Rheological criteria to characterize metallocene catalyzed polyethylenes

Dynamic viscoelastic and capillary extrusion rheometry measurements were ca rried out with a series of 13 metallocene catalyzed polyethylenes and copol ymers of ethene and 1-hexene. The structural parameters were analyzed by si ze: exclusion chromatography (SEC) and C-13 NMR, Showing that the molecular weights range from (M) over bar(w) = 80 000 to 308 000, the polydispersity index from 2 to 3.5 and the degree of short chain branching (SCB) from 0 t o 13.8 SCB/1 000 C. In order to extract the maximum information from the ex perimental data, the following rheological methods were used: a) Viscosity and relaxation time dependence on molecular weight (M) over bar(w). b) Redu ced dynamic viscosity eta'/eta(0) dependence on the product omega . eta(0). c) Frequency dependence of dynamic moduli, storage modulus (G') and loss m odulus (G "). d) log G' versus log G " plots. e) Storage compliance J' depe ndence on storage modulus G'. f) Phase angle delta dependence on complex mo dulus G*. g) Relaxation spectra. h) Dependence of the exponent n of the pow er law model for the viscosity function eta ((gamma) over dot) on of molecu lar weight. i) Analysis of the critical rate for sharkskin. These methods, except the last one, allow to separate the samples into three different gro ups, at least when low frequencies (below 10(-1) Hz) or times higher than 1 0 s are involved. The definition of these groups cannot be undertaken consi dering only the molecular parameters obtained by SEC and C-13 NMR. Analyzin g our rheological results in comparison with long chain branched polyethyle nes (LCB) and looking at the theoretical aspect of the dynamics of long bra nched chains, we assume that among our samples there are five linear (non-L CB, Group I) polyethylenes and two groups of slightly long chain branched p olyethylenes, which differ in the number of branches.