Crystallinity, morphology and elastic behavior of copolymers of syndiotactic polypropilene with α-olefins.

We report a study of the structure and mechanical properties of copolymers of syndiotactic polypropylene (sPP) with different comonomers, as ethylene, butene, hexene, octene etc., up to long branched -olefins. All these copolymers show elastomeric properties even though they present non negligible level of crystallinity at low comonomeric units concentration. The effect of the presence of short or long branches on the crystallization behaviour and elastomeric properties has been analyzed. Incorporation of long branched comonomers, as 1-octadecene and 1-eicosene, allows fast decrease of the glass transition temperature that, associated with the elastic properties, makes these copolymers interesting rubbers. The relationships between structure, crystal morphology 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 between different polymorphic forms of sPP characterized by chains in trans-planar and helical conformations that occurs upon stretching and releasing the tension. These phase transformations provide an enthalpic contribution to the elasticity. Samples with higher comonomer concentrations show very low crystallinity and a typical thermoplastic elastomeric behavior with a classic entropic elasticity. 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, accounting for the development of classic entropic elastic properties. Young modulus and strength of these elastomeric materials can be easily tuned by suitable modification of type and concentration of the comonomeric unit to be copolymerized with propylene that, indeed, affect the degree of crystallinity and the level of entropic and enthalpic contributions to the elasticity. These copolymers represent a new class of elastomeric materials, defined “crystalline elastomers” where the crystallinity plays a fundamental role.