Structural homogeneity

Spatial information about a system can be obtained by analyzing the spatial distribution of PL intensity. Fluorescent tracers may be used to image chemical uptake in biological systems. Luminescence profiles have proven useftil in the semiconductor industry for mapping impurity distributions, dislocadons, or structural homogeneity in substrate wafers or epilayers. Similar spatial infbrmadon over small regions is obtained by cathodoluminescence imaging. 

As has been outlined above, a second, very fruitful synthetic principle for obtaining structurally homogeneous, processable PPP derivatives involves the preparation of soluble PPPs via introduction of solubilizing side groups. The pioneering work here was carried out at the end of the eighties by Schluler, Wegner, et al. [11, 121, who for the first lime prepared soluble poly(2,5-dialkyl-1,4-phenylene)s 6. 

The careful control of electronic properties is, of course, a key motivation of such structural changes the so-called band-gap tuning being a particularly important concern. Efficiency of synthesis and structural homogeneity of the products are essential ingredients of such an approach since failure to achieve e.g. quantitative transformation of precursor polymers or to couple benzene units exclusively in a para-fashion interrupts the extensive -conjugation and hampers a reliable structure-propcrty-relalion. 

In the field of polymer chemistry the regio- and stereoselectivity of the Diels-Alder reaction is used for the concerted synthesis of structurally homogeneous double-stranded ladder polymers [39], which are useful materials with nonlinear optical properties and high electrical conductivity. It has turned out that the repeated Diels-Alder method is superior to an alternative two-step process, in which first an open chain precursor is formed followed by polymer ring closure as structural defects can occur [40]. 

Structural as well as dynamical studies of membrane proteins, constituting an estimated 30% of all proteins, are very important in view of their crucial biological importance. These membrane proteins are obviously dynamically heterogeneous systems, even though they are structurally homogeneous without any cross-links or aggregation so far discussed. 

Recent developments in polymer chemistry have allowed for the synthesis of a remarkable range of well-defined block copolymers with a high degree of molecular, compositional, and structural homogeneity. These developments are mainly due to the improvement of known polymerization techniques and their combination. Parallel advancements in characterization methods have been critical for the identification of optimum conditions for the synthesis of such materials. The availability of these well-defined block copolymers will facilitate studies in many fields of polymer physics and will provide the opportunity to better explore structure-property relationships which are of fundamental importance for hi-tech applications, such as high temperature separation membranes, drug delivery systems, photonics, multifunctional sensors, nanoreactors, nanopatterning, memory devices etc. 

R.D. McCullough, R.D. Lowe, M. Jayaraman, and D.L. Anderson, Design, synthesis, and control of conducting polymer architectures structurally homogeneous poly(3-alkylthiophenes), J. Org. Chem., 58 904-912, 1993. 

Besides purity and structural homogeneity the obtention of crystals suitable for X-ray diffraction experiments depends on many other parameters including pH, temperature, protein concentration and the nature and concentration of the precipitant. It results that many crystallisation experiments and often large quantities of protein... 

The structural homogeneity of the various cellulose triacetate fractions obtained by fractional precipitation was established by both Infrared and nuclear magnetic resonance spectroscopy. 

A preliminary stndy on the viscoelastic behaviour of polyolefin foam sheets with different chemical (PE and PP) and cellular structure by DMA, in the low freqnency and low compression ranges, is presented. DSC and SEM are also used to determine the morphological parameters of the samples. A connection between the morphological properties (apparent degree of crystallinity), type of cellular structure, homogeneity, cell size and shape, cell wall thickness) and the viscoelastic behavionr, a basic key for the development of mechanical and insnlating applications, has been established. 9 refs. 

Expression by transient transfection methods using mammalian cell lines is a convenient and rapid method of producing recombinant proteins when E. coli systems fail to produce correctly folded, structurally homogeneous protein. Moreover, it is a method that is routinely used to produce proteins for crystallization. 

Copolymers are readily prepared by conducting polymerizations of a mixture of monomers. However, to obtain a product having any reasonable, structural homogeneity, it is necessary to take the reaction mechanism into account, and to perform the experiment under conditions consistent with classical, copolymerization theory. With properly controlled experiments, it is possible to determine the relative reactivities of the monomers, and the range of compositions and mer sequence-length distributions in any copolymer produced.81,82... 

A clear crystallochemical interpretation of hardness anisotropy measurement results, especially for monocrystals, makes it possible to estimate the structural homogeneity of crystal. Button et al. (1979) testing the hardness of cubic sodium tungsten bronzes Na W03 (where 0.4 < x < 0.75) with the Knoop indenter, found the hardness of W03 to rise from 450 to 844 within a highly differentiated hardness anisotropy for various values of the Na+ ion. This variation is the outcome of differences in atomic spacings in crystals. 

Finally, compression-extrusion testing involves an extrusion cell commonly used for weakly structured, homogeneous food products. This apparatus consists of a piston that is forced into a cylinder open at one end and containing the product (Figure H2.2.5). Beyond the point of rupture of the food, the compressed material is forced to flow back through the annular space between the piston and the cylinder (Bourne, 1976 Edwards, 1999). The gap between the piston and the cylinder is called the annulus (Bourne, 1982). Variation in the annulus width results in variation in the force required for extrusion (Bourne, 1982). 

It seems clear that for reactions of carbocations with nucleophiles or bases in which the structure of the carbocation is varied, we can expect compensating changes in intrinsic barrier and thermodynamic driving force to lead to relationships between rate and equilibrium constants which have the form of extended linear plots of log k against log K. However, this will be strictly true only for structurally homogeneous groups of cations. There is ample evidence that for wider structural variations, for example, between benzyl, benzhydryl, and trityl cations, there are variations in intrinsic barrier particularly reflecting steric effects which lead to dispersion between families of cations. 

The structural homogeneity of the alkaloids from Pauridiantha can also be connected to a glucolysis step occurring at a late stage of the biosynthetic evolution. As a consequence, glucoalkaloids are abundant in most Pauridiantha species, but their structure remains primitive, e.g., they are close to that of strictosidine, the first of the alkaloids formed in vivo. This biosynthetic dead end can... 

Without stepping into any nomenclature controversies, for the purposes of this review, cocrystal systems will be regarded as those mixed crystal systems where the individual components exist as solids under ambient conditions [11]. Aakeroy has summarized guidelines for cocrystal formation from supramolecular synthons as being constructed from discrete neutral molecular species that are solids at ambient temperatures, and where the cocrystal is a structurally homogeneous crystalline material that contains the building blocks in definite stoichiometric amounts [12],... 

Ackman, R.G. (1964). Structural homogeneity in unsaturated fatty acids of marine lipids. Journal of the Fisheries Research Board of Canada 21, 247-254. 

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