On the rheology of ethylene-octene copolymers

It has previously been shown that the plateau modulus, GNp, and thus the entanglement molecular weight, Me, of flexible polymers can be correlated to the unperturbed chain dimension, <R2>o/M, and mass density, ρ, via the use of the packing length, p. For polyolefins, a method was recently proposed whereby knowledge of the average molecular weight per backbone bond, mb, allows <R2>o/M and consequently GNo and Me to be estimated. This is particularly valuable for polyolefin copolymers since the melt chain dimensions are often unknown. This work corroborates these theoretical predictions by studying the rheology of a series of carefully synthesized ethylene/octene copolymers with varying octene content (19–92 wt%). Furthermore, the results reported herein also allow the advancement of rheological characterization techniques of polymer melts. For instance, based on the analysis of the linear viscoelastic properties of these copolymers, it has been found that several rheological parameters scale with the copolymer comonomer content. Analysis of the viscoelastic material functions in terms of the evolution of the phase angle, δ, as a function of the absolute value of the complex modulus, |G*|, (the so-called van Gurp-Palmen plots), provides a fast and reliable rheological means for determining the composition of ethylene/α-olefin copolymers. The crossover parameters, Gco(=G′=G″) and gco(=1/ωco) also scale with copolymer composition.