Heterogeneous Polymer Blends II. Physical Properties of Binary Elastomer Composites

Abstract

Tensile properties of two-phase elastomer-elastomer vulcanizates were measured and evaluated in terms of the initial number average molecular weight (M_n), the crosslink density (v_eff) and the glass temperatures (Tg's) of the composite. Interpretations of the tensile behavior of elastomers with a single Tg are reasonably clear. Such interpretations are much more complex for heterogenous blends of two materials having dissimilar viscoelastic properties.

Heterogeneous blends of either an EPDM (−60° Tg) with a BR (−60° Tg) or with an EPDM (−60° Tg) with a BR (−5° Tg) were prepared with differing composition, morphology and two levels of radiation crosslinking. The morphology of the samples used in this study has been described in Part 1.[1]

The rupture energies of the blends having −60°/05° Tg's were usually greater than a weight averaged value of the components and for equivalent compositions were higher than that obtained for blends having equal (−60°/−60°) Tg's. This favorable synergism persisted after the data were put into a form which takes into account the large effects of M_n.V_eff and T_g. The effect of domain size in the experimental range of 0.2 to 10 micrometres on rupture energy was small. There is some evidence for a maximum in rupture energy at the 1 micrometre level.

At a given Mooney–Rivlin 2C₁ modulus, a higher rupture energy was observed for vulcanizates from the higher M_n blends. Such higher M_n vulcanizates have a higher ratio of physical entanglements to chemical crosslinks and a lower proportion of free ends in the network.