Thermotropic and Lyotropic Liquid Crystals

Polymers may exhibit the behavior of liquid-crystal order either in melt (thermotropic liquid crystals) or in solution (lyotropic liquid crystals). In thermotropic liquid crystals, the phases of the liquid crystals may be observed to change as temperature is increased. In lyotropic liquid crystals, the ordered crystalline state is 

Thermotropic liquid crystals are controlled by the melting temperature Tm and by the isotropic temperature TJ for the stability of mesophases. In certain cases, a glass temperature Tg rather than a melting temperature may be observed before the isotropic temperature 7 . The mesophases of thermotropic liquid-crystalline polymers are formed in pure compounds or in polymer melt. Their properties are essentially based upon rodlike mesogenic moieties polymerized together with small flexible chains, that is, the main-chain or side-chain polymers. 

Lyotropic liquid crystals are formed by solutions. They undergo a spontaneous transformation from an isotropic to an anisotropic (nematic or smectic) ordered solution above certain threshold concentrations which are a function of their molecular axial ratio (length L to diameter D). 

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Recent solid state NMR studies of liquid crystalline materials are surveyed. The review deals first with some background information in order to facilitate discussions on various NMR (13C, ll, 21 , I9F etc.) works to be followed. This includes the following spin Hamiltonians, spin relaxation theory, and a survey of recent solid state NMR methods (mainly 13C) for liquid crystals on the one hand, while on the other hand molecular ordering of mesogens and motional models for liquid crystals. NMR studies done since 1997 on both solutes and solvent molecules are discussed. For the latter, thermotropic and lyotropic liquid crystals are included with an emphasis on newly discovered liquid crystalline materials. For the solute studies, both small molecules and weakly ordered biomolecules are briefly surveyed. 

Starting with the crystalline state, the mesophase is reached by increasing the temperature or by adding a solvent. Accordingly, a differentiation can be made between thermotropic and lyotropic liquid crystals, respectively. As with thermotropic liquid crystals, a variation of the temperature can also cause a phase transformation between different mesophases with lyotropic liquid crystals. 

Our understanding of lyotropic liquid crystals follows in a similar manner. The action of solvent on a crystalline substance disrupts the lattice structure and most compounds pass into solution. However, some compounds yield liquid crystal solutions that possess long-range ordering intermediate between solutions and crystal. The lyotropic liquid crystal can pass into the solution state by the addition of more solvent and/or heating to a higher temperature. Thermotropic and lyotropic liquid crystals, both turbid in appearance, become clear when they pass itno the liquid and solution states, respectively. 

Moreover, there are amphiphilic molecules that can behave as both thermotropic and lyotropic liquid crystals. 

This volume covers the structural relations between thermotropic and lyotropic liquid crystals (Chapters 1 and 2) and compares them with the micellar systems (Chapter 3). The interfacial aspects and the accompanying stability problems are covered in Chapters 5 and 6. The molecular dynamics in liquid crystals, the importance of water structure and of counter-ion binding for their stability are three essential factors for long range order systems, which are treated in Chapters 7, 8, and 9. The final chapter by E. J. Ambrose illustrates the change of order in a biological system under malignant conditions. 

The self-organization of both thermotropic and lyotropic liquid crystals make these ordered fluids remarkable media for the dispersion and organization (alignment) of CNTs. This subject has been the focus of a recent excellent review by Scalia [231], theoretical work on anchoring at the liquid crystal/CNT interface by Popa-Nita and Kralj [458], and a number of earlier experimental reports on liquid crystal/CNT composites demonstrating that liquid crystal orientational order can be transferred to dispersed CNTs, which is commonly illustrated using polarized Raman spectroscopy [459 -62]. 

The films may be stacked in layers. There are second order parameters describing the molecular array more specifically. The basic smectic arrangement can be found in both thermotropic and lyotropic liquid crystals. 

Order and Mobility are two basic principles of mother nature. The two extremes are realized in the perfect order of crystals with their lack of mobility and in the high mobility of liquids and their lack of order. Both properties are combined in liquid crystalline phases based on the selforganization of formanisotropic molecules. Their importance became more and more visible during the last years in Material science they are a basis of new materials, in Life science they are important for many structure associated functions of biological systems. The main contribution of Polymer science to thermotropic and lyotropic liquid crystals as well as to biomembrane models consists in the fact that macromolecules can stabilize organized systems and at the same time retain mobility. The synthesis, structure, properties and phototunctionalization of polymeric amphiphiles in monolayers and multilayers will be discussed. 

