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.

Crystallization studies of isotactic polypropylene containing nanostructured polyhedral oligomeric silsesquioxane molecules under quiescent and shear conditions

“…This apparatus was recently used to study the behavior of orientation-induced crystallization in neat iPP resins. 17 For each experiment, the sample was cut into a ring (inner diameter ϭ 10 mm, outer diameter ϭ 20 mm, thickness ϳ 500 m) and mounted between the two Kapton™ windows. The temperature-control program for the shear stage was set as follows: (1) samples were first heated at a rate of 10°C/min from room temperature to 190°C (held for 5 min); (2) samples were then cooled at a rate of 30°C/min down to the chosen isothermal crystallization temperature for measurement; and (3) as the temperature reached the target value, a brief impulse of step shear (rate ϭ 50 s Ϫ1 , strain ϭ 1000%) was applied to the supercooled melt.…”

“…We recently gave a detailed explanation of these pattern changes during the shear-induced crystallization of neat iPP. 17 In this study, we investigated the effect of POSS on the shear-induced crystallization behavior in comparison with the neat resin. Sample pp0 showed significantly slower development of the meridional maxima than the POSS-iPP composites.…”

“…Considerable efforts have been devoted to shear-induced crystal structural changes in iPP with wide-angle X-ray diffraction (WAXD). [5][6][7][8][9][10][11][12][13][14][15][16][17] These structural changes usually involve three crystal polymorphs, monoclinic (␣), hexagonal (␤), and orthorhombic (␥), with different unit cell arrangements and a mesophase. For example, Kalay and coworkers 8,11 reported the occurrence of the ␥ phase in injection-molded polypropylene.…”

“…Recently, we have developed a unique in situ SAXS technique with synchrotron radiation that is suitable for monitoring morphological changes during shear-induced crystallization in real time. 17 In this work, the in situ SAXS method was used to investigate the behavior of crystallization under shear in iPP-based nanocomposites.…”

Modeling Flow-Induced Crystallization

“…The surface area of a kebab on which both parent and daughter morphology can grow is denoted by ψ 1,p . Reproduced with permission of the copyright owner [5] For such experiments, a shear cell is combined with in-situ X-ray diffraction (XRD) [4,[48][49][50][51][52][53], birefringence [18,23,43], or light scattering [46], or examined ex-situ with microscopy [4]. The critical flow conditions are usually expressed in terms of stress [4,44], mechanical work [35,45], or shear rate for a fixed strain [43].…”