Amorphous crystallization process

Nucleation. Crystal nucleation is the formation of an ordered soHd phase from a Hquid or amorphous phase. Nucleation sets the character of the crystallization process, and it is, therefore, the most critical component ia relating crystallizer design and operation to crystal size distributions. 

It is important to note that this second choice is possible because expression (25) includes the smooth background belonging to the crystalline phases, so it can be separated from the background due to the amorphous phase. A typical example, where the amorphous phase is not available, is the study of crystallization process. In this case, the composition and the diffraction pattern of the amorphous phase can change a lot. 

Many polymers solidify into a semi-crystalline morphology. Their crystallization process, driven by thermodynamic forces, is hindered due to entanglements of the macromolecules, and the crystallization kinetics is restricted by the polymer s molecular diffusion. Therefore, crystalline lamellae and amorphous regions coexist in semi-crystalline polymers. The formation of crystals during the crystallization process results in a decrease of molecular mobility, since the crystalline regions act as crosslinks which connect the molecules into a sample spanning network. 

The crystallization process of flexible long-chain molecules is rarely if ever complete. The transition from the entangled liquid-like state where individual chains adopt the random coil conformation, to the crystalline or ordered state, is mainly driven by kinetic rather than thermodynamic factors. During the course of this transition the molecules are unable to fully disentangle, and in the final state liquid-like regions coexist with well-ordered crystalline ones. The fact that solid- (crystalline) and liquid-like (amorphous) regions coexist at temperatures below equilibrium is a violation of Gibb s phase rule. Consequently, a metastable polycrystalline, partially ordered system is the one that actually develops. Semicrystalline polymers are crystalline systems well removed from equilibrium. 

There has been considerable interest recently in an alternative type of ABA triblock structure, where the end blocks could form crystalline domains, by crystallization, rather than amorphous domains by phase separation. It was felt that, since such a crystallization process need not depend on the incompatibility between the blocks, it should be possible to have a homogeneous melt, which should exhibit a much lower viscosity, and hence much easier processing, than the heterogeneous media of the conventional triblock copolymers. Furthermore, thermoplastic... 

Crystallinity often has little if any effect on 7A, but with some polymers crystallized under certain conditions, the 7A value is raised (78,79). The increase appears to be caused either by polymer being restricted to short amorphous segments between two crystallites or by stresses put on the amorphous chain sequences as a result of the crystallization process. In either case the mobility is restricted, so higher temperatures are required to restore it. Thus quench cooling tends to increase 7A whereas annealing reduces TK back to the value typical of the amorphous polymer. 

Many different zeolite structures are already known, but there is permanent need for new or improved ones to satisfy novel and specific industrial and technological applications. To successfully accomplish this task, a deeper understanding of the zeolite crystallization process is certainly needed. One of the important parts in that study is the structural investigations of their amorphous aluminosilicate precursors (gels). 

The amorphous PEN resin pellets are first dried at 180 °C and then extruded at 290-300 °C through a die, formed into a sheet, which is then followed by a two-step orientation (forward draw and sideway draw process) just above the glass transition temperature (Tg) (>120°C). After the orientation process, the PEN film is conveyed between rollers at 210-220 °C to induce crystallization. At the end of the orientation and crystallization process, the film is cut and rolled into widths and lengths to suit individual customers [14-16], Two of the process used to produce such films are shown in Figure 10.3. 

The crystallization process can be illustrated by a phase diagram that shows which state (liquid, crystalline, or amorphous solid [precipitate]) is stable under a variety of crystallization parameters. It provides a means of quantifying the influence of the parameters such as the concentrations of protein, precipitant(s), additive(s), pH, and temperature on the production of crystals. Hence phase diagrams form the basis for the design of crystal growth conditions (McPherson, 1999 Ducruix and Giege, 1992 Ducruix and Giege, 1999 Chayen et ah, 1996 and references therein). 

Additionally, the microwave treatment during the crystallization process at high temperature may cause the metastable mesophase to collapse into the denser or amorphous phase in synthetic mixture as well as provide the favorable condition for the formation of silicalite-1. A summary of parameters obtained by nitrogen sorption is shown in Table 2. In Table 2, pore diameters of major peaks ( ) for sample II-IV are increased from 2.5 to 2.87 nm as extending the microwave irradiation. It implied that the additional space created in the mesoporous channels, as a consequence of the pore size enlargement, that is filled by extra water [16]. 

Bi-phasic porous silicates containing amorphous and crystalline components can be prepared as stable pellets even if different crystallization routes known from the conventional crystallization processes are employed. Thus the inorganic template-free crystallization route and the crystallization in presence of propylamine could be applied and optimized for the crystallization on and into porous glasses, the so-called supported crystallization. 

To illustrate the usage of implicit finite difference schemes, we will solve the cooling process of an amorphous polymer plate. Since an amorphous polymer does not go through a crystallization process we will assume a constant specific heat2. The implicit finite difference for this equation can be written as,... 

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. 

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