Crystallization, secondary crystalline

Secondary crystalline transitions (below Tm) occur if the material transforms from one type of crystal to another. These transitions are, like the melting point, thermodynamic first-order transitions. 

All of the organic matter and part of the aluminosilicates, hydroxyoxides, and silica in soil exist in structures too small or too poorly crystalline to be detectable by x-ray diffraction, These amorphous materials are not well understood, but they should logically be among the most reactive of soil components, because their structure is so open and their surface area so great, They represent a transition state between unweathered parent materials and well-crystallized secondary soil minerals. 

A considerable secondary crystallization, typical for PEEK, is seen in Fig. 6.99. Only about 50% of the total crystallinity can be assigned to primary crystallization. Secondary crystallization starts at about 28 h ( 10 s). At that time, the reversing amplitude of the storage modulus is almost constant. At later times, it decreases slowly, indicating very different processes for primary and secondary crystallization. 

Fig. 11.12. AFM images showing some details on spirals and secondary crystalline structures formed on top of a crystal for a 102 nm thick film crystallized at 57° C and then quenched to room temperature. Besides the spirals which grew at 57°C, we also can identify crystals grown at room temperature. The size of the images is (A) 50 X 50 (im (height range 150 nm), (B) 20 X 20 (im (height range 80 nm), (C) 20 X 20 (im (height range 180 nm)...

X-ray and neutron structure analysis generally deal with crystalline matter, either single crystals or crystalline powders. Nonperiodic objects, such as glasses, play only a secondary role. 

These secondary crystalline phases can easily be differentiated from the primary phase by their XRD lines.An example of the diffraction lines observed is given in Table 6. The lines of this second crystalline phase fit to the three phases listed above. The lattice parameters of these phases are close together, therefore a unique phase could not be identified in the materials. Another problem in this investigation was that the intensity of the diffraction lines could not be measured.As a result, the three secondary crystalline phases found in all the heat treated specimens were also in the as received material. Summarizing, it can be concluded that by the XRD analyses, the state of the crystallization of the amorphous second phase, as a result of the heat treatments, could not be determined. 

Crystollization of Secondary Crystalline Phases on Molecular Crystal Substrates. 

Secondary nucleation requires the presence of crystalline product. Nuclei can be formed through attrition either between crystals or between crystals and solid walls. Such attrition can be created either by agitation or by pumping. The greater the intensity of agitation, the greater... 

Secondary crystallization occurs most readily in polymers that have been quench-cooled. Quenched samples have low degrees of crystallinity and thus have relatively large volumes of amorphous material. A pre-requisite for secondary crystallization is that the amorphous regions must be in the rubbery amorphous state. Increased temperature accelerates the rate of secondary crystallization. The new volumes of crystallinity that form during secondary crystallization are generally quite small, amounting to less than 10% of the crystalline volume created during primary crystallization. 

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