Effect of dimethyl formamide in the synthesis of linear low density polyethylene on its rheological properties

Different amounts of dimethyl formamide (DMF) as an external electron donor have been used for the synthesis of LLDPE samples with different branch structure via gas phase polymerization of ethylene/1-butene copolymer with TiCl4/MgCl2 and triethyl aluminum catalyst system. Then, the effect of DMF (and consequently branch structure) on rheological properties of the synthesized samples, such as complex viscosity, MFI, loss modulus, storage modulus, transition point, power law index and Cole-Cole plot, have been studied. According to the obtained results, all of the curves relating the rheological properties of the samples to the ratio of DMF/Ti passed through an extremum point at about DMF/Ti = 0.25, which can be attributed to the changing of branch characteristic of the samples from heterogeneous viscous liquid-like structure to a uniform elastic three dimensional one at this extremum point.