Critical rubber layer thickness of core-shell particles with a rigid core and a soft shell for toughening of epoxy resins without loss of elastic modulus and strength

Core-shell particles with a rigid silica core and a poly(butyl acrylate) (PBA) rubber shell are used to toughen epoxy resins without loss of elastic modulus and tensile strength. Both the diameter of silica core (D) and thickness of PBA shell (T_s) of silica-PBA core-shell particles are accurately controlled by sol-gel synthesis and seed emulsion polymerization process, respectively. From the results of notched Izod impact tests, a brittle-ductile transition of these epoxy/silica-PBA composites occurs when the rubber shell thickness (T_s) exceeds a critical value (T_sbd). It is found that T_sbd increases with increasing D which implies a critical rubber content (10 wt%) in the core-shell particles or a critical T_s/D ratio (0.0375), above which all composites transit from brittle to ductile failure. The composite elastic modulus decreases with increasing T_s. However, the critical thickness values of T_sm and T_ss, below which composite modulus and tensile strength, respectively, are equal to or higher than the corresponding epoxy matrix values, are both larger than T_sbd. These results prove that brittle polymers can be toughened without loss of elastic modulus and tensile strength. However, the critical surface-to-surface inter-particle distance (L_c) is controlled by both properties of the core-shell particles and polymer matrix. When the rubber shell thickness T_s > T_sbd, L_c is increased when D is decreased. But when T_s < T_sbd, the brittle-to-ductile transition cannot occur, even when L_c is less than 100 nm.