Crystallization behavior and optical properties of isotactic polypropylene filled with α-nucleating agents of multilayered distribution.

Isotactic polypropylene (iPP)-based multilayered composites containing alternating layers of pure iPP and α-nucleating agents (α-NAs)-filled iPP (αPP) were fabricated through layer-multiplying co-extrusion technology. With the manipulation of layer number, a tunable multilayered distribution of α-NAs was achieved and its effect on the crystallization behavior and optical properties of iPP was investigated. When the thickness of an individual iPP layer, which is equal to the distance between adjacent αPP layers, was large, the crystallization process of iPP was governed by homogeneous nucleation, although the crystallization of areas near the layer interfaces was induced by α-NAs. As a result, most iPP formed big spherulites thus a poor clarity. By decreasing the layer thickness via multiplying the layer number, the nucleation of iPP was gradually transformed from a homogeneous mode to a heterogeneous mode because of the more and more strong influence of αPP layers on the crystallization process of adjacent iPP layers. Consequently, iPP exhibited similar crystallization behavior compared with αPP. The polarized optical microscopy results further demonstrated that the size of spherulites in iPP was significantly reduced with the increase of layer number, which contributed to the enhancement of transmittance and decline of haze. Accordingly, this work developed a novel approach to tailor the crystallization behavior and optical properties of iPP.