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.

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In situ synchrotron wide-angle X-ray diffraction (WAXD) was used to monitor crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140 °C after step shear. The melt was subjected to a shear strain of 1430% at three different shear rates (10, 57, and 102 s -1 ) using a parallel-plate shear apparatus. WAXD results were used to determine the type (R-and β-crystals), orientation, and corresponding mass fractions of i-PP crystals. It was found that formation of oriented R-crystals occurred immediately after application of the shear field. Subsequently, growth of primarily unoriented β-crystals was observed. WAXD patterns clearly showed that β-crystals grew only after the formation of oriented R-crystals in the sheared i-PP melt. The contribution of β-crystals to the total crystalline phase was as high as 65-70% at high shear rates (57 and 102 s -1 ) and low (20%) at low shear rates (10 s -1 ), which was attributed to the different amount of surface area of oriented R-crystal cylindrites generated at different shear rates. The growth of β-crystals which is related to the surface area of the oriented R-form crystalline assembly has been proposed earlier. Also, the unoriented nature and fast growth of the β-crystals determined from WAXD experiments provide an explanation for the 2 orders of magnitude increase in the kinetics of crystallization of the unoriented structures, which was previously observed (but not explained) in our crystallization study by small-angle X-ray scattering (SAXS).

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Scanning electron micrographs of a solvent-extracted sheared polyethylene (PE) blend revealed, for the first time, an unexpected shish-kebab structure with multiple shish. The blend contained 2 wt % of crystallizing ultrahigh molecular weight polyethylene (UHMWPE) and 98 wt % of noncrystallizing PE matrix. The formation of multiple shish was attributed to the coil-stretch transition occurring in sections of UHMWPE chains. Synchrotron x-ray data provided clear evidence of the hypothesis that multiple shish originate from stretched chain sections and kebabs originate from coiled chain sections, following a diffusion-controlled crystallization process.

In-situ rheo-SAXS (small-angle X-ray scattering) and rheo-WAXD (wide-angle X-ray diffraction) studies were carried out to investigate the nature of shear-induced precursor structures in isotactic polypropylene (iPP) melt at 165 °C, near its nominal melting point. Immediately upon the cessation of shear, SAXS patterns clearly showed an evolution of oriented structures in hundreds of angstroms, while the corresponding WAXD patterns did not exhibit any crystal reflections. SAXS patterns at later times showed that the shish-kebab morphology was developed, and the kebabs possessed only a small amount of crystallinity (3%). The combined SAXS and WAXD results indicate that, at the early stages of crystallization, a scaffold (network) of oriented structures is formed. These structures contain (1) primary nuclei (through homogeneous nucleation) that may be crystalline or mesomorphic but having linear connectivity along the flow direction, which form the shish entity, and (2) shish-induced layered crystalline lamellae (kebabs) oriented perpendicularly to the flow direction that have poor lateral connectivity. Subsequent polymer crystallization takes place in the framework of the scaffold, which is probably dominated by the lower molecular weight species. Amounts of the crystalline primary nuclei and the layered crystalline lamellae in the precursor structures were estimated. The results verified, quantitatively for the first time, the well-known concept that minor amounts of linear nuclei induce multiple secondary nucleation sites for the growth of a large quantity of lamellae that grow radially outward from the central core. A mechanistic pathway for the early stages of crystallization in polymer melts under flow is proposed.

Development of shear-induced crystallization precursor structure was studied by in-situ rheo-SAXS (small-angle X-ray scattering) and rheo-WAXD (wide-angle X-ray diffraction) techniques using binary polymer blends of high and low molecular weight polyethylenes near their nominal melting temperatures (120 °C). Two low molecular weight polyethylene copolymers, containing 2 mol % hexene, with weight-average molecular weights (M w) of 50 000 (MB-50K) and 100 000 (MB-100K), and polydispersity of about 2, were used as the noncrystallizing matrices. A high molecular weight polyethylene homopolymer with M w of 250 000 (MB-250K) and polydispersity of about 2 was used as the crystallizing minor component. Two series of model blends, MB-50K/MB-250K and MB-100K/MB-250K, each containing weight ratios of 100/0, 97/3, 95/5, and 90/10, were prepared by solution blending to ensure thorough mixing at the molecular level. At the chosen shear conditions (rate = 60 s-1, duration = 5 s, T = 120 °C), while no flow-induced structures were seen in pure MB-50K and MB-100K melts, the blends in both series showed distinct but different shear-induced structures. Results indicate that the high molecular weight component dominates the formation of crystallization precursor structures in the blend under shear, which can act as a template for further crystallization. A “shish-kebab” structure, detected by both SAXS and WAXD, was observed in the MB-100K/MB-250K (90/10) blend, while only a twisted lamellar structure (kebab) was seen in the rest of the blends under the same shear conditions. These findings suggest that the matrix viscosity plays an important role to influence the formation of crystallization precursor structure of the high molecular component under flow. In the MB-100K/MB-250K (90/10) blend, the length of the shish was estimated from the equatorial streak in SAXS, which showed a noticeable decrease with time, while the corresponding scattering intensity was found to increase. The evolution of the shish-kebab structure from SAXS is consistent with the appearance of the (110) peak in WAXD, which can be explained by the coil−stretch transition induced by flow.

Studies of dilute polymer solutions in shear flow suggest that the mean fractional extension of molecules increases gradually with the Weissenberg number (Wi = shear rate × longest relaxation time) and approaches an asymptotic value of 0.4−0.5, while in elongational flow it approaches full contour length above a certain critical strain rate. In an entangled polymer melt, this behavior is more complex due to inter- and intramolecular interactions. In situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) experiments were performed to investigate the effects of shear rate, shear duration, and Wi on the extent of molecular orientation/extension and crystal orientation in an isotactic polypropylene (iPP) melt. Two series of experiments were designed:  (1) variation of shear rate (30, 45, and 60 s-1) at a constant shear duration (5 s) and (2) variation of shear duration (1.3, 3, and 5 s) at a constant rate (60 s-1). The degree of crystal orientation (Herman's orientation function, f) observed at 165 °C and fraction of oriented crystals (X o) observed in a fully crystallized sample at room temperature increased with both shear rate and shear duration. Interestingly, at a constant strain (rate × duration), short-duration shear at a high rate was found to be more effective (i.e., higher f and X o) than long-duration shear at a low rate. The longest relaxation time for the iPP sample and Wi were estimated from the dynamic moduli data. Both f and X o were found to gradually increase with Wi and approached plateau values at high values of Wi. Results indicated that, even under a very intense shear field (or high Wi values), molecules do not extend to full contour length, and there is a limiting value for mean orientation/extension and subsequent crystal orientation in a polymer matrix. Characteristic dimensions of the shish-kebab entity formed in a sheared iPP melt at 165 °C were determined from the rheo-SAXS data. It was found that the average shish length was 700−750 nm and the average spacing between adjacent kebabs was 60−70 nm.