Lyotropic system

M. Allain, P. Oswald, J. M. di Meglio. Structural defects and phase transition in a lyotropic system optical birefringence and order parameter measurements. Mol Cry St Liq Cryst 7625 161-169, 1988. 

Sato et al.11 realized that for these lyotropic systems, whose phase boundaries have little temperature dependence, an investigation of the handedness in the widest possible temperature interval should be carried out. As the cholesteric handedness in a few cases is opposite at different temperatures, the data at a single temperature are meaningless. Using a simple thermodynamic analysis, they proposed a plot of the cholesteric wavenumber qc (the reciprocal pitch) as a function of the reciprocal temperature 1 IT [Eq. (1)]... 

Quasi-nematic or compensated cholesteric phases were formed by CTC dissolved in mixtures of methylpropyl ketone (MPK) and DEME. CTC/MPK has a right-handed twist but CTC/DEME a left-handed one (109). Siekmeyer et al. (IIQ) studied the phase behavior of the ternary lyotropic system CTC/3-chlorophenylurethane/triethyleneglycol monoethyl ether. 

Note 3 In lyotropic systems, biaxial nematic mesophases have been identified from the biaxial symmetry of their tensorial properties. 

As shown above, by shortening the flexible spacer, it is possible to produce rather rigid chains which should probably better be described as main-chain mesogens. Mesophase superstructures can also be achieved with flexible side-chains (Shibaev et al. 88)). Lyotropic systems of this type are given by Finkelmann et al. 89). 

The development is reviewed of liquid-crystalline polymers whose mesophase formation derives from the nature of the chemical units in the main chain. The emphasis lies primarily on highly aromatic condensation polymers and their applications. The general properties of nematic phases formed by such polymers are surveyed and some chemical structures capable of producing nematic phases are classified in relation to their ability to form lyotropic and thermotropic systems. The synthesis, properties, physical structure and applications of two of the most important lyotropic systems and of a range of potentially important thermotropic polymers are discussed with particular reference to the production and use of fibres, films and anisotropic mouldings. 

A major impetus was given to work, both academic and industrial, in the field of lyotropic systems by the development by duPont of commercial fibres having exceptionally high tensile strength and modulus through use of nematic anisotropic solutions of relatively rigid-chain aromatic polyamides. The earliest product to appear, Fibre B, was based upon poly (p-benzamide) (I)10), but was replaced by the fully commercial product, Kevlar, based upon poly (p-phenylene terephthalamide) (II) U). Arenka, from Akzo, also has the latter chemical repeating unit. 

Thermotropic polymers require no solvent for formation of a liquid-crystalline phase, which occurs instead within a defined temperature range. The chemical units useful for thermotropic polymer formation are generally those already exemplified in Figs. 1 and 2, but these units in homopolymer form give rise to crystalline polymers with melting points above their decomposition temperatures. The problem of polymer design is to reduce the melting temperature in order to obtain a liquid-crystalline phase at a temperature below that of decomposition. Whereas the lyotropic systems are... 

The papers appearing in this volume represent the common research areas encountered in lyotropic systems. An effort is made to include articles connecting the lyotropic liquid crystals with biological structures characterized by similar long-range order phenomena. The papers contained in this book should serve not only researchers in the field who want to expand their knowledge of lyotropic liquid crystals but also those who are starting in the field. 

In sharp contrast to the situation for lyotropic systems, dynamic behavior of thermotropic liquid crystals has received considerable research attention. It is useful, therefore, to begin by assessing the implications of this work for lyotropic systems. 

Little work seems to have been done on thin oriented layers of lyotropic liquid crystals although there is one recent report of preparation of such a layer of the lecithin-water lamellar phase (JO). As indicated by Brochard and de Gennes (II), theories of the hydrodynamics of thermotropic smectic materials can be adapted to describe oriented layers of lamellar liquid crystal in lyotropic systems. 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

Experiments by Muller et al. [17] on the lamellar phase of a lyotropic system (an LMW surfactant) under shear suggest that multilamellar vesicles develop via an intermediate state for which one finds a distribution of director orientations in the plane perpendicular to the flow direction. These results are compatible with an undulation instability of the type proposed here, since undulations lead to such a distribution of director orientations. Furthermore, Noirez [25] found in shear experiment on a smectic A liquid crystalline polymer in a cone-plate geometry that the layer thickness reduces slightly with increasing shear. This result is compatible with the model presented here as well. 

