Handbook of Size Exclusion Chromatography and Related ...

the total area into up to 12 sections. Correlations were then attempted using some
or all of the sections as parameters. A good correlation with temperature
susceptibility was obtained using three of twelve sections chosen statistically.
Kim et al. (133) used slices of SEC chromatographs to predict the properties
of dry and water-soaked compacted asphalt, aggregate mixes, and road cores.
Chromatographs of neat and solvent-extracted material were divided into ten slices
and correlated to tensile strength and the resilient modulus of the mixes. Using all
ten slices some very good correlations were obtained and fair correlations were
obtained using three slices in the LMS region for the dry mixes. Viscosity and
penetration of 27 asphalts before and after aging were correlated with ten slices
and reduced sets chosen statistically (62).
Similar studies (32,65) have been reported that include modified asphalts.
Correlations were attempted with a variety of properties including Superpave
performance specifications (134). Up to 12 slices were included in the
correlations, which improved steadily with the number of slices. Correlation
was much better with equal time slices than with equal area slices, but only a few
were good. The inclusion of modified asphalts, which have a large effect on the
LMS region, doubtless affected the results.
3.4 Determination of Asphalt Molecular Weight Distribution
Because SEC responds directly to apparent molecular size, it appears to be a
simple method for obtaining the molecular weight distribution of asphalt.
However, it turns out not to be a straightforward determination for a number of
reasons. The first, already discussed, is that some asphaltic fractions associate in
solution. These same fractions also may tend to be adsorbed in the column. A final
factor is the chemical complexity of asphalt. It is well known that the order of
elution of polar and nonpolar compounds can be considerably altered by changing
solvents, so it is difficult to choose calibrating compounds for such a complex
mixture.
The common calibration procedure for asphalt depends on preparative SEC
fractionation. Fractions thus obtained are then subjected to analytical SEC analysis
to obtain mean elution values, and the fraction molecular weights are determined
by an independent method, such as VPO. In general, a single plot of molecular
weight vs. elution volume holds rather well for most asphalts (12), but upon aging
asphalts by air blowing, a series of such curves is produced for different degrees of
hardening (26).
Molecular weight–elution volume curves are actually very sensitive to
composition. Champagne et al. (73) plotted molecular weight vs. retention time
for a series of pure compounds along with polystyrenes, obtaining separate and
distinct curves for the polystyrenes, long-chain asphaltenes, and nonfused
polyaromatics. For fused polyaromatics scatter was obtained.
© 2004 by Marcel Dekker, Inc.