Linear thermoviscoelasticity and characterization of noncrystalline EPDM rubber networks

Abstract

An experimental study has been made to specify how the time‐temperature superposition and the linear viscoelastic characteristics vary with the degree of crosslinking for a broad class of noncrystalline peroxide‐cured EPDM networks. A new, very sensitive method is applied to determine the horizontal and vertical shift functions in an independent way. All uncrosslinked samples are thermoelasticoviscously simple with horizontal shift functions aT of the WLF type and vertical shift functions almost independent of temperature, in agreement with recent theoretical understanding. Upon crosslinking, these materials become thermoviscoelastically complex networks, but superposition can still be accomplished by assuming different temperature dependences for the relaxational strength and the equilibrium modulus. The aT functions can be taken independent of the degree of crosslinking. The vertical shift functions bT for the relaxational strength vary with the degree of crosslinking between theoretical predictions for uncrosslinked and perfectly crosslinked EPDM networks. The equilibrium moduli of the lightly cured networks decreases with increasing temperature, which is ascribed to the presence of interchain associations between ethylene sequences in the trans state. Upon further crosslinking, these effects gradually vanish and eventually the networks can be described as viscoelastically simple with an energy elastic contribution due to the ethylene trans‐gauche transitions. The linear viscoelastic characteristics, namely the storage and loss moduli and compliances and phase‐angle master curves and the relaxation and retardation spectra are discussed as a function of the degree of crosslinking. A sol/gel analysis and equilibrium swelling measurements complete the experimental characterization of three familes of five EPDM networks each.

Viscoelastic behaviour of model polyurethane networks in the main transition zone

“…In contrast to our work, Scholtens [19] did not obtain a proportionality between shear modulus and temperature from his measurements on EPDM-networks (terpolymers of ethylene, propylene and a diene monomer). His measurements should be compared with the low frequency range of our master curves.…”

Chemical Cross‐linking of Polypropylenes Towards New Shape Memory Polymers

“…One way to achieve such networks is cross‐linking of linear homo‐ or copolymers. Polyethylene (PE), natural rubber (NR), ethylene vinyl acetate (EVA), ethylene propylene diene rubber (EPDM), and many others can be cross‐linked using peroxide, siloxane, or radiation curing . Unfortunately, cross‐linking of polypropylene, a mass polymer with remarkable mechanical properties, is more complicated due to the presence of tertiary C‐atoms .…”

“…Polyethylene (PE), natural rubber (NR), ethylene vinyl acetate (EVA), ethylene propylene diene rubber (EPDM), and many others can be cross-linked using peroxide, siloxane, or radiation curing. [14][15][16][17][18][19] Unfortunately, cross-linking of polypropylene, a mass polymer with remarkable mechanical properties, is more complicated due to the presence of tertiary C-atoms. [ 20,21 ] The main problem consists in the nature of radical reactions since the majority of macro radicals decays by β-scission resulting in a decrease of molecular…”

A critical analysis of the effect of crosslinking on the linear viscoelastic behavior of styrene–butadiene rubber and other elastomers

“…Additional reduction of the data was carried out to obtain master curves, and the heat of dissociation of the entanglement points was determined from the temperature dependence of the new shift factors. Scholtens13 reported that TTS with nonstandard vertical shifting was required for lightly crosslinked ethylene propylene diene monomer (EPDM) elastomers, which was attributed to the “presence of interchain associations between ethylene sequences in the trans‐state.” Scholtens used a modified TTS scheme similar to the McCrum–Morris reduction method,18 where different temperature‐dependent factors were used for the equilibrium contribution and the relaxing contribution to the storage modulus. Using this method, the reduced functions could be superposed and the vertical shifts were found to be dependent on the crosslink density.…”