Liquid-Phase Blending Time

Liquid-phase mixing time measurements can be made using the techniques described in the single-phase mixing section. Again the probes must be protected from interference from the gas bubbles. 

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When gas bubbles, liquid drops, or solid particles are dispersed in a liquid, the blending time for the continuous phase is increased, even if the comparison is made at the same specific power input. The effect increases with viscosity, and for viscous liquids the blending time can be up to twice the normal value when the gas holdup is only 10 percent. 

Another difference is that in blending liquids a well-mixed product usually means a truly homogeneous liquid phase, from which random samples, even of very small size, all have the same composition. In mixing pastes and powders the. product often consists of two or more easily identifiable phases, each of which may contain individual particles of considerable size. From a well-mixed product of tliis kind small random samples will differ markedly in composition in fact, samples from any given such mixture must be larger than a certain critical size several times the size of the largest individual particle in the mix) if the results arc to be significant. 

Phase diagrams have been proposed for blends of polyethylenes where the first component is linear, or less branched, and the second component is more branched. The method involves quench cooling each blend composition from the melt at various temperatures, so that there is insufficient time for liquid-liquid phase separation. Separated blends are considered to be separated at the particular temperature of the melt prior to quenching. TEM and DSC are used to characterize the quenched blends. The phase diagrams exhibit UCST behavior, depending on the difference in branching content of the component polyethylenes. Branch length has been found not to be important since it is the branch points that are excluded from the crystals (20,21). 

Solvent mixing, less relevant commercially, is widely used in scientific studies to determine the natures of blends. By using dilute solutions of the components the polymers, miscible or immiscible in bulk, can be combined homogeneously. Slow removal of solvent from inherently immiscible polymer mixtures allows Hquid-liquid phase separation to proceed and the polymers to segregate. However, rapid solvent removal or co-precipitation into a large volume of non-solvent can result in intimate mixtures of even immiscible polymers results may depend on the solvent used. Thus, non-equilibrium, unstable mixtures of inherently immiscible polymers can be produced. Such mixtures may segregate when heated above the TgS of the samples when molecular mobility permits. This situation is encoimtered many times in studies of PCL blends. 

After [ CJethylene is bound and removed from the source of unbound ethylene, blending of the sprouts causes the release of a substantial amount of bound ethylene very rapidly. Figure 2A shows the amount of [ CJethylene released into the gaseous phase after blending for various time intervals up to 20 min. Much of the ethylene is released during the first minute. Since all of the samples were started at the same time, the amount of ethylene in the gas phase plus liquid phase should total the amount in the sprouts at 1 min. Figure 2B shows the amount of ethylene released into the gas phase after allowing diffusion to occur for the specified time, then... 

Fig. 2. A Ethylene from intact mung bean sprouts (O). and in liquid phase ( ) and gas phase (A) after blending in water. Blending was started at zero time for the number of minutes indicated. Values have been corrected for unbound ethylene ...

If the reaction rate is very slow, the concentration difference between Cjg and Cb grows closer. In the limit, Cb is equal to Cg and the maximum reaction rate is obtained at the saturation composition. It almost all cases it is assumed that the continuous or liquid phase is well mixed, so that no gradients exist. This is true in most equipment because the blend time is usually small compared to the mass ttansfer time. This means that Cb is the same at all places in the vessel. 

To elucidate the spatiotemporal emergence of crystalline structure and liquid-liquid phase separation in the crystalline-amorphous polymer blends, Rathi (80] employed the time dependent Ginzburg-Landau (TDGL Model-C) equations pertaining to the conserved concentration order parameter and the noncrystal order parameter. Model C is a combination of TDGL Model-A and Model-B, viz. ... 

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