Stress-Strain Behaviour, Microhardness, and Dynamic Mechanical Properties of a Series of Ethylene-Norbornene Copolymers

Abstract

Ten copolymers of ethylene and norbornene, synthesised by a metallocene catalyst, covering the composition range from 31 to 62 mol-% of norbornene, have been studied by stress-strain, microhardness and dynamic mechanical measurements. These copolymers show a single glass transition and the correlation between Tg and the norbornene content is quite good. The inclusion in the polymer chain of rigid norbornene units, with the corresponding increase in Tg, leads to considerably higher Young moduli and microhardness values in these cycloolefin copolymers. The same result is found for the yield stress: this parameter shows an increase by a factor of two in the studied copolymers. Two relaxations, α and γ have been found by means of dynamic mechanical analysis of these copolymers. The results show that the γ relaxation has a similar origin to that of polyethylene and therefore its intensity diminishes as the proportion of ethylene segments in the copolymers decreases. On the contrary, the α relaxation of the copolymers has a completely different origin than that of polyethylene. Thus, this relaxation represents the glass transition in the copolymers, associated with the amorphous character, while it is attributed to motions in the crystalline regions of the parent homopolymer, polyethylene.