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Handbook of Size Exclusion Chromatography and Related ...

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SEC with Shodex columns (OH Pak SB803, 804, 805). The overall transition <strong>of</strong><br />

MWdistributioninearlytolatefractionsissimilartotheoneinFig.4a.Thereisa<br />

difference, however. The early fractions (1–3) have agreater leading edge than<br />

trailing edge. Late fractions collected low-MW components, <strong>and</strong> therefore their<br />

MW distribution is narrower than that <strong>of</strong> the original polymer. The difference<br />

between the two separations is ascribed to aweak adsorption <strong>of</strong> poly(vinyl<br />

pyrrolidone) onto the silica surface. Wewill see the effect <strong>of</strong> adsorption more<br />

clearly when we examine HOPC separation in the theta condition.<br />

Aneed to inject aconcentrated solution was demonstrated, substantiating<br />

theseparationmechanism(1).Itwasalsoshownthatuse<strong>of</strong>porouspackingwitha<br />

narrow pore size distribution is essential (3). Performance <strong>of</strong> separation was<br />

compared for columns packed with CPG <strong>and</strong> silica gels that have similar mean<br />

pore diameters. CPG is known to have anarrower pore size distribution. The<br />

resolution was far better when separated by CPG.<br />

Good separation applies to afewearly fractions only.The mass <strong>of</strong> polymer<br />

with anarrowed MW distribution is at best 20% <strong>of</strong> the mass <strong>of</strong> polymer injected.<br />

Often, the number is less than afew percent. Nevertheless, those early fractions<br />

can provide asufficient amount <strong>of</strong> polymer for further purification by HOPC (5)<br />

<strong>and</strong> further spectroscopic analysis (8) <strong>and</strong> thermal analysis. In fact, HOPC was<br />

applied repeatedly to early fractions to prepare st<strong>and</strong>ard-grade polymer samples<br />

(5). Using the preparative capability <strong>of</strong> HOPC, it was verified that multimeric<br />

impurity components in presumably monomethoxy-, monohydroxy-terminated<br />

poly(ethylene glycol) are diol-terminated (8).<br />

7 SEPARATION PRINCIPLE: HOPC IN ATHETASOLVENT<br />

In asolution <strong>of</strong> polymer in agood solvent, the second virial coefficient A2 is<br />

positive. Positive A2 makes the osmotic pressure deviate upward from that <strong>of</strong> an<br />

idealsolution,asillustratedinFig.5.AsolutioninthethetaconditionhasA2 ¼ 0.<br />

Then, the osmotic pressure remains that <strong>of</strong> the ideal solution until a contribution<br />

by the third virial coefficient becomes sufficiently large, which occurs at a<br />

concentration much higher than the overlap concentration c*. In most polymer<br />

solutions, lowering the temperature decreases A2 to zero (upper critical solution<br />

temperature), although it may not be possible in an accessible temperature range.<br />

Exceptions are solutions in which polymer is solubilized by hydrogen bonding.<br />

Examples include poly(ethylene glycol) in water <strong>and</strong> poly(isopropyl acrylamide)<br />

in water. In these solutions, raising the temperature causes the polymer to<br />

precipitate (lower critical solution temperature). For details on the theta solvent<br />

condition, see Ref. 12.<br />

In the theta solvent, the absent second virial coefficient drastically alters the<br />

partitioning at high concentrations. Again, lattice Monte Carlo simulation was used<br />

© 2004 by Marcel Dekker, Inc.

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