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On “modulus shift” and thermorheological complexity in polyolefins

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Abstract

Sparsely branched polyolefins often exhibit a thermorheological complexity, which was reported to be maskable by a modulus shift. However, the only physical background for a modulus shift is a density change, and this influence factor is only small in the relatively narrow temperature regime accessible by polyolefins. This paper deals with the question, how this modulus shift can be caused by experimental artifacts and real effects. The physical background of these two contributions to a vertical activation energy, as well as a meaningfulness of the application of a modulus shift, is found not to be given for polyolefins, when measuring only in a temperature range between 130 and 230 °C.

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Notes

  1. Actually, this dependence has been reported years before by Fulcher (1925a, b), Vogel (1921), and Tammann and Hesse (1926) in a different form—the so-called VFTH-temperature dependence, being equivalent to the WLF equation.

  2. Measuring the thermal expansion coefficient of a geometry is actually relatively easy. Most modern rheometers have a normal force control. All that has to be done is to zero the gap at a given temperature (in the case presented here room temperature) and set the normal force control to a low force (here: 0.5 N). Then the temperature is changed and the values of the apparent gap (the real gap is zero!) are recorded. It is important to wait until the gap has fully stabilized before recording the point at the respective temperature (this is usually later than when the temperature has stabilized, as stainless steel is a rather bad heat conductor). In case of the setup used, the required waiting time increased with temperature (because the temperature gradient inside the geometry increases) and was between 5 and 10 min. In order to be sure to be in equilibrium, the chosen waiting time was >30 min.

    If a rheometer without a normal force controller is used, a thin steel bar should be put in the gap when zeroing it and removing it before increasing the temperature again – this avoids damage to the air bearing due to thermal expansion. At the desired temperature, the gap is to be determined, at which the geometry and the steel bar are locked to each other in the same way as during the initial zeroing. This way the thermal expansion coefficient can be measured with comparable accuracy to the method proposed for rheometers with normal force control.

  3. Hashmi et al. (2012) found that it is possible to use very big samples (about factor 2 larger than necessary to fill the geometry) to use a constant correction factor for getting correct rheological data of hydrogels during swelling. Remmler T (personal communication, 2015) confirmed the small influence of outward bent surfaces, while inward bent surfaces will significantly reduce the viscosity.

  4. The authors’ experience values on this matter are that for samples with acceptably short relaxation times (shorter than 1 min) and thus zero shear-rate viscosity η 0 (<106 Pas), achieving this is not a significant experimental challenge, but for very high viscosity melts achieving such an optimal sample surface proves difficult.

    This is one of the reasons of the inferior reproducibility of such samples. Also the insufficient adhesion of these samples to the geometry and the possible presence of orientations and small fractures play a role (such high viscosity samples tend to be very brittle in the melt, ripping, in some cases, even at a Hencky strain εH below 1 in elongational tests).

    In case of very low viscosity samples (η 0 < 100 Pas) the gravity prevents an outwards curvature of the free sample surface.

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Acknowledgments

The authors would like to thank the Nanshan District Key Lab for Biopolymers and Safety Evaluation (No. KC2014ZDZJ0001A) and Shenzhen City (JCYJ20140509172719311). This paper was inspired by several papers we reviewed and referees of our papers, explicitly asking us for using a modulus shift or questioning this concept.

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Authors and Affiliations

  1. College of Materials Science and Engineering, Shenzhen University, Shenzhen, 518060, People’s Republic of China

    Florian J. Stadler, Shiguo Chen & Shaojun Chen

  2. Nanshan District Key Lab for Biopolymers and Safety Evaluation, Shenzhen, 518060, People’s Republic of China

    Florian J. Stadler, Shiguo Chen & Shaojun Chen

  3. Shenzhen Key Laboratory of Special Functional Materials, Shenzhen, 518060, People’s Republic of China

    Florian J. Stadler, Shiguo Chen & Shaojun Chen

  4. Shenzhen Engineering Laboratory for Advanced Technology of Ceramics, Shenzhen, 518060, People’s Republic of China

    Florian J. Stadler, Shiguo Chen & Shaojun Chen

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  1. Florian J. Stadler
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  2. Shiguo Chen
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Correspondence to Florian J. Stadler.

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Stadler, F.J., Chen, S. & Chen, S. On “modulus shift” and thermorheological complexity in polyolefins. Rheol Acta 54, 695–704 (2015). https://doi.org/10.1007/s00397-015-0864-9

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  • DOI: https://doi.org/10.1007/s00397-015-0864-9

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