| Abstract: | Using highly active peroxide initiators, a number of polyethylene samples were prepared at low temperatures of 70–140°C and a pressure of 1900 bar. The average molecular weight and the melt flow index were adjusted by adding propionaldehyde as modifier. The characteristic properties of the samples as regards the molecular weight distribution, the average molecular weight, the frequency of long chain branching, the crystallinity and the density were determined. As expected, the polymers are characterized by density values of more than 0.955 g/ml and crystallinity values of 65–80% which are characteristic of PE-HD. The molecular weight distribution, too, remains within narrow limits. On the other hand, rather surprisingly, a relatively high frequency of long chain branching typical of PE-LD is obtained. The investigation of the phase behaviour has shown that polymerization under high pressure at temperatures above 115–118°C takes place in the homogeneous range. The samples polymerized in the transition zone between the single and the two phase area exhibit maximum crystallinity and melt enthalpy. The frequency of long chain branching passed through a minimum. |
