Polystyrenes (standards with different molecular weights and broadly distri
buted polymers) and 1.4-cis polybutadienes had been characterized by online
coupling of SEC columns with both light scattering (MALLS) and viscosity d
etectors. SEC columns with differences in separation power had been used.
Investigations were done on the dependence of the substance specific calibr
ation curves and the exponent a the KMH-equation on the columns; the differ
ences between the substance specific calibration curves for narrowly distri
buted standards and for polystyrenes with a broader distribution; the mutua
l influence of the components in polymer mixtures on separation and the eff
ect on resulting molecular weight distribution.
The averages of the molecular weight, calculated from the measured distribu
tion, depend on the resolution of the SEC columns. The reason for this is a
mutual influence of the components in the polymer mixture. A consequence o
f this is a change in the distribution curve: the molecular weight distribu
tion becomes broader despite the polydispersity decreasing (the M-n value i
s overestimated). The maximum of each peak is shifted in mixtures of standa
rds related to single measurements. This fact does not appear in the use of
high resolution columns.
The slope of the substance specific calibration curve also depends on the s
eparation power of the columns in both the SEC light scattering and the SEC
viscosity coupling. It can be neglected only for narrowly distributed poly
mers (M-w/M-n < 1,2). For higher polydispersties we have to reckon on a cle
ar influence, and for those polymers we get wrong values for the number ave
rage molecular weight and for the exponent a in the KMH equation. The probl
em is that the molecular weight sensitive detectors always return weight av
erages of the measured values. In these cases, the measurement values depen
d on the broadness of the distribution in the examined slice. This means th
at the local dispersion of the slices in the elugram becomes important for
the calculation of the desired value.
This effect is not only essential for determination of molecular weight dis
tributions as described; it is also important for polymers with heterogenei
ty in the molecular architecture (LCB) or in copolymers with chemical compo