Morphology and thermal properties of polyethylenes made by metallocene and Ziegler-Natta catalysts

Morphology and thermal behavior of high density polyethylene and ethylene-octene copolymers prepared with classical Ziegler-Natta (zHDPE) and metallocene catalysts (mHDPE and mLLDPE) are comparatively discussed. Wide angle X-ray diffraction (WAXD) data at room temperature showed that mHDPE crystals are more perfect with smaller unit cell dimensions and have higher crystalline density and larger crystallite sizes when compared to zHDPEs. In the case of copolymers, the unit cell dimensions did not change significatively when the number of branches was increased. Average interplanar chain distances in the amorphous phase increased with increasing octene content in the copolymer, showing a marked influence on the packing of the amorphous phase and confirming that the branches are concentrated in this phase. The existence of second phase in the ethylene-octene copolymers prepared with metallocenes was identified by calorimetry. The existence of this phase is time-dependent and was interpreted as the result of hexyl branches ordering in the amorphous phase. A morphological model based on Vile three-phase model, WAXD results and calculations involving empirical equations for fold and extended chain lamellae crystals was proposed to explain the time-dependent thermal behavior of these copolymers.