Prestressed double network thermoset: preparation and characterization

An innovative scheme to prepare Prestressed double network (PDN) epoxies is presented using a judicious combination of tetrafunctional curatives that have similar molecular weights but different reaction kinetics. A diglycidyl ether of bisphenol A epoxy monomer was reacted stoichiometrically and sequentially with various molar ratios of an aliphatic polyetheramine curing agent and an aromatic curing agent. Deformation was imposed on the partially cured resins after the formation of the first network, and postcuring was conducted at 50% compressive strain. Physical properties of the resulting PDN epoxies were examined using thermomechanical analysis, dynamic mechanical analysis, uniaxial tensile test, and plane-strain fracture toughness test. The application of prestress resulted in no changes in glass transition temperature, coefficient of linear thermal expansion, and Young’s modulus. However, a marked increase in fracture toughness is observed, accompanied by strong birefringence and visible roughness on the fracture surface.