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Previous work has shown that the formation of a network structure of nanoparticles within a polymer matrix can significantly reduce nanocomposite flammability and that viscoelastic properties could be utilized to predict their flammability reduction. The present work extends this type of investigation to the study of clay and carbon nanotube nanocomposites. In particular, we study PS/clay, PS/MWNT, PMMA/clay, and PMMA/SWNT nanocomposites. At a clay level of about 10% by mass, the network structure is formed for the PS and the PMMA clay nanocomposites; it requires a level of about 0.5% with the SWNT and 2% with the MWNT. These samples showed significantly reduced mass loss rates of PS and PMMA. However, the solid residues collected from radiative gasification tests of PS/clay and PMMA/clay showed many small cracks, despite the network formation within the initial sample. This is in contrast to the smooth, continuous residues (no cracks or openings) for PS/MWNT and PMMA/SWNT nanocomposites. The cracks in the clay samples are probably formed due to weaker network at elevated temperatures due to weaker bridging interaction between clay platelets as compared to stronger network resulting from dense entanglement and bridging of carbon nanotubes.

The surface tension of 1,2-dimethyl-3-N-hexadecyl imidazolium tetrafluoroborate (Im BF 4) as a function of concentration has been measured. Between 10 −5 and 4 × 10 −4 mol/L, the surface tension decreases with increasing imidazolium concentration. Aggregate formation appears to occur at a concentration near 4 × 10 −4 mol/L for temperatures between 50 and 75 ◦ C. Small angle neutron scattering measurements show the aggregates to be micelles with an aggregation number ≈83. Advancing and receding contact angles of Im BF4 surfactant solutions wetting pre-equilibrated mica surfaces show surfactant adsorption occurring at 10 −6 mol/L. At 7 × 10 −5 mol/L, a maximum in the hydrophobicity of the surface is measured, with a contact angle of 84 ◦ ± 2 ◦ . The results show that the Im BF4 surfactant adsorbs onto the mica surface and produces a hydrophobic surface, similar to the behavior of cetyl trimethyl ammonium bromide (CTAB). However, a hydrophobic surface

The thermal and flammability properties of polyamide 6/clay (2 and 5% by mass fraction) nanocomposites were measured to determine their flame retardant (FR) performance. The gasification process of the nanocomposite samples at an external radiant flux of 50 kW/m 2 in a nitrogen atmosphere was observed, and the residues collected at various sample mass losses were analyzed by thermogravimetric analysis, transmission electron microscopy, and X-ray diffraction to determine the content of the residue and to understand the FR mechanism of the nanocomposites. The analysis of the floccules of blackened residues shows that up to 80% by mass of the residues consists of clay particles and the remainder is thermally stable organic components with possible graphitic structure. Furthermore, clay particles are stacked in the carbonaceous floccule residues and the d-spacing of the clay platelets is in the range of 1.3 – 1.4 nm as compared to the well exfoliated original sample. The accumulation of the initially well-dispersed clay particles in the sample on the burning/gasifying sample surface are due to two possible mechanisms. One is recession of the polymer resin from the surface by pyrolysis with the de-wetted clay particles left behind. Another mechanism is the transportation of clay particles pushed by numerous rising bubbles of degradation products and the associated convection flow in the melt from the interior of the sample toward the sample surface. Numerous rising bubbles may have another effect on the transport of clay particles. Bursting of the bubbles at the sample surface pushes the accumulated clay particles outward from the bursting area and forms the island-like floccules instead of forming a continuous net-like structure of a clay filled protective layer. Therefore, both PA6/clay nanocomposite samples did not produce sufficient amounts of protective floccules to cover the entire sample surface and vigorous bubbling was observed over the sample surface which was not covered by the protective floccules. q 2003 Elsevier Ltd. All rights reserved.