Liquid crystalline phases behaviour

Amphiphiles often have a complex phase behaviour with several liquid crystalline phases These liquid crystalline phases are often characterised by long-range order in one directior together with the formation of a layer structure. The molecules may nevertheless be able tc move laterally within the layer and perpendicular to the surface of the layer. Structura information can be obtained using spectroscopic techniques including X-ray and neutror diffraction and NMR. The quadrupolar splitting in the deuterium NMR spectrum can be... 

The liquid crystalline phase is called a mesophase and is intermediate between solid and liquid. In this mesophase the molecules show liquid-like long-range behaviour, Le. are essentially disordered, but also some crystal-like... 

Although liquid crystalline phases can be formed from entirely rigid units such as rods and ellipsoids, all liquid crystal molecules are partially flexible objects which can change their shape in a variety of ways. The extension of rigid-rod theories to account for flexibility is complex and has been the object of considerable effort for several years. It is now clear that simply relaxing the assumption of molecular rigidity is expected to have profound effects on phase behaviour. 

Iki and Hori discussed the crystal structures of the compounds CPnOB and nOPCB in relation to the mesophase behaviour considering the dimer model proposed by Cladis et al. [99] for this kind of molecule in the liquid crystalline phase. 

During the studies of phase behaviour two types of liquid crystalline phases were identified. LC material was viscous and exhibited intense "white" birefingence. material was apparently homogeneous but of low viscosity and exhibited "multi-coloured" birefringence. The liquid crystalline phases observed in the equilibrium studies of surfactant concentrations up to 25 are unlikely to take part in the self-emulsification process due to the presence of two-phase regions between L2 and liquid crystalline phases however, LC material may account for the improved stability of emulsions formed by 25 surfactant systems (Table II). Figure 4c indicates that by increasing the surfactant concentration to 30 the... 

The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

A low melting point is preferable in order to avoid metastable, monotropic liquid crystalline phases. Low-temperature mesoporphic behaviour in general is technologically more useful, and alkyl terminal groups promote this. 

The unusual optical properties of liquid crystals had been remarked upon and described for several centuries before their uniqueness as a state of matter was recognised. Their early reports described the strange melting behaviour and appearance of some naturally occurring materials, either as pure compounds or as gels in water, which have now been shown to be thermotropic or lyotropic liquid crystals. Thermotropic liquid crystalline phases are formed under the action of heat, see Figures 2.1 and 2.2, and the lyotropic liquid crystalline phases are formed by the action of a solvent, such as water, usually with an amphiphilic compound. However, the nature of these materials, or indeed their exact... 

The critical thicknesses are thus in the range of the dimensions of lamellar, cylindrical or spherical mesophases in block copolymers with ordered morphologies. The question is whether the phase boundary between the amorphous and the liquid-crystalline phase in a block copolymer will exert an ordering effect as assumed in the original theory or rather a disordering influence. The latter case and transitions between the two cases have also been treated recently by an extension of the theory (5). Therefore a theoretical framework exists, within which the transition behaviour of amorphous / liquid-crystalline block copolymers can be described. 

The thermal behaviour of the block copolymer PSPCHOLI is shown in Figure 4. The phase separation into an amorphous and a liquid crystalline phase is apparent by a Tg of the PS phase and a T of the mesophase appearing at the positions of the homopolymer without any significant shift (Table II). Tg of the PCHOL-phase was not clearly resolved. 

The monomer exhibits complex phase behaviour, in particular, a monotropic liquid crystalline phase, which is only apparent on cooling, such materials can only be properly characterized by using differential scanning calorimetry (DSC) in conjuction with optical microscopy and also by X-ray scattering (see Chapter 1). 

This additional factor was introduced to ensure the proper behaviour over both liquid crystalline and solid phases. In simulations of the liquid crystalline phase alone this term may be omitted for computational efficiency. 

The liquid crystalline phase is called a mesopliase and is intermediate between solid and liquid. In this mesopliase the molecules show liquid-like long-range behaviour, i.e. are essentially disordered, but also some crystal-like aspects of short-range order. The type of long-range order in the mesopliase may vary. If the molecules align themselves as layers this is described as a smectic phase if the alignment is as parallel threads it is described as a nematic phase. 

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