Shish Formation and Relaxation in Sheared Isotactic Polypropylene Containing Nucleating Particles

Abstract

The formation and relaxation of shish in molten isotactic polypropylene (iPP) and two samples containing different commercial nucleating particles was studied during and after continuous shear flow. In-situ measurements of the shish were made by simultaneous wide- and small-angle X-ray scattering (rheo-SAXS/WAXS) using synchrotron radiation. In the SAXS patterns, sharp meridional reflections were observed in all three samples during the initial stages of shear flow. Their presence indicates that shish form in discrete steps, by the aggregation of pointlike nuclei into oriented threads. Compared to pure iPP, the addition of both nucleants increased the amount and the dimensions of shish formed. However, the shish also relaxed faster in the presence of both nucleated materials. This behavior was attributed to the particles inducing lower molecular weight chains to participate in shish formation. The particle aspect ratio was observed to greatly influence shish formation. This is explained by modification of the local strain environment near the surface of the different shaped particles.

More Dominant Shear Flow Effect Assisted by Added Carbon Nanotubes on Crystallization Kinetics of Isotactic Polypropylene in Nanocomposites

“…It is considered that a stress amplification of flow usually occurs for the particle-filled systems. 19 As elongated particles, CNTs can be oriented by shear flow, causing a more powerful variant of stress amplification. The local stresses generated around the CNTs during shear flow may be reinforced, leading to an enhanced effect of shear flow on crystallization kinetics.…”

“…18 The typical "shish-kebab" morphology, which can improve the ultimate mechanical properties of shaped products, is usually induced by shear flow through stretching of the high molecular mass chains and the subsequent formation of shish structure, onto which the shorter chains grow epitaxially to form the kebabs. 19,20 The crystallization kinetics for CNT/ polymer composites under shear flow might be quite different, because CNTs can assist in the formation of extended polymer chain fibrils to generate the hybrid shish and disk-shaped polymer crystals (kebabs) growing epitaxially onto the hybrid shish, resulting in the nano-hybrid shish-kebabs (NHSK) morphology. 21 −23 It has been confirmed that the nanohybrid shish forms with an oriented layer of aligned macromolecular chains, induced by the coeffects of CNTs and shear flow, indicating that CNTs play an important role in the formation of oriented nuclei, especially at the shear flow conditions where the oriented nuclei cannot form in the absence of CNTs.…”

Flow-induced crystallization of polypropylene in the presence of graphene nanoplatelets and relevant mechanical properties in nanocompsoite fibres

“…Most of the studies on the flow-induced crystallization have been done on the pure polymers. However, flow-induced crystallization in the presence of nanoparticles is much more complex than that in the pure polymer [29][30][31][32][33][34][35][36]. Since particles disturb the flow filed, a significant effect of presence of particles on the flow-induced crystallization is expected.…”

“…The number of heterogeneous sites available for nucleation is mainly affected by the concentration and dispersion of particles in the polymer matrix. The combined effect of flow and addition of particles have been suggested to yield a synergistic increase in the number of active nuclei, raising crystallization rate further than their individual contribution [31].…”

Polymorphism in Sheared Isotactic Polypropylene Containing Nucleant Particles

“…Nucleants may also alter morphology development under flow‐induced crystallisation conditions. The addition of particles has been shown to bring about a synergistic increase in the number of flow‐induced point‐like nuclei 6–11. Furthermore, the presence of particles has been shown to reduce the critical flow intensity required to form the shish‐kebab morphology 9, 12–16.…”