A rheological study of long branching in polyethylene by blending

Authors

  • Mihailo S. Jaćović,

    1. Materials Research Laboratory, Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, and Arco Polymers, Monroeville, Pennsylvania
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    • On leave from the University of Beograd (Belgrade), Yugoslavia.

  • Dorothy Pollock,

    1. Materials Research Laboratory, Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, and Arco Polymers, Monroeville, Pennsylvania
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    • Arco-Polymers, Monroeville, Pennsylvania.

  • Roger S. Porter

    1. Materials Research Laboratory, Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, and Arco Polymers, Monroeville, Pennsylvania
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Abstract

Steady shear viscosities, dynamic viscosities and moduli, and the corresponding activation energies for flow were examined for a branched polyethylene, a linear polyethylene, and three of their blends at 150° and 190°C. The polyethylenes were chosen to have closely matched molecular weights and distributions. An R-17 Weissenberg rheogoniometer and an Instron capillary rheometer were used. At lower stress, the branched polymer had a higher viscosity than the linear one, possibly because of the contribution of long branches to entanglements. At high stress, this contribution is reduced and the inherently smaller coil dimensions likely become responsible for the lower viscosity of the branched polymer. The activation energy for the branched polymer is high and decreases with stress, in contrast to the low and almost-constant value for the linear polymer. The effects here of pressure on compression are considered. The entanglements of long branches may also decrease with increasing temperature. With decreasing stress, the activation energy for branched polymer tends to become constant, corresponding to an absence of pressure effects and an equilibrium entanglement of long branches for a given temperature range. The linear relationship between activation energy and blend composition problably means that any compressional effects, like free volume, are additive and that long-branch entanglements rearrange with added linear molecules. The linearity may be the result, in part, of a broad distribution for the lengths of long branches.

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