Comparison of Molecular Structure and Rheological Properties of Electron-Beam- and Gamma-Irradiated Polypropylene

Polypropylene (PP) irradiated with electrons or electromagnetic waves (gamma-rays) undergoes chain scission, and the macroradicals generated form branched molecules as shown by size-exclusion chromatography coupled with laser scattering. Information on the irradiation modification of the molecular structure of PP is still limited in the literature, especially with respect to the controlled generation of long-chain branches (LCB). This paper examines the branching structure of an electron-beam- and gamma-irradiated polypropylene. In general, appropriate irradiation conditions lead to the formation of LCB, which were analyzed in detail by rheological means. At smaller doses, the zero shear-rate viscosities eta(0) of the electron-beam-irradiated PP lie above the values for the unmodified PP with the same M-w but are distinctly lower than the linear reference at higher doses. This result can be interpreted by the change from a starlike to a treelike branching topography. The gamma-irradiation leads to eta(0) values smaller than those of the linear reference, giving rise to the assumption of the generation of treelike molecules. The measurements of the elongational viscosities support this molecular picture. An explanation of these findings is given along the line that the electron irradiation conducted at dose rates higher than the gamma-irradiation creates a larger concentration of short-living radicals, which will partially annihilate each other, but favor the growth of some long branches, which get branched themselves at higher doses. The structure of the gamma-irradiated samples is postulated to consist of a blend of linear and treelike molecules.