Linear thermoviscoelasticity and characterization of slightly crystalline EPDM networks

Three EPDM network families with various degrees of chemical crosslinking and slightly crystalline below about 350 K have been characterized by thermoviscoelastic and equilibrium swelling measurements. Between 300 and 450 K the thermoviscoelasticity of these elastomers, whether cured or not, is complex. Important differences from previous results on noncrystalline EPDM networks are their dependence on waiting time and temperature history below 350 K, and an equilibrium modulus for the uncured elastomers. These effects increase with rising ethylene content of the EPDM and are related to the crystallinity in the networks. A previously proposed modified time‐temperature ( t‐T ) superposition scheme is also applicable to these data, and the resulting frequency shift functions a T are of the usual WLF type. The vertical shift functions for the relaxational components b T are different from those for the equilibrium moduli. Experimental data demonstrate that the absolute values of the relaxational components, i.e., G ″ (ω) and H (τ), are not changed by small variations in crystallinity. In addition, larger variations in crystallinity do not affect their frequency dependence, only their absolute values. On the contrary, the equilibrium modulus, and therefore all characteristics containing this as a component, is extremely sensitive to small variations in crystallinity, and hence in temperature. The linear viscoelastic characteristics of these slightly crystalline networks and their deviations from thermoviscoelastically simple behavior are discussed as functions of the ethylene content, temperature, and degree of crosslinking of the elastomer. The data provide evidence for three contributing factors, namely chemical crosslinks, crystallinity, and a more disordered type of crystallinity termed microparacrystallinity.