Small molecule liquid crystals

In recent years, the behaviour of liquid crystalline polymers including elastomers has been a subject of considerable interest 104,105). It is known that small molecule liquid crystals turn into a macroscopic ordered state by external electric or magnetic fields. A similar behaviour seems to occur for liquid-crystalline polymer networks under mechanical stress or strain. 

As briefly mentioned earlier, thermal studies have been used in conjunction with characterization by polarized light microscopy to determine the miscibility of polymeric and small molecule liquid crystals and low molecular weight mesogens, of the same or different types of liquid crystallinity, can also be used as plasticizers or diluents for polymers, as demonstrated in a study involving side chain liquid crystalline polymers... 

Note that the period is inversely proportional to shear rate y hence, the strain period Py is independent of shear rate. When A < 1 the nematic is called a tumbling nematic, while when A > 1, the nematic is flow-aligning. As discussed in Sections 10.2.5 and 10.2.6, both cases (tumbling and flow-aligning) can occur in small-molecule liquid crystals. 

The rheological and flow properties of ordered block copolymers are extraordinarily complex these materials are well-deserving of the apellation complex fluids. Like the liquid-crystalline polymers described in Chapter 11, block copolymers combine the complexities of small-molecule liquid crystals with those of polymeric liquids. Hence, at low frequencies or shear rates, the rheology and flow-alignment characteristics of block copolymers are in some respects similar to those of small-molecule liquid crystals, while at high shear rates or frequencies, polymeric modes of behavior are more important. 

Blends in which both of the components ere capable of forming liquid crystalline phases in the melt have been investigated. Rheological data as well as solid state data is presented which suggests that not all of the blends behave the same. This observation is in contrast to ideas of small molecule liquid crystals. The results are interpreted in terms of present theories regarding blends. 

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. 

The next sections of this paper contain the results of solid state structure investigations of these blend samples. The suggestion which will be advanced is that ideas concerning traditional polymer blend systems are more applicable for providing an understanding of these results than are ideas concerning small molecule liquid crystals. 

The contrast of this work compared to findings for mixtures of small molecule liquid crystals ( ) suggests that entropy plays an important part in determining miscibility in liquid crystal systems. The entropic effect could conceivably enter in two different ways. 

Figure 2. Representative Examples of Small Molecule Liquid Crystals,...

Structure-property relations in small molecule liquid crystals have been the object of systematic investigation for some time and are at least qualitatively well understood. The question naturally arises as to whether these relationships can be transferred to polymeric mesogens. At present the answer seems to be a tentative yes, at least to the level of a first approximation. A few... 

Small molecule liquid crystals suffer a lowering of the mesophase-isotropic transition temperature upon lateral substitution, i,e. replacement of hydrogen on an aromatic ring. As can be seen from the compounds below the same trend is observed for both polymer and the chemically analogous small molecule. It is known that for small molecule mesogens the transition temperature, governed by the relation Tni - A H/AS, decreases upon lateral substitution primarily due to an entropic effect. The... 

An example of recorded curve is shown in fig. 3a. Fig. 3b corresponds to the case of a small molecule liquid crystal (PAA). The intensity transmitted by the sample varies abruptly at the transition temperature the observed difference of transition temperature during cooling and heating is small. Contrary observations are made in a polymer (DDAQ) where there is a relatively large biphasic area. To describe the transition, we trace only the temperature at which the polymer becomes completely isotropic (shown by an arrow). The variations of this temperature can be compared with pseudo-transition temperature as defined by BrochardS and calculated JO,al though deviations are expected for large biphasic separation. 

If the polymer and the solvent are appropriately choosen there is complete miscibility. An example is the case of the mixture of DDA-9 in PAA. Hiscibility is promoted by steric similarity and the mesogenic rigid part of the polymer is identical to the rigid part of the small molecule liquid crystal. The transition temperature of the pure polymer is greater than that of the small molecule liquid crystal and the mixture shows the predicted lO continuous intermediate temperature between the two pure components (fig. 7]. 

