Kinetic analysis on spherulite growth rate of polypropylene catalloys

Abstract

The kinetic analysis on melt-crystallization of polypropylene catalloys (PP-cats) was conducted through measuring their spherulite growth rates. A multiple melting behavior of PP-cats was found through differential scanning calorimetry (DSC) and the corresponding crystalline microstructures of PP-cats were studied by wide-angle X-ray diffraction (WAXD). The calculated Tmo value of propylene homopolymer (PP) suggests an obvious melting point depression of PP-cats. Moreover, it is found that the existence of ethylene–propylene copolymer could result in the changes of crystalline microstructure of PP and the PP crystal is in favor of growing along (040) lattice plane of α-monoclinic crystal. The crystal growth rate of PP-cats decreases with the increase of ethylene–propylene copolymer content in PP-cats. A comparison of crystallization kinetics between PP-cats and virgin iPP through a modified Lauritzen–Hoffman model indicates that there appears a transition from regimes II to III in iPP and PP-cats containing low ethylene–propylene copolymer content. However, for the PP-cats containing high ethylene–propylene copolymer content, crystallization always processes in regime II. In addition, both calculated nucleation parameter (Kg) and the fold surface free energy (σe) for PP-cats increase with the increase of ethylene–propylene copolymer content, implying that the existence of ethylene–propylene copolymer is unfavorable for the surface nucleation of PP and regular folding of the molecule chain. It is believed that an increase in viscosity of the melts induced by different compositions could remarkably slow crystallization growth down, because under this condition surface nucleation dominates as compared with crystal growth.