Studies on Stress Relaxation and Thermomechanical Properties of Poly(acrylonitrile-butadiene-styrene) Modified Epoxy−Amine Systems

Abstract

Epoxy networks based on diglycidyl ether of bisphenol A cured with diamino diphenyl sulfone and modified with poly(acrylonitrile-butadiene-styrene) (ABS) were prepared according to two different cure schedules, one with a single step curing and the other with two step curing. The samples were carefully analyzed by thermomechanical analysis (TMA) to understand the physical aging phenomenon. The TMA runs on samples with single curing step are strained and show “bumps” in the expansion traces indicating that internal stress relaxation takes place during heating. On the other hand, the samples prepared by two-step curing were not strained and hence no bumps occurred. The ABS modified epoxy blends were further characterized by Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), mechanical tests using a universal testing machine, and field emission scanning electron microscopy (FESEM). The FTIR spectroscopy study confirms that the epoxy/amine reaction was complete, irrespective of the cure schedule used. TEM micrographs reveals heterogeneous morphology for all the blends studied. DSC and TGA were employed to evaluate the thermal stability of epoxy/ABS blends. The mechanical properties of both strained and unstrained samples were investigated in detail and are correlated to the blend morphologies. The result shows that the mechanical and morphological properties are affected by blending with the thermoplastic but not with the cure schedule used. The addition of low ABS amounts (≤6.9 wt %) in the epoxy resin resulted in epoxy matrix/ABS particle morphologies leading to more than 100% increase in tensile toughness compared to neat cross-linked epoxy. FESEM micrographs of fractured surfaces proved fracture mechanisms such as nanocavitation, crack path deflection, crack pinning, ductile tearing of the thermoplastic, and local plastic deformation of the matrix. In contrast, when cocontinuous morphologies are formed at higher ABS loadings the mechanical properties are much lower than those formed for the neat epoxy system.