Thermal solvent medium

Shiraishi, N., Matsunaga, T. and Yokota, T. (1979). Thermal softening and melting of esterified wood prepared in an N,0,-DMF cellulose solvent medium. Journal of Amlied Polymer Science, 24(12), 2361-2368. 

The thermal motion of molecules of a given substance in a solvent medium causes dispersion and migration. If dispersion takes place by intermolecular forces acting within a gas, fluid, or solid, molecular diffusion takes place. In a turbulent medium, the migration of matter within it is defined as turbulent diffusion or eddy diffusion. Diffusional flux J is the product of linear concentration gradient dCldX multiphed by a proportionality factor generally defined as diffusion coefficient (D) (see section 4.11) ... 

Pyrolysis reactions are one class of reactions that benefit from the use of an SCF solvent medium since the SCF can solubilize reacted products and remove them from the high-temperature zone, thus avoiding further thermal decomposition. The carbon formation that occurs at the high temperatures normally encountered in pyrolysis reactions can therefore be minimized. Improved yields, selectivities, and product separation have been attained in an SCF reaction medium, compared with conventional pyrolysis methods. There is no doubt that a sufficiently high temperature is needed to provide the thermal energy to break bonds in a pyrolysis reaction. But it is not necessary to operate at excessively high temperatures to increase the vapor pressure of the product materials because the SCF medium has the necessary solvent strength to dissolve the products and remove them from the reaction zone. 

This section discusses the results of reactive molding of CW nanocomposites using FA as a polymerizable solvent medium to produce CW-PFA nanocomposites. Cellulose whiskers are not commercially available, and therefore, they were prepared by hydrolysis of MCC with sulfuric acid. The preparation of the CW was followed by their thorough morphology characterization, and finally, by the polymerization of FA to PFA in their presence. To characterize the polymerization behavior and to investigate how the presence of CW influences the polymerization of FA, FTIR spectra were collected before and during the resinification process. Finally, characterization of the thermal stability of the CW-PNC, as measured by TCA, is discussed and compared to the pure polymer. The results provide a useful qualitative measure of the CW dispersion in the cured PNC. 

Solvent effects on the initiation reaction are primarily on the rate of decomposition of initiator molecules into radicals and in the efficiency factor, f, for polymerization. However, in some instances the solvent plays a significant role in the initiation process, for example, in initiation reactions with t-butoxy radical where the primary radical rarely initiates a chain but instead abstracts a hydrogen atom from the solvent medium, which subsequently initiates the chain.The consequence of this is that the polymer chains contain fragments of solvent. As the stability of the chains to thermal and photochemical degradation is gov-... 

The initiator in radical polymerization is often regarded simply as a source of radicals. Little attention is paid to the various pathways available for radical generation or to the side reactions that may accompany initiation. The preceding discussion (see 3.2) demonstrated that in selecting initiators (whether thermal, photochemical, redox, etc.) for polymerization, they must be considered in terms of the types of radicals formed, their suitability for use with the particular monomers, solvent, and the other agents present in the polymerization medium, and for the properties they convey to the polymer produced. 

From the Arrhenius form of Eq. (70) it is intuitively expected that the rate constant for chain scission kc should increase exponentially with the temperature as with any thermal activation process. It is practically impossible to change the experimental temperature without affecting at the same time the medium viscosity. The measured scission rate is necessarily the result of these two combined effects to single out the role of temperature, kc must be corrected for the variation in solvent viscosity according to some known relationship, established either empirically or theoretically. 

Independently, Caddick et al. reported microwave-assisted amination of aryl chlorides using a palladium-N-heterocyclic carbene complex as the catalyst (Scheme 99) [lOlj. Initial experiments in a domestic microwave oven (reflux conditions) revealed that the solvent is crucial for the reaction. The Pd source also proved very important, since Pd(OAc)2 at high power in DMF gave extensive catalyst decomposition and using it at medium and low power gave no reaction at all. Pd(dba)2/imidazohum salt (1 mol% catalyst loading) in DME with the addition of some DMF was found to be suitable. Oil bath experiments indicated that only thermal effects are governing the amination reactions. 

Thermal solid-state reactions were carried out by keeping a mixture of powdered reactant and reagent at room temperature or elevated temperature, or by mixing with pestle and mortar. In some cases, the solid-state reactions proceed much more efficiently in a water suspension medium or in the presence of a small amount of solvent. Sometimes, a mixture of solid reactant and reagent turns to liquid as the reaction proceeds. All these reactions are called solid-state reactions in this chapter. Solid-state reactions were found to be useful in the study of reaction mechanisms, since it is easy to monitor the reaction by continuous measurement of IR spectra. 

The photochromism of the spiropyran depends on the structure of heterocyclic parts, the medium such as solvent or plastic films, temperature, and light energy. Though the actual mechanisms may be more complex, a simple photochromic behavior in the spiropyrans is illustrated in Scheme 1. Initially, a spiropyran is excited by photoirradiation, and then a cisoid isomer arises after dissociation of the C—O bond. Finally, the cisoid form changes to the thermodynamically stable transoid form. The equilibrium between the cisoid and transoid forms largely depends on the substituent groups. The reversal of the colored form to the colorless spiropyran occurs by thermal or photochemical energy. More detailed mechanisms will be described in Section 1.2.1.6. 

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