Odd spacers

All enamels had excellent adhesion, solvent resistance, and flexibility. Enamels made from LC diols Ib-lg were far superior to those made from control diols 2a-2g and 3a-3g in both hardness (5H-6H vs. H-2H 5H corresponded to about 20 KHN on a Tukon tester) and impact resistance (50 to >80 in-lb vs. 8 to 15 in-lb). Odd spacers lb and Id afforded the best properties in these formulations. Spacer variations did not measurably affect enamel properties in the control oligoesters. 

Self-assembly of triple-stranded helicates 5.9-5.11 with even and odd spacers. 

Mechanical fields can also be deployed to achieve a macroscopic orientation of the helicoidal z-axis of cholesteric elastomers. As discussed above, nematic side chain polymers with odd spacer length exhibit a locally prolate chain conformation... 

Fig. 5.21. (a) A schematic illustration of relative core orientations in members of a covalently connected pair of mesogenic cores for even and odd spacer-chain parities (all-tmns conformation) and (b) dimer (D ) versus monomer (M ) LC core order parameters in the nematic phase (from [62]). 

The steric frustrations have also been detected in LC polymers [66-68]. For example, the smectic A phase with a local two-dimensional lattice was found by Endres et al. [67] for combined main chain/side chain polymers containing no terminal dipoles, but with repeating units of laterally branched mesogens. A frustrated bilayer smectic phase was observed by Watanabe et al. [68] in main-chain polymers with two odd numbered spacers sufficiently differing in their length (Fig. 7). 

Fig. 7. Two-dimensional packing observed for the main-chain polymer with two odd numbered spacers of different length. Solid lines and arrows indicate the two-dimensional rectangular lattice and macroscopic polarization, respectively (Watanabe et al. [68])...

We note that the bilayer smectic phase which may be formed in main-chain polymers with two odd numbered spacers of different length (Fig. 7), should also be polar even in an achiral system [68]. This bilayer structure belongs to the same polar symmetry group mm2 as the chevron structure depicted in Fig. 17b, and macroscopic polarization might exist in the tilt direction of molecules in the layer. From this point of view, the formation of two-dimensional structure of the type shown in Fig. 7, where the polarization directions in neighbouring areas have opposite signs, is a unique example of a two dimensional antiferroelectric structure. 

Figure 5.12 Dependence of peak frequencies in infrared spectrum of glucosamide bolaam-phiphiles NC( )GN-GLC (14) on methylene spacer length n. (a) The CH2 antisymmetric vas and symmetric vs stretching vibrations reveal gauche-included conformation for short chains and an all-trans conformation for longer chains, (b) Amide I and II frequencies show an even-odd effect for n > 10. Reprinted from Ref. 53 with permission of Wiley-VCH.

In a model study aimed at elucidating the behavior of the polymers, Watanabe found that the dimer polymethylene diol diesters of type 7 (Figure 8.18) formed smectic phases.38 When the spacer between the two mesogenic units in the dimer had an odd number of methylene groups (diester 7 has nine methylenes in the spacer), then an intercalated tilted smectic structure with all anticlinic layer interfaces was formed. This structure is often termed SmC2 in the literature. As for the B6 phase, all of the layer interfaces in this structure are equivalent, and the X-ray layer spacing is less than half the fully... 

NMR spectra and Tj measurements at different temperatures. The local polymer chain motion varies over a frequency range of 104-106 Hz in the nematic phase. The activation energy of this motion is found to increase with decreasing number ( ) of methylene units in the spacer, and exhibits odd-even fluctuations. In a study of a homologous series of main-chain LC polyesters, 13C CP/MAS and variable-temperature experiments reveal a conformation-ally more homogeneous and a less dynamic nature for the even-chained than for the odd-chained polymer structures.300... 

The LC nature of diols la-lg was demonstrated by DSC (Fig. 1). Two first order transitions were observed. The lower transition temperature appeared to be the crystalline-mesophase transition (T, ), and the higher transition temperature the mesophase-isotropic transition (T ). Other transitions were not evident in the DSC. The thermal data revealed an odd-even spacer effect for Tj . 

The thermal behavior of la-lg observed by DSC (Fig. 1) confirms the presence of mesophases and is typical of low molecular weight thermotropic LC materials (M). The lower T , for lb and Id are consistent with the higher entropy of activation for crystallization of odd-n spacers, demonstrated in several main chain LC polymers (23). The apparent absence of nematic-smectic transitions in the DSC... 

Note 4 The tilt direction varies in a random manner from layer to layer in conventional smectic C mesophases. However it can alternate from layer to layer, as in an antiferro-electric chiral smectic C mesophase (see Definition 5.9, Note 7) and in the smectic C mesophase formed by certain liquid crystal dimers with an odd-number of carbon atoms in the spacers. The recommended symbol for this type of mesophase is SmCa. 

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 distribution of chain sequence extension, calculated by using RIS models, is compared with isotropic-nematic transition characteristics for a number of thermotropic polymers comprising rigid groups connected by polymethylene spacers. The distribution depends strongly not only on the odd-even character of the number of methylene units of the spacers, but also on the specific groups (or atoms connected at the ends of polymethylene spacers. 

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. 

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