Liquid crystal stability

Liquid crystals stabilize in several ways. The lamellar stmcture leads to a strong reduction of the van der Waals forces during the coalescence step. The mathematical treatment of this problem is fairly complex (28). A diagram of the van der Waals potential (Fig. 15) illustrates the phenomenon (29). Without the Hquid crystalline phase, coalescence takes place over a thin Hquid film in a distance range, where the slope of the van der Waals potential is steep, ie, there is a large van der Waals force. With the Hquid crystal present, coalescence takes place over a thick film and the slope of the van der Waals potential is small. In addition, the Hquid crystal is highly viscous, and two droplets separated by a viscous film of Hquid crystal with only a small compressive force exhibit stabiHty against coalescence. Finally, the network of Hquid crystalline leaflets (30) hinders the free mobiHty of the emulsion droplets. 

Motoyanagi J, Fukushima T, Aida T (2005) Discotic liquid crystals stabilized by interionic interactions imidazolium ion-anchored paraffinic triphenylene. Chem Commun (Cambridge) 1 101-103... 

Liquid Crystal Stabilization. Davies 15) first proposed the formation of a viscous layer around emulsion droplets when he calculated that the electrical forces, interfacial tension, and solvation were not sufficient to explain the stability observed experimentally in many emulsions. Furthermore, he could not explain why many emulsions containing aromatic hydrocarbons formed flocculated clusters that resisted coalescence, whereas the same systems, in which aliphatic hydrocarbons were substituted for the aromatic hydrocarbons, displayed no such stability against coalescence. Davies reasoned that his postulated thick, viscous layers arose from interaction between the liquid molecules and the emulsifier. 

Friberg et al. (17) demonstrated that a sudden increase in emulsion stability arose in the concentration range where a liquid-crystalline phase could be separated from the emulsion. They postulated, with good reason, that this liquid-crystalline phase was the viscous layer around the emulsion droplets that Davies had been seeking. Further, the liquid-crystal stabilization hypothesis explained the difference in stability between emulsions of aromatic and aliphatic hydrocarbons, because aromatic hydrocarbons, because of their large polarizability, are more prone to form lyotropic liquid-crystal structures than aliphatic hydrocarbons. 

Eccleston [19] reviewed the emulsion stability factors on the basis of the HLB and of the DLVO theory. He emphasized the Friberg school results which show that the presence of liquid crystals stabilizes the emulsions by delaying the film-thinning process and, consequently, by reducing the rate of coalescence. He also mentioned stabilization by the formation of a gel network (surfactant-fatty alcohol-water system). Dahms [20] explained the role of fatty alcohol as a viscosity modifier on the basis of lamella-phase generation. 

Dierking I, Komitov L, Lagerwall ST, Wittig T, Zentel R (1999) Horizontal chevrrai dmnain formation and smectic layer reorientation in SmC liquid crystals stabilized by polymta-networks. Liq Cryst 26(10) 1511-1519... 

Murashige T, Fujikake H, Ikehata S, Sato F (2004) Memory effect of ferroelectric liquid crystal stabilized by polymer fibers. Electron Conunun Jpn 87(4) 16-24 Musevic 1, Blinc R, Zeks B (2002) The physics of ferroelectric and antiferroelectric liquid crystals. World Scientific, Singapore... 

Pal Majumder T, Mitra M, Roy SK (1994) Dielectric relaxation and rotational viscosity of a ferroelectric liquid crystal mixture. Phys Rev E 50(6) 4976-4800 Petit M, Daoudi A, Ismaili M, Buisine JM (2006) Electroclinic effect in a chiral smectic-A liquid crystal stabilized by an anisotropic polymer network. Phys Rev E 74 061707 Petit M, Hemine J, Daoudi A, Ismaili M, Buisine JM, Da Costa A (2009) Effect of the network density on dynamics of the soft mode and the Goldstone modes in short-pitch ferroelectric liquid crystals stabihzed by an anisotropic polymer network. Phys Rev E 79 031705 Pirs J, Blinc R, Marin B, Pirs S, Doane JW (1995) Polymer network volume stabilized ferroelectric liquid crystal displays. Mol Cryst Liq Cryst 264 155-163 Polyanin AD, Zaitsev VF (2003) Handbook of exact solutions for ordinary differential equations, 2nd edn. Chapman Hall, Boca Raton... 

