Low-molar-mass systems

The selective reflection of circularly polarized light on radiation of normal light is also exhibited by the cholesteric polymers. Like the 1-l.c. systems, the Grandjean-texture is formed spontaneously, if the polymer is sandwiched between glass plates as shown in Fig. 23. The measurements of indicate no difference to low molar mass systems. In Fig. 24 VHT) is shown for the induced cholesteric polymers, whose birefringence was discussed above (refer to Table 8, copolymers No. 4, m = 6). The different curves refer to different mole fractions of the chiral comonomer. With... 

The first detailed investigations on the phase behavior of a p-l.c. compared with the chemically very similar low molar mass system have been reported recently110). The system investigated is shown in Table 11, No. 3. In order to determine the influence of the linkage of the monomeric surfactant to the siloxane backbone, we will compare the phase behavior of the monomer consisting of the hydrophobic 10-undecenoic acid esterified with the hydrophilic octa-ethylenglycol-monomethyl-ether (x = 8) with that of the corresponding polysiloxane. 

In the cryoscopic method, the freezing temperature of a solution is compared with that of the pure solvent. The polymer must be solvable in the solvent at flie freezing temperature and must not react with the solvent either ehemieally or physically. Difficulties may arise from limited solubility and from the formation of solid solutions on freezing. Application of cryoscopy to polymer solutions is not widespread in literature despite the simplicity of the required equipment. Cryoscopy was reviewed by Glover, who also discussed technical details and problems in concern with application to polymer solutions. A detailed review on cryometers and cryoscopic measurements for low-molar mass systems was recently made by Doucet. Cryometers are sold commercially, e.g., Knauer. Measurements of thermodynamic data are infrequent. Applications usually determine molar masses. Accurate data require precise temperature measurement and control as well as caution with the initiation of the crystallization process and the subsequent establishment of equilibrium (or steady state) conditions. High purity is required for the solvent and also for the solute. 

In low molar mass systems there are maity fascinating trends of mesomorphic behaviour and so this should also be seen in polymeric analogues. The additional ordering on polymerisation causes liquid crystal phases to be more ordered than for the monomeric analogue and transition temperatures and clearing points are higher. Accordingly, if a... 

So far, a large number of low-molar-mass systems have been studied by ultrafast fluorescence techniques in sub-nanosecond time regions [35-39]. Recently, a relatively slow (nanosecond) relaxation process proceeding in mixed low-molar-mass solvents, consisting in redistribution of components of the solvent mixture in the solvate shell of the fluorophore upon the excitation, has also been reported [40-43, 46, 47]. However, an important part of experimental studies is still concerned with relatively slowly relaxing biological systems, such as lipid membranes [48-50], proteins [51, 52], nucleic acids [53], and also colloidal [54] and polymer systems [55-57]. 

In low molar mass liquid crystals, conformational dynamics is typically too fast to be studied by 2D exchange NMR or line shape analysis. Dynamical studies of small molecules belong therefore almost exclusively to the realm of relaxation experiments. There are, however, cases of slow motions even in low molar mass systems, which can be investigated by 2D exchange spectroscopy. A recently investigated example is a discotic tetrabenzocyclophane derivative, which shows very slow motions in the columnar phase compared to other discotic compounds [97]. As illustrated in Fig. 6, deute-... 

This work with polymeric acetylides was extended to the study of related low molar mass systems and the platinum(II) complexes, 16, were found to exhibit thermotropic N and SmA phases [27]. 

Given the presence of mobile counter-ions, the osmotic pressure, which is extremely low in solutions of neutral pol3Tners, again takes on the much higher values of low molar mass systems. 

Even if one solves the indexing problem and then proceeds with the analysis by an evaluation of measured reflection intensities, one cannot expect to achieve an accuracy in the crystal structure data that would be comparable to those of low molar mass compounds. This is not only a result of the lack of single crystals, but also represents a principal property In small crystallites, as they are found in partially crystalline polymers, lattice constants can be affected by their size. Furthermore, disorder is more frequently found as in low molar mass systems, in particular, in the region near the fold surface. 

As in low molar mass systems crystal formation in a polymer melt starts with a nucleation step. Thermal fluctuations form in the melt embryos, i.e., particles with an enhanced inner order. If the size of an embryo surpasses a critical value it turns into the nucleus of a growing crj tal smaller embryos disappear again. It is possible to directly observe this process with an atomic force microscope, as is shown in Fig. 5.19 for a crystalhzing polyether (short name BA-C8, the material crystallizes slowly at room temperature). The encircled dot in the left-hand picture is a nucleus that subsequently develops into a single lamellar crystallite. 

Initially, research focused on the possible display properties of smectic A and nematic, linear side-group polysiloxanes however, the very high viscosity inevitably means that the response times will be slow compared with low-molar-mass systems. Recently, ferroelectric polysiloxane and polyacrylate LCPs, such as (22, 5f 30°C 55°C 120°C I and 23, 15°C S 103°C 124°C /), and composites of them... 

In principle, by lowering the temperature, the orientational correlation between neighboring molecules grows, which in turn is expected to support the formation of a biaxial nematic phase. Because of the onset of crystallization or the formation of smectic phases, a biaxial nematic phase in a low molar mass system could not be achieved experimentally for a long time. These problems paved the way for the concept of a biaxial nematic polymer. 

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