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.

Extracting the phase distribution of the electron wave packet ionized by an elliptically polarized laser pulse

We use an interferometic scheme to extract the phase distribution of the electron wave packet from above-threshold ionization in elliptically polarized laser fields.In this scheme,an electron wave packet released from a circularly polarized laser pulse acts as a reference wave and interferes with the electron wave packet ionized by a time-delayed counter-rotating elliptically polarized laser field.The generated vortex-shaped interference pattern in the photoelectron momentum distribution enables us to extract the phase distribution of the electron wave packet in the elliptically polarized laser pulse with high precision.By artificially screening the ionic potential at different ranges when solving the time-dependent Schördinger equation,we find that the angle-dependent phase distribution of the electron wave packet in the elliptically polarized laser field shows an obvious angular shift as compared to the strong-field approximation,whose value is the same as the attoclock shift.We also show that the amplitude of the angle-dependent phase distribution is sensitive to the ellipticity of the laser pulse,providing an alternative way to precisely calibrate the laser ellipticity in the attoclock measurement.