Crystallization behavior, morphology and mechanical properties of copolymers of syndiotactic polypropylene with branched monomers

We report a study of the structure and mechanical properties of copolymers of syndiotactic polypropylene (sPP) with different comonomers from ethylene to branched a-olefins. The effect of the presence of short or long branches on the crystallization behaviour and elastomeric properties of sPP has been analyzed. Incorporation of long branched comonomers, as 1-octadecene and 1-eicosene, allows fast decrease of the glass transition temperature and development of interesting elastomeric materials. The relationships between structure and stress-induced phase transformations and mechanical properties have been clarified. In samples with low comonomer content the elastic properties are associated with a reversible polymorphic transition that occurs upon stretching and releasing the tension, which provides an enthalpic contribution to the elasticity. Samples with higher comonomer concentrations show very low crystallinity and a typical thermoplastic elastomeric behavior. The study of the crystal morphology shows the presence of small bundles of rod-like lamellar crystals in all copolymers whose size decreases with increasing content and size of comonomeric units. The small needle-like crystals act as knots of an elastomeric lattice, explaining the development of elastic properties.