Malik MK, Deshmukh RR (2014) Electro-optics of homogeneously aligned nematic liquid crystals stabilized by a polymer network. Int J ChemTech Res 6 1833-1835 Manohar R, Tripathi G, Singh AK, Srivastava AK, Shukla JP, Prajapati AK (2006) Dielectric and optical properties of polymer-liquid crystal composite. J Phys Chem Solids 67 2300-2304 Mei E, Higgins DA (1998) Polymer-dispersed liquid crystal films studied by near-field scanning optical microscopy. Langmuir 14 1945-1950... 

Kavaliunas DR, Frank SG. 1978. Liquid crystal stabilization of multiple emulsions. J Colloid Interface Sd 66 586-588. 

The nematic to smectic A phase transition has attracted a great deal of theoretical and experimental interest because it is tire simplest example of a phase transition characterized by tire development of translational order [88]. Experiments indicate tliat tire transition can be first order or, more usually, continuous, depending on tire range of stability of tire nematic phase. In addition, tire critical behaviour tliat results from a continuous transition is fascinating and allows a test of predictions of tire renonnalization group tlieory in an accessible experimental system. In fact, this transition is analogous to tire transition from a nonnal conductor to a superconductor [89], but is more readily studied in tire liquid crystal system. 

Clark N A, Handschy M A and Lagerwall S T 1983 Ferroelectric liquid crystal electro-optics using the surface stabilized structure Molec. Cryst. Liq. Cryst. 94 213-34... 

As with the polysulphones, the deactivated aromatic nature of the polymer leads to a high degree of oxidative stability, with an indicated UL Temperature Index in excess of 250°C for PEEKK. The only other melt-processable polymers in the same league are poly(phenylene sulphides) and certain liquid crystal polyesters (see Chapter 25). 

Liquid crystal polymers (LCP) are a recent arrival on the plastics materials scene. They have outstanding dimensional stability, high strength, stiffness, toughness and chemical resistance all combined with ease of processing. LCPs are based on thermoplastic aromatic polyesters and they have a highly ordered structure even in the molten state. When these materials are subjected to stress the molecular chains slide over one another but the ordered structure is retained. It is the retention of the highly crystalline structure which imparts the exceptional properties to LCPs. 

Table 3.1-5 Melting points and heats of fusion for isomeric [BMIM][PFg] and [PMIM][PFs] ionic liquids, showing melting point and crystal stability increasing with the degree of branching in the alkyl substituent.

Structured laundry liquids are currently available in Europe and were recently introduced in the United States [50,51]. These products typically contain high levels of surfactants and builder salts, as well as enzymes and other additives. In the presence of high ionic strength, the combination of certain anionic and nonionic surfactants form lamellar liquid crystals. Under the microscope (electron microscope, freeze fracturing) these appear as round droplets with an onion-like, multilayered structure. Formation of these droplets or sperulites permits the incorporation of high levels of surfactants and builders in a pourable liquid form. Stability of the dispersion is enhanced by the addition of polymers that absorb onto the droplet surface to reduce aggregation. 

The raw materials from which di-D-fructose dianhydrides can be obtained in appreciable yield are readily available from comparatively inexpensive agricultural feedstocks. Thus, these compounds are attractive as chiral-starting materials for chemical synthesis. Their stability to acid and heat, and their relative rigidity, because of the conformational constraints covered here, are also features that might be exploited during syntheses.119 A series of variously substituted di-D-fructose dianhydrides has been prepared,119 starting from 6,6 -dideoxy-6,6 -di-halosucroses. The properties of these and other derivatives of di-D-fructose dianhydrides are summarized in Tables XIV-XX. Two of these derivatives, 48 and 56, exhibit thermotropic liquid-crystal properties.119... 