Both thermotropic and lyotropic liquid crystal polymers exhibit characteristic features with regard to their microstructureJ Anisometrical monomers such as rods or discs are connected to chains in an appropiate manner. These anisometrical monomers are considered to be the mesogens and may be part of main chain LCP, side chain LCP, or of both types together (Fig. 6). Between the mesogens are located flexible spacers of non-mesogenic character. Sufficient flexibility is a prerequisite for liquid crystal formation, with an increase in either temperature or solvent concentration. 

Whether quasicrystalline structures are limited to alloys remains an open question. It is possible that their occurrence is much more widespread than had been previously thought. Indeed there is evidence for quasicrystallinity in both thermotropic and lyotropic liquid crystals. Diffraction patterns of decagonal symmetry have been recorded in lyotropic liquid crystals [K. Fontell, private communication], (Fig. 2.19), and there is theoretical evidence for the existence of a quasicrystalline structure within the blue phase of cholesterol (Chapters 4, 5). (The decagonal structure has quasisymmetry perpendicular to the tenfold axes, and translation symmetry along them.) Viruses crystallise in icosahedral clusters and the list continues to grow. In addition to five-fold symmetry, it has been shown that eight and ten- fold quasisymmetry is possible. ... 

In another example, Sarcocine-NCA prepared by phosgenation of sarcosine, followed by cyclisation in the presence of triethyl amine is used for the preparation of lipopeptide-based branched polymers forming thermotropic and lyotropic liquid crystals as shown in scheme 197 (Ref. 248). 

The extrapolated line of log S-log C crossed each other at a critical concentration Cq at which S stays constant and independent of temperature. These results suggest that the temperature dependence of the cholesteric pitch would inflect at the concentration higher than Cq This is analogous to the behavior of thermotropic liquid crystals composed of cholesteric solute and nematic solvent, where the sign of dS/dT reverses at a critical concentration. It is understood that the behavior of both thermotropic and lyotropic liquid crystals is comparable provided that the nematic substances of the former are substituted with the solvents of the latter. The critical concentration Cq is about 0.41 vol/vol and this value is very close to the concentration at which the side chains on neighboring molecules of the polymer come to contact each other ( refer to fig.5 ). From these results, it is expected that the origin or mechanism of twist would change at this concentration Cq. The... 

Non-polymeric, liquid crystals are divided into thermotropic and lyotropic liquid crystals. Compounds which have liquid crystalline behavior in solution are called lyotropic liquid crystals. The amount of solvent is then the most important variable. Mainly thermotropic liquid crystals will be discussed here. 

This chapter presents a summary of manuscripts published in the perissod of June 2011-June 2012 focusing on the use of NMR techniques to elucidate the microstructure and dynamics of self-assembling systems. In section 2 reviews and articles on general methods and models have been included. In section 3 the papers on thermotropic and lyotropic liquid crystals, phospholipids, vesicles and bicelles have been covered. Section 4 has been devoted to micellar solutions including ionic and non ionic surfactant systems, polymer amphiphiles and mixed amphiphiles systems. 

From a historical point of view as well as due to their applications, thermotropic and lyotropic liquid crystals have always been treated separately. While thermotropics and the concept of liquid crystallinity in general were discovered as late as in 1888 [3], lyotropic phases were known to mankind since the Bronze Age [4], as they occur during the soap-making process. Due to this, lyotropic liquid crystals find their main applications in the detergent industry and in cosmetics. As various biological systems, e.g. cell membranes, take a lyotropic liquid crystalline form, they also possess some medical and pharmaceutical importance [5]. In contrast, thermotropic liquid crystals are used for completely different applications, e.g. for displays, thermography, tunable filters or lasers [6]. Thus, it is not astonishing, that two distinct fields of research evolved for the two types of liquid crystals. However, thermotropic and lyotropic liquid crystals share a common state of matter with many similarities. For example, many mesophases which occur in thermotropics can also be found in lyotropics. Still, there are some thermotropic phases which do not seem to have a lyotropic counterpart. 

