Rheologically determined negative influence of increasing nucleating agent content on the crystallization of isotactic polypropylene

Abstract

Dibenzylidene sorbitol (DBS) exists in the form of fibril and usually acts as an effective nucleating agent to facilitate crystallization of polyolefin during manufacturing. In this research, the isothermal crystallization of isotactic polypropylene (iPP) containing different amounts of DBS was followed by dynamic rheometry, and described upon a viewpoint of viscoelastic property evolution. Since the adopted temperatures within the entire sample preparation and characterization process were below the melting point of DBS, the DBS additives played a role of only solid nucleating agent, thus the possible effect of a changed miscibility between iPP and DBS on the viscoelastic properties as change of temperature could be ignored. Although saturation of nucleating iPP was observed at 0.1% DBS concentration, a negative influence with further increasing DBS content on the crystallization of iPP was determined for the first time via time sweep of G′ upon a single angular frequency and application of “inverse quenching” protocol within a wide range of angular frequency. Even more, a largely increased G′ and viscosity were observed in the melt as decreasing the temperature of iPP containing 0.1% DBS, suggesting a strong quasi-solid like behavior before iPP crystallization. A complement for well understanding the crystallization of iPP containing nucleating agent was discussed based on the formation of the DBS fibrils' network, the interfacial tension between crystalline/amorphous phase, and the consistency of crystallizing lamellae. Our study demonstrates clearly that the crystalline characteristic upon saturation of heterogeneous nucleating is difficult to be detected by traditional means, but could be followed reasonably by rheological measurements which is much sensitive to the microstructural changes.