The Intrinsic Mechanical Nonlinearity ³Q₀(ω) of Linear Homopolymer Melts
Miriam Angela Cziep ^{1, b)}, Mahdi Abbasi ^{1, c)} and Manfred Wilhelm ^{1, a)
1)} Institute of Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany

^a) Corresponding author: manfred.wilhelm@kit.edu
^b) miriam.cziep@kit.edu
^c) mahdi.abbasi@partner.kit.edu

Abstract. Medium amplitude oscillatory shear (MAOS) in combination with Fourier Transformation of the mechanical stress signal (FT rheology) was utilized to investigate the influence of molecular weight, molecular weight distribution and the monomer on the intrinsic nonlinearity ³Q₀(ω). Nonlinear master curves of ³Q₀(ω) have been created, applying the time-temperature superposition (TTS) principle. These master curves showed a characteristic shape with an increasing slope at small frequencies, a maximum ³Q_{0, max} and a decreasing slope at high frequencies. ³Q₀(De) master curves of monodisperse polymers were evaluated and quantified with the help of a semi-empiric equation, derived from predictions from the pom-pom and molecular stress function (MSF) models. This resulted in a monomer independent description of the nonlinear mechanical behavior of linear, monodisperse homopolymer melts, where ³Q₀(ω,Z) is only a function of the frequency ω and the number of entanglements Z. For polydisperse samples, ³Q₀(ω) showed a high sensitivity within the experimental window towards an increasing PDI. At small frequencies, the slope of ³Q₀(ω) decreases until approximately zero as a plateau value is reached, starting at a PDI around 2 and higher.