Polymers, liquid crystal

Liquid crystals were discovered by the botanist Friedieh Reinitzer, although the first observation of liquid crystals can be likely ascribed to George-Luis LeClerc. After their discovery, liquid crystals were considered for a long time as a curiosity. 

Engaged with the constitution of cholesterol, for cholesteryl benzoate Reinitzer reported two phase transitions in the eourse of melting. Melting starts with the formation of a cloudy liquid, which transforms on further heating into a clear liquid. This type of liquid crystal is addressed as thermotropic. In addition, there are lyotropic liquid crystals that change their behavior as both a function of concentration in a solvent and of temperature. 

Liquid crystals originate from the phenomenon that the crystal structure is partly maintained in the liquid phase above the melting point. In particular, they exhibit a long range orientational order, but not a positional order. The various types of liquid crystal phases, also called mesophases can be classified according to their type of ordering into  

Various subclasses of liquid crystals have been described. From the molecular view, Uquid crystals are built up of rod-hke stiff moieties in the molecule. This is also true for liquid crystal polymers (LCP)s. 

Discotic Uquid crystalline molecules are disc-shaped molecules. Examples are Hexaalkanoyloxy benzenes, hexaalkoxy triphenylenes, bis-(4-n-decylbenzoyl)methanato copper (II), hexa-n-alkanoates of truxene and octa substituted phthalocyanines.  

Liquid crystal polymers (LCP) are polymers that exhibit liquid crystal characteristics either in solution (lyotropic liquid crystal) or in the melt (thermotropic liquid crystal) [Ballauf, 1989 Finkelmann, 1987 Morgan et al., 1987]. We need to define the liquid crystal state before proceeding. Crystalline solids have three-dimensional, long-range ordering of molecules. The molecules are said to be ordered or oriented with respect to their centers of mass and their molecular axes. The physical properties (e.g., refractive index, electrical conductivity, coefficient of thermal expansion) of a wide variety of crystalline substances vary in different directions. Such substances are referred to as anisotropic substances. Substances that have the same properties in all directions are referred to as isotropic substances. For example, liquids that possess no long-range molecular order in any dimension are described as isotropic. 

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. 

Poly(l,4-oxybenzoyl) (LII) [IUPAC poly(oxy-l,4-phenylenecarbonyl), obtained by selfreaction of p-hydroxybenzoic acid, and the various aramids (Sec. 2-8f) were among the first LC polymers studied. The experience in commercializing poly(l,4-oxybenzoyl) and the 

Many of the polymers discussed in the following sections exhibit liquid crystal behavior. 

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. 

Typical Characteristics of Some Important Plastics (a) Semi-crystalline plastics 

Low density polyethylene (LDPE). This is one of the most widely used plastics. It is characterised by a density in the range 918-935 kg/m and is very tough and flexible. Its major application is in packaging him although its outstanding dielectric properties means it is also widely used as an electrical insulator. Other applications include domestic ware, tubing, squeeze bottles and cold water tanks. 

High Density Polyethylene (HDPE). This material has a density in the range 935-965 kg/m and is more crystalline than LDPE. It is also slightly more 

Metallocene-based polyethylene does not offer the lower production costs associated with LLDPE. Hence there will be a price premium for the new materials but this is felt to be justified in view of their improved property profile. 

The term liquid crystal is in some ways unfortunate, since materials in this state are not crystalline and they may but need not be liquid. A preferable term that is often used is mesophase, which means middle phase and attempts to indicate that the order in this state is between that of the liquid state and that of the crystalline state. Mesogens are then molecules that tend to form mesophases and the attribute of being liquid-crystallinelike is called being mesomorphic. 

Before describing LCPs it is useful to describe briefly the various forms of liquid crystallinity exhibited by small molecules. 

Several types of LCPs are available commercially, all consisting of copolymers composed of molecules with rigid and flexible monomers. The rigid part, called the 

For linear polyethylene fibers made in this fashion, the tensile modulus (a measure of the strength of a material, which is defined more rigorously in Chapter 16) of 44GPa (6.4 X lO psi) compares well with steel (which is about 206 GPa or 30 x lO psi) but has a density 7.6 times as great PE fibers with a tensile strength of 1.8 GPa (2.6 x 10 psi) have also been claimed [9]. Compare these figures with those for ordinary, quiescently crystallized HOPE in Table 4.1. 

Extended-chain crystals have also been identified as tying together lamellae in bulk-crystallized polymers and forming the core ( skewed ) of the interesting shish kebab structures grown from dilute solutions subjected to shearing [10]. 