Another peculiar property of LCPs is shown in Fig. 15.47, where the transient behaviour of the shear stress after start up of steady shear flow is shown for Vectra A900 at 290 °C at two shear rates. We will come back to this behaviour in Chap. 16 for lyotropic systems where this behaviour is quite common and in contradistinction to the transient behaviour of conventional polymers, as presented in Fig. 15.9. This damped oscillatory behaviour is also found for simple rheological models as the Jeffreys model (Te Nijenhuis 2005) and according to Burghardt and Fuller, it is explicable by the classic Leslie-Ericksen theory for the flow of liquid crystals, which tumble, rather than align, in shear flow. Moreover, it is extra complicated due to the interaction between the tumbling of the molecules and the evolving defect density (polynomial structure) of the LCP, which become finer, at start up, or coarser, after cessation of flow. 

Solutions of the aromatic polyamides (PpBA, PpPTA and PmPTA), the polybenzazoles (PBT and PBO), poly(benzyl glutamate) (PBG) and hydroxypropylcellulose (HPC) are the most studied main chain lyotropic systems and our understanding of the behaviour of lyotropics is based on investigations of this relatively small number of materials (Moldenaers, 1996). They form main chain liquid crystals because of their rigid molecular structure in the appropriate solvents. Two kinds of solvents are used (Collyer, 1996) ... 

Moldenaers P, "Time-dependent Effects in Lyotropic Systems", in Acierno D and Collyer AA (Eds) "Rheology and Processing of Liquid Crystal Polymers", Chapmann Hall, London, 1996, Chap. 8. 

The virial expansion has enjoyed greater appeal, especially as applied to lyotropic systems. Onsager s classic theory rests on analysis of the second virial coefficient for very long rodlike particles. It was the first to show that a solution of hard, asymmetric particles such as long rods should separate into two phases above a threshold concentration that depends on the axial ratio of the particles. One of these phases should be anisotropic (nematic), the other completely isotropic. The former is predicted to be somewhat more concentrated than the latter, but it is the alignment (albeit imperfect) of the solute molecules that is the predominent distinction. The phase separation is a consequence of shape asymmetry alone intervention of intermolecular attractive forces is not required. 

Binary Lyotropic Systems Consisting of Hard Rodlike Particles and a Diluent... 

Binary Lyotropic Systems. Comparison of Theory with Experiment... 

Lyotropic systems in which the nematogenic component is a poly(N-alkyl isocyanate) (PIC),... 

While the possibility of this dispersion of micro-domains of the nematic phase in an isotropic phase cannot be dismissed, concrete evidence for morphologies of this kind in nematogenic copolymers is not prominently in evidence. The longer sequences of rigid units undoubtedly are responsible for promotion of liquid crystallinity but, as theory suggests they appear to be uniformly dispersed Sequences of units that include many flexible members and hence are not rodlike may assume a role analogous to that of the solvent in a lyotropic system The nematic copolymer should, on this basis, consist of a single phase. 

This conclusion was reached, tentatively, by Frenkel, Shaltyko and Elyashevich A phenomenological analysis presented by Pincus and de Gennes predicted a first-order phase transition even in the absence of cooperativity in the conformational transition. These authors relied on the Maier-Saupe theory for representation of the interactions between rodlike particles. Orientation-dependent interactions of this type are attenuated by dilution in lyotropic systems generally. In the case of a-helical polypeptides they should be negligible owing to the small anisotropy of the polarizability of the peptide unit (cf. seq.). Moreover, the universally important steric interactions between the helices, regarded as hard rods, are not included in the Maier-... 

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