Handbook of Size Exclusion Chromatography and Related ...

SEC. One of the reasons is that they are generally easily soluble in SEC solvents
and need no specific SEC experimental condition, such as high temperature.
In some cases, however, it is difficult to perform SEC analysis, especially
when attempting SEC of new rubbers. An example is polyorganophosphazene
rubber (30,31). For SEC, the choice of SEC conditions should be made first. The
SEC/low-angle laser light-scattering (LALLS) or SEC/LALLS/viscosity
detector coupling systems give effective results. By these techniques, the dilute
solution properties of the rubber polymer, which are closely related to their
behavior in SEC, are understood simultaneously.Cooperative data from SEC and
dilute solution properties give information on molecular branching, molecular
weight distribution, and compositional heterogeneity so that more precise
molecular characterization can be obtained.
3.1 Solvents of Rubber for SEC
SEC is aseparation technique based on differences in molecular sizes in solution.
The most essential condition in the SEC of rubbers is that they be dissolved
completelyinSECsolvents.SolventsusedfortheSECofrubbersaresummarized
in Table 2. The most common solvent is tetrahydrofuran (THF).
So-called organic solvent-resistant rubbers and heat-resistant rubbers exist.
Also, there are rubbers that contain microcrystalline parts or molecular
associations even in their solution state. NBR, CR, Z, Q, FKM, and EPDM (see
Appendix) are examples. An example of SEC analysis of these rubbers is seen in
phosphazene rubbers (30,31). In the SEC of such rubbers, difficulties arise in
finding suitable SEC solvents. In principle, such methods as increasing the
temperature to enhance the solubility are needed for these rubbers. In EPDM or
EPM (copolymers of ethylene and propylene), for instance, normal room
temperatureSECwasonceused,buttodayuseofahigh-temperatureSECismost
commonlyusedbecausetheymaycontainsomecrystallinepartsdependingonthe
block of C2 or C3 segments. Choice of other solvents depends on the required
sensitivity of the detectors.
Care should be taken when handling rubber solutions because rubbers have
considerable amounts of unsaturated double bonds and are prone to oxidation by
the peroxide in THF or even by the oxygen in air. The addition of suitable antioxidants
is very common to reduce the incidence of such oxidative degradation.
Also, the solution should not be exposed to light or high storage temperature.
Common antioxidants used in SEC for rubbers are shown in Table 3.
3.2 Presence of Gel
Both natural and synthetic rubbers normally have a gel component, which is a part
that remains undissolved in a solvent (61,62). The gel component is probably
produced by chain branching during the polymerization process or by slight
© 2004 by Marcel Dekker, Inc.