Thermotropic liquid crystals viscosity

The viscosity of thermotropic liquid crystals increases following the sequenee nematic< smectic A < smectic C. 

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

For thermotropic liquid crystals, the viscosity increases in the following sequence ... 

For a thermotropic liquid crystal, its physical properties, such as birefringence, viscosity, dielectric anisotropy, and elastic constant, are all dependent on the operation temperature -except at different rates. Polymer-stabilized BPLC is no exception [45]. Figure 14.10 shows... 

Aromatic thermotropic liquid crystal polyesters (Ar-TLCP s) and TLCP s containing aliphatic linkages can be compatibilized as binary-TLCP blends by transesterification. The morphology and physical properties of the resultant binary-TLCP blend are dependent on the blockiness, composition and viscosity ratios of the two TLCP components. Polycarbonate (PC) can also be blend compatibilized with either TLCP s or binary-TLCP blends, by transesterification of aliphatic linkages from the TLCP s into the PC. In this work, the degree of selective transesterification is quantified and its effect on TLCP blend compatibility is described... 

A typical shear thinning behavior as well as a decrease in viscosity and an increase in the melt flow index while increasing the content of thermotropic liquid crystal copolyester was recorded. In addition, the oriented and ordered fibril structure of thermotropic liquid crystal copolyesters generated in the poly(ethylene 2,6-naphthalate) matrix is illustrated in Figure 6.2. 

Thermotropic liquid crystals were studied for the first time in the late 19th century. Initially, research focused on the structural characterization and classification of liquid crystals [11]. Also, various theories [12] were implemented on viscosity [13], elasticity constant [14], etc. Major progress has been recorded in the year 1960, driven by practical applications of liquid crystals with main [15, 16] and side chains [16, 17]. 

For an isolated spin-1 system, it is convenient to define sum and difference magnetizations [Eqs. (2.84)-(2.85)] in the J-B experiment. The decay of the difference (quadrupolar order) proceeds exponentially at a rate T q, while the sum (Zeeman order) recovers exponentially towards equilibrium at a different rate. The J-B experiment allows simulataneous determination of these rates from which Ji uJo) and J2 2ujo) can be separated. Table 5.1 briefly summarizes thermotropic liquid crystals in which spectral density measurements were reported. Figure 5.4 illustrates the temperature and frequency dependences of spectral densities of motion (in s by including the interaction strength Kq factor) for 5CB-di5. The result is fairly typical for rod-like thermotropic liquid crystals. The spectral densities increase with decreasing temperature in the nematic phase of 5CB. The frequency dependence of Ji uJo) and J2(2a o) indicate that molecular reorientation is likely not in the fast motion regime. However, the observed temperature dependence of the relaxation rates is opposite to what is expected for simple liquids. This must be due to the anisotropic properties (e.g., viscosity) of liquid crystals. 

A review of the literature demonstrates some trends concerning the effect of the polymer backbone on the thermotropic behavior of side-chain liquid crystalline polymers. In comparison to low molar mass liquid crystals, the thermal stability of the mesophase increases upon polymerization (3,5,18). However, due to increasing viscosity as the degree of polymerization increases, structural rearrangements are slowed down. Perhaps this is why the isotropization temperature increases up to a critical value as the degree of polymerization increases (18). 

The rheology of low molecular weight thermotropic compounds has been a subject of considerable theoretical and experimental analysis In general, liquid crystals are easily oriented by surfaces, electromagnetic fields and mechanical stress or shear, and the degree of orientation, in turn, affects their melt viscosity. The rheological behavior of a liquid crystal is known to be greatly dependent on the nature and also on the texture of its mesophase. 

One of the main features of nonionic water-soluble cellulose derivatives is that they exhibit, like some other polyethers, an inverse solubility-temperature behavior, i.e. there is phase separation on heating above the so-called lower critical solution temperature (LCST). The temperature at which a polymer-rich phase separates is normally referred to as the cloud point (CP). For ideal solutions, this temperature corresponds to the theta-temperature. Actually, for some derivatives, the cloud point may be preceded, if the concentration is not too low, by a sol-gel transformation with an increase in viscosity and possibly formation of liquid crystals (see Sect. 3.5). As it will be seen later, this reversible thermotropic behavior may be detrimental to the performance of the derivatives or can be advantageneously utilized to develop applications. 

More recently,thin walled articles have been fabricated by blow-molding composites of liquid crystal polymer (LCP) and expanded porous polydetrafluoroethylene sheetingl l material. Container application examples include food and pharmaceuticals, automotive gas tanks, bottles, and other vessels. Unlike most other thermoplastic polymers, thermotropic LCP forms high-viscosity melts that have thixotropic characteristics. Applying shear force to the melt substantially alters the melt viscosity of LCP and the orientation of its polymer domains. These attributes are useful to... 

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