Microdeformation and fracture mechanisms in polyamide-6/organoclay nanocomposites

A study on the mechanisms for embrittlement of polyamide-6 (PA-6) by nanometer-sized clay in nanocomposites is reported in this paper. Tensile modulus and yield strength in these composites were found to increase with clay concentration, while the strain at rupture decreased. Similar to the strain at rupture, fracture toughness also decreased dramatically. Investigation by transmission electron microscopy alone indicates that multiple crazing appears to be the only significant toughening mechanism for the nanocomposite with 2.5 wt % clay which does not explain why composites with more clay are significantly more brittle. The deformation behavior was therefore further studied using wide (WAXS) and small (SAXS) angle X-ray scattering, scanning electron microscopy (SEM), and optical microscopy. WAXS detected the existence of the gamma-phase of PA-6, probably due to the presence of clay. However, the occurrence of this crystalline phase cannot explain the observed changes in mechanical behavior. SAXS study indicated that crazing occurred in the PA-6 matrix in tension and that the craze concentration depended on clay loading. The highest craze concentration was observed at 2.5 wt % clay. The existence of crazes in nanoclay composites was also confirmed by optical microscopy and transmission electron microscopy. In addition, SAXS showed that microcracking also occurred in these nanocomposites along with crazing, with the highest concentration of microcracks observed at 2.5 wt % clay loading. TEM showed that microcracks occurred near the interface of clay and matrix. The existence of a high concentration of crazes and microcracks in the nanocomposite with 2.5 wt % of clay is the likely cause for its high toughness relative to those composites containing higher clay loading. At higher clay loadings the high incidence of microcracks probably prevented the crazing mechanism from operating to its fullest possible extent, thus resulting in low toughness.