Homoionic inorganic montmorillonites as fillers for polyamide 6 nanocomposites

Abstract

The homoionic montmorillonites Ca-MMT, Mg-MMT, Ba-MMT and Ca-MMT intercalated with ε-caprolactam – Ca-MMT·CL were prepared from commercial Na-MMT and characterized by WAXS and TGA. They were used as fillers for nanocomposites of polyamide 6 synthesized either by anionic polymerization of ε-caprolactam (monomer casting) or by melt blending. WAXS analysis showed that the intercalation of MMT by the polyamide was complete for all nanocomposites, with only a very small fraction of exfoliated platelets being detected by TEM. The decrease in the number of layers in the MMT tactoids suggests that tactoid splitting was lower for the blended nanocomposites than for the polymerized ones. Both the rate of polymerization and the polyamide yield in the nanocomposites were comparable to those of an unfilled system. The MMT fillers, the density of which was more than twice that of the ε-caprolactam in which they were suspended, sedimented during the first stage of polymerization. TGA was used to determine the degree of sedimentation at various levels of the resulting mold. In line with the coordination of polyamide chains to the surface cations of MMT particles, the sedimentation level increased in the following sequence: Mg-MMT < Ba-MMT < Ca-MMT·CL << Na-MMT. Ca-MMT was found to be the only non-sedimenting filler suitable for use in the synthesis of polyamide nanocomposites by either monomer casting or the use of reactive injection molding (RIM) technologies.