Glass-forming materials, nematic

Pc macrocycle. Some of them are mesomorphic and show a Colh phase (Fig. 79 I-Gl G 115 Coin 270 I II-Gl G <-20 Colh > 320 h H-G2 G 115 Colh 250 I II-G3 94 Colh 108 I) for which the structure of the columnar mesophase is frozen at room temperature (anisotropic glasses). The functionalization of these dendrons in the 3 and 5 position of the terminal rings by oligo(ethyleneoxy) chains (0CH2CH2)30CH3, and their subsequent grafting onto the phthalocyanine of type I (Fig. 79) led to amphiphiUc materials [333]. In concentrated ethanol solutions ( 20-40% by mass), the compound with the fimctionaUzed G1 dendron behaves as a discotic amphiphUe forming a coliunnar nematic lyotropic phase. In addition, it possesses a columnar mesophase stable from room temperature up to 260 °C. Systems of type III are not mesomorphic. 

The simplest PPEs are the dialkyl-PPEs 4 made by either alkyne metathesis of dialkyldipropynylbenzenes or by Pd-catalyzed couplings. The materials are thermally and photochemically very stable and form thermotropic liquid crystalline phases of the smectic variety [55], Optical textures under crossed polarizers resemble those of nematic phases due to the homeotropic alignment of the chains on glass... 

Similar materials could be obtained by an emulsification method [253]. Nematic liquid crystal is emulsified into an aqueous dispersion of a water-insoluble polymer colloid (i.e., latex paint). An emulsion is formed which contains a droplet with a diameter of a few microns. This paint emulsion is then coated onto a conductive substrate and allowed to dry. The polymer film forms around the nematic droplets. To prepare an electrooptical cell a second electrode is laminated to the PDLC film [253]. In the phase separation and solvent-casting methods the chloroform solutions of liquid crystal and polymer are also used [254, 255]. The solution is mixed with the glass spheres of the required diameter to maintain the desired gap thickness and pipetted onto a hot (140 °C) ITO-coated glass substrate [255]. After the chloroform has completely evaporated another ITO-coated glass cover is pressed onto the mixture and then it is cooled down. Structural characteristics of the PDLC films are controlled by the type of liquid crystal and polymer used, the concentration of solution, the casting solvent, the rate of solvent evaporation, perparation temperature, etc. [254]. 

In the liquid crystalline phase, the molecules lose their positional order, but their mesogenic units still retain their orientational order. When the liquid crystalline polymers solidify, polymer glasses are formed in which the liquid crystalline structure may become frozen in. The glassy state may have the orientational order of a nematic phase or a smectic phase depending on the liquid crystalline phase present before the glass transition. This gives the material properties not normally found in amorphous glassy... 

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