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Handbook of Size Exclusion Chromatography and Related ...

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Table 3 Comparison <strong>of</strong> Mn Results by Various Methods<br />

Sample<br />

By<br />

titration a<br />

By NMR<br />

(500MHz)<br />

By GPC b1<br />

(PS)<br />

that a, the exponential constant in the Mark–Houwink–Sakurada equation <strong>of</strong><br />

two polymers, is the same. Fortunately,the Mn is normally more important than<br />

Mw forlowMWmaterial.Aslightdeviation<strong>of</strong>thecalibrationcurveonthehigher<br />

MWsidecannormallybetolerated.TheMWs <strong>of</strong>polyolsamples calculated using<br />

this calibration curve agree well with other primary methods, such as NMR <strong>and</strong><br />

titration (Table 3).<br />

It is important to note, during the above procedure, that the peak maximum<br />

MW (Mp) <strong>of</strong> MALDI/MS is different from that <strong>of</strong> SEC for two reasons: (1) the<br />

peak height in MALDI/MS is approximately proportional to the number <strong>of</strong><br />

polymermolecules,instead<strong>of</strong>theconcentration,weightbyvolume,asinSEC;(2)<br />

thex-axisinMALDI/MSislinearinMW,instead<strong>of</strong>roughlylog(MW)asinSEC.<br />

The Mp needs to be converted before creating the calibration curve. The dead<br />

volume between the RI detection cell <strong>and</strong> the outlet for collection, which is<br />

normally significantly large, should be adjusted for acorrect calibration.<br />

It is not uncommon that the MW distribution <strong>of</strong> alow MW material covers<br />

the solvent mismatch peaks. As mentioned earlier, the concentration effect is<br />

normallynotsignificantforlowMWsamples.Thesolventmismatchproblemcan<br />

be reduced by increasing sample load, which means higher sample concentration<br />

<strong>and</strong> larger sample volume.<br />

5 SPECIAL SUBJECT:ANALYSIS OF POLYMER ADDITIVES<br />

USING SEC<br />

By GPC b2<br />

(Polyol)<br />

Polyol-1A 974 966 1808 1079<br />

Polyol-1B 1032 1088 1933 1121<br />

Polyol-2 2370 2590 3283 2311<br />

Polyol-4 3993 4520 4599 3573<br />

a Determined using ASTM St<strong>and</strong>ard Test Method D1957-86.<br />

b GPCconditions:seeFig.11.Mn results in column b1 were calculated using a PS calibration. Those in<br />

column b2 were calculated using the polyol absolute MW calibration.<br />

Owing to improved resolution in low MW SEC, many polymer additives, if they<br />

are soluble in acommon solvent with the polymer, can be directly analysed using<br />

SEC without tedious extraction steps. Acrawax C(N,N 0 -ethylene-bis-stearamide)<br />

as an additive in polyurethane is agood example, because <strong>of</strong> the difficulty in<br />

finding agood solvent for HPLC analysis. Both Acrawax C<strong>and</strong> polyurethane are<br />

© 2004 by Marcel Dekker, Inc.

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