Epoxy/thermoplastic blends have been extensively studied because of the importance in toughening application and the fundamental academic interests.Morphological structure formation through reaction-induced phase separation in the diglycidyl ether of bisphenol A (DGEBA) and polysulfone (PSF) blends are investigated with various amounts of PSF and different kinds of curing agents.The fractured surfaces are studied with the scanning electron microscope

on the cross section or normal to the film surface of the sample film.Several morphological structures are observed at different PSF concentrations.Sea-island and inter-connected nodular structures are respectively obtained at lower and higher PSF concentrations.At appropriately middle PSF compositions

usually around 0.15 weight fractions for the samples

an unusual three-layered structure is formed.The upper and lower layers are similar

with PSF-rich particles sparsely dispersing in the epoxy-rich matrix; most of the PSF-rich domains are located in the middle layer

where bicontinuous structure is formed.It is found that the three-layered structure is developed from an initially bicontinuous structure

which will retract (PSF-rich network shrink) toward the middle region of the sample film with the proceeding of reaction and phase separation.Driving force for this phenomenon is the change of the Gibbs free energy

shift of the coexistence curve

volume fraction decrease of the PSF-rich network and the dynamic asymmetry between the components.At middle PSF compositions

the layered structure can be formed at various curing temperatures.When different curing agents are used

reaction rate will change

however

the reaction rate is not an essential cause for the formation of the layered structure.When the curing agent has a higher reactivity with the epoxy (for the DGEBA

2-ethyl-4-methylimidazole (2E4MZ) and PSF system)

reaction kinetics may be better competitive with the phase separation kinetics

and the thickness ratio of the middle layer will increase

compared with the DGEBA

4

4'-diaminodiphenyl sulfone (DDS) and PSF system.However

better compatibility on the molecular level is introduced when the medium reactive curing agent

4

4'-diaminodiphenyl methane (DDM) is used to react with the DGEBA.The consequence is that the components are more dynamically symmetric during the phase separation

and the bicontinuous structure may not (or need not) develop to become a layered structure.