Types of liquid-crystal polymers

In true thermotropic systems the liquid-crystalline phase exists over a range of temperatures between the melting point, or glass-transition tern- 

18 Some examples of main-chain liquid-crystal polymers (a) poly(p-phenylene benzobisoxazole) (PBO), lyotropic (b) poly(4-carboxybenzene-propionic acid), thermotropic and (c) a thermotropic polymer with a long flexible decamethylene spacer. 

There is a further very important class of main-chain liquid-crystal polymers that differ from those discussed above by being random copolyesters based on polymers such as poly(hydroxybenzoic acid) -fO— —C=0), where — — represents a para-disubstituted benzene ring (para-phenylene unit). The random copolymers include units of various other forms containing phenylene or naphthylene rings and their introduction reduces the crystallinity and melting point considerably without necessarily lowering the value of Polymers of these types have 

So far both melting and transition temperatures have been referred to without giving any actual values. Table 12.2 lists these important parameters for all the polymers mentioned in this section. 

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Two types of liquid crystal polymers (LCP s) can be distinguished lyotropic and thermotropic. The first type is formed in a solution this is done when the polymer has such a high melting point (low AS, see 4.2), that it cannot be handled in the molten condition without being degraded. If this limitation is not present, then the orientation can be brought about in the melt in such a case we have a thermotropic LCP. 

Figure 2.18 (a) Some typical mesogen groups, (b) Schematic representation of types of liquid crystal polymers according to the location of the mesogen groups in the main chain (right) or as side substituents (left). 

Fig. 1.4 Schematic representations of the principal types of liquid-crystal polymers (LCPs) ...

Figure 1. Schematic diagram illustrating the structures of two types of liquid crystal polymer and a liquid crystal dendrimer.

Fig. 1. Examples of three types of liquid crystal polymer molecule I, chain made up from rigid aromatic units joined by relatively stiff ester groups II, chain containing rigid units separated by flexible aliphatic sequences DI, chain with a flexible backbone but rigid side-groups.

Chirality in side chain liquid crystal polymers can also be introduced by constructing a chiral centre as part of the spacer moiety or as part of the polymer backbone, but these types of liquid crystal polymer are relatively rare. 

Definition and Main Types of Liquid Crystal Polymers.80... 

A common feature of different types of liquid crystal polymers (LCPs), e.g., thermotropic side-chain or main-chain (either stiff or with flexible spacers) polymers, is their slowed-down dynamics compared to low molecular weight liquid crystals (LCs). Often polymers can be quenched to a glassy state in which the liquid-crystalline order is preserved but motions are completely frozen out. Such liquid-crystalline glasses provide a unique opportunity to determine, in principle, the full orientational distribution function, whereas only its second moment is available from motionally averaged NMR spectra. Thus LCP studies have made fundamental contributions to LC science. 

In one type of liquid crystal polymer (LCP) the rigid cores (or even disks) are connected head-to-tail leading to a main-chain LCP in the second type they are connected via a linking group onto a polymer backbone giving a side-group LCP (Figure 2.14). 

Some of the nematic type of liquid crystal polymers are rigid solids at room temperature and, on the basis of an outstanding combination of properties and processing... 

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. 

Shibaev, V. P., Moiseenko, V. M., Plate, N. A. Thermotropic liquid crystalline polymers, 3, Comb-like polymers with side chains simulating the smectic type of liquid crystals. Makromol. Chem. 181, 1381 (1980)... 

By analogy with small molecule liquid crystals, where the type of liquid crystal formed is used as a test for miscibility, it is expected that all polymer molecules that form the same type of liquid crystalline phase will be miscible (4). This is in contrast to more traditional polymers where miscibility is the exception rather than the rule. The present work will suggest which of these concepts is applicable to liquid crystal polymer blend systems. 

The suggestion that two liquid crystal polymers are incompatible with each other is contrary to ideas which are well established for small molecule liquid crystals (4). In fact, miscibility with other liquid crystals is one of the criteria sometimes used to establish the type of liquid crystal being dealt with. On the other hand, if the rheological criteria established for other polymer blend systems are valid for liquid crystal polymer blend systems as well, the two materials being discussed in the present work must be incompatible. 

Side-chain type polymeric liquid crystal with side-on attachment Fig. 4.8 Basic types of liquid crystalline polymers... 

