Effect of Molecular Weight, Polydispersity, and Monomer of Linear Homopolymer Melts on the Intrinsic Mechanical Nonlinearity 3Q0(ω) in MAOS

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• 作者：Miriam A. Cziep ; Mahdi Abbasi ; Matthias Heck ; Lukas Arens ; Manfred Wilhelm
• 刊名：Macromolecules
• 出版年：2016
• 出版时间：May 10, 2016
• 年：2016
• 卷：49
• 期：9
• 页码：3566-3579
• 全文大小：705K
• 年卷期：0
• ISSN：1520-5835

文摘

Linear mono- and polydisperse homopolymer melts have been investigated with Fourier transformation rheology (FT rheology) to quantify their nonlinear behavior under oscillatory shear via mechanical higher harmonics, i.e., I_3/1(ω,γ₀). Master curves of the zero-strain nonlinearity, ³Q₀(ω) ≡ lim_γ0→0 I_3/1/γ₀², have been created for these linear homopolymer melts, applying the time–temperature superposition (TTS) principle. The quantity ³Q₀(ω) is examined for its dependence on molecular weight, molecular weight distribution, and monomer. The investigated nonlinear master curves of ³Q₀(ω) for polymer melts with a polydispersity index (PDI) of about 1.07 or smaller display the expected scaling exponent of ³Q₀(ω) ∝ ω² at low frequencies until a maximum, ³Q_0,max, is reached. This maximum ³Q_0,max was found to be in the magnitude of the longest relaxation time τ₀ and the value of ³Q_0,max to be weakly dependent on molecular weight, or the number of entanglements Z, with ³Q_0,max ∝ Z^0.35. Within the measured experimental window, the initial slope at low frequencies of nonlinear master curves is very sensitive toward the molecular weight distribution, as quantified through the PDI. The slope of ³Q₀(ω) decreases until approximately zero as a limiting plateau value is reached at a polydispersity of around PDI ≈ 2. The experimental findings are also compared to ³Q₀(ω) predictions from pom-pom and molecular stress function (MSF) constitutive models. Analytical solutions of ³Q(ω,γ₀) for diminishing small strain amplitudes (γ₀ → 0) are presented for each model, and an asymptotic solution for ³Q₀(ω) is derived for low and high frequencies for monodisperse samples. This simplified equation is a function of Deborah number (De = ωτ₀) in the general form of ³Q₀(De) = aDe²/(1 + bDe^2+k). This equation was fitted to our experimental data of monodisperse homopolymer melts. It is shown that under these conditions the parameters a and b are only functions of the number of entanglements Z and are independent of the investigated monomers. Consequently, a and b can be linked to general polymer properties. With this article, the mechanical nonlinear response of linear polymer melts with regard to molecular weight and distribution as well as monomer type is quantified. The here presented results should be of great interest toward constitutive model development and toward computational polymer physics, e.g., molecular dynamic simulations, as our results seem to quantify general features in nonlinear rheology of linear homopolymer melts.