Nematic structure, polymerization

The fact that the structure of a solid monomer influences its polymerization substantially now seems obvious. It is not as clear whether structural phenomena can effect polymerization if the monomer is a liquid. It has long been known that ordered regions or clusters exist in liquids, and several years ago it was assumed that in some cases these regions in liquid monomers can influence the polymerization. One of the most vivid examples—namely, polymerization in the liquid-crystalline state—was accomplished by Krentzel and co-workers (I, 2, 3). The object of their study was p-methacrylylhydroxybenzoic acid, which forms conventional crystals in the pure state and does not polymerize in the solid state. However, when mixed with alkoxybenzoic acid, it forms liquid crystals of both smectic and nematic forms. Polymerization of p-meth-acryllylhydroxybenzoic acid in various forms of liquid crystals was compared with polymerization of the same substance dissolved in dioxane and dimethylformamide (DMF). 

The present polymeric system is divided into no equivalent partial regions. There are only three different states for a partial region, that is, nematic structure, smectic structure and pre-crystalline structure. Every partial region takes one of the three states. The number of the nematic regions, smectic regions and pre-crystaUine regions at time t are denoted by n (t), ns t), and ritit), respectively. 

Liquid crystal polymers have phase transitions similar to conventional polymers. Polymeric crystals may form either smectic or nematic structures upon melting, but can become a completely nematic phase once the temperature is further increased. If the temperature is extremely high, an isotropic melt is formed. However, because LCPs decompose at high temperature, the isotropic transition may not take place before the composition. As a result, it is sometimes very difficult to determine the exact structure of potential LCPs. 

For structures with a high curvature (e.g., small micelles) or situations where orientational interactions become important (e.g., the gel phase of a membrane) lattice-based models might be inappropriate. Off-lattice models for amphiphiles, which are quite similar to their counterparts in polymeric systems, have been used to study the self-assembly into micelles [ ], or to explore the phase behaviour of Langmuir monolayers [ ] and bilayers. In those systems, various phases with a nematic ordering of the hydrophobic tails occur. 

On the other hand, polymer B.3.2 (Table 9), which according to the X-ray analysis, does not exhibit a layer structure, displays fan texture, typical for low-molecular and polymeric smectics. It might be suggested, that this polymer also possesses some elements of layer ordering, which are different from those known for low-molecular sibotactic nematics. 

The only general statement that might be made is that the specific features of the structure of polymeric nematics are provided for by the ordering effect of the main chain on the packing of mesogenic groups in LC phase. 

The fusion of LC phases above Tcl causes a sharp change in the character of flow and the values of Ef for nematic and smectic polymers become closer. In an isotropic phase Ef for a polymeric smectic ( 140 kJ/mole) is only twice as large as Ef for a polymeric nematic (70—80 kJ/mole). In other words, the transition from LC phase to isotropic melt, accompanied by the liberation of mesogenic groups from the mesophase levels the differences in the character of flow of smectic and nematic polymers. The differences in Ef for isotropic phase are determined only by the differences in chemical nature of the main chain of smectic and nematic polymers. The values of Ef, in this case, are close to the Ef values for poly(butylmethacrylate) and poly(butylacrylate), respectively, which are structurally similar to polymers XI and XII except that they do not contain mesogenic groups. 

As another example of a helical polyacetylene, the single-handed helical polyacetylene fibril, whose structure was studied by SEM, was prepared by the polymerization of acetylene within a chiral nematic liquid crystalline phase.192... 

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