391765 Liquid Crystallinity in PAH Oligomers Isolated from Carbonaceous Pitches VIA Supercritical Extraction
Using supercritical extraction to effect the separation from a broad molecular weight distribution carbonaceous pitch, the first unsubstituted, monodisperse polycyclic aromatic hydrocarbon (PAH) to form a liquid crystalline phase (100% mesophase) has been isolated. With a molecular weight of 598 Da and consisting of only 14 aromatic rings, this pyrene trimer is also the lowest molecular weight (mol wt) PAH species for which the existence of liquid crystallinity has been reported. Multiple isomers of the pyrene trimer exist, providing the melting-point depression (mpt = 290 °C) necessary for the existence of a liquid phase and the possibility of mesophase formation. The trimer cut of an M-50 petroleum pitch (mol wt = 645-890 Da; mpt = 330 °C) was also isolated via supercritical extraction and was found to consist of ~40% mesophase. This trimer is the lowest average mol wt carbonaceous pitch for which significant mesophase formation has been reported.
Both trimers were isolated from their starting pitches via multistage, packed-column supercritical extraction (SCE), using toluene and N-methylpyrrolidone (NMP)/toluene mixtures as the extractive solvents. Mass spectrometry and UV-vis and fluorescence spectroscopy were used for molecular characterization and validation.
The results of this study indicate that for PAHs, the molecular weight for which liquid crystallinity occurs can be significantly reduced by creating PAH oligomers of lower polydispersity and increased monomer-unit homogeneity. The inherently isomeric nature of such systems will provide the melting-point depression required for existence of a liquid crystalline phase. Potential materials applications for these materials will also be briefly discussed.
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