Crystallization and rheological behaviors of metallocene polyethylene

Metallocene catalysts enable precise synthesis of polyolefins including metallocene polyethylene(mPE) with uniform molecular weight distribution, controlled branching, and tailored co-monomer incorporation, yielding polymers with superior properties, thus advancing high-performance applications while optimizing production efficiency and sustainability. A zirconium dichloride complex, three intermediates and methylaluminoxane were chosen as a metallocene catalyst for synthesis of the mPE samples with different molecular weights M_ws. The long-chain branches and fractal architecture of the mPE chains were confirmed by investigations of the rheology and crystallization behaviors. The branch content of the mPE samples was obtained by small angle neutron scattering(SANS) technology and analyzed according to the local scattering laws and mass-fractal power laws. The SANS results showed that all the four mPE samples have the same persistence length l_p and the mole fraction branches Φ_br increased with the weight-average molecular weight (determined by gel permeation chromatography, GPC) M_w,GPC of the samples. From melt linear viscoelastic rheology, the M_ws of the main chain of the mPEs had been determined. It was found that the M_w,GPC of mPE is overestimated at low M_w but underestimated at high M_w with a critical actual M_w value of around 2 × 10⁵ g $·$ mol⁻¹. Thus, by combination of melt rheology and SANS, the more accurate M_ws of the mPEs were estimated to provide details of the long-chain branched (LCB) molecular structures of the mPEs. The results would shed lights on experimental characterization of LCB architectures of polymers for further investigations on relations between their structures and properties.