STRUCTURE AND STRESS-STRAIN BEHAVIOR OF P ROPYLENE COPOLYMERS WITH LOW CONTENT OF ETHYLENE UNITS

Propylene copolymers containing 2.2 - 12.5 mol % of ethylene units wer e prepared using a modified titanium-magnesium catalyst. The structure and stress-strain behavior of the copolymers were studied. It was sho wn that the content of alpha-crystalline modification decreases and th e fraction of gamma-crystalline modification of polypropylene (PP) inc reases with an increase in the content of ethylene units and a decreas e in mean length of crystallizable PP blocks in copolymers. The modulu s of elasticity and yield stress at room temperature linearly decrease with the decreasing content of alpha-modification of PP. When the con tent of ethylene units increases, the values of ultimate elongation a break and the impact strength of copolymers rise. The value of tensile strength is maximum for the copolymer containing 2.2 mol % of ethylen e units. Necking occurs both in homopolymer and copolymers with low co ntent of ethylene units. In the case of copolymers containing more tha n 6 mol % of ethylene units, the increasing content of gamma-crystalli ne phase of PP makes the plastic flow more homogeneous.