In this chapter, the structural properties of thermotropic and lyotropic liquid crystals will be compared. In a first step, the driving forces for the formation of the mesophases, as well as the building blocks of the two types of liquid crystals will be analyzed. Afterwards, the structures and properties of the most important liquid crystalline phases will be described, as far as they are important in the context of this thesis. 

Phase Sequences of Thermotropic and Lyotropic Liquid Crystals... 

In Sect. 5.1.2 it was shown, that two things are necessary for the formation of the lyotropic analog of the SmC phase. Firstly, the surfactant molecule has to exhibit a very balanced structure inbetween the structure of conventional thermotropic and lyotropic liquid crystals. Especially, the lyotropic part has to incorporate a polar chain with oxygen atoms connecting the diol head group to the rest of the molecule. Secondly, the solvent has to possess at least two hydrogen bond donor atoms... 

The plethora of liquid crystal structures and phases is categorized into two main classes thermotropic and lyotropic liquid crystals. While thermotropic liquid crystals are formed by, e.g., rod- or disc-shaped molecules in a certain temperature range, lyotropic liquid crystals are liquid crystalline solutions, built up by, e.g., aggregates of amphiphilic molecules in a certain concentration range. Many liquid crystal phases are found in thermotropic as well as in lyotropic systems. In some cases, however, the lyotropic analog of a thermotropic phase has never been observed. The probably most interesting of these missing link cases is the thermotropic chiral smectic C (SmC ) phase, which has become famous as the only spontaneously polarized, ferroelectric fluid in nature. 

The formation of transient domain patterns aligned perpendicular to the initial director during the relaxation process of the Frederiks transition has been known since the earliest observations by Carr [20] and Guyon et al. [21]. After these observations some other transient domain structures were found in thermotropic and lyotropic liquid crystal with perpendicular [22], parallel [23], oblique [24], and two-dimensional [25] striped patterns relative to the initial orientation of the director in both the electric and magnetic fields. 

The terms liquid crystal, mesophase, or mesomorphic state are used synonymously to describe a number of different states of matter in which the molecular order lies between the almost perfect long-range positional or orientational order of solid crystals and the long-range disorder found in ordinary isotropic liquids. Two main classes of liquid crystals are usually distinguished lyotropic and thermotropic. In lyotropic mesophases, the combination of order and mobility can be achieved by using a solvent thermotropic mesophases are based on the temperature-induced mobility of form-anisotropic molecules in the melt. Surfactants can often form both thermotropic and lyotropic liquid crystals i.e., they possess amphitropic properties [2]. 

Cellulose and its derivatives have the ability to behave both as thermotropic and lyotropic liquid crystals. As mentioned above, several specific phases of liquid crystals occurs, depending on the structure or combination of molecules. In the nematic phase, the molecules have only orientational ordering (making the liquid crystal phase less ordered), while in the smectic phase, the molecules have both orientational and positional ordering [75]. In addition, the optically active molecules can form a chiral nematic phase (or cholesteric phase). In this case, the molecules are helix-oriented generating some spectacular optical properties. 

E. M. Andresen and G. R. Mitchell, Orientational behaviour of thermotropic and lyotropic liquid crystal polymer systems under shear flow. Europhys. Lett. 43, 296-301 (1998). 

Table 1. Diffusion constants and anisotropy ratios of typical thermotropic and lyotropic liquid crystals considered in the text (Extensive data and references are collected in Kruger 24], Lindblom et al. [25], and Karger et al. [38],...

Relaxation measurements provide another way to study dynamical processes over a large dynamic range in both thermotropic and lyotropic liquid crystals (see Sec. 2.6 of Chap. Ill of Vol. 2A). The two basic relaxation times of a spin system are the spin-lattice or longitudinal relaxation time 7] and the spin-spin or transverse relaxation time T2. A detailed description, however, requires a more precise definition of the relaxation times. For spin 7=1, for instance, two types of spin-lattice relaxation must be distinguished, related to the relaxation of Zeeman and quadrupolar order with rates 7j"2 and Jfg. The relaxation rates depend on spectral density functions which describe the spectrum of fluctuating fields due to molecular motions. A detailed discussion of spin relaxation is beyond the scope of this... 

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