Crystallisation suppression

Physical Properties. Colourless crystalline solid, soluble in boiling water, very sparingly soluble in cold water crystallises 2H2O. The strongly acidic — SO3H group suppresses the normal basic properties of the — NHj group the acid therefore dissolves readily in alkalis, but not in dilute mineral acids. 

An amorphous material sometimes referred to as amorphous poly(ethylene oxide), aPEO, consists of medium but randomly-variable length segments of poly(ethylene oxide) joined by methyleneoxide units. Fig. 5.13 (Wilson, Nicholas, Mobbs, Booth and Giles, 1990). These methyleneoxide units break up the regular helical pattern of poly(ethylene oxide) and in doing so suppress crystallisation. The aPEO host polymer and its salt complexes can crystallise below room temperature, but this is not detrimental to the properties of the polymer-salt complexes at or above room temperature. Similarly, dimethyl siloxy units have been introduced between medium length poly(ethylene oxide) units to produce an amorphous polymer. Fig. 5.14 (Nagoka, Naruse, Shinohara and Watanabe, 1984). 

The rubbery plateau can be "stabilised" by cross-linking, the regions of rubbery flow and liquid flow are completely suppressed if enough chemical cross-links are introduced to serve as permanent network junctions in place of the temporary chain entanglements. Crystallisation is a kind of physical cross-linking with (numerically) many junctions. It is understandable that the amorphous state is more or less "stabilised" by crystallisation, so that the transition becomes less pronounced. 

Some polymerisations do not continue in a linear fashion, and branching occurs, as shown in Fig. 1.7(a). When branching is prevalent, it can have a serious effect on properties. Thus the polymerisation of ethylene under high-pressure conditions gives a product that has so many side chains on each main chain that crystallisation is appreciably suppressed. This material is softer than the highly crystalline linear form from a catalytic low-pressure process. The two forms of polyethylene may be distinguished by a difference in density between them, the more crystalline material being denser. 

First studies were undertaken with Periwinkle plants from Jamaica. The roots, stems, and leaves all contain active material the leaves contain the highest proportion of activity, but in the seeds almost no activity is found.Today we know that four of the pure alkaloids, crystallised from extracts of Vinca rosea plants, namely vinblastine [(169), = vincaleukoblastine)], vincristine [(170), = leurocristine], vinleurosine [= leurosine, (183)], and vinrosidine [= leurosidine, (181)] show good effects in suppression of tumour growth. In the last decade about fifty-five alkaloids have been found in Vinca rosea extracts and tested for their pharmacological effects. Besides the four mentioned none of the... 

The rapid cooling rate and small particle size of powders condensed from a plasma may also result in suppression of crystallisation either completely or partly. The powder then consists of an amorphous phase as observed in the case of the AljOj—SiOj system in which completely amorphous particles may be prepared with AI2O3 contents up to = 80wt An analysis of the nucleation kinetics... 

The above facts do not favour protein or peptide quantitation using MALDI-MS. Some problems are associated with MALDI-MS quantification i) low shot-to-shot reproducibility, ii) various signal suppression effects, and iii) strong influence of sample preparation and matrix crystallisation. Nevertheless, it is possible to use MALDI-MS to obtain absolute or relative quantitation. In most cases, the idea is to use an internal standard for an absolute quantitation, but this standard must have the same physico-chemical characteristics as the quantified peptide. The use of a different peptide in terms of sequence may result in different desorption and ionisation properties. Usually, the internal standard is the same peptide labelled with a stable isotope to modify slightly the mass. 

The isolabile proteins do flocculate, coacervate or crystallise at their isoelectric point. For a large group of these proteins, which have received the name of globulins, it is characteristic that they only separate out at their isoelectric point, when the medium contains no, or sufficiently little, indifferent salt. The protein separated out at the LE.P. has the property of dissolving in dilute salt solutions. These properties give us a direct proof for the idea that the separation of the globulins at the I.E.P. is based on the existence of pronounced intermolecular complex relations. Indeed the property of indifferent salts of suppressing complex relations has already been met with very frequently in this chapter. It is then to be expected... 

If no complications were involved, one would expect the plastic flow to increase continuously with the degree of extension, but for crude rubber it appears that the flow decreases after surpassing a limit of about 450% elongation. This can be explained by assuming that, above this limit, crystallisation results in the chains fitting so strongly in the crystalline lattice that plastic flow is more and more suppressed. 

For hard spheres with (j) = 0.74 and = 0.545, (4.40) provides g , = 0.11, so 11%. In dispersions with large q, small colloids are needed in practice because it is difficult to synthesize model polymer chains with sizes > 200 nm in solution. Small colloids (<100nm) are often quite polydisperse. Therefore, systems studied with large q in general tend to be relatively polydisperse in colloidal sphere size. This implies that in experimental systems with large q, crystallisation is suppressed or absent. 

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