Long-chain branched polypropylenes by electron beam irradiation and their rheological properties

The aim of this paper is to investigate how the molecular structure of linear polypropylenes can be modified by electron beam irradiation. For this purpose a linear precursor was irradiated with different doses. The samples were analyzed by size-exclusion chromatography coupled with a light scattering detector. With increasing doses, a reduction of molar mass and an increasing number of long-chain branches were found. Moreover, conclusions with respect to the topography of the long-chain branches obtained were drawn from the molar mass dependence of the zero shear viscosity of the irradiated samples, which deviates significantly from that of linear polypropylenes. The experimental results can be interpreted in a way that at low doses very few but long branches occur. At higher doses more and shorter branches per molecule exist. The elongational experiments clearly exhibit a change of the strain-hardening behavior with irradiation dose, which is in agreement with the structural changes concluded from the shear measurements. The results from rheology and their interpretation demonstrate two features. First, rheological experiments conducted on irradiated polypropylenes are much more sensitive with respect to long-chain branching than the classical characterization methods based on size-exclusion chromatography coupled with light scattering. Second, from a comparison of the rheological behavior of linear and irradiated polypropylenes some conclusions can be drawn regarding the topography of the long-chain branches generated.