Mechanical properties and elastic behavior of syndiotactic propene-butene copolymers

The mechanical properties of syndiotactic propylene-butene copolymers (sPPBu), prepared with a metallocene catalyst, have been studied in the whole range of comonomer composition. Meltcrystallized unoriented films and oriented fibers of sPPBu copolymers show good elastic properties even though the samples present non-negligible level of crystallinity at any comonomer composition. These materials behave as thermoplastic elastomers with remarkable rigidity and mechanical strength that can be tuned by changing the comonomer composition. In samples with low butene concentration the elastic properties are associated with the reversible transformation between the trans-planar form III of syndiotactic polypropylene, obtained by stretching, and the helical form II, formed by releasing the tension. The occurrence of reversible phase transitions assists the elasticity of these materials through a non-negligible free energy contribution that is added to the conventional entropic contribution.