Packings bulkiness

Apart from these simple silanes, derivatives witli aromatic groups at different places in tire chain have also been investigated [136, 137], It was found tliat tire average tilt angle of tliese molecules depends on tire specific functional entities contained in tire chains. It is likely tliat apart from packing considerations—important for bulky groups, for example—otlier factors also influence tire resulting tilt. 

In the foregoing discussions of theoretical models and experimental results, we have focused on linear polymers. We have seen the effect of chain substituents on viscosity. All other things being equal, bulky substituents tend to decrease f and thereby lower 17. The effect is primarily due to the opening up of the liquid because of the steric interference with efficient packing arising from the substituents. With side chains of truly polymeric character, the picture is quite different. 

Properties. One of the characteristic properties of the polyphosphazene backbone is high chain dexibility which allows mobility of the chains even at quite low temperatures. Glass-transition temperatures down to —105° C are known with some alkoxy substituents. Symmetrically substituted alkoxy and aryloxy polymers often exhibit melting transitions if the substituents allow packing of the chains, but mixed-substituent polymers are amorphous. Thus the mixed substitution pattern is deUberately used for the synthesis of various phosphazene elastomers. On the other hand, as with many other flexible-chain polymers, glass-transition temperatures above 100°C can be obtained with bulky substituents on the phosphazene backbone. 

There is one exception to the rule that requires bulky hydrophobic residues to fill the interior of eight-stranded a/p barrels in order to form a tightly packed hydrophobic core. The coenzyme Biz-dependent enzyme methylmalonyl-coenzyme A mutase, the x-ray structure of which was determined by Phil Evans and colleagues at the MRC Laboratory of Molecular... 

Use of acids or amines containing large bulky side groups which prevent close packing of the molecules. 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

Introduction of bulky lateral substituents on monomer units to increase interchain distance and prevent close packing in polymer crystal. The use of unsymmetrically substituted monomers, resulting in a random distribution of head-to-head and head-to-tail structures in polymer chains, further helps in disrupting regularity. Some examples of substituent effects are given in Table 2.16. 

Fig. 3.—The structure of trisilver cobalticyanide. There are three mutually interpenetrating rhombohedral frameworks, distorted from the cubic frameworks of Fig. 2 in such a way as to achieve increased density and closer approximation to the optimal packing of the bulky cobalticyanide groups.

The pneumatic pump has a relatively large flow capacity but, today, is largely used for column packing and not for LC analyses. It can provide extremely high pressures and is relatively inexpensive, but the high pressure models are a little bulky. A diagram of a pneumatic pump is shown in figure 4. 

Molecules aim at being packed as closely as possible in the crystal 26 28). Bulkiness of a molecule, however, is an impediment. It presupposes a molecular constitution rather unbalanced in its spatial dimensions and a certain extent of conformational rigidity. Here we encounter a parallel to an everyday occurrence trunks can easily be arranged to a compact ordered stack, unlike bulky root stocks which cannot. The... 

In some cases, planar chain structures become impossible when replacing a hydrogen atom on the backbone of a macromolecule by a bulky group. Packing problems arise due to steric hindrance if the trans orientation of each bond is... 

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