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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. 

Polyamide 66 (PA66) and polyamide 6 (PA6) share many comparable properties due to their similar chemical structures. However, their crystallization kinetics and morphological differences are not as well understood as other properties. This work establishes the crystallization kinetics and morphology of additive-free PA66 and PA6 at high undercooling conditions using a modified fast scanning calorimetry technique. Two polyamides show similar kinetics profile and morphology, but the transitions associated with polymorphs occur at different temperatures. Regarding kinetics, PA66 always crystallizes faster than PA6 regardless of the polymorphs formed, supported by the temperature-dependent Avrami kinetics coefficients k. Both PA66 and PA6 show a bimodal kinetics profile with a local crystallization rate minimum at 135 and 110 °C, respectively. Apart from the crystallization rate, a sudden broadening of the exothermic crystallization peak is found near the rate minimum. The broadening is described by a drastic change of the Avrami index n from 3 to 2. The morphology at the micro- and nanoscales of polyamides was followed by a polarized optical microscope (POM) and atomic force microscopy (AFM). The POM reveals that both polyamides turn translucent from transparent near the rate minimum. The temperature-dependent AFM micrographs show multistep transitions from amorphous-like morphology, cauliflower-like crystal, crystal aggregates, and lamellar structure after Tc changes from near Tg to above the kinetics break temperature. Although two polyamides have similar molecular weight and the same content of amide groups, the morphological transition in PA66 is found to always be 20 °C higher than in PA6, suggesting a difference in their thermodynamic drive to nucleate. The conclusions drawn from the Avrami analysis in the final part of this study provide a universal explanation of the drastic peak broadening observed in many previously studied thermoplastics.