Chemistry Reference
In-Depth Information
according both polydispersities can be characterized by a high effectivity, which
can been seen in Figs.
34
and
35
.
In summary, both fractionation methods using columns can be applied for the
fractionation of copolymers; however, the separation efficiency for the fraction-
ation with respect to the molecular weight is lower than for the fractionation of
homopolymers. For the development of further fractionation methods for copoly-
mers, it is suggested that the CPF column is also used for fractionation in two
directions, similar to the cross-fractionation. This can be realized experimentally
very simply by changing the solvent mixture in the different runs, necessary to
produce different fractions.
4 Summary
For the first time, a theoretical framework for the fractionation of statistical copo-
lymers using successive fractionation methods and columns is introduced, taking
into account the polydispersity with respect to molecular weight and chemical
composition.
The application of this theoretical framework based on continuous thermody-
namics allows the investigation of operating parameters (solvent gradient, temper-
ature gradient, features of the fractionation column, fractionation strategy) on the
efficiency of the fractionation, where the two-dimensional distribution of statistical
copolymers is completely taken into account. From the thermodynamic point of
view, copolymer fractionation is the successive establishing of LLE for suitable
solutions of the polymer to be fractionated. Similar to the theoretical description of
distillation or extraction columns in chemical engineering, the column is divided
into theoretical stages. Assuming an LLE on each theoretical stage, the polymer
fractionation can be modeled using phase equilibrium thermodynamics.
From the results of calculations carried out for the successive cross-fractionations,
where in principal four different fractionation strategies are possible, it can be
concluded that the fractionations in both solvent mixtures should be performed
using SSF, meaning that the obtained sol phase should always be taken as a fraction.
During simulation of the fractionation in columns, such as the BW column or the
CPF column, the influence of the operative conditions on the fractionation effectiv-
ity was investigated. For the simulation of the BW column, the main focus was the
analytical purpose and in the simulation of the CPF column, the focus was the
preparative purpose. Similar to the results found for the stepwise fractionation,
the most important feature for an effective fractionation in column 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.
Acknowledgment Sincere thank is given to Dr. Heike Kahl for support in preparation of the
manuscript.
