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Handbook of Size Exclusion Chromatography and Related ...

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many cases, the diluted phase contains only pure solvent (32). Alternatively,solid<br />

particles <strong>of</strong> aprecipitate appear when the quality <strong>of</strong> solvent rapidly deteriorates<br />

<strong>and</strong> also if macromolecules tend to crystallize. Solubility <strong>of</strong> macromolecules in a<br />

solvent is affected by all molecular characteristics <strong>of</strong> polymers. It depends rather<br />

strongly also on temperature <strong>and</strong> pressure. Before SEC was introduced, phase<br />

separation phenomenawere extensively used for polymer fractionation (3). In the<br />

case <strong>of</strong> copolymers, solvent–nonsolvent systems were sought in which effect <strong>of</strong><br />

either molar mass or composition was suppressed (36). In the case <strong>of</strong> crystalline<br />

polymers,suchaspolyolefins,thesolubility-basedphenomenaformabaseforthe<br />

important methods,TREF (37) <strong>and</strong> CRYSTAF(38). Phase separation phenomena<br />

are directly used in high-performance precipitation/redissolution liquid<br />

chromatography <strong>of</strong> macromolecules (3) termed also gradient polymer elution<br />

chromatography (GPEC R )(39). Phase separation processes <strong>of</strong> macromolecules,<br />

precipitation,<strong>and</strong>(re)dissolution<strong>of</strong>polymerspecies,areusuallyslowcomparedto<br />

adsorption <strong>and</strong> enthalpic partition. Slow redissolution processes are rather<br />

frequent with very high molar mass polymers, <strong>and</strong> with species that may undergo<br />

crystallization (40). Thus, the kinetics <strong>of</strong> phase separation may contribute to<br />

chromatographic b<strong>and</strong> broadening <strong>and</strong> splitting. Moreover, if both separated<br />

phases contain macromolecules, b<strong>and</strong> broadening <strong>and</strong> splitting may be very<br />

important.Complexpolymersmayevenformmultiphase systems.Inmanycases,<br />

phase separation is accompanied by adsorption <strong>and</strong>/or partition phenomena. The<br />

resulting hybrid separation mechanism may be difficult to control.<br />

For the above reasons, the phase separation retention mechanism is mainly<br />

used in HPLC <strong>of</strong> nonpolar polymers <strong>and</strong> oligomers <strong>and</strong> under conditions that<br />

prevent sample crystallization.<br />

4 MATERIALS FOR TWO-DIMENSIONAL HPLC OF<br />

MACROMOLECULES<br />

4.1 Column Filling Materials<br />

Some general features <strong>of</strong> HPLC column fillings were briefly outlined in Sec. 1.<br />

4.1.1 Column Packings for <strong>Size</strong> <strong>Exclusion</strong> <strong>Chromatography</strong><br />

Particulate column packings dominate SEC because sizes <strong>of</strong> separation pores in<br />

monoliths are difficult to control. Silica gels exhibit high mechanical stability<br />

<strong>and</strong> advantageous pore structure with fast mass transfer kinetics. Their limited<br />

stability in basic mobile phases is not important in most HPLC applications to<br />

synthetic polymers. However, silica gels are rather active, largely due to the<br />

presence <strong>of</strong> surface silanols, <strong>and</strong> it is difficult to reach 1 0 for polar polymers<br />

with bare silica column fillings. Therefore heterogeneously crosslinked<br />

poly(styrene-co-divinylbenzene) (PS/DVB) resins represent the most common<br />

© 2004 by Marcel Dekker, Inc.

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