Fractionation of Polymers
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In the working group of Professor Wolf two different kinds of polymer fractionation techniques were developed. They allow the production of large amounts of material with narrow molecular weight distribution (MWD) starting from material with a much broader MWD. The fractionation principle is for both methods the same. It will be explained for the special case of a single solvent and for the (in real life) easier to deal with mixed solvent systems. Basing on the earlier invented Continuous Polymer Fractionation (CPF), the so called Continuous Spin Fractionation (CSF) was developed in 2002. For the CPF a filled column is used to carry out a counter-current extraction. The main feature of the CSF is the use of spinning nozzles. For both methods we will show a schematically apparatus. Some practical examples of different polymers will be listed below.
A homogeneous solution of the polymer is used as feed (FD) and the pure theta solvent as extracting agent (EA). The flow rates of these two liquids are chosen in such a manner that the total composition of the mixture within the apparatus corresponds to a point inside the miscibility gap (WP: working point of the fractionation). Then, the two phases formed in the column coexist throughout the process, and the polymer originally contained in the feed spreads into the phase originating from the extracting agent. During this process the polymer species of lower molecular weight are preferentially removed from the feed. Because of the counter-current nature of the process, the partition of the polymeric species takes place repeatedly, until the feed essentially contains only components of higher molecular weight. This phase, a concentrated solution, denoted as gel (GL) leaves the apparatus at its lower end. The phase mainly formed from the extracting agent (dilute solution, denoted as sol (SL)) has taken up the corresponding material of lower molecular weight and leaves the apparatus at its upper end.
If the desired narrow MWD of the fractions cannot be obtained in a single run, it is possible to use the gel directly as feed for the next fractionation step.
Figure 2 shows the situation of the polymer fractionation for the normally used mixed solvent. Here the polymer is dissolved in a solvent / non-solvent mixture and this solution (FD) is extracted by a liquid (EA) which contains the same mixed solvent as the feed.
With this method, as compared with conventional counter-current extraction, the alternative for the polymer molecules of different molecular weight is not to be dissolved in either of two solvents, but to be enriched in the more dilute or the more concentrated phase, both of which contain the same solvent.
Continuous Spin Fractionation (CSF)
In the case of the CSF the feed is pressed through a spinning nozzle into a properly chosen extracting agent. The threads of the viscous polymer solution disintegrate shortly after their exit into tiny droplets from which the better soluble macromolecules can be easily extracted because of the short distances of transport and successful fractionation becomes possible even with rather concentrated polymer solutions. The droplets, freed from the low molecular weight material, contain the gel fraction whereas the saturated continuous phase comprises the sol fraction.
Continuous Polymer Fractionation (CPF)
CPF can, in principle, be performed with any apparatus for counter-current extraction. So far, the experiments have been mainly carried out with a modified commercially available sieve bottom column with pulsator, a packed column, a mixer-settler system, or a centrifugal separator.
Some typical results are reported for two following polymers:
With the linear low density polyethylene
it is necessary to perform the experiments in the region of 135 °C, to overcome
the crystallization tendency of PE. In this special case, diphenylether (DPE)
was chosen to demonstrate that CPF can be performed with only one single solvent
used to prepare the feed.
This polymer has a high tendency to crystallize
and is also rather susceptible to chemical degradation. Systematical study of
mixed solvents found a suitable combination (dichloromethane/diethyleneglycol)
where these problems can be avoided.
The follow schematic representtion of the results of consecutive CPF runs gives an idea how to get material with the needed molecular weight and non-uniformity. The data indicated in the boxes below are the phase and CPF run number and the weight percentage of this fraction. In the second line the first number gives the molecular weight in kg/mol and the second is the non-uniformity of the polymer.
The result of the CPF was checked gainst GPC data of the feed and the resulting sol and gel. The most striking feature is the good removal of low molecular weight chains.
So far the following polymers have been fractionated by means of CPF in our group:
poly(vinyl chloride) (PVC)
How to apply CPF or CSF to new polymers?
First, one has to find a suitable solvent pair or single solvent with which liquid-liquid phase equilibria can be realized. Criteria for the choice are thermodynamic and kinetic aptness plus availability and price. The next important step is the determination of the limits of solubility (phase diagram at different temperatures) as shown in Figs. 1 and 2 schematically. After that, it is important to select the best working conditions.
Since these procedures (CPF/CSF) are quite universal, technically interesting polymers with narrow MWD are now available for basic research and special applications. Please note that we hold patents on this subject.