Currently, the effect of the molecular quadrupoles on liquid crystal properties is not clearly understood though there is some evidence to suggest that they influence surface properties and phase stability [28, 60]. 

Electro-optic materials can be made using liquid crystal polymer combinations. In these applications, termed polymer-stabilized liquid crystals [83,86], the hquid crystal is not removed after polymerization of the monomer and the resulting polymer network stabilizes the liquid crystal orientation. 

Liquid crystals based on aliphatic isocyanides and aromatic alkynyls (compounds 16) show enantiotropic nematic phases between 110 and 160 °C. Important reductions in the transition temperatures, mainly in clearing points (<100 °C), areobtained when a branched octyl isocyanide is used. The nematic phase stability is also reduced and the complexes are thermally more stable than derivatives of aliphatic alkynes. Other structural variations such as the introduction of a lateral chlorine atom on one ring of the phenyl benzoate moiety or the use of a branched terminal alkyl chain produce a decrease of the transition temperatures enhancing the formation of enantiotropic nematic phases without decomposition. 

It is interesting to note the influence of the counteranions on the thermal behavior. Irrespective of the isocyanide used, all the nitrate gold derivatives show low thermal stability and undergo extensive decomposition at relatively low temperatures (only the low melting trialkoxyphenyl derivative shows liquid crystal behavior). In contrast. 

The preparation and study of metal nanoparticles constitutes an important area of current research. Such materials display fascinating chemical and physical properties due to their size [62, 63]. In order to prevent aggregation, metal nanoparticles are often synthesized in the presence of ligands, functionalized polymers and surfactants. In this regard, much effort has focused on the properties of nanoparticles dispersed into LCs. In contrast, the number of nanoparticles reported that display liquid crystal behavior themselves is low. Most of them are based on alkanethiolate stabilized gold nanoparticles. 

Several mixtures of hexanethiol capped gold nanopartides and triphenylene based discotic LCs have been studied. These mixtures display liquid crystal behavior (columnar mesophases) and an enhancement in the DC conductivity, due to the inclusion of gold nanoparticies into the matrix of the organic LC [70]. Other studies of mixtures of gold nanoparticies with mesogens indude a series of cholesteryl phenoxy alkanoates. The inclusion of the nanopartides does not change the inherent liquid crystal properties of the cholesteryl derivative but the mesophases are thermally stabilized [71]. 

Mixtures of a nematic liquid crystal (LC or LC ) with small quantities of gold nanoparticles coated with alkylthiolates (<5 wt%) including an alkylthiolate functionalized with a chiral group have been studied (Figure 8.29) [72]. All mixtures show nematic mesophases with transition temperatures and phase stability very similar to those oftheliquid crystal precursors LC or LC. The introduction ofachiral center into the mixtures (mixtures of Au ) produce chiral nematic mesophases. A similar result is obtained in mixtures of Au and LC doped with the chiral dopant (s)-Naproxen. 

Serrette, A.G. and Swager, TM. (1994) Polar Superstructures Stabilized by Polymeric Oxometal Units Columnar Liquid Crystals Based on Tapered Dioxomolybdenum Complexes. Angewandte Chemie (International Edition in English), 33, 2342-2345. 

A review of the literature demonstrates some trends concerning the effect of the polymer backbone on the thermotropic behavior of side-chain liquid crystalline polymers. In comparison to low molar mass liquid crystals, the thermal stability of the mesophase increases upon polymerization (3,5,18). However, due to increasing viscosity as the degree of polymerization increases, structural rearrangements are slowed down. Perhaps this is why the isotropization temperature increases up to a critical value as the degree of polymerization increases (18). 

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