RELATIONSHIPS BETWEEN SEPARATION POWER AND THE MW AVERAGES IN SEC MEASUREMENTS WITH MOLECULAR MASS SENSITIVE DETECTORS

Abstract

Polystyrenes (standards with different molecular weights and broadly distributed polymers) and 1.4-cis polybutadienes had been characterized by online coupling of SEC columns with both light scattering (MALLS) and viscosity detectors. SEC columns with differences in separation power had been used. Investigations were done on the dependence of the substance specific calibration curves and the exponent α the KMH-equation on the columns; the differences between the substance specific calibration curves for narrowly distributed standards and for polystyrenes with a broader distribution; the mutual influence of the components in polymer mixtures on separation and the effect on resulting molecular weight distribution. The averages of the molecular weight, calculated from the measured distribution, 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 of this is a change in the distribution curve: the molecular weight distribution becomes broader despite the polydispersity decreasing (the Mn value is overestimated). The maximum of each peak is shifted in mixtures of standards 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 separation power of the columns in both the SEC light scattering and the SEC viscosity coupling. It can be neglected only for narrowly distributed polymers (Mw/Mn < 1,2). For higher polydispersties we have to reckon on a clear influence, and for those polymers we get wrong values for the number average molecular weight and for the exponent α in the KMH equation. The problem is that the molecular weight sensitive detectors always return weight averages of the measured values. In these cases, the measurement values depend on the broadness of the distribution in the examined slice. This means that 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 distributions as described; it is also important for polymers with heterogeneity in the molecular architecture (LCB) or in copolymers with chemical composition distributions.