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Handbook of Size Exclusion Chromatography and Related ...

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where C<strong>and</strong> Dare constants for aparticular polymer–eluent system in agiven<br />

column, or<br />

VR ¼E Flog[h]M (8)<br />

(universal calibration dependence) (51), where E <strong>and</strong> F are constants for a<br />

given column provided enthalpic interactions are negligible (1 0) <strong>and</strong><br />

macromolecules form isolated, flexible coils.<br />

According to Eqs (7) <strong>and</strong> (8), retention volumes decrease with increasing<br />

polymer molar mass. For VR <strong>of</strong> macromolecules retained in the strong interaction<br />

regime (1 .1cr) we have<br />

VR ¼GþHexpM (9)<br />

where G<strong>and</strong> H are constants for agiven polymer species in agiven HPLC<br />

column/eluent system <strong>and</strong> at agiven temperature. It is attractive to intentionally<br />

combine, to couple entropic <strong>and</strong> enthalpic retention mechanisms so that they<br />

mutuallycompensate<strong>and</strong>themolarmassdependence<strong>of</strong>retentionissuppressedor<br />

even absent.<br />

Severalapproachestosuch coupling <strong>of</strong>retention mechanismswererecently<br />

described in areview (56). Therefore, we shall abridge the present discussion on<br />

this matter.<br />

The result <strong>of</strong> isocratic coupling <strong>of</strong> exclusion <strong>and</strong> enthalpic interaction<br />

retention mechanisms, which leads to molar mass independent retention <strong>of</strong><br />

polymer analytes is evident from Fig. 4, curve 6, for 1¼1cr.<br />

5.1 Liquid <strong>Chromatography</strong> <strong>of</strong> Macromolecules Under<br />

Critical Conditions (LC CC)<br />

Depending on the retention mechanism applied, the method can also be termed<br />

liquid chromatography at critical adsorption point or liquid chromatography at<br />

critical partition point. LC CC should not be confused with supercritical liquid<br />

chromatography <strong>of</strong> polymers. Recently, critical chromatography in supercritical<br />

fluids was also attempted (6,57). At this point, tribute should be paid to three<br />

groups <strong>of</strong> Russian authors. One group discovered the “critical approach” in the<br />

1970s <strong>and</strong> applied it to high polymers (for a review see, for example, Ref. 58),<br />

another independently applied critical chromatography for the successful<br />

separation <strong>of</strong> various oligomers (59), <strong>and</strong> the third elaborated theory <strong>of</strong> LC CC<br />

<strong>and</strong> proposed its application to various complex polymer systems (60). After a<br />

longer period, LC CC was applied to numerous complex polymers <strong>and</strong> oligomers<br />

including binary polymer blends, <strong>and</strong> block-copolymers (3–5). LC CC was also<br />

© 2004 by Marcel Dekker, Inc.

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