Effects of Blending on the Melting and Crystallization of Polyethylene ahd Polypropylene under High Pressure

Abstract

The melting and crystallization behavior of polyethylene (PE) and polypropylene (PP) blends under high hydrostatic pressures up to 500 MPa was studied by differential thermal analysis (DTA). These high pressure crystallized blends were also investigated at 1 atm by DTA, X-ray diffraction, and electron microscopy. The melting temperature Tm of PE in the blend under high pressure was determined from the peak temperature of a DTA curve of melting. The Tm versus pressure curve for PE in the blend shifts to the high pressure side with decreasing PE content. In the DTA curve of melting at 1 atm for a blend crystallized under 450 MPa, an endothermic peak that could not be assigned to the melting of folded PE chain crystals (FCC) appeared on the low temperature side of the melting peak for extended chain crystals (ECC), indicating that small PE crystals different from FCC were formed in the molten PP during crystallization. The X-ray diffraction pattern of this blend showed that no eutectic crystallization occurs even in the isothermal crystallization under high pressure. Electron microscopic observations of the fracture surface of the high pressure crystallized blend showed a clear boundary between the PE and PP domains, suggesting that no interdiffusion of the two polymers occurs.