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Handbook of Size Exclusion Chromatography and Related ...

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size information is derived from universal calibration. Equation (27) can be<br />

rewritten as<br />

g 0 ¼ M*<br />

Mb<br />

aþ1<br />

(30)<br />

where M* is the molecular weight <strong>of</strong> the linear molecule with the same<br />

hydrodynamic volume as the branched molecule calculated from universal<br />

calibration <strong>and</strong> a is the Mark–Houwink exponent for the linear molecule. Figure 12<br />

illustrates the effect <strong>of</strong> branching on the molecular weight calibration curve.<br />

The value <strong>of</strong> Mb in Eq. (30) is a number-average molecular weight, <strong>and</strong><br />

because light scattering measures the weight-average molecular weight, values <strong>of</strong><br />

g 0 do not agree with those measured by viscometry if there is significant<br />

polydispersity at each elution slice. This occurs when species with different<br />

degrees <strong>of</strong> branching have the same hydrodynamic volume.<br />

Figure 12 Typical SEC calibration curves for linear <strong>and</strong> branched polymers.<br />

© 2004 by Marcel Dekker, Inc.

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