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Handbook of Size Exclusion Chromatography and Related ...

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macromolecules in the column effluent, one can directly calculate mean molar<br />

mass values (mass, number, z, z þ 1, ...averages) <strong>of</strong> the analysed polymer sample<br />

<strong>and</strong> also determine its molar mass distribution function. SEC measurements are<br />

easy to automate <strong>and</strong> SEC results are usually highly repeatable. Moreover, highspeed,<br />

high-throughput SEC systems allow at least semiquantitative on-line<br />

characterization <strong>of</strong> samples in polymer production plants <strong>and</strong> in combinatorial<br />

polymer laboratories. These features make SEC extremely popular in both<br />

polymer science <strong>and</strong> technology. As a result, SEC has almost fully substituted or at<br />

least suppressed the use <strong>of</strong> various bulk methods such as membrane <strong>and</strong> vapor<br />

pressure osmometry, light-scattering measurements, <strong>and</strong> even viscometry.<br />

Unfortunately, SEC cannot be applied directly to molar mass determination<br />

<strong>of</strong> many complex polymers which, as mentioned, exhibit more than one single<br />

distribution <strong>of</strong> their molecular characteristics. This situation is schematically<br />

represented in Fig. 1, which shows a typical SEC chromatogram that is a<br />

dependence <strong>of</strong> polymer concentration in the effluent on retention volume.<br />

In the case <strong>of</strong> complex polymers, size <strong>of</strong> macromolecules usually depends<br />

on all molecular characteristics, that is, not only on molar mass but also on<br />

chemical structure (for example, the composition <strong>of</strong> copolymers) <strong>and</strong> on physical<br />

architecture (for example, the long-chain branching) <strong>of</strong> macromolecules. To<br />

convert VR values into particular local molar mass (M) values, functional<br />

dependence between size or molar mass <strong>and</strong> composition or architecture <strong>of</strong><br />

macromolecules must be known. This last condition is only rarely fulfilled.<br />

Therefore various interpolation approaches are used in which, for example,<br />

Figure 1 SEC chromatogram <strong>of</strong> a statistical binary copolymer. Each slice contains<br />

macromolecules <strong>of</strong> similar sizes; however, polymer species in each slice have different<br />

molar masses, chemical compositions, <strong>and</strong> sequence lengths.<br />

© 2004 by Marcel Dekker, Inc.

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