The flexible spacer

In a series of comb LCPs where spacer length is the only variable, there may be two reasons for any observed differences. Firstly, we may be seeing the result of changing the effective length of the mesogen (i.e. at least a part of the spacer is involved in the meso-genic structure) which is known to affect both the nature and thermal stability of mesophases (Section 7.3.3.1). Secondly, we may be seeing 

A comprehensive programme of synthesis and characterization by the Bordeaux group has provided a wealth of comparative data on which Table 7.6 is based note that the polymers are well defined, monodisperse systems. The general points to emerge from these and comparable data are as follows  

Effect of spacer length on thermal properties of siloxane combs Me 

The data also illustrate the polymer effect, viz. tethering a mesogen to a polymer tends to increase the mesophase ordering. Thus, of the small-molecule species (Fig. 7.8) used in the synthesis of the smectic combs (Table 7.6), all are nematic apart from (X = CN, n=ll), and only the cyanomesogens with long tails (n 8) exhibit smectic as well as nematic phases. 

2 Lateral attachment of mesogens parallel (biparallel) systems While we have not otherwise looked at connections between spacer and mesogen, there has been particular interest in LCPs with laterally attached mesogens since Hessel and Finkelmann reported the first example in 1984. Table 7.8 summarizes published data on several polymers that have been prepared since then. In general the phase 

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The side chains in these structures can rearrange into positions either parallel or perpendicular to the deformed chain backbone. [178]. The outcome depends particularly on the nature and length of the flexible spacer connecting the mesogenic groups to the chain backbone. As expected the physical properties can become strongly anisotropic [179]. 

Note 2 The mesogenic groups ean be, e.g., rod-like or dise-like, and can be attached laterally or longitudinally to the flexible spacers. 

Steady-state fluorescence spectra are obtained for dilute solutions of 16 aromatic polyesters with the repeating unit ABm. The chromophore, A, is isophthaloyl or phthaloyl, and the flexible spacer, B, Is methylene or oxymethylene. The number of methylene units Is 2 - 6, and the number of oxyethylene 1 - 4. Results obtained are interpreted within the frame of the RIS method. 

The prospensity for the formation of bends in the flexible spacer in polyesters containing naphthyl units is examined by the study of the fluorescence of a series of diesters. The dependence of the degree of intramolecular excimer formation on the length of the aliphatic spacer, under circumstances where the dynamics of rotational isomerism in the flexible spacer is suppressed, is evaluated by extrapolation of the measurements to infinite viscosity n. The extrapolated results exhibit an odd-even effect, with the more intense excimer emission being observed when the number of methylene groups is odd. The odd-even effect is rationalized by an RIS analysis of the diesters. 

The fluorescence is measured in dilute solution and in glassy PMMA for polyesters in which 2,6-naphthalene dicarboxylate is the rigid unit, and (CF iy+t is the flexible spacer. The anisotropy in the rigid medium demonstrates the existence of intramolecular energy migration, which becomes more important as y decreases from 5 to 1. The Forster radius is about 12 A in the bichromophoric compounds and 14 A in the polyesters. 

Fluorescence is measured in dilute solution of model compounds for polymers of 2,6-naphthalene dicarboxylic acid and eight different glycols. The ratio of excimer to monomer emission depends on the glycol used. Studies as functions of temperature and solvent show that, in contrast with the analogous polyesters in which the naphthalene moiety is replaced with a benzene ring, there can be a substantial dynamic component to the excimer emission. Extrapolation to media of infinite viscosity shows that in the absence of rotational isomerism during the lifetime of the singlet excited state, there is an odd-even effect In the series in which the flexible spacers differ in the number of methylene units, but not in the series in which the flexible spacers differ in the number of oxyethylene units. 

The magnetic susceptibilities of dimer liquid crystals such as NC-Ph—Ph—O—(CH2>n—Ph—Ph—CN(n 9, 10) are measured by a SQUID magnetometer. The results obtained are interpreted within the framework of the RIS approximation, the effect arising from the conformational anisotropy of the flexible spacer being strictly taken into account. The order parameters of the mesogenic core axis thus estimated are found to be consistent with those directly observed at just below 7N) by the ZH NMR technique using mesogen-deuterated samples. 

The fluorescence In dilute solution is measured for five polyesters with terephthalate as the rigid aromatic unit and diols derived from cyclohexane as the flexible spacer, A conformational analysis concludes that the spacers most conducive to excimer formation are the 1,3-c/s-cyclohexanediol and 1,4-e/s-eyclohexanedimethanol. This result from calculations is compatible with experimental results. 

Schematically the arrangement of a macromolecule with side-chain mesogenic groups in the liquid crystalline state is shown in Fig. 13. Flexible spacers give the mesogenic group its mobility. It is of interest to note that in such liquid crystals all positional mobility of the mesogen is based on conformational motion of the flexible spacer and backbone. Restricting this mobility either prohibits ordering, or freezes the order into the glassy state.

As shown above, by shortening the flexible spacer, it is possible to produce rather rigid chains which should probably better be described as main-chain mesogens. Mesophase superstructures can also be achieved with flexible side-chains (Shibaev et al. 88)). Lyotropic systems of this type are given by Finkelmann et al. 89). 

Shortening the flexible spacer must ultimately lead to rigid chain macromolecules. Many examples of such macromolecules have been reviewed by Preston99. An interesting example is the series of polyfethylene terephthalate-co-oxybenzoate)s... 

The nematic phase stability is higher the greater the length and length diameter ratio of the rigid-rod unit, and the shorter the flexible spacer unit, and rises ultimately to values above the temperature of decomposition of the polymer. 

While for m-l.c. s the state of order is only determined by the anisotropic interactions of neighbouring molecules, for the polymers additionally a disturbing effect of the backbone via the flexible spacer on the anisotropic order of the mesogenic side chains is to be expected and vice versa. Therefore it is of interest to investigate whether... 

Using this method in Fig. 15, the order parameters vs. temperature of the polymers are compared with the corresponding monomer mixture Ml. The monomer Ml exhibits a higher value of S than the polymer by about 10 % and confirms the previous linear dichroism measurements. If we compare the magnitude and temperature dependence of S for the polymers Cl to C3, which differ in the length of the flexible spacer, no difference in S can be found within the experimental error. 

From these experiments we can conclude that the length of the flexible spacer has no measurable influence on the orientational long range order of the mesogenic side chains. If on the contrary l-l.c. s are attached to the polymer backbone, the state of order is reduced. 

From electric field effects another important information can be obtained concerning the efficiency of the flexible spacer. The chemical construction of l.c. side... 

These measurements clearly confirm the consideration that the length of the flexible spacer directly influences the elastic constants. This could also be obtained by other authors 69>. 

In conclusion, it has to be emphasized, that the l.c. side chain polymers in principle exhibit field effects like l-l.c. s, assuming that the flexible spacer is sufficiently long. The applicability of polymers in high performance display technology seems to be less practicable due to the high viscosity. On the other hand, the polymer specific glass transition enables the realization of storage elements an information, put in by an electric field in the l.c. state, can be durably frozen in. 

These experiments clearly confirm the theoretical concept of the helical twist of cholesteric phases developed by Goossens and Vertogen 79,86), which is based on the hindered rotation of chiral molecules around their long molecular axis and the introduction of the order parameter D. Furthermore they support the concept of the flexible spacer, described in Chap. 2.1. 

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