Crystallization behavior and mechanical properties of regiodefective, highly stereoregular isotactic polypropylene: effect of regiodefects versus stereodefects and influence of the molecular mass

Highly isotactic polypropylene samples, containing a very low amount of rr stereodefects (0.1-0.2%) and slightly higher concentration of defects of regioregularity (0.8-0.9% of 2,1 erythro units), with different molecular masses have been prepared with an isospecific but not fully regioselective metallocene catalyst. The effects of the presence of rr defects and of 2,1 regiodefects, and the effect of the molecular mass on the mechanical properties and crystallization behavior of polypropylene have been analyzed. Samples containing 2,1 regiodefects are very stiff and much more fragile than the samples containing only rr stereodefects. The presence of rr defects, even for low concentrations, produces instead increase of ductility and improvement of drawability at room temperature. The different effect of stereo- and regio-defects is probably related to their different levels of inclusion inside the crystalline phase. The uniform inclusion of rr defects in the crystals makes the stereodefective and regioregular samples more homogeneous, where crystalline and amorphous phases have the same composition. The study of the crystallization behavior has shown that the molecular mass strongly influences the amount of alpha and gamma forms that crystallize from the melt. Samples containing regiodefects and with high molecular masses (higher than 200000) show the same amount of gamma form and the same crystallization rate. A lower amount of gamma form and higher crystallization rates are instead observed for the sample having similar microstructure but lower molecular mass. This indicates that the crystallization of the gamma form is favored over the alpha form when the crystallization is slow, that is, for samples with high molecular mass. When the molecular mass is lower than 100000 the crystallization is faster and the formation of the alpha form is kinetically favored. The comparison with regioregular samples containing only rr stereodefects has shown that the effects of rr stereodefects and 2,1-regiodefects on the crystallization properties of polypropylene are very similar, at least when the concentration of defects is small (1% of rr stereodefects or 2,1 erythro units). Both defects produce a shortening of the regular crystallizable isotactic and regioregular sequences, inducing crystallization of the gamma form.