Rheological properties of two polypropylenes with different molecular structure

Two polypropylenes with different molecular structure were investigated in shear and elongational flow. One resin is an ethylene-propylene copolymer with linear molecules and the other is a long-chain branched propylene homopolymer. The elongational flow behavior was investigated in stressing and creep experiments. For the linear material no strain hardening can be observed whereas the strain-hardening behavior of the long-chain branched material is more pronounced than it was found for any other polyolefin up to now. The branched resin also exhibits high recoverable strains. The strain-hardening behavior is analyzed with respect to its strain and strain rate dependence. The results are discussed with regard to the molecular structure. The coexistence of a long-chain branched structure and a high molecular weight component is assumed to be the reason for the outstanding elongational flow behavior of the branched polypropylene. (C) 1999 The Society of Rheology. [S0148-6055(99)00402-2].