Processing temperature-dependent nucleation and growth behaviors of polypropylene nucleated by a bisamide nucleating agent

Abstract

It has been widely reported that the processing temperature significantly influenced the effectiveness of a β-nucleating agent (β-NA) in polypropylene (PP) and thus leads to variations in the final relative content of β-phase (kβ). To comprehensively understand the changing kβ with processing temperature, PP nucleated by a bisamide β-NA, N,N′-dicyclohexylterephthalamide (DCHT), was heating-treated at different temperatures (Tf) in the range of 200–250 °C, and the impact of Tf on the nucleation and subsequent growth behaviors of the α- and β-phases were separately investigated. Under low Tf, DCHT induced a high relative density of β-nuclei to α-nuclei (Nβ/Nα) with high total nucleation density (N), and both the α- and β-phases grew competitively until the crystallization ended. Under middle Tf, the Nβ/Nα became low with middle N induced by DCHT, and the faster-growing phase dominated the subsequent growth process. Under high Tf, there was a middle Nβ/Nα with low N, and the faster-growing phase became more dominant during the growth process. Based on the observed morphology of DCHT, the decreasing N with Tf could be associated with the specific surface area, the varying Nβ/Nα could be attributed to the variations in both the energy barrier and the surface area of β-nucleation under diverse DCHT morphologies, and the varying growth behavior was probably associated with the outer surface shape of diverse DCHT morphologies. Combining the thorough analysis of the changing nucleation and growth behaviors with Tf, a possible mechanism was proposed to explain the effect of processing temperature on the final polymorphic composition.