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Influence of molecular parameters on the stress dependence of viscous and elastic properties of polypropylene melts in shear

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Abstract

The stress dependencies of the steady-state viscosity η and, particularly, that of the steady-state elastic compliance J e of various linear isotactic polypropylenes (PP) and one long-chain branched PP are investigated using creep-recovery tests. The creep stresses applied range from 2 to 10,000 Pa. In order to discuss the stress-dependent viscosity η and elastic compliance J e with respect to the influence of the weight average molar mass M w and the polydispersity factor M w/M n the PP are characterized by SEC–MALLS. For the linear PP, linear steady-state elastic compliances \(J_{\rm e}^0 \) in the range of 10 − 5–10 − 3 Pa − 1 are obtained depending on the molar mass distribution. \(J_{\rm e}^0 \) of the LCB-PP is distinctly higher and comes to lie at around 10 − 2 Pa − 1. \(J_{\rm e}^0 \) is found to be independent of M w but strongly dependent on polydispersity. η and J e decrease with increasing stress. For the linear PP, J e as a function of the stress τ is temperature independent. The higher M w/M n the stronger is the shear thinning of η and the more pronounced is the stress dependence of J e. For the LCB-PP, the strongest stress dependence of η and J e is observed. Furthermore, for all PP J e reacts more sensitively to an increasing stress than η. A qualitative explanation for the stronger stress dependence of J e compared to η is given by analyzing the contribution of long relaxation times to the viscosity and elasticity.

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Notes

  1. According to the literature stereo defects generated by the synthesis with metallocene catalysts are responsible for interrupting the length of the crystallisable, isotactic sequences and, thus, lowering T m (Phillips and Wolkowicz 1996).

  2. Creep-recovery experiments at higher stresses could not be performed because edge fracture of the sample occurs during the experiment.

  3. The \(J_{\rm e}^0 \) values given by Patham and Jayaraman lie in the order of 5·10 − 4 Pa − 1 for all the three mLLDPE investigated, and thus, in the range of LCB-mLLDPE (Resch et al. 2009). All other quantities presented in their paper confirm the hypothesis that their materials contain LCB: 1) The δ(∣G*∣)-plots show a deflection point, 2) η 0 is higher than expected for linear products at the given M w, and 3) the activation energy E a lies in the range of 37 to 41 kJ mol − 1, whereas for mLLDPE values around 33 kJ mol − 1 are expected (e.g. Wood-Adams 2001).

  4. The spectrum is calculated by the method of Stadler and Bailly (2009) from a frequency sweep extended by recovery data according to the method of Kaschta and Schwarzl (1994a, b).

  5. In the nonlinear stress regime Eqs. 8 and 9 are not strictly valid anymore, as they are derived from the linear viscoelastic theory. However, a similar dependence of η and J e on the relaxation spectra as for η 0 and \(J_{\rm e}^0 \) can be assumed.

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Acknowledgements

The authors would like to thank the German Research Foundation (DFG) for the financial support of this work. The contributions of Mrs. Inge Herzer, Mrs. Marika Sturm, and Mrs. Michelle Malter regarding the SEC–MALLS, the IR-spectroscopy, and the DSC measurements are gratefully acknowledged. Prof. Dr. Florian Stadler (School of Semiconductor and Chemical Engineering, Chonbuk National University) deserves acknowledgement for the calculation of the relaxation spectrum and its evaluation concerning η 0 and \(J_{\rm e}^0 \).

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  1. Institute of Polymer Materials, Friedrich-Alexander-University Erlangen-Nürnberg, Martensstr. 7, 91058, Erlangen, Germany

    Julia A. Resch, Joachim Kaschta, Friedrich Wolff & Helmut Münstedt

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  1. Julia A. Resch
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  2. Joachim Kaschta
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  3. Friedrich Wolff
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  4. Helmut Münstedt
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Correspondence to Helmut Münstedt.

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Resch, J.A., Kaschta, J., Wolff, F. et al. Influence of molecular parameters on the stress dependence of viscous and elastic properties of polypropylene melts in shear. Rheol Acta 50, 53–63 (2011). https://doi.org/10.1007/s00397-010-0502-5

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  • DOI: https://doi.org/10.1007/s00397-010-0502-5

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