Mesomorphic gels

The state of organization of the PS-PEO block polymers in the swollen, dispersed, or dissolved states has been shown by Sadron, Skoulis, and others (Finaz et ai, 1961 Gervais and Gallot, 1970 Kovacs et a/., 1966 Lotz, 1963 Lotz and Kovacs, 1966 Lotz et a/., 1966 Manson and Kovacs, 1965 Sadron, 19626, 1963 Skoulios and Finaz, 1962) to depend primarily on three factors the relative affinity of the solvent for both polymer sequences, the polymer concentration, and the temperature. One may obtain a wide variety of forms, ranging from mesomorphic gels to spherulites and single crystals, depending on the above conditions. 

The term mesomorphic gel refers to a swollen material exhibiting a type of supermolecular organization, such as crystallinity (Battaerd and Tregear, 1967, pp. 138,139 Mabis, 1962). 

When a block copolymer is dissolved in a solvent that is a good one for one set of units and a poor one for the other a micellar structure forms.( 183,284) The ability to form micelles is a distinguishing feature of block and graft copolymers. Homopolymers and random type copolymers do not form micellar structures in solution. A micelle usually consists of a swollen core of the insoluble block connected to and surrounded by the soluble blocks. As the copolymer concentration is increased the micelles aggregate and organize into structures that have been termed mesomorphic gels. It is from this organized structure, where the chains themselves are in nonordered conformation, that crystallization takes place. 

As pointed out in Chapter 1, supramolecular chemistry comprises two broad, partially overlapping areas covering on the one hand the oligomolecular supermolecules and, on the other, the supramolecular assemblies, extended polymolecular arrays presenting a more or less well-defined microscopic organisation and macroscopic features depending on their nature (layers, films, membranes, vesicles, micelles, microemulsions, gels, mesomorphic phases, solid state species, etc.). 

A mixture of soap crystals in a saturated solution in which the soap crystals produce a gel-like consistency. The soap crystals in this case are referred to as curd-fibres . Soap curd is not a mesomorphic (liquid-crystal) phase. 

The above-mentioned physicochemical properties of phospholipids lead to spontaneous formation of bilayers. Depending on the water-lipid ratio, on the type of phospholipids, and the temperature, the bilayer exists in different, defined mesomorphic physical organizations. These are the La high-temperature liquid crystalline form, the Lp gel form with restricted movement of the hydrocarbon chains, and an inverted hexagonal phase, Hn (see Sections 1.3.1 and 1.3.2). 

Water and isobutyric acid (IBA) have been chosen as solvents because they form a mesophase with HPC and they have a critical temperature, approximately 26 C for an IBA proportion of approximately 0.4. The phase diagram shows different regions one- or two-phase, mesomorphic, or not. The two-phase region can be segregated. The mesomorphic phase can form a gel or show cholesteric colors (25). The most important Information given by this phase diagram is that the behavior of phase separation of IBA/H2O/HPC system is governed by the extreme preference of HPC for IBA as compared with water. This results in the formation of the ordered phase in dilute HPC solutions as the IBA content in the solvent is reduced. Mesophases can be obtained for low polymer concentrations of a few percent. 

Lipids constitute a diverse and important group of biomolecules. Most lipids can behave as lyotropic liquid crystals. In the presence of water, they self-assemble in a variety of phases with different stmcture and geometry. The lipid polymorphic and mesomorphic behavior, i.e., their ability to form various ordered, crystalline, gel, or liquid-crystalline phases as a function of water content, temperature, and composition, is one of the most intriguing features of lipid-water systems. The mutual transformations between these phases and their physiologic implications are the subject of this article. 

Lipid polymorphic and mesomorphic phases generally are characterized by their 1) symmetry in one, two, or three dimensions, 2) hydrocarbon chain ordering and specific chain arrangements in the ordered gel and crystalline phases, and 3) type (normal or inverted) for the curved mesomorphic phases. For four decades, the nomenclature introduced by Luzzati (40) has been used to designate lipid phases. Because of the rapid growth of the number of... 

From a biologic viewpoint, of greatest interest are the transitions that involve the physiologically important lamellar liquid-crystalline phase, namely, the gel- liquid-crystalline (melting) transition, and the lamellar- nonlamellar mesomorphic transitions. 

The amphiphilic molecules can occur in lyotropic and thermotropic mesomorph, and the organization of lipid bilaycrs such as liposomes can exist in a temperature range where all the mesophases from gel state to liquid crystal state are favored. In the gel state the phospholipid acyl chains are closely packed and the molecular movements are deteriorized, while in the liquid crystalline slate, the fatty-acid moieties are in a more fluid stale and are able to move more freely. 

Liquid-crystalline materials of types A-I and A-II exhibit homogeneous (non-phase separated) mesophases by the association of identical or different molecules. A new class of mesomorphic H-bonded materials, anisotropic liquid-crystalline gels (Fig. 2, Type B), has recently been prepared by the selfaggregation of H-bonded molecules in non-hydrogen-bonded normal liquid crystals [29, 30]. These materials are macroscopically soft solids and form heterogeneous (phase-separated) structures consisting of liquid crystals and fibrous solids. 

First I want to draw attention to polymers wherein specific solute-solvent interactions do promote mesophase formation. There are two general classes of polymer-solvent systems wherein instances of mesomorphism result from intermolecular interactions that are closely related to those that stabilize the amphiphilic MLC lyotropics DSidechain polymers with amphiphilic sidechains appended to the polymer backbone may form lyotropic PLCs in water, 2) Block copolymers exhibit organized, gel-like phases when one block is preferentially solvated. The former PLCs are intimately related to lyotropic MLCs, however, compared to the monomer amphiphiles, new phases and often phase stability is influenced by the attachment of the amphiphilic core to the chain. Finkelmann and co-workers have recently considered such amphiphilic sidechain PLCs.(17)... 

Fig. 4 Ar,AT-bisGauroyl)-fran5-l,2-cydohexylamine 20 and intermolecular hydrogen bonding motif between self-complementary cyclohexylbisamide units in gel aggregates gelator 21 with mesogenic cyanobiphenyloxy units and the mesomorphic compoimds 22 and 23 giving room-temperature liquid crystals...

Figure 15. Wide angle x-ray diffraction patterns for gel I, gel II and mesomorphic phases of SDHP.

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