Polymer crystallization secondary nucleation

Interestingly, this barrier does not depend on chain length. This result coincides with experimental observations on the primary nucleation rate of bulk polymers [128-130]. For secondary nucleation of crystallization on a smooth growth front, a similar free-energy expression can be obtained for 2D nucleation ... 

Secondary nucleation is essentially a crystal growth process. Secondary nucleation occurs by the deposition of a stem of the polymer molecule on a preexisting crystal-face as shown in Figure 15. The overall rate of this process is given by the following expression [58],... 

Note Nucleation may be classified as primary or secondary. Primary nucleation can be homogeneous or heterogeneous if heterogeneous nucleation is initiated by entities having the same composition as the crystallizing polymer, it is called self-nucleation. Secondary nucleation is also known as surface nucleation. 

In polymers crystallized from the melt, in most cases spherulitic structures are observed spherical agglomerates of crystals and amorphous regions, grown from a primary nucleus via successive secondary nucleation (Figure 4.18). The dimensions of the spherulites are commonly between 5 pm and 1 mm. When spherulites grow during the crystallization process, they touch each other and are separated by planes. In a microtome slice they show a very attractive coloured appearance in polarized light. 

Walstra, P. 1998. Secondary nucleation in triglyceride crystallization. Prog. Colloid Polym. Sci. 108, 4-8. 

Once primary nuclei are formed the ensuing spherulites grow radially at a constant rate. Primary crystallization, which occurs initially on the surface of the primary nucleus and then on the surface of the growing lamellar, also involves a nucleation step, secondary nucleation. It is this step that largely governs the ultimate crystal thickness and which forms the focus of most kinetic theories of polymer crystallization. 

Polymer crystal growth is predominantly in the lateral direction, because folds and surface entanglements inhibit crystalliza- 4 don in the thickness direction. Neverthe-1 less, there is a considerable increase in the fold period behind the lamellar front during crystallization from the melt and, as we have j seen, polymers annealed above their crys-tallization temperature but below Tm also irreversibly thicken. Nevertheless, in most theories of secondary nucleation, the most i widely used being the theory of Lauritzen and Hoffman,28 it is assumed that once a part of a chain is added to the growing crystal, its. fold period remains unchanged. 

Although the overall phenomena observed during polymerization seem to be accounted for, several interesting questions about the crystal morphology, primary and secondary nucleation mechanism, directiveness of polymer chain and crystal growth, and location of ion and counter-ion... 

Crystal growth can only start after the crystalline phase has been nucleated, in contrast to a chemical reaction which may proceed on any single encounter of two reaction partners. This difference between the chemical reaction which sets the primary bonds and the crystallization which sets the secondary bonds in crystals of flexible linear high polymers makes the crystal nucleation to the central problem. From the different examples of crystallization during polymerization collected here one can derive five different nucleation processes A. an intermolecular nucleation followed by simultaneous polymerization and crystallization dose to the ceiling temperature, B. an intermolecular nucleation followed by simultaneous polymerization and crystallization far from the ceiling temperature, C. an intermolecular nucleation followed by successive pol3uner-ization and crystallization, D. and E. an intramolecular nudeation which may also show simultaneous or successive polymerization and crystallization. 

The local resolution of laser-induced reactions depends on primary effects, i.e., the laser light, and secondary effects induced by the system. Laser-induced metal nucleation and crystal growth and the relevant mechanisms depend mainly on the electronic properties of the substrate, but also on interfacial and electrolyte properties. Depending on the system parameters, focused laser light can influence overvoltage-dependent terms particularly by local heat formation or by local activation of the solid state/electrolyte interface. As the electric properties of the substrate material is of strong influence, the effects will briefly be discussed for metal, semiconductor and polymer substrates. 

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