Design of the ultimate behavior of tetrafunctional epoxies modified with polysulfone by controlling microstructure development

The reaction-induced phase separation in a tetrafunctional epoxy-cyclic anh ydride system modified with polysulfone (PSF) was followed by optical micro scopy (OM), light scattering (LS), and scanning electron microscopy (SEM). The selected system was N,N,N',N'-tetraglycidyl-4,4'-diamino diphenylmethan e cured with methyl tetrahydrophthalic anhydride, in the presence of variab le PSF concentrations. The different experimental techniques allow us to es tablish the phase separation mechanism. For modifier concentrations close t o the critical point, 10 and 15 wt% PSF, phase separation proceeded by spin odal demixing (SD). For a modifier concentration much lower than the critic al point, 5 wt% PSF, phase separation occurred via the nucleation and growt h (NG) mode. For 7.5 wt% PSF, depending on the cure temperature, SD or NG w as observed. Dynamic mechanical behavior of the resulting materials had bee n discussed based on fractionation of different species during the phase se paration process. The fracture toughness increased significantly when bicon tinuous (10 wt% PSF) or phase-inverted (15 wt% PSF) structures were generat ed. For mixtures containing 15 wt% PSF, the dependence of fracture toughnes s on the stoichiometric ratio (anhydride groups/epoxy groups) was analyzed. (C ) 1999 John Wiley & Sons, Inc.