Spontaneous Three‐Layer Formation in the Curing of Polyimide/Epoxy Blends

Morphological structures of epoxy resin toughened by a fluorinated polyimide were observed by SEM. Three layers formed spontaneously parallel to the film surface in the range of polyimide weight fraction w = 0.14–0.20, when the samples were cured at 130 °C. The two outer layers consisted mostly of the epoxy‐rich phase. The middle layer consisted of bicontinuous domains of polyimide‐ and epoxy‐rich phases with a characteristic domain size much smaller than the outer layer thickness. At w = 0.10–0.13, aggregates of polyimide‐rich particles formed. Observation of the morphological development at w = 0.15 suggested that polyimide‐rich domains were distributed over the whole sample in the early period. magnified image