Mesophase classes

This allows identifying two large classes of solid mesophases class ii) where long-range periodicity along three-dimensions may be still defined, in spite of the presence of large amounts of disorder and class iii) where periodicity may be defined only in one or two dimensions. 

Before leaving the topic of LCP/LCP blends, one additional point needs to be stressed. Low molecular weight liquid crystals (LMWLC s) of the same mesophasic class often show miscibility (Gray and Windsor 1974). On the other hand, TLCP s that form the same mesophase, usually nematic, do not necessarily exhibit complete miscibility. Thus, in order to be able to implement the ideas of the last several paragraphs, caution must be exercised in choosing the two LCP s to form the miscible blend. 

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. 

Since the discovery of discotic liquid crystals [121], the mesophases formed by rod-like and disc-like molecules have been considered as belonging to different liquid crystalline classes. Indeed, the conventional rod-like and disc-... 

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. 

Fig. 7 Persistence length P is plotted vs. the chain diameter D both for polymers giving Class I and for polymers giving Class II mesophases. (From ref [11], see also Table 1). Reproduced with permission from [11]. Copyright 2004 Am Chem Soc...

The members of Class II in Table 1 present very small enthalpies of the mesophase-liquid transition [ AHml < 0.5 kJ/(mol of chain bonds)], suggesting that their mesophase is hardly stabilized by specific interatomic interactions. By contrast, we point out that in all cases the crystal-mesophase transition has a significant enthalpy value, mostly AHqm > 1 kJ/(mol of chain bonds). Consistent with their relatively flexible character, the polymers listed in the Tables have their glass transition below ambient temperature. 

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... 

Note 1 The mesophase formerly designated as smectic D (see Definition 3.1.5, Note 3) belongs to this class. 

Note Depending on the order in the molecular stacking in the columns and the two-dimensional lattice symmetry of the columnar packing, the columnar mesophases may be classified into three major classes hexagonal, rectangular and oblique (see Definitions 3.2.2.1. to 3.2.2.3). 

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. 

There have been only a few reports on crown ethers laterally attached to rod-like molecules in the last few years. Nevertheless, very interesting compounds have been synthesized in the past. The general problem of this compound class is the flexible crown ether itself that can destroy the mesomorphic properties when attached laterally. Making the rigid rod longer can circumvent this problem. Another way to obtain stable mesophases is the complexation with suitable salts. 

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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