Unmixing process

Entanglements severely influence the viscosity which in turn affects significantly the dynamics of the unmixing process. The interfacial layer of polymer-polymer systems is more extended than for small-molecule systems, but, is smaller than the size of a chain molecule. Consequently, the interfacial tension in polymer systems is significantly influenced by the loss of conformational entropy of chain... 

Blend solutions. Solutions of blends comprising immiscible polymers Pj and P2 in a nonselective solvent have miscibility gaps as shown schematically in Fig. 14. When the polymer concentration increases by solvent evaporation the polymer coils start to interpenetrate above a certain concentration. As a consequence, interactions between the polymers become operative and phase separation must start above a critical polymer concentration p. The composition of the new phases will be situated on the branches of the coexistence curve. Finally, the unmixing process is arrested owing to enhanced viscosity. This simple scheme reveals the factors directing morphology evolution in blend solutions ... 

Special features of the isothermal unmixing process are the following ... 

AH a This step involves the separation of 2 kg of a 15% LiCl solution into its pure constituents at 25°C. This is an unmixing process, and the heat effect is the same as for the corresponding mixing process, but is of opposite sign. For 2 kg of 15% LiCl solution, the moles of material entering are... 

The occurrence of liquid immiscibility is determined by phase equih-brium thermodynamics, as developed and reviewed by Gibbs (1961). The driving potential for the unmixing process is a reduction in the system free energy. That is, phase separation can occur whenever the free energy for the polyphase system is lower than that for the homogeneous single phase. 

To calculate the enthalpy of mixing, it is usefid to consider another hypothetical path. In this case, we need to recognize the thermochemical data are in the form of the difference between the pure species (unmixed state) and the mixture in the form of the given equation for A/jot. However, both the initial state and the final state contain mixtures of more than one species. Therefore, we pick the hypothetical path shown in Figure E6.9C where we first unmix the initial state into its pure species components and then mix all three components to arrive at the final state. As a procedural note, in this case it is useful to formulate the problem in terms of extensive properties (i.e., AH, ) rather than intensive properties (i.e., Ah ) because the unmixing process and the mixing process contain different numbers of moles (8 vs. 10 mol). 

Diffusion is important in reactors with unmixed feed streams since the initial mixing of reactants must occur inside the reactor under reacting conditions. Diffusion can be a slow process, and the reaction rate will often be limited by diffusion rather than by the intrinsic reaction rate that would prevail if the reactants were premixed. Thus, diffusion can be expected to be important in tubular reactors with unmixed feed streams. Its effects are difficult to calculate, and normal design practice is to use premixed feeds whenever possible. 

Running an LLDPE Resin on a Cast Film Process with Unmixed Gels in the Extrudate... 

A single-phase (unmixed) Mg(NH3)3 decomposes during continuous heating up to 500-600°C in a two-step process [155, 156, 158]... 

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