Bulk mesophase

Two simple thermodynamic considerations are suggested upon examination of Fig. 8. The first is that at temperatures below Tml the free energy of the bulk mesophase G m is in general bound to be lower than Gl, the free energy of the amorphous. In the limit of Class II mesophases, since AHml = 0, we will have Gm = Gl at T = 0 K while Gm < Gl at temperatures 0 < T < Tml since it is Sm > Sl at temperatures low enough as compared to Tml (Sect. 3.1). In the case of Class I mesophases AHml > 0. he., mesophases are enthalpically stabilized with respect to the liquid state, while Sm < Sl, so it will be Gm < Gl at temperatures T, with 0 < T < Tml- Note that the above consideration will in... 

Fig. 8 Free energy per chain bond of the liquid (L), of the bulk mesophase (M), and of the crystalline (C) polymer. Txy is the transition temperature from X to Y (X, Y = L, M, C). a In this case the mesophase is stable between 7cm and 7ml while Tcl is virtual. (From [11]) b The mesophase is virtual, and so are 7cm and 7ml melting of the crystal into the liquid is only observable transition occurring at Tcl...

The spatial geometry of disclination reactions in bulk mesophase has recently been presented by Zimmer and Weitz (29) Working with coarse-structured mesophase prepared by lengthy pyrolysis of A240 petroleum pitch at 400°C, they defined the disclination arrays on a succession of polished sections spaced at about 7 ym. In this way a +tt disclination was traced through a branching point (i.e., a reaction point) to form a -tt and a +2tt disclination. Thus a reaction... 

Figure 16. Spatial sketch of a disclination reaction in bulk mesophase.

For materials containing only C, H, and 0, oxygen and hydrogen have opposite effects. Therefore, an important parameter is the atomic ratio 0/H (1,2,6,7). As oxygen content increases, the size of the elemental domains of bulk mesophase (local molecular orientation, LMO) decreases. As hydrogen content increases, LMO... 

After the measurement of the NMR spectra of Kureha pitch at 723 K and ethylene tar pitch at 743 K, the samples were immediately quenched to room temperature and observed by polarized-light microscopy. It was confirmed that the bulk mesophase was produced. 

The difference in orientation between the domains creates a complex array of polarized light extinction contours in the bulk mesophase corresponding to various types of linear discontinuity in molecular alignment. 

Pitches are thermoformable and suitable, in the molten state, to be melt extruded through the orifices of a spinneret. Upon heating, the apparent melt viscosity of a pitch initially decreases within a narrow temperature range (the softening range), then passes through a broad minimum, and increases sharply above 450-500 C as bulk mesophase is formed [23]. 

Union Carbide [243] claims that a pitch suitable for spinning should preferably contain 40-90% mesophase, with domain sizes greater than 200 pm, which will then produce a fiber with a highly oriented structure. Domain sizes below 100 pm produce stringy clumps, unsuitable for spinning. When the droplets coalesce to form the bulk mesophase, some... 

BULK MESOPHASE is a continuous anisotropic phase formed by the coalescence of mesophase spheres. BULK MESOPHASE retains fluidity and is deformable in the temperature range up to about 770K, and transforms into GREEN COKE by further loss of hydrogen or other low molecular weight species. 

CARBONACEOUS MESOPHASE is a liquid crystaUine state of PITCH which shows the optical birefringence of disc-like (discotic) nematic crystals. It can be formed as an intermediate phase during thermolysis (p5Tolysis) of an isotropic molten PITCH, or by precipitation from PITCH fractions prepared by selective extraction. With continuous heat treatment, the CARBONACEOUS MESOPHASE coalesces to a state of BULK MESOPHASE before solidification to GREEN COKE, with further loss of hydrogen or other low molecular weight compounds. 

MESOPHASE PITCH is a PITCH with a complex mixture of numerous, essentially aromatic hydrocarbons containing anisotropic liquid crystalline particles (CARBONACEOUS MESOPHASE), detectable by optical microscopy and capable of coalescence into the BULK MESOPHASE. 

This bulk mesophase can sometimes be formed directly from the isotropic pitch without observation of intermediate spheres. 

In the formation of carbonaceous mesophase by thermolysis (pyrolysis) of isotropic molten pitch, the development of a liquid-crystalline phase is accompanied by simultaneous aromatic polymerization reactions. The reactivity o/pitch with increasing heat treatment temperature and its thermosetting nature are responsible for the lack of a true reversible thermotropic phase transition for the bulk mesophase in most pitches. Due to its glass-like nature most of the liquid-crystalline characteristics are retained in the super-cooled solid state. 

See Brooks and Taylor structure in the carbonaceous, mesophase, bulk mesophase, pitch... 

See bulk mesophase, carbonaceous mesophase, mesogenic pitch, pitch... 

---