Crystallisation, induction

Therefore, the crystallisation kinetics study of uniaxially stretched PCP has shown that a crystalline phase with a mixed type morphology is formed in it. A fibrillar crystallites fraction in this morphology increases with both growth of the drawing ratio X and raising of the crystallisation duration t (for X>3). The increase in X results in a reduction in the crystallisation induction period and the rate of this process increases. The crystallisation rate constant is an exponential function of A. in the case of one-dimensional linear growth of crystallites [40]. 

Fig. 2 Crystallisation induction time as a function of isothermal annealing temperature...

The maximum rates of crystallisation of the more common crystalline copolymers occur at 80—120°C. In many cases, these copolymers have broad composition distributions containing both fractions of high VDC content that crystallise rapidly and other fractions that do not crystallise at all. Poly(vinyhdene chloride) probably crystallises at a maximum rate at 140—150°C, but the process is difficult to foUow because of severe polymer degradation. The copolymers may remain amorphous for a considerable period of time if quenched to room temperature. The induction time before the onset of crystallisation depends on both the type and amount of comonomer PVDC crystallises within minutes at 25°C. 

With the camphanic acid ester derivative 2-89 b a reasonable asymmetric induction was obtained to afford the corresponding dihydropyran after crystallisation in 42% yield as a single diastereomer in enantiopure form (Fig. 2-24) [124]. 

When a polymer sample cools down from the molten state to the temperature of measurement, crystallisation is very slow at first. After an "induction" period the process speeds up to a maximum rate and slows down again as it approaches the final equilibrium state. 

The above disadvantage of the lack of spatial information can be overcome by a combination of NMR data and other techniques. From the aH pulse NMR, the fraction and the molecular mobility of different molecular environment can be obtained as free induction decays (FIDs) within a short time, which is suitable for a practical, better understanding of the morphology-property relationship. Wide angle X-ray diffraction (WAXD) and small angle X-ray diffraction (SAXD) as well as electron microscopy provide direct information between the nano- and micrometre scale. A combination of NMR data with those from X-ray diffraction and electron microscopy should be able to analyse the structure from the atomic level to the macro scale. In this review, the morphology-property relationship, the dynamics of morphological transition, the kinetics of crystallisation, etc. analysed by a combination of NMR and other tools are introduced. 

The same authors also showed that the 2-cyano-2-methyl- and 2-cyano-2-phenylsparteine derivatives crystallise in a cisoidal form, as monosalts, probably due to the reduction of the basic nature of the N-1 atom by the negative inductive effect of the cyano substituent. The protonation of the N-16 atom leads to its inversion and establishment of an intramolecular hydrogen bond, as in the case of the 2-methyl- and 2-phenylsparteine derivatives [197]. 

GH 2. The tetrameric lacZ (3-galactosidase of Escherichia coli has been intensively investigated for over 40 years, initially because of its relationship to Monod s classic work on enzyme induction. A very large set of data accumulated by Wallenfels and Weil unfortunately had to be discarded because it was obtained in inhibitory Tris buffers.The first glycosyl-glycosidase intermediate to become kinetically accessible was with this en-zyme the 2-fluoro-2-deoxy-a-D-galactopyranosyl enzyme, the parent enzyme and ES complexes have since been crystallised and its structure determined. 

The synthesis of crystalline microporous solids is a complex reaction-crystallisation process, usually involving a liquid phase and both amorphous and crystalline solid phases. Described below (section 6) are the main elements of this transformation the induction period, nucleation processes and crystal growth. Having established this background, the mechanism underlying these changes is then examined in more detail (section 7). 

In a typical zeolite synthesis, the first definite evidence for a successful reaction is the appearance of crystals of the product. As noted above (section 6.1), this signal for the end of the induction period is dependent upon the method of detection most commonly a combination of visual inspection or microscopy with X-ray diffraction. Thereafter, crystal growth can be monitored by the same techniques and the resulting S-shaped growth curve of bulk crystallinity against time is by far the most commonly reported measurement of zeolite crystallisation kinetics (fig- 2). 

Not least of the many difficulties in the modelling of crystallisation fouling is the generally unknown extent of the induction or initiation period. The problem is illustrated by data published by Ritter [1981]. Table 8.5 gives the ranges of experimental induction times he obtained for CaSO and Li O precipitation. The... 

If these oxidized sections were flexed the cracks ran apparently at random. Similar samples which were crystallised and oxidised as thin films tended to crack along spherulite radii. If the sample was first oxidised to sibout the induction time and then crystallised there was again no evident relationship between oxidation and morphology. However the staining process had clearly extracted material from the spherulite boundaries leaving them as channels in the film. If the unstained sections were flexed they frequently cracked along the spherulite boundaries. 

The next stage in the crystallisation curve after the induction period is the initial period of rapid increase in growth rate. Pioneering studies by Zhdanov on... 

A highly attractive enol ether/chlorosulfonylisocyanate-cycloaddition approach was developed by Sagami Ltd. [56] The stereoselectivity of the cycloaddition is controlled by double chiral induction with (S)-benzyloxypropanal. In course of the following steps, the small amount of the undesired diastereo-mer is removed by crystallisation. Finally, after an elegant Baeyer-ViUiger oxidation and rearrangement, the chiral auxiliary remains in the molecule as an acetate group. 

The silyl-protected hydroxy-group provides in the diastereoselective [2 + 2]-cydoaddition the asymmetric induction of the ]S-lactone, and is in the Mitsunobu reaction by inversion ofthe stereogenic centre converted into the desired absolute configuration. The end-product, a slightly yellowish oil, stiU contains around 10% of unwanted diastereomers, which cannot be separated off. However, enantiomericaUy pure tetrahydrolipstatin can be obtained by hydrogenation and crystallisation. 

In the nucleation stage, small clusters of solute molecules are formed some of these clusters may grow sufficiently to form stable nuclei and subsequently form crystals. Others fail to reach adequate dimensions before they dissolve again. Within the metastable zone width (MSZW), the induction time to the onset of crystallisation has an inverse relationship with the supersaturation [44-47]. 

For the seeded MSMPR experiment, the rate of de-supersaturation was also veiy fast for the first 15 minutes of the ciystallisation run, as can be seen in Figure 5. The rapid decrease in Ca also indicated that there was no induction time. After the first residence time (15 minutes), the reduction was gradually slow and eventually the curve levelled off. This is obviously due to nucleation and growth of calcium sulphate crystals at the expense of Ca. Finally, the concentration of Ca in the crystallising solution was steady after seven residence times, i.e. after 7x15 minutes or after 105 minutes. 

In contrast with the two crystallisation studies discussced previously, for the single pipe flow experiment, the induction time was clearly observable and varied from 100 minutes to 400 minutes for the temperature range fiom 20 C to 40 C. The lower the temperature the longer the induction time. Thus the induction time of the calcium sulphate scale formation depends strongly on temperature. A correlation was formulated [15] as follows ... 

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