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Handbook of Size Exclusion Chromatography and Related ...

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in Fig. 4a. The middle fractions (8–12) are indistinguishable from the original<br />

PCL10K. Later fractions have alower MW,but fractions 15 <strong>and</strong> 16 return to a<br />

distribution not much different from that <strong>of</strong> the original PCL (recoiling). In the<br />

theta solvent, early fractions have quite ahigh MW.Middle fractions maintain a<br />

narrower distribution than that <strong>of</strong> the original PCL10K. Late fractions have<br />

enriched low-MW components. Recoiling was absent.<br />

The advantage <strong>of</strong> HOPC in the theta solvent is obvious. The resolution is<br />

better from early to late fractions, in agreement with the result <strong>of</strong> the computer<br />

simulation study.Furthermore, the theta solvent had ahigher loading capacity.<br />

Nevertheless, the amount <strong>of</strong> fractions with a narrow MW distribution, say<br />

Mw=Mn ,1:2, is less than 1% <strong>of</strong> the polymer injected, which is still greater than<br />

thecounterpartinthedioxaneseparation.Theamountwasdrasticallyincreasedby<br />

separating with aweakly adsorbing medium as shown below.<br />

9 HOPC IN ATHETASOLVENT WITH WEAKLY<br />

ADSORBING MEDIA<br />

Using porous media that weakly adsorb the polymer may improve the separation<br />

performance in HOPC. In the past, SEC in the theta condition was attempted,but<br />

adsorption impaired the separation (20). All the polymer injected failed to come<br />

out. In HOPC, in contrast, adsorption <strong>of</strong> some <strong>of</strong> the polymer injected does not<br />

preventmost<strong>of</strong>thepolymer from elutingfrom thecolumn,separatedbythe pore,<br />

unless the adsorption is too strong.<br />

The theta solvent <strong>of</strong>fers an excellent environment for fine-tuning the degree<br />

<strong>of</strong> adsorption. In the theta solvent, polymer chains are on theverge <strong>of</strong> associating<br />

each other. Aslight decrease in A2 will lead to precipitation or phase separation.<br />

Therefore, in the solution placed near asurface, weak attractive interactions<br />

between the polymer <strong>and</strong> the surface will be sufficient to adsorb the polymer.<br />

Wecompare the separation performance for the same PCL10K. When the<br />

pore surface weakly adsorbs the polymer, the performance <strong>of</strong> HOPC is better.<br />

TMS-120B (CPG120B modified with TMS), TMS-75B [CPG75B (mean pore<br />

diameter 81A ˚ )modified with TMS], <strong>and</strong> C8-75B (CPG75B modified with C8)<br />

weaklyadsorbPCL10Kintoluene.None<strong>of</strong>thesemediaadsorbsthesamepolymer<br />

in dioxane. HOPC was conducted using acolumn packed with one <strong>of</strong> the three<br />

media at 308C. A25wt% solution <strong>of</strong> PCL10K in toluene was injected into a<br />

toluene-filled column. Injection volumes were 3.45, 3.41, <strong>and</strong> 2.16mL,<br />

respectively, much greater than the injection volumes in separation with the<br />

nonadsorbing medium (C8-120B). Figure 9compares HOPC retention curves for<br />

the three separations. When the surface was TMS, the polymer did not elute until<br />

nearly twice as much volume as the typical injection volume in a good solvent was<br />

loaded into the column. The tailing <strong>of</strong> the retention curve is obvious. In the smaller<br />

© 2004 by Marcel Dekker, Inc.

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