Chain architecture of LDPE as a function of molar mass using size exclusion chromatography and multi-angle laser light scattering (SEC-MALLS)

Abstract

Long chain branching causes a decrease in molecular size and hence in the radius of gyration and the hydrodynamic radius, as compared to polymers having a linear structure and having the same molar mass. In principle there is a simple relation between the ratio of the radii of gyration and the hydrodynamic radii: g′ = g b . At constant temperature and one solvent/polymer combination, the b-value is mainly dependent on the chain architecture. In the classical approach, determination of b as a function of molar mass requires time consuming fractionation with subsequent characterization of the fractions to determine the hydrodynamic radius and the radius of gyration. Fortunately, by application of size exclusion chromatography (SEC) in combination with only multi-angle laser light scattering (MALLS) and using the universal calibration principle, it appeared possible to determine the g and g′ as a function of the molar mass. Utilization of this method revealed that low density polyethylene (LDPE) exhibites a continuous decrease with increasing molar mass of the b-value from 1.8 to 1.2 for a tubular product, and from 1.5 to 1 for an autoclave product. This was also expected from the polymerization conditions.