Mechanical properties of thermosets

Abstract

The tensile properties of a DGEBA (diglycidylether of bisphenol A)-norbornene anhydride network (T _g≈130±5 °C), were studied in the range (220 K-T _g); 4×10⁻⁴ to 14×10⁻³S⁻¹. The viscoelastic spectrum (1 Hz) reveals a low β transition at 220 K. The bulk modulus is practically constant between 200 K andT _g — 20 K. The Poisson's ratio increases very slowly untilT _g — 30 K. Then it increases rapidly to reach its asymptotic value (0.5) near toT _g. The tensile (E) and shear (G) moduli display the classical behaviour linked to viscoelasticity. Plastic yielding occurs atT ≥ 80 °C, the elongation at yield is almost temperature and strain rate independent (G3_y = 0.035), whereas the yield stress obeys Kambour's relationship: σ_y = 1.1 (T _g —T) and Eyring's law (activation volume = 914cm³mol⁻¹). Physical ageing at 120 °C strongly affects the yield stress and the ductility. The maximum draw ratio, obtained atT ≥T _g, is Λ_RC = 1.35, which seems to be consistent with the network's crosslink density.