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Handbook of Size Exclusion Chromatography and Related ...

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detectors depend on the oligomer molar mass <strong>and</strong> chemical composition (end<br />

group effect) <strong>and</strong> the data must be corrected (4). An evaporative light-scattering<br />

detectormayproduceerroneousresultsduetoevaporation<strong>of</strong>verylowmolarmass<br />

sample constituents (72). Mass spectrometry <strong>of</strong> fractions obtained by coupled or<br />

2D-HPLC separations <strong>of</strong> oligomers produces, as a rule, a very valuable,<br />

unequivocal set <strong>of</strong> information.<br />

12.2 Polymer Mixtures<br />

As mentioned, all synthetic polymers are multicomponent in their nature <strong>and</strong><br />

contain macromolecules exhibiting different sizes (molar masses), <strong>and</strong> possibly<br />

also different architectures, <strong>and</strong> compositions. Thus, in a general sense, all<br />

synthetic polymers represent mixtures <strong>of</strong> different macromolecules. By<br />

convention, however, the terms polymer mixtures <strong>and</strong> polymer blends denote<br />

only those multicomponent macromolecular systems in which the distributions in<br />

molar mass, architecture, <strong>and</strong> chemical composition exhibit pronounced<br />

discontinuities. In many papers, the term “polymer mixtures” is used as ageneral<br />

one while the term “polymer blends” is reserved for multicomponent systems in<br />

which two <strong>and</strong> more macromolecular substances are combined intentionally.<br />

Polymer blends are frequently encountered in both technology <strong>and</strong> everyday life.<br />

Amacromolecular additivemay improvevarious processing <strong>and</strong> utility properties<br />

<strong>of</strong> the major polymer component <strong>and</strong>/or decrease the price <strong>of</strong> the resulting<br />

material. Polymer mixtures may,however, come into existencewhere they are not<br />

wanted. For example, parent homopolymers are <strong>of</strong>ten formed in the course <strong>of</strong><br />

copolymer syntheses <strong>and</strong> also many chemical transformations <strong>of</strong> polymers<br />

includingoxidizationleadtopolymermixtures.Arather particulargrouppolymer<br />

mixtures represent some technological scraps <strong>and</strong> (municipal) waste.<br />

Analysis <strong>of</strong> polymer mixtures <strong>and</strong> molecular characterization <strong>of</strong> their<br />

constituents belong to the most important applications <strong>of</strong> coupled <strong>and</strong> twodimensional<br />

(or quasi two-dimensional) high performance chromatographic<br />

methods. This is also due to the fact that conventional SEC <strong>of</strong>ten cannot produce<br />

evensemiquantitativedataonpolymer mixturesbecauseparticularconstituents<strong>of</strong><br />

similar sizes cannot be discriminated by an exclusion retention mechanism.<br />

Binary polymer mixtures can be separated applying “critical” or “barrier”<br />

HPLC procedures (Secs 5.1 <strong>and</strong> 5.2). One component is eluted according to an<br />

exclusion mechanism while another remains unseparated <strong>and</strong> elutes near VM. The<br />

latter component can be further characterized, for example, by the second<br />

dimension SEC column.<br />

Various multicomponent polymer mixtures can be discriminated using full<br />

retention–elution procedures (Sec. 7) <strong>and</strong> their constituents may be successively<br />

characterized by SEC or, if needed, by appropriate coupled or two-dimensional<br />

HPLC methods. FRE methods allow reconcentration <strong>of</strong> diluted polymer solutions<br />

© 2004 by Marcel Dekker, Inc.

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