Crystallization and polymerization

Another characteristic of this solution is its proneness to crystallization and polymerization. When parts of the exhaust system are constantly welted by Adblue on the same spot, undesired urea crystals or polymers may form if the exhaust line temperature is lower than 300°C. This phenomenon will result in uncontrolled ammonia production when the crystals or polymers melt or sublimate after being heated at significantly higher temperatures (T > 350°C). This may result in ammonia release. Furthermore, the crystals or polymers can also have an impact on the SCR catalyst cells by reducing the catalyst surface and thus reducing the catalyst performances. 

Figure 1.13—occur because of simultaneous crystallization and polymerization at 150°C. This temperature is near the maximum crystallization rate temperature ( 145°C) of nylon 6 homopolymer [66]. The presence of solid crystallites increases the complex viscosity of the polymerizing system because of a filler effect. 

Modelling non-isothermal crystallization is the next important step in a quantitative description of reactive processing. This is particularly important, because crystallization determines the properties of the end product. Therefore, the development the spatial distribution of crystallinity, a, and temperature, T, with time throughout the volume of the reactive medium must be calculated. It is also noteworthy that crystallization and polymerization processes may occur simultaneously. This happens when polymerization proceeds at temperatures below the melting point of the newly formed polymer. A typical example of this phenomenon is anionic-activated polymerization of e-caprolactam, which takes place below the melting temperature of polycaproamide. 

Figure 2.26. Heat effects, observed during anionic activated polymerization of E-caprolactam at 190°C (a) and 160°C (b). Curves 1 and 2 are components related to crystallization and polymerization, respectively. Curves 3 and 4 are calculated from Eq. (2.36) and from a simple additive rule, respectively.

During the initial polymerization of trioxane with (C4H9)2OBF3 in melt or solution, no solid polymer is formed, and the reaction medium remains clear. Using a high resolution NMR spectroscope, C. S. H. Chen and A. Di Edwardo observed the appearance of soluble linear polyoxy-methylene chains. In the cationic copolymerization of trioxane with 1,3-dioxolane, V. Jaacks found also that a soluble copolymer forms first and turns later into a crystalline copolymer of different composition. Crystallization and polymerization proceed simultaneously in the solid phase. 

All monomer epitaxial crystallization and polymerization is controlled by the substrate surface and results in new, highly oriented crystalline films with new structures and properties which are presently under investigation. 

Polymerization in the crystalline state is, as stated above, thermodynamicaUy different from the same polymerization in the homogeneous medium. Polymerizations, which would not be allowed under homogeneous conditions, may be accomplished in the crystalline state, i.e. monomer may add directly to the active species in the crystal and polymerization is accompanied by simultaneous crystallization. In this process each propagation step is equivalent to an increase of the crystal size. 

The applications of liquid crystals and polymeric liquid crystals are under constant... 

The problems of interaction of crystallization and polymerization of Sect. 3.2.2 are reviewed in Wunderlich B (1976) Macromolecular Physics, Vol 2, Crystal Nucleation, Growth, Annealing. Academic Press, New York, Chap VI. 

Both crystallization and polymerization reactions are temperature-activated phenomena, allowing the transformation of the system from a liquid-like to an almost solid-like material. The structure evolution can be suitably monitored by a degree of conversion parameter, which follows an analytical kinetics model. 

There are other physical measurements which reflect molecular mobility and can be related to relaxation times and friction coefficients similar to those which characterize the rates of viscoelastic relaxations. Although such phenomena are outside the scope of this book, they are mentioned here because in some cases their dependence on temperature and other variables can be described by reduced variables and, by means of equation 49 or modifications of it, free volume parameters can be deduced which are closely related to those obtained from viscoelastic data. These include measurements of dispersion of the dielectric constant, nuclear magnetic resonance relaxation, diffusion of small molecules through polymers, and diffusion-controlled aspects of crystallization and polymerization. 

In Figs. 6.8-6.10 some typical TMA results are reproduced. The TMA data are usually taken as linear expansivity or length of the sample. For isotropic solids and liquids, it is shown in Fig. 2.15 that the linear and volume expansivities are simply related. For nonisotropic crystals and polymeric films or fibers, this is not so, and data on linear expansivity must be augmented for full interpretation with information on sample orientation and structure. Eleven TMA curves are reproduced in Fig. 6.8. The top left figure shows the abrupt... 

The Mixed Crystallizer. One problem that concerned our research for many years was the understanding of the cyclic behavior of some crystallization and polymerization reactors (27. 28) Under certain conditions the particle size undergoes strong cyclic fluctuations, which severely upset operation and control. To understand this behavior we set up a simple model proposed by Hulburt and Katz for a stirred tank crystallizer, which is simply a particle balance of the crystallizer as well as a mass balance of the solute For our purposes it is not necessary to go into the details of the model, but rather to deal directly with its implications. The only important kinetic parameters of this model are the linear crystal growth rate G and the nucleation rate B. The simplest assumption we can make about B and G are to assume that they are functions of supersaturation only. 

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