Polyaromatic mesophase

Keywords Polyaromatic Mesophase / Silicon-Alloyed Carbon Fibers / Co-pyrolysis / Monomeric and Polymeric Precursors... 

The polyaromatic mesophase (PA-MP) is a nematic, discotic, chemotropic liquid crystal. Owing to its high density (about 1.5 gcm ), its high carbon yield of about 90 %, and its thermoplasticity, it is unique as a precursor of structure carbons. An important application is the manufacture of high modulus (HM) and ultra-high modulus (UHM) carbon fibers [1]. By alloying with silicon, physical and chemical properties of the materials, such as strength, hardness and oxidation resistance, can be improved. These modified carbons were available by chemical vapor deposition (CVD) processes only up to now. The preparation by liquid phase pyrolysis is novel, economic, and thus opens a completely new field of applications. 

Mesophase. The general carbonization mecheinism of polyaromatic hydrocarbons is relatively simple, at least in theory, since it proceeds by the rupturing of the carbon-hydrogen bonds and the removal of the hydrogen. Some of these hydrocarbons first go through an intermediate liquid or plastic stage which occurs at temperatures above approximately 400°C. This stage is the so-called mesophase , in which the material shows the... 

Polymerization. The pitch is heated to approximately 400°C and is transformed from an isotropic to a mesophase (or liquid crysteil) structure consisting of large polyaromatic molecules with oriented layers in parallel stacking. This structure is similar to the needle-coke stage of molded carbons described in Ch. 4, Sec. 2.3. 

At the first view, therefore, a carbon fibre made from mesophase pitch seems to be a contradiction itself. The solution of this problem may be seen in the stabilization treatment of the fibre after spinning which is not only necessary for conservation of the fibre morphology during subsequent heat treatment (the same as with the thermoplastic PAN) but also for crosslinkages between the preformed polyaromatic compounds, by which formation of graphitizing carbon should be inhibited. Thus again, the stabilization treatment in air... 

Until 1977, it was believed that only rod-like molecules, having a considerably higher length than width, could show liquid crystalline properties (calamitic mesogens). However, in 1977, Chandrasekhar realized that not only rod-like molecules, but also compounds with a disc-like molecular shape are capable of forming mesophases (Chandrasekhar et al. 1977 Sabine et al. 2007) and these are referred to as discotic liquid crystals (DLCs). The first examples of this kind of mesomorphism were observed in hexa-alkanoyloxybenzenes, hexaalkoxy- and -aUcanoyloxy-triphenylenes (Fig. 16.2), and were established by X-ray studies (Kumar 2010). Generally, the disc shaped molecules have central core as benzene or polyaromatic, such as triphenylene or phthalocyanine. 

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