Many theoretical models have been suggested for the origin of three-region flow behavior. Some bases of the theories are discussed here. LCPs contain many disinclinations in chain orientation, which are called defects, as they are commonly found in small molecule liquid crystals [25]. A good review for the defects in LCPs can be found in the literature [26]. The directional vector of... 

Interest, academic and Industrial, In Liquid Crystal Polymers (LCP s) was sparked by the commercialization of Kevlar aromatic polyamide fiber In the early 1970 s. [1,2] This fiber can be made almost as stiff and as strong as steel, at one fifth of the density of steel. In addition. It has good resistance to chemical attack and outstanding resistance to heat. From a scientific point of view, LCP s are Interesting because they. In addition to displaying a variety of phenomena and properties seen with conventional Isotropic polymers, also exhibit many of the complex physical properties of small molecule liquid crystals.[3]... 

The second section of the talk is concerned with liquid crystals that have defects or texture, for whose flow behavior a theory is not yet available. The effects of these upon orientation development during flow of LCP s is significant, because normal processing operations do not lend themselves to the techniques developed for the control of orientation and of texture of small molecule liquid crystals. Also the texture of LCP s seems to be more persistent, if for no other reason than the time scale necessary to affect it, than that of small molecule LC s. A number of physical problems related to texture are described in closing. 

S is zero for a random distribution of directors it is unity if all the directors are aligned perfectly. Measurements of S for small-molecule liquid crystals generally give values in the range of 0.4 to 0.7, corresponding to RMS orientation angles of 39° to 27°. 

Experimentally, both small molecule liquid crystals and LCP s display all sorts of director orientation patterns at high shear rates, and the existence of these Is of obvious Importance to control of orientation In processing (see Page 656 of Reference 2 for some discussion and references). It is conjectured that these may correspond to instabilities of the solutions of L-E theory. If feasible, a study of these instabilities - classification of patterns, prediction of conditions for occurrence, etc. - would be useful to relate to experimental observations. 

Fig. 7. A texture of a main-chain liquid crystal polymer, X, as seen in transmitted light between crossed polars. By analogy with small molecule liquid crystal parlance, it is known as a Schlieren texture. The fine granular nature of the background is caused by crystallization on cooling otherwise the microstructure is unchanged from that in the liquid crystal state. (Photograph courtesy of T. J. Lemmon, Department of Materials Science and Metallurgy, University of Cambridge.)...

Fig. 9. The preferred orientation, 5 = P icos or)) measured as a function of temperature by proton NMR. The data are for polymer VI and are compared with results from the small molecule liquid crystal PAA (adapted from...

Since the first discovery of the liquid crystalline phase over one hundred years ago, the classification of the distinct liquid crystalline phases in small-molecule liquid crystals has been well established (7,2). As shown in Figure 1, the least ordered liquid crystalline phase is the nematic phase that only possesses molecular orientational order due to the anisotropy of the molecular geometric shape. The next ordering level introduced is the layer structure in addition to the molecular orientation to lorm a smectic A (S/J or a smectic C (Sc) phase. Following the phase the hexatic B (Ho), smectic crystm B (So) and smectic crystal E (S ) phases are observed. In this series the long axis of the molecules is oriented perpendicular to the layer surface while order is increasingly developed from positional order normal to the layer in bond... 

In summary, TPPs show complicated phase transition behaviors. Their phase diagrms are established and various phases are identified via the thermodynamic transition properties obtained from DSC, the structural order and symmetry determined by WiOCD, and morphology and defects observed under PLM and TEM. In particular, the WAXD fiber patterns in different phases play the most important role in determining the phase structures and symmetry. It is evident that the concepts of highly order smectic phases developed in small-molecule liquid crystals can also be utilized in the main-chain liquid crystalline polymers. 

A representative small molecule liquid crystal with these features, 4-pentyloxyphenyl 4 -pentyloxybenzoate, is shown on the following page. This molecular structure can be pictorally represented by the figure below it. Back in the 1920 s the prolific German synthetic... 

One of the most interesting phenomena of the liquid crystalline polymers (LCPs) that cannot be expected in flexible or small-molecule liquid crystals is the band texture. 

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