Stepwise fractionation

In CRYSTAF, the analytical process is followed by monitoring the polymer solution concentration during crystallization by temperature reduction. Aliquots of the solution are filtered (through an internal filter inside the vessel) and analyzed by a concentration detector at different temperatures, as shown in Fig. 21. The whole process is similar to a classical stepwise fractionation by precipitation with the exception that, in this new approach, no attention is paid to the polymer being precipitated but to the one that remains in solution. 

Only a few papers can be found in the literature that deal with theoretical questions on this topic. With the help of continuous thermodynamics, a theory to model stepwise fractionation of homopolymers was developed [45-47]. This theoretical framework could be extended to fractionation in columns [48-50]. The application of the developed theory was able to contribute to the improvement of the fraction technique [49]. Folie [51] studied the fractionation of copolymers. In his theoretical framework, the polymer was described by pseudocomponents with respect to the molecular weight however, the polydispersity with respect to the... 

The stepwise fractionation procedures (SSF and SPF) are one-direction fractionations and form the basis of cross-fractionation, where first the original polymer is fractionated in intermediate fractions using one solvent system and afterwards each intermediate fraction is further fractionated yielding the final fractions using another solvent system. There are four different possibilities for a fractionation strategy ... 

From the calculation results of the stepwise fractionation methods, it is known that the amount of polymer should by equal in all fractions. In CPF, the amount of polymer segments depends on the chosen working point and the temper-... 

During simulation of the fractionation in colunms, such as the BW column or the CPF colunm, the influence of the operative conditions oti the fractionation effectivity was investigated. For the simulation of the BW colunm, the main focus was the analytical purpose and in the simulation of the CPF colunm, the focus was the preparative purpose. Similar to the results found for the stepwise fractionation, the most important feature for an effective fractionation in colunm is also that the polymer is equally distributed in the corresponding fractions. This can be achieved by a suitable chosen concentration or temperature gradient, or both. 

The strong stepwise fractionation of ratios is reflected in the partition of REE into condensing solids. The... 

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