Crystallisation mechanism

Recent developments on research into a bacterial C-F bond forming enzyme are reviewed. The fluorinase enzyme was isolated from Streptomyces cattleya in 2002 and shown to catalyse the conversion of fluoride ion and S-adenosyl-L-methionine (SAM) to 5 -fluoro-5 -deoxyadenosine (5 -FDA) and L-methionine. Subsequently, the enzyme has been the subject of cloning, crystallisation, mechanism and substrate specificity studies. This review summarises the current status of this research. 

Addition of a wax modifier to crude disrupts the normal crystallisation mechanism of wax, thereby reducing wax deposition. Since the chemical reacts during the onset of crystallisation by co-crystallisation, it is only effective when introduced at temperatures above the cloud point. 

Weibullian/sine function is introduced. Applications of this method to polyethylenes prepared by various conditions successfully explain the morphology and physical properties. Crystallisation mechanism of nylon 46 by high-resolution NMR in the solid state is also introduced. 

Qualitatively it may be noted that the constant values of n for both types of silica fillers at all experimental terperature (with the one exception) indicates a similar crystallisation mechanism by both the high and low energy surface silica conposites. For the pure polymer, there operates a different mechanism, showing either a change in heterogeneous nucleation time dependence or in growth morphology. 

Eet us consider crystallisation mechanism changes for HDPE/EP nanocomposites in comparison with the initial HDPE, which define the degree of reduction in crystallinity with an increase in c p. As it is known [8], the crystallisation kinetics of polymers is often described with the aid of the Kolmogorov-Avrami equation, obtained for low-molecular substances (Equation 4.19), in which the exponent n value can be changed within the range of 1-4 [9]. 

The dependence K c p), calculated according to Equation 8.2, is shown in Figure 8.3 by a dashed line. As one can see, this dependence demonstrates much slower reduction in K with increase in observed experimentally. This means that for the considered nanocomposites the value of K is defined not so much by the polymer chain flexibility, characterised by parameter C, as by the variation of the characteristics of nucleation and crystallisation mechanisms, from which the exponent n is the most important. 

Hence, the results stated above have shown that reduction in the degree of crystallinity of HDPE/EP nanocomposites is due to variation of the characteristics of nucleation and crystallisation mechanisms. The change of polymer molecular characteristics has less influence on the crystallinity degree. These effects can be accurately described within the frameworks of the fractal model. 

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