Structure Development during Shear Flow-Induced Crystallization of i-PP: In-Situ Small-Angle X-ray Scattering Study

Abstract

In-situ synchrotron small-angle X-ray scattering (SAXS) was used to follow orientation-induced crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140 °C after step shear under isothermal conditions. The melt was subjected to a shear strain of 1428% at three different shear rates (10, 57, and 102 s -1 ) using a modified Linkam shear stage. The SAXS patterns showed strong meridional reflections due to the rapid development of oriented polymer crystallites within the melt. On the basis of the SAXS data, a schematic representation of nucleation and growth in orientation-induced crystallization of i-PP is proposed. During flow, orientation causes alignment of chain segments of polymer molecules and results in the formation of primary nuclei in the flow direction. These nuclei facilitate the growth of oriented crystal lamellae that align perpendicular to the flow direction. The half-time of crystallization was calculated from the time evolution profiles of the total scattered intensity. The crystallization kinetics was found to increase by 2 orders of magnitude as compared to quiescent crystallization. A method was used to deconvolute the total integrated scattered intensity into contributions arising from the isotropic and anisotropic components of the crystallized chains. The fraction of oriented crystallites was determined from the ratio of the scattered intensity due to the oriented (anisotropic) component to the total scattered intensity. At low shear rates (∼10 s -1 ) the oriented fraction in the polymer bulk was lower than at high shear rates (57 and 102 s -1 ). It was shown that only the polymer molecules above a “critical orientation molecular weight” ( M *) could become oriented at a given shear rate (γ̇). The M * values at different shear rates were determined from the area fractions of the molecular weight distribution of the polymer. The observed dependence of M * on shear rate was fit to the relationship M * ∝ γ̇ -α, with α being an exponent. Analysis of results suggests that the value of M * is sensitive at low shear rates (below 60 s -1 ) but not at high shear rates. Experimental results are shown to be in agreement with theoretical predictions having the α value of 0.15.