High-molecular-weight mesophases

High molecular weight mesophases were first studied during the late 1930 s using suspensions of tobacco mosaic virus (TMV). Bawden and Pirie (26) reported a solution of TMV separated into two phases as the concentration was increased, one of which was birefringent. 

A high molecular weight mesophase pitch with no side groups will not spin, since it decomposes before it is sufficiently fluid to flow. Hence, commercially, a mixture of high molecular weight with a few side groups is used. A mesophase pitch is more difficult to spin than an isotropic pitch. 

D. M. Riggs, The Characterisation andKinetic Mechanism of Mesophase Formation in High Molecular Weight Carbonaceous Materials, Ph.D. dissertation, Rensselaer Polytechnic Institute, Troy, N.Y., 1979. 

Preliminary room temperature x-ray data of 0.65 Me4C00-PECH indicates that the sample presents a highly ordered smectic mesophase which was not yet completely assigned. The textures seen by polarized optical microscopy are also typical of smectic phases. Due to the very high molecular weights involved, textures specific to mesophase in thermodynamic equilibrium could not be developed within a reasonable amount of time by annealing. 

Type 1 gels are mesophases that are so highly ordered that they resist disruption of their structure and are thus extraordinarily viscous, to the point of appearing solid-like, even though no high molecular weight species need be present in the system. Surfactants, both synthetic (e.g., sodium dodecylsulfate) and natural (e.g., phospholipids), and clays are typical representatives of this class. 

TABLE II. Ratio of Theoretical to Experimental Volume Fractions at Mesophase Separation in the High Molecular Weight Limit... 

Mesophases containing high molecular weight components have been known for many years. The earliest studied involved polyelectrolytes such as tobacco mosaic virus (1). Approximately 30 years ago mesophases were observed... 

Even for polymerizable surfactants, curvature control is still very important, and Eastoe et al. describe mixtures of polymerizable surfactants that retain the same preferred curvature before and after cross-linking [44, 45]. Current approaches to the one-to-one replication of bicontinuous or other surfactant mesophases without the use of polymerizable surfactants involve the use of surfactants of high molecular weight [46] or highly viscous cubic phases featuring slower rearrangement dynamics. 

Quite unusually for homologous series of thermotropic polymers, the samples with odd-numbered segments, except 2 (m=3), did not show a mesomorphic behaviour. While acknowledging that liquid crystal properties might be highly molecular weight dependentJ this appears as a clear indication that in polymer, or oligomer, systems the onset of a mesophase may be limited to discrete values of m. 

Riggs DM, The characterization and kinetic mechanism of mesophase formation in high molecular weight carbonaceous materials, PhD Thesis, Rensselaer Polytechnic Inst, Troy, NY, 1979. 

Carbon fibers from anisotropic mesophase pitch High molecular weight aromatic pitches that are mainly anisotropic in nature are referred to as mesophase pitches. 

According to the previous discussion it is found that increasing the degree of polymerization decreases the entropy of the system and if the monomeric structural unit exhibits a virtual or monotropic or mesophase, the resulting polymer should most probably exhibit a monotropic or mesophase. Alternatively, if the monomeric structural unit displays a mesophase, the polymer should display a mesophase that is broader. It is also possible that the structural unit of the polymer exhibits more than one virtual mesophase and therefore at high molecular weights the polymer will inerease the number of its mesophases. All these effects have been observed in various systems. 

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