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Handbook of Size Exclusion Chromatography and Related ...

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tomeasurethepolystyrenecontent,<strong>and</strong>therefractometerwasusedtomeasurethe<br />

total change in refractive index. The UV signal was then used to correct for<br />

changes in polymer refractive index <strong>and</strong> allow the concentration <strong>of</strong> both<br />

components at each elution volume to be calculated. Figure 9shows the weight<br />

fraction <strong>of</strong> styrene for two samples as afunction <strong>of</strong> elution volume.<br />

Another approach is to use amethod proposed by Goldwasser (18). This is<br />

applicable to copolymers <strong>and</strong> polymer blends <strong>and</strong> allows the number-average<br />

molecular weight to be calculated if the sample injected mass is known without a<br />

concentration detector. Figure 10 shows chromatograms from blends <strong>of</strong> equal<br />

concentrations <strong>of</strong> polystyrene <strong>and</strong> polymethyl methacrylate (20). The measured<br />

Mn is in good agreement with the value calculated from the known molecular<br />

weight <strong>of</strong> the two components. Note that the refractometer response is twice as<br />

sensitive to the polystyrene because <strong>of</strong> the larger dn=dc.<br />

4.1.3 Branching<br />

One <strong>of</strong> the most important applications <strong>of</strong> molecular weight sensitive detectors is<br />

in the characterization <strong>of</strong> branched polymers. A branched molecule in solution has<br />

Figure 9 Weight fraction <strong>of</strong> polystyrene vs. elution volume for two samples <strong>of</strong><br />

polystyrene-b-methyl methacrylate. Sample 1 contains residual polystyrene homopolymer<br />

in the low-molecular-weight region <strong>of</strong> the distribution. (From Ref. 66, with permission from<br />

Springer-Verlag Publishers.)<br />

© 2004 by Marcel Dekker, Inc.

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