Model linear metallocene-catalyzed polyolefins: Melt rheological behavior and molecular dynamics

Abstract

The linear viscoelastic properties of two families of metallocene-catalyzed polyolefins were determined in the "plateau" and terminal regions. Low values of the plateau modulus G were observed for linear polyethylene relative to values cited for conventional and model hydrogenated polyolefins in several studies. This diminished G value is discussed in terms of a possible decrease in entanglement density compared to that of other polymers synthesized through different polymerization processes [Aguilar et al. (2001)]. For atactic polypropylene the situation becomes more complex, but the values obtained are within the broad range observed in the literature for isotactic and atactic species (0.30–0.86 MPa) [Aguilar et al. (2003)]. When the viscoelastic response is inferred from molecular data, the result may vary depending on the physical entanglement state. It would thus appear that great care should be taken to ensure that measurements are performed on samples in suitably entangled reference states. The viscoelastic variables determined here, were verified according to the different molecular dynamic models available, all based on reptation concepts. Results showed exceptional agreement when the effect of polydispersity was modeled either through an average relaxation time for each molecular species or through the double reptation concept.