Reaction-induced phase separation technique

Figure 9.2 Morphologies obtained by a reaction-induced phase separation technique from a homogeneous medium (reactive monomer-tporogenic miscible additive) possible trajectories and morphologies in a conversion-concentration phase diagram. Subscript mono indicates monomer, and subscript adit indicates additive.

The initial mixture is homogeneous, and phase separation takes place during the cure of the thermoset. This second technique is called reaction-induced phase separation (Williams et al., 1997) and may lead to several types of morphologies a dispersion of modifier-rich particles in a thermoset matrix a dispersion of thermoset-rich particles in a modifier matrix (phase-inverted morphology), or two bicontinuous phases. 

In the first section it was shown that a) the reaction-induced phase separation process is a competitive technique with that using a two-phase dispersion from the beginning of the reaction (there are advantages and disadvantages with both procedures) b) the purpose of modifying a thermosetting polymer may be very different for one or another application (desired morphologies are very... 

The experimental description of the reaction-induced phase separation at different times and scales by the use of different techniques would be useful to... 

A common technique for separating the water and the oil in a microemulsion is a temperature-induced phase separation, yielding an excess water phase (increasing the temperature) or an excess oil phase (decreasing the temperature). It is the simplest way to separate the oil and the water. Nevertheless this method is quite time consuming and often not complete. In case of enzymatic reaction, a change in temperature can lead to a loss of enzyme stabiUty. 

The Ruhrchemie/Rhone-Poulenc process is performed annually on a 600,000 metric ton scale (18). In this process, propylene is hydroformylated to form butyraldehyde. While the solubility of propylene in water (200 ppm) is sufficient for catalysis, the technique cannot be extended to longer-chain olefins, such as 1-octene (<3 ppm solubility) (20). Since the reaction occurs in the aqueous phase (21), the hydrophobicity of the substrate is a paramount concern. We overcame these limitations via the addition of a polar organic co-solvent coupled with subsequent phase splitting induced by dissolution of gaseous CO2. This creates the opportunity to run homogeneous reactions with extremely hydrophobic substrates in an organic/aqueous mixture with a water-soluble catalyst. After C02-induced phase separation, the catalyst-rich aqueous phase and the product-rich organic phase can be easily decanted and the aqueous catalyst recycled. 

Among these various phase separation techniques, pressure-induced phase separation is particularly important since pressure changes can be brought about uniformly and very fast throughout the bulk of a solution. This would not be so in other techniques due to for example heat (in TIPS) and mass transfer (in SIPS) limitations. The technique therefore opens up new opportunities for formation of microstructured materials with potentially more uniform morphologies. It is also important to recognize temperature, solvent, reaction, or field-induced phase separation may all be carried out at elevated pressures if so desired, as such all modes of phase separation methods are of interest when working with near-critical or supercritical fluid systems. 

Wahl and Deck were able to obtain an estimate of an assumed second-order rate coefficient ( 10 l.mole" .sec at 4°C) using a separation procedure based on the extraction of Fe(CN)e by a chloroform solution of Ph AsCl, in the presence of the ions Co(CN)g and Ru(CN)6, to reduce the exchange between the iron species in the two liquid phases. A similar estimate was obtained using a precipitation method in the presence of the carrier Ru(CN)6. A direct injection technique was used as short reaction times were necessary. Wahl has reviewed the large induced exchanges occurring in the chemical separation methods. The extraction procedure when the carriers Co(CN)6 and Ru(CN) are present provides the most satisfactory method of separation. ... 

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