Classes of Mesophases

The condis crystal motion results in a similar rotation as in the plastic crystal, but the molecules do not rotate as a whole, rather they undergo segmental motion and 

The two main liquid-crystal polymorphs, nematic and smectic, of disc- and rodlike mesogens are compared in Fig. 5.134 to their ideal crystals. Possible flexible 

Going from left to right represents the change of a single chain with time, ora snapshot of different chains in time. 

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In addition to the true smectic phases, there is another class of mesophases, formerly also called smectic phases, which do possess extra positional order and which are derived from the true smectics and referred to as crystal smectic phases. These phases are characterized by the appearance of interlayer correlations and, in some cases, by the loss of molecular rotational freedom. Thus, the (crystal) B, G, and J phases are SmB, SmF, and SmI phases, respectively, with interlayer correlations, whereas the E, H, and K phases are B, G, and J phases which have lost rotational freedom (note that, confusingly, there are both smectic and crystal B phases). These phases possess considerable disorder and are, therefore, properly intermediate between the solid and liquid states. 

Mesogens are compounds which generate mesophases, intermediate between the crystal phase and the liquid or solution phase. Mesophases possess some of the order of crystals, but are fluid like liquids. There are different classes of mesophases, principally thermotropic where temperature is the principal variable in phase changes and lyotropic where there is additional solvent and concentration is an important variable. The different classes of thermotropic phases relate to the different types of partial ordering of the molecules in nematic phases there is orientational order of the molecules, and in smectic phases there is translational order. 

There are two principal categories of mesophases, thermotropic and lyotropic. Thermotropic liquid crystals are formed within a particular range of temperature in a molten material, with no solvent present, whereas lyotropic liquid crystals are formed by some substances when they are dissolved in a solvent. Within each of these categories there are three distinct classes of mesophases, which were first identified by Friedel in 1922. The simplest of these to describe are the nematic and smectic classes, illustrated schematically in fig. 12.16. These phases are formed by long thin rigid molecules which tend to line up parallel to each other. 

In addition to the true smectic phases, there is another class of mesophases, formerly also called smectic phases, which do possess extra positional order and which are derived from the true... 

The term mesophase also includes ordered liquids (nematic, smectic, cholesteric and discotics), which present long-range orientational order like in a solid, but positional disorder like in a liquid [2]. In these materials, large-scale molecular motion is possible, which is a characteristic of the liquid state rather than of the solid state. The term liquid crystals is conventionally used to address them. This sub-class of mesophases will not be treated in this context. 

Because of their unique blend of properties, composites reinforced with high performance carbon fibers find use in many structural applications. However, it is possible to produce carbon fibers with very different properties, depending on the precursor used and processing conditions employed. Commercially, continuous high performance carbon fibers currently are formed from two precursor fibers, polyacrylonitrile (PAN) and mesophase pitch. The PAN-based carbon fiber dominates the ultra-high strength, high temperature fiber market (and represents about 90% of the total carbon fiber production), while the mesophase pitch fibers can achieve stiffnesses and thermal conductivities unsurpassed by any other continuous fiber. This chapter compares the processes, structures, and properties of these two classes of fibers. 

A relatively new class of high-performance carbon fibers is melt-spun from mesophase pitch, a discotic nematic liquid crystalline material. This variety of carbon fibers is unique in that it can develop extended graphitic crystallinity during carbonization, in contrast to current carbon fibers produced from PAN. 

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... 

Turning to polymers giving thermodynamically stable mesophases we must assume that, since we have described bundles as an inherent structural feature of undercooled polymer melts, such structures should occur, at least in principle, also in such systems, to the extent that attractive interchain interactions which account for bundle formation play a significant role. On the other hand, rigorously speaking Class II mesophases are entropy-stabilized and inter-chain... 

Liquid Crystalline Polymers. One class of polymers that requires some special attention from a structural standpoint is liquid crystalline polymers, or LCPs. Liquid crystalline polymers are nonisotropic materials that are composed of long molecules parallel to each other in large clusters and that have properties intermediate between those of crystalline solids and liquids. Because they are neither completely liquids nor solids, LCPs are called mesophase (intermediate phase) materials. These mesophase materials have liquid-like properties, so that they can flow but under certain conditions, they also have long-range order and crystal structures. Because they are liquid-like, LCPs have a translational degree of freedom that most solid crystals we have described so far do not have. That is, crystals have three-dimensional order, whereas LCPs have only one- or two-dimensional order. Nevertheless, they are called crystals, and we shall treat them as such in this section. 

Mesophase materials are possible for all three classes of molecules. Biological mesophases were already discovered in the middle of the 19th Century. Reinitzer later described the special two-stage melting of cholesteryl benzoate. These materials were then named liquid crystals by Lehmann in 1904 5). Small molecule mesophase materials will be referred to from time to time in this review as reference materials. 

Mesophases of rigid macromolecules have not found much attention as such. There should be an increase in mechanical properties due to partial order. One may expect that more attention will be paid to these materials in the future, as the mesophases of the other two classes of molecules are better understood. 

The term condis crystal , which is a contraction of the term conformationally disordered crystal , was coined to designate the most important mesophase for flexible, linear macromolecules. We are not aware of prior naming of this class of mesophases4. 

Although PMOs and related materials have many potential applications, including adsorbants,49,50 sensors,51 and catalysts,52,53 we focus here on two specific classes of functional materials, nanovalves and polymer-silica nanocomposites formed via the co-assembly of hybrid precursors (or polymer-monomer species) with surfactant mesophases. 

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