Toughness enhancement of polyimides by in situ generation of silica particles

Films of particulate polyimide–silica hybrids were produced by the sol–gel process from solution mixtures of a commercial polyamic acid and pre-hydrolysed tetraethoxysilane. Mechanical properties were evaluated at different temperatures, varying from 20 to 250 °C. The fracture toughness characteristics were studied by the ‘essential work of fracture’ method using double edge-notched specimens of different ligament lengths. The essential (we) and the non-essential or plastic work of fracture (²wp) were estimated from plots of the specific total work of fracture versus ligament length. The results showed that not only the modulus and yield strength but also the fracture toughness increased when submicron silica particles, produced by the sol–gel method, were dispersed in the polyimide matrix. In particular, it was found that both we and ²wp increased with temperature to a larger extent than the parent polyimide. This enhancement in the fracture toughness was attributed primarily to extensive cavitations originating at the particle interface with the matrix.