Crystallization, fractional inducing

The total crystal fraction, oriented crystal fraction (/oc). and unoriented crystal fraction (/uc) during extension and retraction have been analyzed. Results can be described as follows. At strain zero, the total crystal fraction was the same as the unoriented crystal fraction, which was about 14%, and both values decreased with strain. This indicates that a fraction of the original crystals was destroyed at the initial deformation stages. (This phenomenon was also reported in ethylene-based ethylene-propylene copolymer (34).) At strain 0.7, the total crystal fraction decreased about 4% (from 14% to 10%) at strains above 0.7, both total crystal fraction and oriented crystal fraction increased with strain, indicating the occurrence of strain-induced crystallization, whereas the unoriented crystal fraction decreased continuously with strain. The increase of the total crystal fraction was slower than that of the oriented crystal fraction, suggesting that some unoriented crystals were reoriented by... 

FIGURE 3.18 Total anisotropic fraction, crystal fraction, and oriented amorphous fraction of sulfur vulcanized NR stretched slowly at ambient temperature. The upper points correspond to extension, and the lower to retraction. For strains >200%, crystallization is induced, resulting in a decrease in the amorphous phase orientation. 

Supercritical fluids can be used to induce phase separation. Addition of a light SCF to a polymer solvent solution was found to decrease the lower critical solution temperature for phase separation, in some cases by mote than 100°C (1,94). The potential to fractionate polyethylene (95) or accomplish a fractional crystallization (21), both induced by the addition of a supercritical antisolvent, has been proposed. In the latter technique, existence of a pressure eutectic ridge was described, similar to a temperature eutectic trough in a temperature-cooled crystallization. 

Another route to enantiomcrically pure iron-acyl complexes depends on a resolution of diastereomeric substituted iron-alkyl complexes16,17. Reaction of enantiomerically pure chloromethyl menthyl ether (6) with the anion of 5 provides the menthyloxymethyl complex 7. Photolysis of 7 in the presence of triphenylphosphane induces migratory insertion of carbon monoxide to provide a racemic mixture of the diastereomeric phosphane-substituted menthyloxymethyl complexes (-)-(/ )-8 and ( + )-( )-8 which are resolved by fractional crystallization. Treatment of either diastereomer (—)-(/J)-8 or ( I )-(.V)-8 with gaseous hydrogen chloride (see also Houben-Weyl, Vol 13/9a, p437) affords the enantiomeric chloromethyl complexes (-)-(R)-9 or (+ )-(S)-9 without epimerization of the iron center. 

It should be noted that the fraction of ECC in samples obtained by other methods described in Sect. 2 is approximately as small as that of the framework in the orientation-ally crystallized samples. These methods differ in details but depend on the mechanical treatment of the crystallizing system and are therefore given the common name stress-induced crystallization . Although the structure of the samples obtained by these methods has some features in common with that of orientationally crystallized samples, the thermodynamics and kinetics of orientational crystallization are fundamentally different from the mechanism of stress-induced crystallization. 

Under all ordinary circumstances the average lengths of terminal chains and of internal chains will be the same, or nearly so hence represents the weight fraction of the structure which is active in deformation. For certain applications, as for example to crystallization induced by stretching, 8a is an appropriate measure of the effective portion of the... 

Changes in concentrations induced by fractional crystallization are much more visible for compatible than for incompatible elements, o... 

A kinetic model for single-phase polymerizations— that is, reactions where because of the similarity of structure the polymer grows as a solid-state solution in the monomer crystal without phase separation—has been proposed by Baughman [294] to explain the experimental behavior observed in the temperature- or light-induced polymerization of substimted diacetylenes R—C=C—C=C—R. The basic feature of the model is that the rate constant for nucleation is assumed to depend on the fraction of converted monomer x(f) and is not constant like it is assumed in the Avrami model discussed above. The rate of the solid-state polymerization is given by... 

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