Monomer liquid crystals

In the case of copolymer LCs there is speculation that the alternating primary structure. .. core-spacer-core-spacer... restricts inter-core translations and thereby stabilizes smectic phases. While we remain cognizant of the potentially important role that primary and secondary molecular structure of mesogens can play, in the remainder of this chapter, however, we will focus on aspects of mesomorphism that, for the most part, can he described in terms of the idealized prolate (oblate) shape of calamitic (discotic) mesogens (Fig. 5.1). We consider the nature and implications of the local molecular order in the nematic state after briefly reviewing the molecular crystal and the isotropic liquid. 

Here aligned nematic implies that the director n has the same orientation throughout the diffracting volume element ( 1 mm ). 

The molecular quantity of interest, the average orientation of 1 relative to n - the nematic order parameter 5 - is defined in terms of 

W(P) sin p dp is the (normalized) probability of finding 1 in the range dp about the direction p with respect to the director W P) is independent of 6 in a normal isotropic liquid. In Eq. (5.2) the order parameter S, the average of the second Legendre polynomial p2(cos P) = l/[2(3cos — 1)], assumes the value unity in a perfectly ordered system (when 1 11 n as in the molecular crystal idealized in Fig. 5.11), and the value zero when 1 is isotropically distributed (Fig. 5.12(a)). In order to extract S from the experimental observable /(x) one can use Eq. (5.1) (recognizing that cos P = cos x sin co) with an assumed form for W P) to fit 7(x) (numerically) [44]. If, for example, one assumes the validity of a Gaussian distribution. 

In nematic phases of MLCs, S typically ranges from about 0.25 to 0.75. We will pursue the meaning of these magnitudes for S in order to obtain a better feel for the nature of orientational order in nematics. First, however, we should recognize that we are able to describe nematic order with a single number (the scalar S) because we have made assumptions about the molecular symmetry in this uniaxial phase. 

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For liquid crystalline polymers, the elastic constants are determined not only by the chemical composition but also by the degree of polymerization, i.e., the length of the molecular chain. One main aim of this section is to address the effects of molecular chain length on the elastic constants of liquid crystalline polymers. Figure 6.1 shows the three typical deformations of nematic liquid crystalline polymers. The length and flexibility of liquid crystalline polymers make the elastic constants of liquid crystalline polymers quite different from those of monomer liquid crystals. 

For the purpose of this article, we focus our attention on the nematic mesophase smectic orders are more crystal-like and thus are beyond our scope. Typical nematic liquid crystals are characterized by a uniaxial order, though imperfect, along the preferred axis of the domain. No such long-range order exists in directions transverse to the domain axis. In most examples, low molar mass (monomer) liquid crystals carry flexible tails. Conformational ordering of these tails in the mesophase has been extensively studied in relation to the odd-even character of the phase behavior with the number of constituent atoms of the pendant chain. Various statistical models and theories have been presented [52-57]. In most cases, however, the ordering of the tail is relatively weak [58,59]. 

The values of y were similarly obtained for dimer CBA- (n = 9,10) and trimer CBA-Tn (n = 9,10). These compounds exhibit the nematic LC phase over a limited temperature range, hampering an accurate estimation of y by the extrapolation from this phase. Accordingly the y values were estimated by method 1 only from higher-temperature phases i.e., y i values are estimated from the isotropic phase, and ycN values from the nematic phase [95]. The ytr values thus derived are all accommodated in Tables 2 and 3, respectively, for the NI and CN transitions. Thermal pressure coefficients of monomer liquid crystals such as 4-cyano-4 -alkylbiphenyls ( CB) and 4-cyano-4 -alkoxybiphenyls ( OCB) are available in the article by OrwoU et al. [112]. The y values applicable to the NI transition of these compounds are cited in Table 4 for comparison. As shown in these tables, use of the volume change A Vtr at the transition (column 4) leads to the estimate of the volume-dependent entropy ASy (column 5) according to Eq. 3. 

As comparison of Tables 2 and 4 indicates, the changes in volume and entropy at the NI transition obtained for the mainchain dimer and trimer liquid crystals are much larger than those reported for conventional monomer liquid crystals [112]. In Tables 2 and 3, the constant-volume transition entropies (AStr)v are expressed in terms of joules per mole per kelvin. The conformational entropy changes estimated on the basis of the NMR quadrupolar splitting data observed in the LC state are as follows = 13.3... 

At the NI transition, an orientation-dependent term such as AS " " must play a role [26]. In practice, however, the contribution from this source seems to be comparatively small by inference from those of the monomer liquid crystals [112] (cf. Table 4). In treating the transition entropy of real systems, contributions from the so-called communal entropy as well as other residual entropies are often considered by introducing an extra term ASj in Eq. 1 [15,17,34,35,120] ... 

Creation of liquid crystalline phases a comparative view emphasizing structure and shape of monomer liquid crystals... 

To obtain comb-like or side chain-like architecture, a monofunctional, low molar mass mesogen (or monomer liquid crystal, MLC) can be complexed to a polymer possessing complementary groups. The non-covalent bond may then be located within or near the polymer... 

So-called banded structures, shown in Figure 10.9, are formed in PLCs. The banded structures do not appear in monomer liquid crystals... 

Monomer liquid crystals (MLCs) and polymer liquid crystals (PLCs)... 

Link, D.R., Clark, N.A., Ostrovskii, B.I., Soto Bustamante, E.A. Bilayer-by-bilayer antiferroelectric ordering in freely suspended films of an achiral polymer-monomer liquid crystal mixture. Phys. Rev. E 61, R37-R39 (2000)... 

For the purpose of discussing mesophase formation in polymers it is conveiuent to partition the polymers into two categories and introduce abbreviations that refer to these categories. Polymerized liquid crystals, here abbreviated PLCs, are derived from known, low molecular weight monomer liquid crystals (MLCs) that contain polymerizable functionality (e.g. vinyl units). We designate liquid-crystalline polymers (LCPs) to be semiflexible, linear polymers that are structurally related to conventional engineering thermoplastics, i.e. polymers derived from poly(ester)s, poly(amide)s, poly(imide)s, etc. We will examine the attributes of polymerized liquid crystals first, stressing the similarities between their properties and those of MLCs. 

Photo-Alignment of Monomer Liquid Crystals by Directional Photosensitive Side-Chain Polymers... 

With nematic or cholesteric monomer liquid crystals surface alignment effects are a precondition of their use for electro-optic displays. With smectic monomeric materials addressed using low or high frequency fields... 

To begin with, we divide liquid-crystalline materials into monomer liquid crystals (MLCs) and polymer liquid crystals (PLCs). This convenient terminology is due to Samulski, who also specified that a compound is classified as an MLC irrespective of the fact whether it can or cannot polymerize. Not long ago we had an explosion of new applications of MLCs. We might well be entering now into a similar period with PLCs. 

In order to understand dielectric relaxation in polymer liquid crystals and in liquid crystal polymers it is necessary to review DR phenomena in monomer liquid crystals and in polymers, and this will be done in turn in the next two sections. We begin with liquid crystals. 

Ferroelectric side-chain liquid crystal polymers (fLCPs) have been synthesized only recently. (see Chapter 8). Because of the dilution of chiral side chains in the backbone matrix, the spontaneous polarization is weaker than in the monomer chiral smectics, although Kapitza and others recently reported polysiloxane-based fLCPs with spontaneous polarization comparable to that observed for monomer liquid crystals. 

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