Nematic gels are very interesting systems, thus deserving further study. Actually, these systems are being studied experimentally for applications. Examples are polymer dispersed liquid crystal displays are sometimes dispersed not in a polymer, but in a polymer network. Displays by means of the polymer stabilized cholesteric texture change, are also achieved in crosslinked systems. In addition, the chiral smectic phase has been obtained in such systems as well. Other types of liquid crystal gels have been applied or are expected to be applied in such devices. 

We see in these examples that great effect can be created by substitutions on the two ends of the mesogenic cores. The consequence includes not only the formability of liquid crystal phases and transition temperatures, but also the types of the liquid crystalline phase. It is well known that substitution on the end with longer linear alkyls and alkoxyls favor the formation of smectic phases. One example for this remark has been given in Table 3.3. In the design of liquid crystal polymers, especially in the cases of side-group type and molecular shish-kebabs, the concept of end-substitution has been fully utilized. 

The liquid crystalline material in the LCD is not necessarily a LCP. However, in order to fix the liquid crystalline material conventional polymers are used for embedding and LCP are also used. Side chain LCPs combine the properties of liquid crystals and polymers with flexible main chains. Since the liquid crystal moieties are fixed on the backbone of the main chains of the polymer, this type of liquid crystals may not flow away, as it may be the case in monomeric liquid crystals. 

In all of the devices mentioned above, the polymer network is used to influence the alignment of the liquid crystal, sometimes to stabilize order, other times to introduce disorder. Different types of network structure are desired in each case. Since these polymer networks seem to advantageously alter the electro-optic properties of many types of liquid crystal displays, it is imperative to understand the structure of these polymer networks for modifying, ftirther improving, and even developing entirely new types of liquid crystal displays. 

There are two types of disparities between the steady-state and dynamic flow behavior, one related to the interlayer slip (Eq. (2.56)) and the second to the flow engendered migration of the low viscosity component to the high stress location. Blends of liquid crystal polymer (LCP) with polycarbonate (PC) or poly(ethylene-terephthalate) (PET) may serve as an example of the first type [321,322], whereas those of EPDM (ethylene-propylene-diene terpolymer) with poly(vinylidene-co-hexafluor-opropylene), Viton , exemplify the second [323-325]. In both cases the steady-state shearing was performed in a capillary viscometer- the viscosity ratio of the dynamic to the steady-state data for these two blends was about two and six, respectively. 

Several reactive monomers have been designed, synthesized and characterized for the polymer stabilized liquid crystals. The orientation of anisotropic polymer networks formed in different types of liquid crystals have been studied... 

Some types of liquid crystal displays combine liquid crystal material and polymer material in a single device. The first of these involves micrometre sized droplets of nematic hqnid crystal in a solid, isotropic polymer matrix. There are two requirements on the polymer for this device to work. First, the index of refraction of the polymer, n must... 

Similar materials could be obtained by an emulsification method [253]. Nematic liquid crystal is emulsified into an aqueous dispersion of a water-insoluble polymer colloid (i.e., latex paint). An emulsion is formed which contains a droplet with a diameter of a few microns. This paint emulsion is then coated onto a conductive substrate and allowed to dry. The polymer film forms around the nematic droplets. To prepare an electrooptical cell a second electrode is laminated to the PDLC film [253]. In the phase separation and solvent-casting methods the chloroform solutions of liquid crystal and polymer are also used [254, 255]. The solution is mixed with the glass spheres of the required diameter to maintain the desired gap thickness and pipetted onto a hot (140 °C) ITO-coated glass substrate [255]. After the chloroform has completely evaporated another ITO-coated glass cover is pressed onto the mixture and then it is cooled down. Structural characteristics of the PDLC films are controlled by the type of liquid crystal and polymer used, the concentration of solution, the casting solvent, the rate of solvent evaporation, perparation temperature, etc. [254]. 

Another type of liquid crystalline polymers are those having rod-shaped side chains. TTius, the backbone may be a random coil, but the side chains are organized into one- or two-dimensional liquid crystals. The stiff backbone types make very high modulus fibers and high temperature plastics. The side chain types are useful for their action in magnetic and electrical fields. (Note the behavior of battery-driven liquid crystalline watches, for example. The liquid crystalline material is held between crossed Nicols the orientation of the molecules is controlled by an electric field.) In the following, the terms mesophase and liquid crystal phase are used interchangably. 

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