polymer-dispersed liquid crystals (PDLC) (Doane et al., 1988) or the nematic curvilinear aligned phase (NCAP) (Fergason, 1985). In addition, there are many polymers in nature exhibiting the cholesteric phase such as PBLG, cellulose, DNA, etc. 

A cholesteric liquid crystal contains both nematic and chiral moieties in a single molecular entity or as a binary mixture. Consisting of a helical stack of quasinematic layers, a well-aligned cholesteric film can be characterized by handedness and helical pitch length, p, as depicted in Figure 5 ... 

The static theory of nematic and cholesteric liquid crystals employs the unit vector field n, frequently referred to as the director, to describe the mean molecular alignment at a point x in a given sample volume V, Hence... 

Structure of thermotropic liquid crystals is rather well understood. There are three main structural types nematic, cholesteric, and smectic. In nematic liquid crystals molecules are aligned approximately in the same direction, but positionally molecules are disordered. An axis of preferable molecular orientation is called a director. More precisely, the director is defined as a unit vector n(r) that is parallel to the molecular orientation at the point r. If we use the long axis of the molecules as a reference and denote it as k, the microscopic scalar order parameter 5 is defined [16,17] as follow ... 

Systems that lack inversion symmetry (for example, because of addition of chiral constituents) can form liquid crystal phases with a spontaneous helical distortion to the direction of molecular alignment. One of these phases is the cholesteric, also called a twisted nematic. It is similar to a nematic, but the alignment... 

The photochromic reaction in liquid ctystals (LCs) is one of the effective stimuli inducing modificaticm of the LC ( tical prqralies. For example, photochromic reactions in cholestoic liquid crystals (CLCs) can induce dianges in the cholesteric iridescent reflections (reflected colors) due to the helical molecular alignment.(i, 2) Since the molecular aligrunoit of the CLC... 

Figure 1.11. Molecular alignments of liquid crystals (a) nematic and (b) cholesteric or chiral nematic.

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