Characterization of Liquid Crystals

Different experimental techniques have been used to characterize structural and phase behavior of liquid crystalline materials. Polarizing optical microscopy is one of the essential tools for the characterization of newly synthesized mesogenic materials, together with differential scanning calorimetry (DSC) and x-ray investigations, while DSC provides information on phase transition temperatures and order of transitions. X-ray investigations for actual structural evaluation, i. e. determination of phase type, have to be performed on macroscopically well oriented samples, which is often time consuming and sometimes hard to realize. Therefore 

X-ray studies are often carried out on unoriented samples, while the results only allow limited characterization of the structural features. X-ray studies are very much useful especially to identify the ordered smectic phases. Polarizing microscopy on the other hand can provide a determination of both phase transition temperatures and phase type [5-7]. 

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Another example is the interdisciplinary laboratory developed at Harvey Mudd College (147) in which eight different interdisciplinary experiments, ranging from thermal properties of an ectothermic animal to synthesis and characterization of liquid crystals, are carried out over two semesters. 

The characterization of liquid crystals by polarized light microscopy is the most straightforward method available and, whenever possible, it should be carried out in the initial stages of an investigation on new polymers. Thermal analyses alone can be misleading. In this procedure, a thin layer of the melt is kept at constant temperature on a hot-sta and obsawed between crossed polars. The appearance or texture of the melt is dependent on the structure of the mesophase, and, therefore, it is often possible to directly identify the type of mesophase present by this method. A good review of the microscopy of liquid crj b ajqjears in the books by Hartshome and by Demus and Richter... 

Despite the apparent advantages of such a technique only very few references are found in the literature. This is surprising in view of the very large number of publications in recent years on liquid crystal polymers and of course also on low molecular weight liquid crystals. We wish here to illustrate this thermo-optical analysis (70A) as a tool for characterization of liquid crystal polymers. 

Mao G, Wang J, Clingman SR, Ober CK, Chen JT, Thomas E. 1997. Molecular design, synthesis, and characterization of liquid crystal coil diblock copol5uners with azoben zene side groups. Macromolecules 30 2556 2567. 

Polarizing optical microscopy is one of the most powerful tools for the characterization of liquid crystals. With this method phase transition temperatures as well as phase types can be determined [4, 5]. 

Finally, we note that the further development of TBA was not restricted exclusively to the phase characterization of liquid crystals. Since 1988, a more exended use in... 

From the beginning of the 20th century to 1970 the dielectric behavior of liquid crystals was more of academic interest. Especially dielectric anisotropy at low frequencies, A o = o- j o was one of many qualities for the characterization of liquid crystals. Results of 18 samples measured by Specht, Vaupel, Sulze, Errera, Jezewski, Schulwas-Sorokina, Maier and the respective coworkers are summarized in the tables by Landold-Bdrnstein [30]. Based on these data Maier and Meier could give a connection between the dielectric constants o, e iq and molecular parameters [28]. In this model the dielectric anisotropy... 

One of the most powerful and simple techniques in the characterization of liquid crystal phases is polarized optical microscopy (POM). Using this technique, we can take advantage of the birefringence of liquid crystals to identify different phases. The sample is placed between crossed polarizers on the optical microscope and viewed in transmission. The illuminating light passes through a linear polarizer, the sample, and then a second linear polarizer crossed with the first. 

Vasanthi BJ, Ravikumar L (2013) Synthesis and characterization of poly(azomethine ester)s with a pendent dimethoxy benzylidene group. Open J Polym Chem 3 70-77 Zifa L, Zhang S, Shaokui CAO, Chen Z, Zhou Q (1996) Synthesis and characterization of liquid crystal polymers with T-shaped two-dimensional mesogenic units. Chin J Polym Sci 14 (l) 71-79... 

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