Characterization of the molecular structure of two highly isotactic polypropylenes

Two polypropylenes, PP1 and PP2, produced with different heterogeneous Zieg ler-Natta catalytic systems were studied in this work. Preliminary characte rization of the non-fractionated materials showed that a low difference in their average tacticity (PP2 > PP1) leads to an important modification of t heir rigidity properties. In order to establish correlation between the mol ecular structure parameters and the rigidity properties of these polymers, fractionation of the materials according to crystallizability was performed by means of temperature rising elution fractionation (TREF). Analysis of t he fractions of both PP1 and PP2 was carried out by means of C-13 NMR, size exclusion chromatography (SEC), differential scanning calorimetry (DSC) an d atomic force microscopy (AFM). The results first showed that TREF does no t strictly fractionate PP according to tacticity, but according to the long est crystallizable sequence in a chain. C-13 NMR, SEC and DSC analysis of t he fractions demonstrated that the inter-chain tacticity distributions of t he polypropylenes is affected by the change of the polymerization condition s, which, in turn, modifies the rigidity properties of the materials. Some results also seem to indicate that the intrachain tacticity distributions a re different for the two PP. An AFM study of the elastic modulus was carrie d out for the first time on the TREE fractions. It showed that the rigidity or the fractions strongly increases as the TREF elution temperature increa ses in accordance with a concomitant increase of isotacticity and the cryta llinity of the fractions. PP2 TREE fractions were, moreover, found to exhib it a higher elastic modulus than PP1 TREF fractions at all elution temperat ures. This study allowed us to further identify the TREF fractions that wer e responsible for differences in rigidity. To summarize, it is shown how th e experimentally observed increase of the average rigidity of one of these two polypropylenes can be rationalized via information collected from a TRE F fractionation. (C) 2000 Published by Elsevier Science Ltd.