Creep recovery of random ethylene-octene copolymer melts with varying comonomer content

This paper presents an investigation of the effects of varying comonomer content from 20 to 38 wt % in random ethylene-octene copolymers made with a metallocene catalyst on the melt rheology including transients in shear creep recovery from high strains. The copolymer with 20% octene appeared to be the same as that reported to be long-chain branched in the literature and was used as a reference material. The materials were characterized in oscillatory shear and shear creep at stresses ranging from 5 to 30000 Pa. Van Gurp-Palmen plots prepared from dynamic modulus data and zero-shear viscosity values from creep tests at very low stress confirmed that the reference material was long-chain branched and established that long-chain branching was absent from the copolymer with the highest comonomer content. The steady state recoverable shear compliance values for the three copolymers were very close. However, following large shear deformations brought about at higher strain rates, strain recovery was reduced most significantly for the copolymer with the highest comonomer content where long-chain branching was absent. The A reduction in recovery at short times was also most significant for this copolymer. (C) 2005 The Society of Rheology.