Synchrotron X-ray scattering studies of the nature of shear-induced shish-kebab structure in polyethylene melt

Abstract

Synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) have been utilized to elucidate the nature of shear-induced shish-kebab structures at the early stages of crystallization in a binary blend, containing 5 wt% of high-density polyethylene (HDPE) and 95 wt% of linear low-density polyethylene (LLDPE. The HDPE component possessed a bimodal molecular weight distribution with weight-average molecular weights Mw of 99 kg/mol and 1,100 kg/mole, respectively. X-ray results indicated that the crystallization kinetics and molecular orientation of the blend were significantly enhanced under shear as compared to LLDPE, even though the blend contained only a low concentration of high molecular weight HDPE chains (ca. 1 wt% of = 1,100 kg/mol above the overlap concentration, c*=0.5 wt%). The Avrami exponent of the blend, n=1.9, derived from the WAXD crystallinity evolution under shear, suggested that the two-dimensional (2D) kebabs are developed under diffusion-controlled and spontaneous nucleation conditions. The Avrami exponents were found to decrease with the increase in shear duration time (strain). The corresponding SAXS patterns showed a meridional streak, which can be modeled by a shish-kebab structure containing cylindrical symmetry along the shish axis and polydispersities in diameter, thickness and long period of kebabs. The shish-kebab model was formulated in a closed analytical form under the assumption of independent statistics. Furthermore, Ruland’s method to separate the effects of size and orientation, usually used for the equatorial streaks, was applied to the meridional streak for the first time, under the assumption that the widths of the distributions add as in Lorentzian-type distributions. The fitted mean diameter of kebabs using our shish-kebab model was found to be consistent with the value obtained by using the Ruland’s streak method. The radial growth rate, G, calculated from the kebab evolution initially followed the relationship G ∝ t-1/2, which confirmed the diffusion-controlled growth with spontaneous nucleation in the polymer blend.