VISCOELASTIC PROPERTIES OF POLY(ETHYLENE-CO-STYRENE) COPOLYMERS

The viscoelastic properties of narrowly distributed linear poly( ethyl ene-co-styrene) copolymers with different mole fractions of styrene (x (s) = 0-20.5 mol%) and molecular weights (M-w = 64-214 kg/mol) were an alyzed in the molten state at different temperatures by means of oscil latory rheometry. Analyzing the thermorheological properties of the po lymers, we found that the time temperature superposition principle is fulfilled. The corresponding shift factors follow up to 16.5 mol% of s tyrene units the Arrhenius behavior of neat polyethylene. For a styren e content of about 20 mol%, the polymers no longer crystallize and a t ransition from Arrhenius to WLF behavior of pure polystyrene was obser ved. The zero shear viscosity, eta(0), of the polymers was derived fro m the master curves. The determination of the plateau modulus by the w ell-known tan delta-min criterion is not possible due to the beginning crystallization in the corresponding temperature range. An approximat e calculation of this value is based on the characteristic relaxation time lambda(x) = 1/w(x), corresponding to the crossover of G' and G '' . Indeed, the characteristic modulus G(px) calculated as eta(0)/lambda (x) is a good approximation for the plateau modulus G(p). The viscosit y-molecular weight and relaxation time-molecular weight scaling relati ons were established for three copolymers with different molecular wei ghts and nearly the same styrene content. For both material parameters , the scaling exponent is around 3.4, confirming the linear architectu re of the investigated polymers. The mixing rules describing the chang e of such material parameters like zero shear viscosity or plateau mod ulus independent of styrene content are of logarithmic linear characte r using the weight fraction of styrene units instead of the mole fract ion. The relations found allow the prediction of melt state properties for polymers with arbitrary styrene content. In the future, when cata lysts with sufficient activity for the synthesis of high styrene conte nt copolymers are available, these predictions will have to be checked . (C) 1997 John Wiley & Sons, Inc.