Ming Xiang

Influence of the stereo-defect distribution on the crystallization behavior of Ziegler-Natta isotactic polypropylene

The relationship between the stereo-defect distribution and the crystallization behavior of Ziegler-Natta isotactic polypropylene (ZN-iPP) is an important issue, which has not been clearly studied up to now. In this work, the crystallization behavior of a series of iPP samples with similar average isotacticity but different stereo-defect distribution, polymerized with the same Ziegler-Natta catalyst system, was studied by means of differential scanning calorimetry (DSC) and polarized optical microscopy (POM) observation. The results of isothermal crystallization kinetics indicated that, as the distribution of stereo-defects becomes more uniform, the overall crystallization rate decreases gradually. Meanwhile, the results of self-nucleation isothermal crystallization kinetics showed that, the crystal growth rate decreases gradually and the energy barrier of crystal growth increases. Moreover, the POM observation illustrated that not only the crystal growth rate, but also the nucleation rate decrease gradually as the stereo-defect distribution becomes more uniform. The results above indicated that for iPP polymerized with the same Ziegler-Natta catalyst system, stereo-defect distribution plays an important role in determining the nucleation kinetics, crystal growth kinetics and the overall crystallization kinetics of the resin.

Degradation and Stabilization of Co-POM

This paper focuses on the degradation and stabilization of unstable terminals of Co-POM. As observed by the concentration of formaldehyde generated at 220°C in N2 and the thermogravimetric analysis in air, higher percent of unstable terminals would generate more formaldehyde and lead to serious degradation of Co-POM. With the increase of the percent of unstable terminals, the thermal stability of Co-POM decreases. Terminal stabilization for crude Co-POM were carried out during the polymer melting process under vacuum. The correlation between MFR and TGA indicates that the stability of treated Co-POM depends on the temperature and duration of terminal stabilization.

Effect of Stereodefects Distribution on Rheological Behavior of Isotactic Polypropylene

A series of five isotactic polypropylenes (iPP) used commercially for producing biaxial oriented polypropylene film (BOPP), which had similar average degrees of isotacticity but different stereodefects distribution, was studied. Xylene solubles (XS) at different temperatures and high temperature solution 13C NMR were used to characterize the stereodefects distribution of the five samples. These samples gradually became more random following the order of sample A to E. The plateau modulus (G 0 N ≈ 0.2 MPa at 190°C) of these samples was lower than those of metallocene-catalyzed polypropylenes with very narrow molecular weight distributions (MWD) (G 0 N ≈ 0.4 MPa at 190°C), indicating that the influence of MWD on G 0 N cannot be neglected. The storage modulus and complex viscosity of the five samples decreased gradually with the widened stereodefects distribution, due to the formation of a less constrained network that finally caused the reduction of G 0 N and the increase of the flow activation energy. The rheological behavior of the BOPP resin samples could be well related to the results of XS and 13C NMR, implying that the rheological measurement can be adopted as a simple and efficient method to characterize the stereodefects distribution of isotactic polypropylene.