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.