Entanglement networks of 1,2‐polybutadiene crosslinked in states of strain. IV. States of ease and stress–strain behavior

Rick L. Carpenter; Ole Kramer; John D. Ferry

摘要

AbstractLinear 1,2‐polybutadiene, glass transition temperature (Tg) −18°C, is crosslinked at −10°C, to −20°C by γ irradiation while strained in simple extension, with extension, ratios (λ0) from 1.2 to 2.7. After release, the sample retracts to a state of ease (λs) at room temperature. From equilibrium stress–strain measurements up to a stretch ratio relative to the state of ease (Λ) of 1.2, together with λ0and λs, the concentration of network strands terminated by trapped entanglements (νN) is calculated. For this purpose, a three‐constant Mooney–Rivlin formulation is used, in which the entanglement network is described by Mooney–Rivlin coefficientsC1NandC2N, whereas the crosslink networks is described by the coefficientC1xonly. The ratio ψN=C2N/(C1N+C2N) is estimated from parallel studies of nonlinear stress relaxation of the uncrosslinked polymer, taking into account the thermal history before and during irradiation. For substantial degrees of crosslinking, i.e., forR0′= νx/νN>0.4 (where νNis the concentration of network strands terminated by crosslinks), and for λ0<1.8,C2Nagrees rather well with the value obtained from stress relaxation of the uncrosslinked polymer in the range of time scale where it is nearly independent of time (1.87 X 105pascals). The corresponding value of νNis 2.3 × 10−4moles/cm3, in good agreement with that obtained from viscoelastic measurements of the uncrosslinked polymer in the plateau zone (2.5 × 10−4). However, forR0′≅ 0.2, smaller values ofC2Nand νNare obtained, indicating that for low degrees of crosslinking the entanglements are not completely trapped. Also, for higher values of λ0,C2Nand νNturn out to be somewhat smaller. Similar, less extensive results were obtained previously on a 1,2‐polybutadiene with somewhat higher vinyl content and a higherTg. Crosslinked samples of both these polymers were subjected to equilibrium stress–strain measurements in simple elongation from the state of ease at higher strains up to Λ = 1.7. The results agreed closely with calcul

Entanglement networks of 1,2‐polybutadiene crosslinked in states of strain. IV. States of ease and stress–strain behavior

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