Kevlar is an aromatic polyamid ( aramid ) with the repeat unit  

TABLE 4.2 Comparison of Liquid-Crystalline Kevar 49 Fibers with Nylon 6/6 Fibers [13] 

Several thermotropic aromatic copolyesters have also been commercialized. Vectra A is reported to have the repeating units 

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We are all familiar with tire tliree states of matter gases, liquids and solids. In tire 19tli century the liquid crystal state was discovered [1 and 2] tliis can be considered as tire fourtli state of matter [3].The essential features and properties of liquid crystal phases and tlieir relation to molecular stmcture are discussed here. Liquid crystals are encountered in liquid crystal displays (LCDs) in digital watches and otlier electronic equipment. Such applications are also considered later in tliis section. Surfactants and lipids fonn various types of liquid crystal phase but this is discussed in section C2.3. This section focuses on low-molecular-weight liquid crystals, polymer liquid crystals being discussed in tire previous section. 

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

Chemical Resistance of LGPs. Ceitain liquid crystal polymers (eg, Vectra) have extremely high chemical resistance to a variety of aggressive chemicals and solvents. Table 18 shows the chemical stabiUty of Vectra test-bars to various agents (244). 

NHydroxyben2oic acid is of significant commercial importance. The most familiar application is the use of several of its esters as preservatives, known as parabens. Also of interest is the use in liquid crystal polymer applications. 

A very high, price and performance family of polymers called liquid crystal polymers (LCPs) exhibit extremely high mechanical and thermal properties. As their ease of processing and price improve, they may find appHcation in thin-waH, high strength parts such as nails, bolts, and fasteners where metal parts cannot be used for reasons of conductivity, electromagnetic characteristics, or corrosion. 

The liquid crystal polymers consist of rod-like molecules which, during shear, tend to orient in the direction of shear. Because of the molecular order the molecules flow past each other with comparative ease and the melts have a low viscosity. When the melt is cooled the molecules retain their orientation, giving self-reinforcing materials that are extremely strong in the direction of orientation. 

Crystalline polymers primarily of interest as fibres, including some grades which may be considered as liquid crystal polymers. 

The Kevlar polymer may be regarded as a liquid crystal polymer (see Chapter 25) and the fibres have exceptional strength. They are thus competitive with glass, steel and carbon fibres. 

Yet another recent development has been the alloying of polycarbonates with liquid crystal polymers such as Vectra (see Section 25.8.1). These alloys are notable for their very good flow properties and higher strength and rigidity than conventional bisphenol A polycarbonates. 

In Chapter 3 it was pointed out that certain rod-like polymers showed many of the attributes of liquid crystals in the melt. In particular, these molecules were oriented in shear to such an extent that interchain entanglement was small and the melts had a low viscosity. On cooling of the melt these rod-like molecules remained oriented, effectively self-reinforcing the polymer in the direction of flow. The essential differences in the properties of liquid crystal polymers... 

Figure 25.24. Difference in behaviour between liquid crystal polymers and conventional crystalline polymers in the melt at rest, during shear and when cooled after shearing...

Figure 25.25. Typical sequences found in intractable liquid crystal polymers with >400°C...

It was estimated that in 1987 about 25 companies were involved in the development of liquid crystal polymers, but by 1995 only Hoechst and Amoco were sharing the 3000 t.p.a. global market in the ratio of about 70 30. 

Over the years polymers have been produced suitable for use at progressively higher temperatures. Where this is a requirement, it is usual first to decide whether a rubbery or a rigid material is required. If the former, this has been dealt with by the author elsewhere." If the latter, it is usually convenient to look in turn at polycarbonates, PPO-based materials, polyphenylene sulphides, polysul-phones, polyketones such as PEEK and PEK, polyamide-imides, poly-phthalamides, fluoropolymers, liquid crystal polymers and polyimides. 

Liquid crystal polymers Cross-linked Structures Polyblends... 

Figure 13 Schematic diagram of the dependence of <7 on pressure. (F) Denotes folded-chain nucleus, (B) denotes bundle-like nucleus and (B ) denotes addition of ethyl cellulose liquid crystal polymer. (From Refs. 104, 110, 111, and 117.)...

Linear chain polymers with repeating sequences of hard and soft segments Possibility of formation of liquid crystal polymers and thermoplastic elastomers... 

N. A. Plate (ed.). Liquid Crystal Polymers, Plenum Press, New York (1993). 

D. Aciemo and M. R. Nobile, Thermotropic Liquid Crystal Polymer Blends, (F. P. La Mantia, ed.), Technomic Publishing, Lancaster (1993). 

Muller, K., Kothe, G., and Wassmer, K.-H. Dynamic Magnetic Resonance of Liquid crystal Polymers Molecular Organization and Macroscopic Properties. Vol. 95, pp. 1 — 56. 

High-temperature TPs are available to compete with TSs, metals, ceramics, and other nonplastic materials. The heat-resistant TPs include polyetheretherketone (PEEK) and polyethersulfone (PES), polyamideimide, liquid crystal polymer (LCP) and others. 

Liquid crystal polymer (LCP) Excellent Commercial LCPs excellent natural LCPs not stable. 

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