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Appraisal of the Cox-Merz rule for well-characterized entangled linear and branched polymers

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Abstract

We present a critical assessment of the range of validity of the empirical Cox-Merz rule for a wide range of model entangled polymer samples with a well-defined molecular structure, from linear monodisperse and polydisperse polymers, to branched model polymers (i.e. stars, H-polymers, and combs) and blends of linear polymers of the same chemistry. We focus on melts and concentrated solutions. Overall, we find that the simple empirical rule is obeyed rather well for the investigated cases. As often reported in the literature, relatively small systematic failures occur with the steady viscosity being below the complex one at high rates for most polymers, with linear polydisperse polymers (with a polydispersity index of about 2) being a notable exception. For the latter polymers, the rule is obeyed identically within experimental error. More unusual failures, with the steady shear viscosity being higher than the complex viscosity, are found for branched polymers with more than one branch point. More specifically, these unusual failures are observed at very high branching levels, when the backbone of the polymer is being stretched at low rates due to the motion of the branch points. The extra stress coming for the stretch renders the steady viscosity higher than the complex one. Due to the well-characterized nature of the combs, we can state that failures of the latter type are only apparent when the branches comprise more than 70 % of the molecular structure of the comb. This estimation could serve as a rough guideline in applications, although it is only a necessary and not sufficient condition for these failures to occur.

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Acknowledgments

We thank J. Roovers, K. Ratkanthwar, and N. Hadjichristidis for generously providing the model branched polymers used in this work. We also thank S. Coppola for generously providing polydisperse linear PS samples. We further thank G. Marrucci, G. Ianniruberto, and R. Pasquino for stimulating discussions. Support from the EU (FP7 ITN DYNACOP, grant 214627 and from the FP7 infrastructure ESMI, GA-262348) is gratefully acknowledged.

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  1. Foundation for Research and Technology—Hellas (FORTH), Institute of Electronic Structure and Laser (IESL), Heraklion, 71110, Crete, Greece

    Frank Snijkers & Dimitris Vlassopoulos

  2. Department of Materials Science and Technology, University of Crete, Heraklion, 71003, Crete, Greece

    Dimitris Vlassopoulos

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  1. Frank Snijkers
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Correspondence to Frank Snijkers.

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Snijkers, F., Vlassopoulos, D. Appraisal of the Cox-Merz rule for well-characterized entangled linear and branched polymers. Rheol Acta 53, 935–946 (2014). https://doi.org/10.1007/s00397-014-0799-6

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  • DOI: https://doi.org/10.1007/s00397-014-0799-6

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