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Handbook of Size Exclusion Chromatography and Related ...

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partitioncoefficientsinthebimodalmixture.Onlyathighconcentrationsdoesthe<br />

enhancement <strong>of</strong> KL <strong>and</strong> the suppression <strong>of</strong> KH compared with the monodisperse<br />

counterparts occur. When aconcentrated solution <strong>of</strong> apolydisperse polymer in<br />

theta solvent is partitioned between the pore <strong>and</strong> the surrounding space, the high<br />

osmoticpressurewilldrivelow-MWcomponentsintotheporemorethanitdoesin<br />

asolution<strong>of</strong>amonodispersepolymer<strong>of</strong>thelow-MWcomponents.Thispartisthe<br />

same as in agood solvent, except that the concentration needs to be much higher.<br />

More importantly,though, the flatness <strong>of</strong> the plots <strong>of</strong> the partition coefficients,<br />

especiallyKH,canhelpimprovetheresolution<strong>of</strong>HOPC.ThelowKHoverabroad<br />

range <strong>of</strong> concentrations indicates that the purity <strong>of</strong> low-MW components in the<br />

pore remains high, unchanged from that at low concentrations, until KH starts to<br />

increase at quite ahigh concentration. This means that, in HOPC, only low-MW<br />

components will be able to enter the pores in nearly all steps <strong>of</strong> partitioning in all<br />

theoretical plates in the column during the separation. This property may help<br />

narrow the MW distribution in late fractions. In the good solvent condition, in<br />

contrast, some <strong>of</strong> high-MW components can enter the pores at a lower<br />

concentration, degrading the purity <strong>of</strong> the polymer partitioned to the pore <strong>and</strong><br />

eluting later. Thus, the theta solvent may <strong>of</strong>fer superior separation in HOPC,<br />

especiallyforlatefractions,aslongasstrongadsorptionbytheporesurfaceinthe<br />

unfavorable solvent condition is avoided.<br />

8 COMPARISON OF SEPARATIONS IN AGOOD SOLVENT<br />

AND ATHETASOLVENT<br />

The difference in the separation performances in the two solvent conditions was<br />

demonstrated for poly(1-caprolactone) (PCL), a biodegradable polymer (9).<br />

DioxaneisagoodsolventforPCL.Toluenegivesanear-thetasolventconditionat<br />

308C.PCL–toluenehasanupper-criticalsolutiontemperature<strong>of</strong>around158C(9).<br />

The column used (3:9 300mm) was packed with octyldimethylsilanol (C8)modified<br />

CPG (pore diameter 130A ˚ ,120=200 mesh). In each separation, the<br />

solution <strong>of</strong> PCL10K (Mw ¼1:02 10 4 g=mol, Mn ¼0:61 10 4 g=mol,<br />

Mw=Mn ¼1:66) was injected into the column at 308C until the whole column<br />

was filled with solution. The concentration <strong>of</strong> the solution was 0.228g=mL<br />

(21.9wt% for dioxane; 25.0wt% for toluene). The injection amount was 1.95mL<br />

<strong>and</strong> 2.53mL, respectively.Table 1shows the number <strong>of</strong> drops, the volume <strong>of</strong> the<br />

solution,<strong>and</strong>themass<strong>of</strong>thepolymerineachfractionfortheseparationintoluene.<br />

Asimilar collection schedule was employed in the separation in dioxane.<br />

Figure 7 shows the concentration <strong>of</strong> the eluent as a function <strong>of</strong> the<br />

cumulative volume <strong>of</strong> the eluent since the polymer solution was injected in the two<br />

separations. If one drop were collected in each fraction, then the curve would be<br />

smooth. We call this curve an HOPC retention curve. In dioxane, the eluent<br />

© 2004 by Marcel Dekker, Inc.

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