Monitoring the chemical crosslinking of propylene polymers through rheology

The chemical crosslinking of propylene polymers is described; the reaction has been performed under dynamic conditions, using a peroxide and a furan or bis-maleimide-based coagent as crosslinking promoter; the reaction mechanisms have been investigated through FT-IR spectrometry. Crosslinking experiments have been carried out while keeping constant the amount and the nature of peroxide and increasing the coagent concentration; four different coagents have been used. The samples have been studied, in the molten state, by measuring the melt flow rates (MFR) and the values of G′ (storage modulus), G″ (loss modulus), Tg(δ) (ratio G″/G′), and η^∗ (complex viscosity), in the frequency range 10⁻²-10² rad s⁻¹. Another set of measurements has been performed on the same materials, collecting for each sample the values of η⁰ (zero shear viscosity) and J_e⁰ (steady state creep compliance) by creep-creep recovery experiments. The effects of crosslinking degree estimated by gel content and through rheological measurements allows one to evaluate the efficiency of the coagent both in terms of prevention of β-scission and promotion of the crosslinking reactions. These results are discussed with reference to the selection of proper conditions of the crosslinking process in order to achieve a material having desired MFR, rheological behavior and melt elasticity for selected application.