Effect of additives, catalyst residues, and confining substrates on the fractionated crystallization of polypropylene droplets

摘要

Fractionated crystallization of Ziegler-Natta PP (ZN-PP) droplets produced from ZN-PP/polystyrene (PS) multilayered films resulted in four crystallization exotherms at 40, 64, 90, and 103°C. The origins of these crystallization peaks were investigated in this article. The nucleation of ZN-PP droplets at various crystallization exotherms was attributed to (1) various additives in ZN-PP and PS, (2) catalyst residues present in ZN-PP, or (3) substrate nucleation by confining matrix. Effect of various additives on the fractionated crystallization an additive-free grade of ZN-PP (cPP) was investigated. Crystallization of cPP droplets from 200-nm layers showed that Irganox was responsible for crystallization of droplets at 103°C. Irgafos and ZnS did not affect the crystallization of cPP droplets. We also investigated three grades of PP, which were synthesized using different catalysts. The PP grades were ZN-PP, developmental PP (D-PP) and metallocene PP (M-PP). D-PP droplets from 200-nm layers showed crystallization at 100°C, while ZN-PP resulted in crystallization at 90 and 103°C. It was suggested that the 90°C peak for ZN-PP droplets was most likely due to the presence of Ziegler-Natta catalyst residues. Crystallization of M-PP droplets produced from 200 nm showed multiple crystallization exotherms (60 to 90°C). The effect of confining substrates on the fractionated crystallization of ZN-PP droplets revealed heterogeneous nucleation by polycarbonate (PC) substrate. When the confining matrix was changed from polystyrene (PS) to polymethyl methacrylate (PMMA), the crystallization thermograms of ZN-PP droplets were unaffected.