Chemistry Reference
In-Depth Information
called two-direction fractionation. The original copolymer is fractionated first in a
precipitant solvent system such that the mean content of A units of fractions, say,
increases with an increase in their molecular weights. Then, every intermediate
fraction is separated in another precipitant solvent system, where the mean content
of A units of fractions diminishes with an increase in their molecular weight.
Rosenthal and White [
64
] assert that the final fractions separated in such a way
are highly homogeneous both in molecular weight and in chemical composition.
For example, this method was carried out for various copolymers, namely
styrene methyl methacrylate copolymer [
65 67
], epoxide resins [
68
], styrene
acrylic acid copolymer [
69
], styrene 2-methoxyethyl methacrylate copolymer
[
70
,
71
], ethylene
a
-olefin copolymer [
72
], partially modified dextran ethyl carbonate
copolymer [
73
], vinyl chloride vinyl acetate copolymer [
43
], styrene acrylonitrile
copolymer [
74
], and styrene butadiene copolymer [
75
].
The stepwise fractionation procedures (SSF and SPF) are one-direction fractio-
nations 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:
(a) SSF/SPF
(b) SSF/SSF
(c) SPF/SSF
(d) SPF/SPF
The theoretical framework introduced above can also be applied to cross-
fractionation. This can be achieved by combination of the equations according
to the selected fractionation strategy.
2.3 Baker-Williams Fractionation
Precipitation fractionation as developed by Baker and Williams [
40
] is one of the
best-known column fractionation procedures. The fractionation is performed in a
glass-bead-filled column with a temperature gradient down the column (Fig.
4
). To
start the fractionation, the total polymer is precipitated on the glass beads in a
section at the entry of the column (or in a separate vessel). In a mixing vessel, a
nonsolvent and a solvent are mixed to form a mixture with progressively increasing
solvent power through continuous enrichment of the solvent. The polymer is
dissolved by adding the solvent mixture. The resulting sol phase moves relatively
slowly in the column, and the polymer in a given increment of the liquid sol stream
becomes less soluble due to the temperature gradient and precipitates partially on
the glass beads as a gel phase. The fractionation is achieved by the repeating
exchange of polymer molecules between the stationary gel phase and the mobile
sol phase. The superposition of a solvent + nonsolvent gradient and a temperature
gradient leads to a very high separation efficiency.
