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Handbook of Size Exclusion Chromatography and Related ...

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The idea <strong>of</strong> making time-resolved measurements per se, for example, with<br />

time-dependent static light scattering (TDSLS), has been explored by a number <strong>of</strong><br />

groups. Aggregation (1,2), gelation (3), degradation (4–6), dissolution <strong>of</strong> dry<br />

polymers (7), <strong>and</strong> phase separation have been followed with TDSLS.<br />

The notion <strong>of</strong> following polymerization reaction kinetics has been around at<br />

least since Flory’s original, manual measurements (8), <strong>and</strong> is now an active field<br />

including the use <strong>of</strong> near infrared (9–13), rheology (14–16), electron spin resonance<br />

(17,18), ultraviolet absorbance (19–21), <strong>and</strong> pulsed laser techniques (22,23).<br />

Automatic continuous online monitoring <strong>of</strong> polymerization reactions<br />

(ACOMP) was first introduced by the present author <strong>and</strong> his colleagues in 1998<br />

(24). ACOMP allows simultaneous monitoring <strong>of</strong> weight-average molecular mass<br />

Mw, certain measures <strong>of</strong> polydispersity (25), monomer conversion, <strong>and</strong> intrinsic<br />

viscosity, <strong>and</strong> will be discussed in detail below.<br />

1.1 Automatic Continuous Mixing (ACM)<br />

Automatic continuous mixing (ACM) differs from SEC <strong>and</strong> flow injection analysis<br />

in that it provides a continuous, diluted stream <strong>of</strong> sample to the detectors. Hence,<br />

there are no detector signal peaks, but rather a continuous record <strong>of</strong> the sample<br />

behavior. There is, however, a lag-time between the extraction/dilution <strong>and</strong><br />

the detection. This is strictly dependent on flow rates <strong>and</strong> “plumbing,” <strong>and</strong> can<br />

range from tens to hundreds <strong>of</strong> seconds. Likewise, there is a finite response time for<br />

ACM. It is normally the integrated product <strong>of</strong> a Gaussian spreading <strong>of</strong> an extracted/<br />

mixed volume on its way to the detector, <strong>and</strong> an exponential “mixing chamber”<br />

response time, related to any mixing chambers associated with ACM.<br />

Several configurations have been used for ACM. For ACM in equilibrium<br />

characterization, the simplest is the use <strong>of</strong> a simple syringe pump, or even a gravity<br />

feed, to dilute continuously a sample reservoir. The sample issuing from the<br />

detector lines can optionally recirculate to the sample, usually to conserve sample,<br />

or flow to waste. More refined methods can use a commercial tertiary or quaternary<br />

mixing pump, such as from ISCO (Lincoln, Nebraska, U.S.A.) or Waters (Milford,<br />

Massachusetts, U.S.A.). These were designed to provide gradients <strong>of</strong> different<br />

solvents for use in HPLC, but can also provide mixing <strong>of</strong> any desired, lowviscosity<br />

solutions for ACM (e.g., dilute polymer solutions, electrolytes,<br />

surfactants, colloids, <strong>and</strong> so on). Some mixing pumps allow the time pr<strong>of</strong>ile <strong>of</strong><br />

the gradient to be chosen, which can be very useful when samples respond in a<br />

logarithmic fashion to a component, for example, the reaction <strong>of</strong> polyelectrolyte<br />

conformations, interactions, <strong>and</strong> hydrodynamics to ionic strength.<br />

For situations where polymer reactions are to be monitored, the use <strong>of</strong> a<br />

programmable mixing pump is viable, as long as the solution viscosity in the<br />

reactor is not too high, that is, where solution viscosities do not exceed a few<br />

hundred centipoise (cP). Degradation <strong>and</strong> aggregation reactions are <strong>of</strong>ten carried<br />

© 2004 by Marcel Dekker, Inc.

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