Liquid crystal polymers preparation methods

Applications of this method include among others the preparation of unsym-metrical diaryldiynes [27] and of l,4-bis[2-(4 ,4"-dialkoxyphenyl)ethenyl]benzenes for polymer [28] and liquid-crystal synthesis [29]. Such one-pot acetylene double couplings are also possible between (substituted) 6-bromoazalenes and substituted bromobenzenes [30], and even macrocyclic polyenepolyynes have been made in this way ([31, 32], Scheme 3). 

Orientation of the polymer chains can be obtained by different methods. Highly oriented films of poly acetylene have been made by performing polymerization in a liquid crystal 11,2]. It has also been shown that it is possible to attain highly oriented films of polyacetylene 13] and poly(p-phenylene vinylene) [4] by stretching their precursors. Kaneto et al. 15] have oriented polythiophene by stretching a film, prepared electrochemically on In-Sn oxide (ITO)-coated poly(ethyleneterephthalate) films. The processability of the recently developed poly(3-alkylthiophenes) [6-8] makes this class of polymers suitable for stretching. Yoshino et al. [9] have shown that it is possible to orient poly(3-alkylthiophene), both as free-standing films and in a blend with an elastomer. 

The diffusion of liquid crystals into polymers is particularly intriguing given that the liquid crystal may be in an organized state or an isotropic state depending on the temperature. The diffusion of a liquid crystal (5CB) into a PBMA matrix was studied by using the contact method to prepare a gradient [114]. Concentration profiles were obtained as a function of time and temperature. The presence of an anomalous diffusion process was detected. It was shown that fast FTIR was able to correctly identify the diffusion process as anomalous. As opposed to this, a bulk... 

The liquid crystal polymerization method was modified to prepare a vertically aligned poly acetylene film [58]. As a result, a film with a very curious morphology was formed that was composed of two layers. The layer on the solvent side had a vertically oriented fibrillar structure, whereas the layer on the acetylene gas side had a randomly oriented one. The modified method is very promising for visualizing the polymer growth process and clearly demonstrates that PA chains grow in a liquid crystal solvent. 

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]. 

One of the standard techniques often used for this purpose is the preparation of a perfectly aligned smectic A sample. Liquid crystal often undergoes the following phase sequence isotropic phase (nematic) smectic A => smectic C. The nematic phase (if it exists) could be oriented by traditional methods, such as the rubbing of polymer films or the evaporation of inorganic films at an oblique angle (< 10 ) [135]. The quality of orientation improves for lower values of the specific heat at the nematic smectic A transition and the minimum amount of crystallization centers. 

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