Crystalline materials, liquid separations

From the analysis described above, one can deduce the viscoelastic and surface anchoring parameters of the hquid crystalline material phase separated from the polymer matrix, which in our VIS samples have the values K/rj = (0.3 0.06) 10 °m s and A = 65 35 nm. The observed diffusivity K/t] is almost four times lower than the diffusivity of a pure liquid crystal used to prepare the mixture. This result demonstrates that to properly explain the dynamic properties of the H-PDLC gratings, such as switching-on and -off times, it may not be relevant to take viscoelastic parameters of the liquid crystal substances as used for the mixture, but one needs to measure the properties of a nematogenic material formed in the phase separation process. Since the scattering experiments require no special sample treatment, it is one of the most convenient techniques to perform this task. The observed decrease of K/rj is attributed to the increased viscosity of a liquid crystal medium due to the presence of dissolved parts of the polymer chains [26,40]. The observed value of A results inW = (2.1 1.1) 10 J m , which corresponds to a quite strong anchoring. 

A belief that solid interfaces are easier to understand than liquid ones shifted emphasis to the former but the subjects are not really separable, and the advances in the one are giving impetus to the other. There is increasing interest in films of biological and of liquid crystalline materials because of the importance of thin films in microcircuitry (computer chips ), there has been in recent years a surge of activity in the study of deposited mono- and multilayers. These Langmuir-Blodgett films are discussed in Section XV-7. 

Crystallizers with Fines Removal In Example 3, the product was from a forced-circulation crystallizer of the MSMPR type. In many cases, the product produced by such machines is too small for commercial use therefore, a separation baffle is added within the crystallizer to permit the removal of unwanted fine crystalline material from the magma, thereby controlling the population density in the machine so as to produce a coarser ciystal product. When this is done, the product sample plots on a graph of In n versus L as shown in hne P, Fig. 18-62. The line of steepest ope, line F, represents the particle-size distribution of the fine material, and samples which show this distribution can be taken from the liquid leaving the fines-separation baffle. The product crystals have a slope of lower value, and typically there should be little or no material present smaller than Lj, the size which the baffle is designed to separate. The effective nucleation rate for the product material is the intersection of the extension of line P to zero size. 

Table 5.1 Survey of liquid separations using crystalline materials aromatic applications.

The most commonly employed crystalline materials for liquid adsorptive separations are zeolite-based structured materials. Depending on the specific components and their structural framework, crystalline materials can be zeoUtes (silica, alumina), silicalite (silica) or AlPO-based molecular sieves (alumina, phosphoms oxide). Faujasites (X, Y) and other zeolites (A, ZSM-5, beta, mordenite, etc.) are the most popular materials. This is due to their narrow pore size distribution and the ability to tune or adjust their physicochemical properties, particularly their acidic-basic properties, by the ion exchange of cations, changing the Si02/Al203 ratio and varying the water content. These techniques are described and discussed in Chapter 2. By adjusting the properties almost an infinite number of zeolite materials and desorbent combinations can be studied. 

The amino acid is recrystallized by dissolving all the crude material in 12.5 1. of water heated to 950 on a steam cone. The hot solution is treated with 20 g. of Norite for thirty minutes and filtered hot. An equal volume of 95 per cent alcohol is added immediately, and the flask is placed in the icechest overnight. The crystalline material is collected on a filter and washed with 200 cc. of 95 per cent alcohol. The yield of pure leucine in this fraction is 290-300 g. An additional crop is obtained by evaporating the mother liquors under reduced pressure until considerable solid separates (liquid volume about 1 1.), adding an equal volume of alcohol, and cooling. This crop is washed with 100 cc. of cold water and then with 200 cc. of alcohol it amounts to 60-65 g- The total yield of pure leucine is 350—365 g. (43-45 per cent of the theoretical amount). It decomposes at 290-292° (uncorr.) in a sealed capillary (Note 4). 

As we have seen, the phase behaviour of block copolymers consisting of flexible polymer coils is remarkably rich. If one of the blocks is rigid, the copolymer would be expected to exhibit even more complex phase behaviour. For example, the rigid block could be mesogenic. This leads to the possibility of self-assembly of structures consisting of domains of liquid crystalline material within a microphase-separated block copolymer superstructure. Diblock copo-... 

The following table lists the liquid crystalline materials that are useful as gas chromatographic stationary phases in both packed and open tubular column applications. In each case, the name, structure, and transition temperatures are provided (where available), along with a description of the separations that have been done using these materials. The table has been divided into two sections. The first section contains information on phases that have either smectic or nematic phases or both, while the second section contains mesogens that have a cholesteric phase. It should be noted that each material may be used for separations other than those listed, but the listing contains the applications reported in the literature. 

The air-dried product is crystallized by dissolving it in chloroform, (approximately 120 ml.), followed by dilution of the filtered boiling liquid (Note 10) with hot petroleum ether (boiling range 60-80°, 60-80 ml.). The crystalline product, which separates rapidly, is collected at 0°, rinsed on the filter with a mixture of chloroform and petroleum ether (1 .3), and dried. The yield of magnificent deep-scarlet lustrous prisms, m.p. 127-128°, varies between 28.9 and 30.4 g. (54-57% of the theoretical). Concentration of the combined filtrates and wash liquids under reduced pressure to a small volmne (50-80 ml.) yields an additional small quantity (1.5-3.0 g., 3-6%) of material of satisfactory purity, m.p. 121-124°. 

Upon release of supersaUiration, the initially dissolved compound will be separated from the solution and form a secondary phase, which could be either oil, amorphous solid, or crystalline solid. Crystalline materials are solids in which molecules are arranged in a periodical three-dimensional pattern. Amorphous materials are solids in which molecules do not have a periodical three-dimensional pattern. Under some circumstances with very high supersaturation, the initial secondary phase could be a liquid phase, i.e., oil, in which molecules could be randomly arranged in three-dimensional patterns and have much higher mobility than solids. Generally, the oil phase is unstable and will convert to amorphous material and/or a crystalline solid over time. At a lower degree of supersaturation, an amorphous solid can be generated. Like the oil, the amorphous solid is unstable and can transform into a crystalline solid over time. Even as a crystalline solid, there could be different solid states with different crystal structures and stability. The formation of different crystalline solid states is the key subject of polymorphism, which will be mentioned below and... 

Distinct subsections of the molecules comprising liquid crystals have also been observed using the rate of MQC development in a mixture of liquid crystalline materials. Separate MQ signals were observed for effectively isolated spin systems (phenyl rings) and weakly coupled multi-spin clusters (alkyl tails)80 in a mixture of 4-cyanophenyl 4 -butylbenzoate and 4 cyano-phenyl 4 -heptylbenzoate. 

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