Handbook of Size Exclusion Chromatography and Related ...

Figure 10 Schematic representation of polymer coil dimensions as function of
thermodynamic quality of solvent, M1 . M2.
through changing their sizes. Figure 10 shows a typical course of polymer size
changes with changing thermodynamic quality of the solvent (Fig. 10).
Swollen coils of macromolecules usually extensively shrink in the vicinity of
theta conditions where mutual interactions between polymer segments are equal to
interactions between solvent molecules and polymer segments (9). Important
changes in polymer–column filling interactions, both adsorption and enthalpic
partition, are expected in the vicinity of the theta point (29,30). If the thermodynamic
quality of solvent further deteriorates, macromolecules may collapse and
assume less than 50% of their theta dimensions. The instable, collapsed state of a
macromolecule may last for several minutes (31), which is comparable with the
duration of normal HPLC experiments. The deteriorated solvent quality, however,
inevitably leads to a phase separation (32). The macro-phase separation in polymer
systems is often preceded by a micro- or nano-phase separation that is by
complexation of macromolecules (association or aggregation). The latter
processes, whether solute concentration dependent (association) or not
(aggregation), further complicate HPLC elution of polymer analytes because of
their slow kinetics. Although SEC was used for polymer association studies (see,
for example, Refs. 33–35), it seems that attaining repeatability of retention volumes
and peak shape values for complexing macromolecules is rather difficult. Block-,
graft-, and star-copolymers dissolved in selective solvents (liquids that dissolve one
kind of polymer chain but precipitate another kind) create micellar systems. The
core of micelles is formed by aggregated insoluble chains while the soluble chains
create a protective cloud that prevents macrophase separation. Sizes of micelles
depend on thermodynamic quality of solvent and may change with time.
During phase separation of polymer solutions, at least two phases are
formed. One of them is concentrated (“gel phase”) and contains larger macromolecules.
The other, diluted (“sol”) phase contains smaller polymer species. In
© 2004 by Marcel Dekker, Inc.