Investigation of failure mechanisms in polymer composites by simultaneous measurement of ultra-small-angle scattering and acoustic emission during the deformation. II. Evaluation of the interface strength

The failure mechanism in polystyrene (PS) filled with glass beads was investigated with the methods of ultra-small-angle X-ray scattering (USAXS) and acoustic emission (AE) analysis simultaneously. The interface in the composites was tailored using carboxyl-terminated PS (PS-COOH). Monolayers of PS-COOH were prepared on the glass surface in a wide range of grafting densities, from single chains to a polymer brush. The layers were characterized by ellipsometry, scanning probe microscopy, and the wetting method. The morphology of the layers is in good agreement with the theoretical predictions. USAXS experiments identify submicrovoids (SMVs) at the interface in the composites even before a tensile deformation. The uniaxial loading of the specimens is followed by a debonding (dewetting) process that is identified by simultaneous increase of integral scattering intensity and AE signals. The stress of debonding is proportional to the grafting density of PS-COOH. The amplitude of the AE signals decreases as the grafting density increases. Sizes of SMVs do not depend on the draw ratio, but increase as the interface strength increases. From the analysis of all experimental data, a failure mechanism of the composites is suggested. It is proposed that the debonding stress characterizes the interface strength, while the AE amplitude gives information about the location of microdefects.