(395g) Exploring Liquid Crystalline Phase Behavior of Pyrene Pitch Oligomers Isolated Via Supercritical Extraction

Discovered in the mid-1960s, liquid crystalline (also known as mesogenic/mesophase) pitches became the foundation for a new class of highly graphitic, carbon-based materials. Due to their high carbon content and molecular order in the liquid phase, mesogenic pitches can be melt-processed into carbon materials that possess a valuable combination of mechanical strength, modulus, thermal conductivity, and temperature resilience. However, not all mesophase pitch (MP) precursors are made equal and they must be optimized based on processing capabilities. For instance, higher amounts of mesophase may yield a better final product, but also make the melt more prone to fracturing while being drawn. Furthermore, the softening point of the MP must be low enough so that it will flow before thermal degradation becomes significant, but if the softening point is too low, the MP can undesirably liquefy in downstream processing, wiping out any macroscale shaping. Thus, a thorough understanding of the pitch precursor and its properties/phase-behavior is necessary for successful processing.

Although an improved fundamental understanding of petroleum-based pitches is of great interest, investigations are severely hindered by the innumerable array of individual compounds that comprise these pitches. As a model system, pyrene pitch is ideal for two reasons: 1) the relatively low extent of fragmentation means that pyrene pitches are chiefly composed of only a handful of oligomers, and 2) pure pyrene trimer (MW = 598 Da) forms 100% mesophase with a melting point below 300°C.

Supercritical extraction has proven to be a valuable tool for investigating the properties and phase behavior of pyrene pitches, as it is the only known tool capable of isolating individual pitch oligomers on at least a gram-scale. With these oligomers isolated, they can then be recombined into mixtures of well-defined composition, and the mesophase/isotropic phase behavior of these mixtures characterized. This allows for a systematic exploration of phase behavior, where each “designed mixture” is characterized by the mesophase content (as a function of temperature), softening point, and char yield. Because smaller oligomers depress and larger oligomers elevate the softening point of a mixture, understanding their impact is critical for successfully optimizing the properties and mesogenicity of a pyrene pitch precursor.