The Influence of Ethyl Branch on Formation of Shish-Kebab Crystals in Bimodal Polyethylene under Shear at Low Temperature

Formation of shish-kebab crystals using a bimodal polyethylene system containing high molecular weight(HMW)component with different ethyl branch contents was investigated.In situ small-angle X-ray scattering(SAXS)and wide-angle X-ray diffraction(WAXD)techniques were used to monitor the formation and evolution of shish-kebab structure sheared at low temperature in simple shear mode and low rate.Only the bimodal PE with no branch formed shish-kebab crystals at the shear temperature of 129℃,and the shish length increased with the crystallization time,while bimodal PE with branch has no observable shish under the same conditions.The degree of crystallization for bimodal PE with no branch increased with time up to above 7%,while those with ethyl branch increased continually up to above 23%.Furthermore,bimodal PE's Hermans orientation factor with no branch increased to 0.60,while those with ethyl branch only increased to a value below 0.15.This study indicated that the shish-kebab crystal formed at the low temperature of 129℃is due to the stretch of entangled chains under shear for the bimodal PE with no branch.Only partly oriented lamellar crystals were formed for the bimodal PE with ethyl branch.All the results at the shear temperatures higher,closed to,and lower than the melting point,the modulation of shish crystals formation owing to different mechanisms of the coil-stretch transition and the stretched network by changing shear temperature was achieved in the bimodal PE samples.

Molecular Mobility in the Amorphous Phase Determines the Critical Strain of Fibrillation in the Tensile Stretching of Polyethylene

The microstructural development of bimodal high density polyethylene subjected to tensile deformation was investigated as a function of strain after annealing at different temperatures by means of a scanning synchrotron small angle X-ray scattering(SAXS)technique.Two different deformation mechanisms were activated in sequence upon tensile deformation:intralamellar slipping of crystalline blocks dominates the deformation behavior at small deformations whereas a stress-induced crystalline block fragmentation and recrystallization process occurs at a critical strain yielding new crystallites with the molecular chains preferentially oriented along the drawing direction.The critical strain associated with the lamellar-to-fibrillar transition was found to be ca.0.9 in bimodal sample,which is significantly larger than that observed for unimodal high-density polyethylene(0.4).This observation is primarily due to the fact that the bimodal sample possesses a greater mobility of the amorphous phase and thereby a reduced modulus of the entangled amorphous network.The conclusion of the mobility of the amorphous phase as a determining factor for the critical strain was further proven by the 1H-NMR T2 relaxation time.All these findings contribute to our understanding of the excellent slow crack growth resistance of bimodal polyethylene for pipe application.

Macrocyclic Binuclearα-Diimine Nickel Catalysts for Ethylene Polymerization

Polyolefins are globally important plastics.Molecular weight and molecular weight distribution are two key parameters for determining the properties of polyolefin materials.In this contribution,we develop a strategy for combining the macrocyclic framework and the binuclear effect into the benchmarkα-diimine late transition metal catalysts,and thus macrocyclic binuclearα-diimine nickel catalysts(Ni_(2)-Me and Ni_(2)-iPr)are prepared.Compared to the classical Brookhart's acyclic mononuclearα-diimine nickel analogues(Ni_(1)-Me and Ni_(1)-iPr),these nickel catalysts exhibit enhanced thermostability(up to 110℃)and produce polyethylenes with higher molecular weights(up to 7 times)and lower branching densities(as low as 9 branches/1000C)in methylaluminoxane(MAO)activated ethylene polymerization.This translates into the ability of the catalyst to afford more linear high molecular weight polyethylenes.In particular,bimodal polyethylenes with broad molecular weight distributions(Mw/Mn=8.08-14.66)are generated by the sole catalyst.This work affords diverse polyethylenes.