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Intervals of shear flow stretch polymer chains and form flow-induced precursors, which accelerate crystallization and transform the crystalline morphology from isotropic spherulites to anisotropic structures. The flow-induced crystallization of two commercial samples of isotactic polypropylene with nearly identical molecular weight distributions, differing in concentrations of catalyst residue particles, was investigated using dynamic rheology and ex situ Synchrotron X-ray scattering. Upon the application of flow, the sample with higher particle concentration crystallized at faster rates relative to the sample with lower levels of heterogenous impurities. The nucleation ability of these particles was particularly pronounced at lower levels of deformation, while flow effects became prominent as larger deformations were applied. For sufficiently strong flows ((γ ) ̇≤145 s-1), a lower critical shear stress (~0.096 MPa) was observed for the formation of shish-kebab structures in the sample with higher concentrations of particles. In this work, we have also identified the formation of shish-kebab structures in the presence of weak flow ((γ ) ̇ ≤ 0.3 s-1) when sheared for long durations of time. For equivalent levels of specific work within both flow regimes, the morphologies of these anisotropic structures were found to be characteristically distinct from one another. The long period and degree of crystallinity were also found to increase with shear stress above the stress level needed for the formation of shish-kebab structures. 

Abstract Shear-induced formation of crystal nuclei in polyamide 11 (PA 11) was studied using a conventional parallel-plate rheometer. Crystallization of PA 11 after shearing the melt at different rates for 60 s was followed by the evolution of the complex viscosity. The sheared samples showed in an optical microscope a gradient structure along the radius, due to the increasing shear rate from the center to the edge. The critical shear rate for shear-induced formation of nuclei was identified at the position where a distinct change of the semicrystalline superstructure is observed, being at around 1 to 2 s −1 . Below this threshold, a space-filled spherulitic superstructure developed as in quiescent-melt crystallization. Above this value, after shearing at rates between 1 and 5 s −1 , an increased number of point-like nuclei was detected, connected with formation of randomly oriented crystals. Shearing the melt at even higher rates led to a further increase of the nuclei number and growth of crystals oriented such that the chain axis is in parallel to the direction of flow. In addition, optical microscopy confirmed formation of long fibrillar structures after shearing at such condition. The critical specific work of flow of PA 11 was calculated to allow a comparison with that of polyamide 66 (PA 66). This comparison showed that in the case of PA 11 more work for shear-induced formation of nuclei is needed than in the case of PA 66, discussed in terms of the chemical structure of the repeat unit in the chains. Graphical abstract 

The engineering thermoplastic poly(ether ether ketone) (PEEK) has a rigid backbone that crystallizes relatively slowly upon cooling the melt. In this study, fast scanning chip calorimetry (FSC) was used to analyze isothermal crystallization between 170 and 285 °C, a range from about 27 K above the glass transition temperature up to the melting temperature. Incorporation of carbon nanotubes (CNT) enhances nucleation at all crystallization temperatures, including low temperatures. FSC also was employed to study crystallization at cooling rates spanning 0.33 to 8000 K/s, important as PEEK is subject to these conditions during melt processing. The critical cooling rate to produce a vitrified sample was increased from 500 K/s in the neat PEEK to 4000 K/s in a 5% CNT/PEEK nanocomposite due to faster nucleation rate caused by heterogeneous nucleation.