Flexible Spacer Effects

Less frequently, functional spacers such as diamino-alkylene [106] or disulfide-alkylene segments [109] have been used. Specific reactions performed on the reactive sites of both spacers allow for the chemical modification of liquid-crystalline polymers [114]. The possibility of establishing correlations between the liquid-crystalline behavior, as indicated by the transition temperatures, and the nature and relative properties of the mesogenic and flexible sequences is, therefore, a central issue in designing and developing liquid-crystalline polymers of this type. 

The regular insertion of segments of flexible molecules to separate the mesogenic units along the polymer chain preserves the chemical periodicity of the molecule, al- 

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Hydrosilation reactions have been one of the earlier techniques utilized in the preparation of siloxane containing block copolymers 22,23). A major application of this method has been in the synthesis of polysiloxane-poly(alkylene oxide) block copolymers 23), which find extensive applications as emulsifiers and stabilizers, especially in the urethane foam formulations 23-43). These types of reactions are conducted between silane (Si H) terminated siloxane oligomers and olefinically terminated poly-(alkylene oxide) oligomers. Consequently the resulting system contains (Si—C) linkages between different segments. Earlier developments in the field have been reviewed 22, 23,43> Recently hydrosilation reactions have been used effectively by Ringsdorf 255) and Finkelmann 256) for the synthesis of various novel thermoplastic liquid crystalline copolymers where siloxanes have been utilized as flexible spacers. Introduction of flexible siloxanes also improved the processibility of these materials. 

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. 

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. 

Investigations in the past years have proved that applying the concept of flexible spacer, polymers can be synthesized systematically, which exhibit the l.c. state. Owing to the flexible linkage of the mesogenic molecules to the polymer main chain, very similar relations can be expected with respect to 1-l.c., like chemical constitution and phase behavior, or dielectric properties and field effects for the l.c. side chain polymers. This will be in contrast to main chain polymers, where the entire macromolecule, or in case of semiflexible polymers parts of the macromolecules, form the l.c. structure. The introduction of a flexible spacer between backbone and mesogenic group can be performed in a broad variety of chemical reactions. Some arbitrarily... 

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... 

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

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. 

In addition to the effect of the experimental conditions, the influence of the nature of the arms and of the divinyl compound was also studied. It was shown that bulkiness of the arms strongly influences the yield of star polymer for instance, arms of poly(cetyl vinyl ether) were linked in very low yield as compared with poly(IBVE). The influence of the structure of the divinyl ether was investigated and appears to be of great importance. Coupling with 3 and 4 led to low yield of star polymer, while the efficiency of 1 and 2 was much higher. The explanation provided by the authors was that compact and flexible spacers between the two vinyl groups of 3 and 4 could lead to smaller cores where further reaction of incoming chains would be sterically hindered. 

Figure 31.1 shows the structure of EL monomers having a flexible spacer. IL monomers having an ethylene oxide (EO) or a hydrocarbon (HC) spacer between the vinyl group and the imidazolium cation were synthesized, and the effects of the structure and length of the spacer groups on both the ionic conductivity and thermal properties were investigated. 

To investigate flie effect of the flexible spacer on the physical properties of the polyanion-type ILs, we synthesized polyanion-type ILs having a hydrocarbon or an EO spacer. Figure 31.10 shows the structure of the IL monomers having a flexible spacer between polymerizable group and anion. The effect of the HC spacer on the ionic conductivity of the poly anion-type ILs was analyzed. Figure 31.11 shows the Arrhenius plots of the ionic conductivity for EImVS. ElmCSS. and their... 

Several different structural factors influence the properties of the mesophase in these polymers, including dipolar effects, the planarity and rigidity of the mesogenic unit, and its length-to-width ratio among others. These factors are difficult to quantify, either absolutely or relatively, but some idea of their influences can be obtained by comparing the properties of polymers with different mesogenic units when combined with the same flexible spacer. This comparison has already been made for the dyad and triad esters in Table 2, and in this section it will be extended to other types of liquid crystalline polymers which contain a common decamethylene spacer. 

The effect of reversing ester groups, which has already been briefly discussed, should be mentioned again here. In the case of the azo and azoxy polymers made from carboxy-terminated flexible spacers, polymers with stable mesophases were formed. In contrast, polymers made from oxy terminated spacers had no mesophase. While this behavior is an extreme case, when comparisons are possible it seems that polymers made with the carboxy-terminated spacers almost always have wider ranges... 

More recent studies on the effect of flexible spacers now include the use of either poly(ethylene oxide) or polysiloxane segments. Each such spacer brings with it certain unique characteristics to the final polymer, and it is clear that the role of the spacer... 

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