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Handbook of Size Exclusion Chromatography and Related ...

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where hn is the hydrodynamic volume at each slice from the calibration curve, m is<br />

the total sample amount injected, <strong>and</strong> DV is the retention volume increment<br />

between data points.<br />

A special case <strong>of</strong> this approach is the method <strong>of</strong> calculating the numberaverage<br />

molecular weight from the viscometer output, the universal calibration<br />

curve, <strong>and</strong> the sample amount injected (18):<br />

Mn ¼<br />

P ci<br />

P ( ln hr) i=hni<br />

(15)<br />

In this case the Mark–Houwink exponent is not required, <strong>and</strong> thus this method can<br />

be used when the Mark–Houwink exponent is unknown or when it may vary with<br />

elution volume, as for copolymers <strong>and</strong> polymer blends.<br />

3.1.3 Intrinsic Viscosity Distribution<br />

Another approach to the SEC-viscometry data is that <strong>of</strong> Kirkl<strong>and</strong> et al. (19). The<br />

intrinsic viscosity is a fundamental property <strong>of</strong> the polymer sample in solution, <strong>and</strong><br />

thus polymers may be characterized in terms <strong>of</strong> their intrinsic viscosity distribution<br />

(IVD) without attempting to convert this into a molecular weight distribution.<br />

Moments <strong>of</strong> the IVD may be calculated similar to those for the MWD (20). The<br />

advantage is that the intrinsic viscosity distribution is directly measured <strong>and</strong> is not<br />

subject to the errors introduced when universal calibration is used to calculate<br />

molecular weight.<br />

If the Mark–Houwink coefficients for the polymer–solvent system are<br />

known, then the IVD measured by SEC-viscometry can be converted into the<br />

molecular weight distribution using the Mark–Houwink relation. This should give<br />

greater precision in the measurement <strong>of</strong> molecular weight distribution than SECviscometry<br />

with universal calibration, because the IVD measurement is much less<br />

sensitive to experimental conditions than a calibration curve.<br />

3.1.4 Radius <strong>of</strong> Gyration Measurement<br />

If universal calibration is used with SEC-viscometry, it is also possible to calculate<br />

the radius <strong>of</strong> gyration for linear polymers at each elution volume using the Flory–<br />

Fox equation (21),<br />

where<br />

© 2004 by Marcel Dekker, Inc.<br />

Rg ¼ 1<br />

p 6<br />

M[h]<br />

F<br />

1=3<br />

(16)<br />

F ¼ 2:55 10 21 (1 2:63e þ 2:86e 2 ) (17)

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