Handbook of Size Exclusion Chromatography and Related ...

Table 2 Solution Volume, Polymer Mass, and Average Molecular Weights in Each of
Fractions Collected in Separation of PCL10K by a C8-75B Column in Toluene
Fraction Drops
Volume of
solution (ml)
Mass of
polymer (g)
Mw=10 4
(g=mol)
Mn=10 4
(g=mol) Mw=Mn
1 20 0.231 0.0016 2.39 2.18 1.10
2 20 0.230 0.0053 2.13 1.92 1.11
3 20 0.231 0.0093 1.91 1.71 1.12
4 20 0.235 0.0164 1.71 1.50 1.14
5 20 0.238 0.0239 1.53 1.31 1.17
6 20 0.242 0.0321 1.38 1.16 1.19
7 20 0.244 0.0409 1.29 1.06 1.22
8 20 0.246 0.0443 1.10 0.88 1.25
9 20 0.244 0.0379
10 20 0.242 0.0307
11 40 0.476 0.0474
12 40 0.469 0.0346 0.95 0.72 1.32
13 100 1.147 0.0521
14 100 1.150 0.0189 0.78 0.57 1.36
15 300 3.460 0.0172 0.81 0.54 1.51
16 300 3.460 0.0084 0.76 0.48 1.59
Total 1080 12.545 0.4210
AcloserlookatthechromatogramsinFig.10,inparticularFig.10c,reveals
that the middle fractions have asmaller trailing edge compared with the leading
edge,although theoriginalPCLhasthemtheotherwayaround.Inseparationina
good solvent or in anonadsorbing medium, in contrast, SEC chromatograms of
separated fractions have always agreater trailing edge, as shown in Fig. 4a and
Figs 7a and b. Cutting the tail increases Mn, and thus decreases Mw=Mn.
Curtailing the low-MW components was explained by the following
mechanism (9). As the polymer is introduced to the column, it starts to coat the pore
surface, thus decreasing the pore size. The coating will remove the polymer from the
transported solution. This is why an excess solution needs to be injected before the
first polymer comes out of the column. If the pore is sufficiently small (as in
CPB75B), the coated layer will consist mostly of low-MW components. The
coating will occur beyond the monolayer coverage to further narrow the pore. The
increasing layer thickness will force later-eluting polymer to partition with a
narrower pore size. The adjustable pore size results in a narrowed MW distribution
even for late fractions; in the absence of adsorption, late fractions are almost
indistinguishable from the original polymer injected, because the pore size
© 2004 by Marcel Dekker, Inc.