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Handbook of Size Exclusion Chromatography and Related ...

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Figure 2 Raw ACOMP signals from multiple detectors during the free radical<br />

polymerization <strong>of</strong> AAm. (From Ref. 33.)<br />

along with the UVabsorbance, when AAm is incorporated into a polymer chain. The<br />

increase in the RI during this period is due to a dn=dc <strong>of</strong> 0.153 for AAm. Neither the<br />

viscometer nor TDSLS respond to the presence <strong>of</strong> the dilute monomer.<br />

At 1700s the persulfate initiator was added, <strong>and</strong> the onset <strong>of</strong> the<br />

polymerization reaction is quickly seen; the decrease in the UV monitors AAm<br />

conversion, <strong>and</strong> the increase in TDSLS <strong>and</strong> viscosity indicate the presence <strong>of</strong> an<br />

increasing amount <strong>of</strong> polymer. The decrease in RI is due merely to the fact that the<br />

ISCO mixing pump used in the low-pressure mixing scheme for this experiment<br />

could not maintain the initial 4% withdrawal rate as the reactor liquid viscosity<br />

increased. This poses no problem for exact determination <strong>of</strong> Mw, conversion, <strong>and</strong><br />

so on, since the RI signal, together with the UV, allow the exact concentration <strong>of</strong><br />

monomer (<strong>and</strong> hence polymer from mass balance) <strong>and</strong> the true withdrawal rate to<br />

be computed. A high-pressure mixing technique developed subsequent to Ref. 33,<br />

using two isocratic pumps, maintains a fixed withdrawal rate, <strong>and</strong> hence avoids<br />

“wasting” a detector signal solving an equation for withdrawal pump rate. This<br />

feature becomes crucial in copolymerization, where the RI signal can be used to<br />

determine the concentration <strong>of</strong> a comonomer.<br />

© 2004 by Marcel Dekker, Inc.

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