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We have shown that the addition of certain nanoparticles mixtures can enhance flame retardancy to a greater degree than the addition of either of the nanoparticles alone. The effect is particularly efficient in polymer blends, where more variables need to be considered in the flame behavior. We studied PS/PMMA blends and the respective homopolymers. In this paper we focused on the combination of multiwall carbon nanoutubes (MWCNTs) with clays. We found that the flame retardant (FR) particles segregate to the MWCNTs preferentially, thereby allowing the clays to segregate to the blend interfaces. In this manner both phase stabilization and good dispersion can be achieved. Both long (0.5−40 μm) and short (1−2 μm) MWCNTs were studied and the results indicated that the s-MWCNTs produced the superior flame retardant properties. Addition of all types of nanoparticles, including the standard FR formulations, decreased the time to ignition. On the other hand, the combination of s-MWCNT and clay significantly reduced the heat release rate (HRR) and mass loss rate (MLR) relative to compounds with only one type of nanoparticles. Electron microscopy images of the nanocomposites and the chars showed that after heating the s-MWCNTs were able to diffuse and form a distinct phase in the nanocomposite. Exclusion of the clays allowed the s-MWCNTs to achieve better physical contacts, thereby improving the thermal conductivity. In contrast, the l-MWCNTs were entangled and therefore unable to move. The clays formed a barrier between the tube contacts, resulting in an increase of the specific heat and the HRR and MLR relative to the unfilled or the compound with only clays. We therefore conclude that one must consider the organization of nanoparticles, as well as their chemical nature when designing flame retardant nanocomposites, since specific synergies may be established which can reduce the overall concentration of fillers. [ABSTRACT FROM AUTHOR]