Crystallization perfecting process

In reality, polymer crystaHizatiOTi is too complex to be described by a simple expression such as the Avrami equation. For example, the assumption in Avrami s expression that the volume does not change is inaccurate because the specimen tends to shrink during crystallization. In addition, secondary crystallization and crystal perfecting processes are not taken into account. 

High concentration of nucleating agent may depress the nucleating efBdency and reduce the duration of spherulite growth during crystallization perfecting process. ... 

ZnO contauns excess metal which is accommodated interstitially, i.e. at positions in the lattice which are unoccupied in the perfect crystal. The process by which ZnO in oxygen gas acquires excess metal may be pictured as follows. The outer layers of the crystal are removed, oxygen is evolved, and zinc atoms go into interstitial positions in the oxide. We represent interstitial zinc by (ZnO). However, the interstitial zinc atoms may ionise to give (Zn O) or even (Zn O). The extra electrons produced in this way must occupy electron levels which would be vacant in the perfect crystal. We represent them by the symbol (eo), and refer to them as free electrons. They can be pictured as Zn ions at normal cation sites. We see therefore that three reactions can be written, each giving non-stoichiometric ZnO ... 

Fontaine et al. [81] concluded that the increase in crystallinity by further heating material, crystallized at 200 °C, to 215 °C involves a crystal (lamellae) thickening process which is probably due to crystal perfection at the boundary layers. Further annealing of this material at temperatures above 215 °C led to a melting temperature increase that was attributed to crystal perfection alone and not to crystal thickening. 

The condition of a surface is of importance not only in studies and processes involving the surface itself but also in many investigations involving the bulk properties of materials With the recent advances in crystal growth and purification, more rigorous demands are being made on crystal perfection, orientation, and special surface treatments. Etching plays a vital role in the preparation and characterization of crystals. 

In Fig. 21 DCH crystals are shown before polymerization and at an intermediate conversion. It is typical for the thermal reaction that more perfect monomer crystals require longer reaction times than defect-rich crystals. There is evidence that in the radiation polymerization of DCH the polymer crystal perfection increases with decreasing temperature, i.e., the nucleation process requires a rather high thermal activation energy. 

The large number of reactions which lead to crystallization during polymerization illustrate the importance of the study of these reactions. The chemical reaction is in many of these cases coupled to the cooperative crystallization process which leads to certain effects foreign to separate polymerization reactions. This last section of the review will summarize the conclusions which can be drawn about nucleation and crystal growth, about crystal perfection, about the difference between crystals grown during polymerization and crystals grown after polymerization, and about polymerization in the solid state. 

The DSC trace also gives information on the range of lamellar crystal perfection, since the thinnest, lowest molecular weight, lamellae melt some 30 °C below the final melting point. If a rapidly cooled polyethylene is subsequently annealed in this temperature range, the lamellae will thicken by a process of partial melting and recrystallisation, and the shape of the DSC trace will change. 

---