Tailoring the Mechanical Properties of Isotactic Polypropylene by Blending Samples with Different Stereoregularity

The structure and properties of some blends obtained by mixing two semicrystalline samples of isotactic polypropylene (iPP) characterized by different degree of stereoregularity have been analyzed in the whole range of composition. The two components are a highly isotactic and crystalline sample (iPP1) and a stereoirregular and less crystalline sample (iPP2), and have been synthesized using two different metallocene catalysts. The blends show separate and independent crystallization and melting of the two components, and crystallinity content decreasing almost linearly with increasing the concentration of the low stereoregular component. A continuous change of tensile properties with the composition occurs, with properties ranging from those of thermoplastic materials with high stiffness, typical of the neat iPP1, to those of high-strength elastomers, typical of neat iPP2. The mechanical properties of these blends reflect the complex lamellar morphology that develops upon crystallization, characterized by a high degree of inclusion of both components in the same stacks, with lamellae interconnected by the amorphous portions of chains of the components mixed at molecular level. Similar properties intermediate between stiff plastic materials and elastomers can also be btained with a series of stereodefective iPP samples by tailoring the concentration of stereodefects, using different metallocene catalysts. The advantage of preparing binary blends of the two iPP samples having different concentration of stereodefects for obtaining the same properties is that only two different catalysts must be used.