Effect of the molecular structure of the polymer and nucleation on the optical properties of polypropylene homo- and copolymers.

Two soluble nucleating agents were used to modify the optical properties of nine PP homo- and random copolymers. The ethylene content of the polymers changed between 0 and 5.3 wt%. Chain regularity was characterized by the stepwise isothermal segregation technique (SIST), while optical properties by the measurement of the haze of injection molded samples. Crystallization and melting characteristics were determined by differential scanning calorimetry (DSC). The analysis of the results proved that lamella thickness and change in crystallinity influence haze only slightly. A model was introduced which describes quantitatively the dependence of nucleation efficiency and haze on the concentration of the nucleating agent. The model assumes that the same factors influence the peak temperature of crystallization and optical properties. The analysis of the results proved that the assumption is valid under the same crystallization conditions. The parameters of the model depend on the molecular architecture of the polymer. Chain regularity determines supermolecular structure and thus the dependence of optical properties on nucleation

RHEOLOGICAL AND STRUCTUREINVESTIGATION OF SHEAR-INDUCED CRYSTALLIZATION OF ISOTACTIC POLYPROPYLENE

In the present work, the influence of well-defined simple shear flow histories on the isothermal crystallization of an isotactic polypropylene (i-PP) has been investigated. At first, the research of the flow conditions in terms of temperature, shear rate and shear strain has been performed by means of the rheological technique. The continuous shearing analysis enabled us to build the flow curve at 144°C showing a Newtonian region as well as a shear-thinning zone. Indeed, for a shear rate above a critical value, the molecular orientation occurring during flow provides a kinetic promotion of the crystallization process. In the rheological step-shear flow analysis, an increase of the flow sensitivity parameter, kS/kQ, with increasing the shear rate at a constant strain ( strain = 150) is observed. The structure of the crystallized samples has been investigated by differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS) methods. In agreement with the DSC, the WAXS results show that crystals with a certain bimodal distribution are generated in the samples crystallized under step-shear flow conditions. A small orientation of the (110) plane of the i-PP -phase crystals is also detected. POLYM. ENG. SCI., 45:153–162, 2005. © 2005 Society of Plastics Engineer