Syndiotactic polypropylene and its copolymers with alpha-olefins. Effect of composition and length of comonomer

Abstract

A set of copolymers of propene and alpha olefins (1-hexene, 1-octene and 1-octadecene) and the corresponding homopolymer (sPP) have been synthesized using a syndiotactic metallocene catalyst. The effect of either the incorporation or length of these conomomeric units on the structure and final properties exhibited has been analyzed. As expected, there is a considerable decrease in crystallinity with the increase of comonomer content. Thus, a completely amorphous copolymer is obtained if the molar fraction is high enough. The structural variations drastically influence the viscoelastic and mechanical behaviors of these copolymers. Three or four relaxation processes can be observed depending on composition and length of comonomer. At temperatures above the process associated with the glass transition (β relaxation), a deep drop (in one or two steps) in E′ and a shoulder in E″, overlapped with that β mechanism, are observed. The existence of a relaxation involving crystalline regions is postulated because of its variation with crystal characteristics. Moreover, a relaxation related to internal motions within the comonomeric units is seen in the range of very low temperatures. This process, primarily ascribed to movement of methylenic segments within the comonomer, is strongly depending on composition and length of the pendant aliphatic chains on incorporated units. On the other hand, a decrease in stiffness and microhardness as well as the brittle–ductile transition are observed by simply varying composition when deformation takes place at room temperature.