Shear-Induced Precursor Relaxation-Dependent Growth Dynamics and Lamellar Orientation of β-Crystals in β-Nucleated Isotactic Polypropylene

Abstract

Although a shear flow field and β-nucleating agents (β-NAs) can separately induce the formation of β-crystals in isotactic polypropylene (iPP) in an efficient manner, we previously encountered difficulty in obtaining abundant β-crystals when these two factors were applied due to the competitive growth of α- and β-crystals. In the current study, to induce the formation of a high fraction of β-crystals, a strategy that introduces a relaxation process after applying a shear flow field but before cooling to crystallize β-nucleated iPP was proposed. Depending on the relaxation state of the shear-induced oriented precursors, abundant β-crystals with a refined orientation morphology were indeed formed. The key to producing these crystals lay in the partially dissolved shear-induced oriented precursors as a result of the relaxation process's ability to generate β-crystals by inducing the formation of needlelike β-NAs. Therefore, the content of β-crystals gradually increased with relaxation time, whereas the overall crystallization kinetics progressively decreased. Moreover, more time was required for the content of the β-phase to increase to the (maximum) value observed in quiescent crystallization than for the effect of flow on crystallization kinetics to be completely eliminated. The c-axis of the oriented β-lamellae was observed to be perpendicular, rather than parallel, to the fiber axis of the needlelike β-NAs, as first evidenced by the unique small-angle X-ray scattering patterns obtained. The significance of the relaxation process was manifested in regulating the content and morphology of oriented β-crystals in sheared, β-nucleated iPP and thus in the structure and property manipulation of iPP.

Suppression of the hierarchical structure of water-assisted injection moulded iPP in the presence of a β-nucleating agent and lamellar branching of a β-crystal

“…Of note, molecular orientation is manifested by the diffraction arcs on the patterns; and a decrease in molecular orientation corresponds to the weakening of the arcs into circles. 12 Therefore, in order to estimate the molecular orientation in different zones, the azimuthal proles of the a(040) and b(110) 13 reections are illustrated in Fig. 3.…”

Exploring the polymorphic behavior of a β‐nucleated propylene‐ethylene random copolymer under shear flow

“…The (110) β is always uniform under different shear strains, indicating that the orientation of β crystals is independent of shear. Li et al found that relaxation of oriented molecular chains was beneficial to the formation of β crystals and the oriented β lamellae can be obtained by epitaxial growth on the needle‐like β ‐NA. In our case, the Ca‐Pim particles are nearly spherical, thus the β crystals are always isotropic under shear flow.…”

Greatly improved toughness of isotactic polypropylene blends with traces of carbon nanotubes

“…1D‐SAXS profiles were obtained from the circular average of 2D‐SAXS patterns, in which scattering intensity was plotted as a function of the reciprocal space vector, s (| s | = 2 sin θ/λ , where λ is the wavelength of the incident beam and 2 θ the scattering angle). The long period ( L B ) that defines the statistical average of the distance between two lamellae is determined by the position of the first intensity maximum ( s m ) in 1D‐SAXS profiles by L B = 1/ s m . The thickness of crystalline lamella ( L c ) can be estimated by L c = X c(WAXD) × L B , and the thickness of the confined amorphous phase L a = L B − L c .…”