Thermoplastic elastomers from binary blends of syndiotactic polypropylenes with different stereoregularity

A study of the structure and properties of binary blends of syndiotactic polypropylene (sPP) having different degrees of stereoregularity and synthesized by different metallocene catalysts, is reported. The blends have been prepared by mixing a more stereoregular and crystalline sample with [rrrr] = 78% with low stereoregular and poorly crystalline samples having [rrrr] = 54.6 and 45.8%, and a nearly atactic fully amorphous sample with [rrrr] = 26.5%. All blends show outstanding elastomeric properties with rigidity and tensile strength that can be tailored by opportunely changing the stereoregularity of the components and blend composition. A continuous change of properties with composition is observed, from those of high-modulus thermoplastic elastomers for blends having high concentration of the most syndiotactic sample sPP1, to those of low-strength elastomers for blends with high content of the less syndiotactic component. The addition of the stereoirregular component to the more stereoregular sample produces an effect on the crystallization behavior and mechanical properties analogous to that produced by incorporation of defects of stereoregularity. Blends behave as a single sPP component having an average degree of stereoregularity intermediate between those of the two components. All data indicates a good miscibility the sPP chains of different stereoregularity in the melt and in the amorphous state.