Tensile moduli of co-continuous polymer blends

doi:10.1016/S0032-3861(98)00874-X

Polymer

Volume 40, Issue 24, November 1999, Pages 6645-6650

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Abstract

Co-continuous blends of polyethylene with polypropylene or polystyrene are shown to exhibit high tensile moduli. The experimental results exceed predictions of the moduli using existing models for co-continuous morphologies, and approach the level of the parallel model. These high moduli are found to be isotropic to a good degree, in contrast to fibrillar polymer blends which can have such high moduli only in one direction.

Introduction

Blending of polymers is an effective way of tailoring materials for specific applications. Most polymers are immiscible and blending usually leads to heterogeneous morphologies. The type and scale of these morphologies determine the properties of the blend [1]. This holds especially for mechanical properties such as the tensile modulus. In the case of a droplet–matrix morphology the tensile modulus of a blend will largely be determined by the modulus of the matrix phase. The modulus of a fibrous blend can, however, be largely determined by the modulus of the dispersed (fibrous) phase, especially in oriented samples [2], [3]. Co-continuous blends are expected to fall in between these extremes: neither of the blend components is expected to dominate the moduli of the blends, however, relatively high and isotropic values may be expected because of their interpenetrating phase structure.

The purpose of this study is to demonstrate that co-continuous morphologies are characterised indeed by high isotropic tensile moduli, exceeding values as predicted by existing models for co-continuous structures. In previous papers [4], [5], [6], [7] we have discussed formation, conditions for existence and stability of co-continuous polymer blends. In the present paper we report tensile properties of polyethylene–polypropylene (PE–PP) and polyethylene–polystyrene (PE–PS) blends as a function of composition and morphology, and compare these with existing theoretical models. In a forthcoming paper results for blends with thermoplastic elastomers will be shown and a new model for the moduli of co-continuous structures will be introduced [8].

Section snippets

Theory

Tensile moduli of polymer blends are strongly dependent on the composition and morphology. In literature several models can be found which describe tensile moduli of blends as a function of the composition [9], [10], [11], [12], [13], [14], [15]. Most of these models are valid for a given morphology, others leave the morphology unspecified. Changes of morphology with composition are common in polymer blends, such as the transformation of a dispersed morphology into a co-continuous morphology.

Experimental

Two grades of PS, two grades of low density PE and one grade of PP, shown in Table 1, were used to prepare the PE–PS and PE–PP blend series shown in Table 2. Nine different compositions (9, 17, 27, 35, 46, 56, 67, 78 and 88 vol.% PS in PE and 9, 18, 28, 37, 47, 58, 69, 79 and 89 vol.% PE2 in PP) were made by extrusion at 200°C. The mixing equipment consisted of a 20 mm Collin laboratory extruder equipped with a transport screw, and a static mixer in series with the extruder containing 10 Ross ISG

Results and discussion

The blend systems PE–PS (series I, II and III) and PP–PE (series IV) were chosen for their differences in composition range for full co-continuity [5], [6]. Apart from checking the validity of , , , this enabled a comparison of blends with the same composition but with different morphologies. The results of the extraction experiments are shown in Table 3 and the corresponding lower limiting volume fractions together with the estimates for the upper limits are shown in Table 2. From this table

General discussion

Fully co-continuous blends are found to be characterised by values of their Young’s moduli which are high and isotropic. These large values exceed the predictions of existing models and are nearer to the upper bound for mixtures. This is illustrated in Fig. 7 which summarises the results for the PE–PS blends. These high moduli of co-continuous blends are probably the result of a very effective stress transfer in the fully interpenetrating phase networks. This is not accounted for by existing

Conclusion

Blends of polyethylene–polystyrene and polyethylene–polypropylene with a co-continuous morphology show high isotropic tensile moduli. These high moduli exceed predictions by the models valid for co-continuous morphologies and approach the parallel model. Changing the structure from a droplet–matrix to a co-continuous structure at a given composition can result in a quite significant increase in modulus.

Acknowledgements

Thanks are expressed to the Dutch Ministry of Economic Affairs (IOP/Recycling) for the financial support of this research.

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Tensile moduli of co-continuous polymer blends

Abstract

Introduction

Section snippets

Theory

Experimental

Results and discussion

General discussion

Conclusion

Acknowledgements

Polymerer

Polymer

Polymer

Polym. Engng Sci.