Our study of migration of clay upon annealing of polymeric nanocomposites continued, and an effort was made to understand the mechanism of the migration phenomenon and its dependence on the structure of the nanocomposite. We found that in the case of polypropylene-organolayered silicon (PP-OLS) there was no migration of clay to the surface upon annealing of intercalated samples under a stream of nitrogen. The intercalated structure of the nanocomposite, which occurs in relatively large multi-layer stacks, is too heavy to migrate to the surface. When the annealing is carried out in the presence of air, the polymeric matrix is oxidized and becomes more polar. Due to the increased polarity additional polymeric chains intercalate and bring about exfoliation of the clay. The exfoliated clay particles are nanosize foils composed of aluminum silicate layers to which surfactant and polymeric chains are attached. These exfoliated units which are dispersed in the polymeric matrix migrate to the surface. Migration (R2 in the figure) increases with the extent of oxidation, i.e. with the percentage of air in the nitrogen stream used in the annealing. The degree of oxidation shown as carbonyls was monitored spectroscopically by ATR/FTIR (see figure). The finding that the exfoliated particles migrate was confirmed by annealing fully exfoliated PA6-OLS under a stream of nitrogen not containing air. This was evidenced by small angle x-ray diffraction, ATR/FTIR and high resolution TEM. At annealing temperatures above 250C, the case of the PP-OLS is different from that of PA6-OLS. In the case of PP-OLS the intercalated structure is converted to a mixture of non-colloidal microcomposite and exfoliated clay particles. In the case of PA6 no microcomposite was formed.
The migration and the change of carbonyl of samples annealed at 190°C under different concentration of air for 60min
We believe that the extent the of migration can be used for the determination of the amounts of exfoliated particles in the nanocomposite. This is a new method of characterizing a nanocomposite, and will be of great importance for determining the structural elements of polymeric nanocomposites, for understanding the mechanisms governing the behavior of nanocompositesHigh Temperature and Flammability Behavior of Nanocomposites and Polymer Blends Menachem Lewin (Polytechnic University) DMR-0352588
and for enhancing the mechanical and thermal properties of the nanocomposites. It is believed today that the improvements in the properties, and the performance-in-use increase in proportion with the exfoliated nanocomposite moiety. Such determinations are at present being endeavored only with the use of a large number of high resolution TEM images which is tedious and inaccurate. This new method may have a significant impact on the industrial utilization of polymeric nanocomposites, and may bring about the development of new nanocomposite-based products with highly improved properties.
1. Lewin, M., Mey-Marom, A., Frank, R , Surface Free Energies of Polymeric Materials, Additives, and Minerals. Polym. Adv. Technol., 16. 429-441, (2005)
2. Lewin, M.; Pearce, E.M.; Levon, K.; Mey-Marom, A.; Zammarano, M.; Wilkie, C.W and Jang, B.N. Nanocomposites at Elevated Temperatures: migration and structural changes. Polym. Adv. Technol. 17 226, (2006)
3. Zammarano, M., Gilman, J. W., Nyden, M., Pearce, E. M., Lewin, M., The Role of Oxidation in the Migration Mechanism of Layered Silicate in Poly(propylene) Nanocomposites. Macromol. Rapid Commun. 27 693–696, (2006)
4. Tang, Y., Lewin, M., Pearce, E.M., Effects of annealing on the migration behavior of PA6/clay nanocomposites. Macromol. 2006, accepted
5. Hao, J., Lewin, M., Wilkie, C., Wang, J., Additional evidence for the migration of clay upon heating of clayepolypropylene nanocomposites from X-ray photoelectron spectroscopy (XPS). Polym Degrad. Stab., 91 2482-2485, (2006)
Our studies on the effect of dispersing organo-layered silicates (OLS) together with flame retardant additives in polyamide 6 (PA6) yielded interesting results. The addition of 5% clay to a flame retardant polymeric composition had a profoundly negative effect on the flame retardant rating. We found that in the case of PA-6 flame retarded with ammonium sulfamate (AS) and dipenta erythritol (DI ), a lower percentage of clay preserved the high flame Retardancy rating while decreasing the rate of heat release upon forced combustion in the cone calorimeter. The f.r. rating of this system with 5% clay also could be largely restored by adding a hydrophilic polymer which is strongly absorbed by the clay.
This indicates a new way of combining the f.r. effects of clay with those of conventional flame retardants, and to open the way for the production of highly flame retardant polymers with enhanced mechanical properties. Until now, very few systems exist in which clay can be applied in flame retardant compositions. Our new findings create new and interesting possibilities for further developments in materials.
6. Lewin., M., Reflections on Migration of Clay and Structural Changes in Nanocomposites. Polym. for Adv. Technolgies. 2006, accepted
7. Tang, Y., Lewin., M., Polypropylene OMMT nanocomposite: annealing, structural changes, exfoliation, and migration. Polym. Degrad. Stab., 2006, accepted
8. Lewin, M., Zammarano, M., Tang, Y., Pearce, E. M., Nanocomposites and Flame Retardants:Migration, Oxidation, and Structure of Polymer Organic Layered Silicates (POLS) Rec. Adv. in Flame Retard. of Polym. Mater.17, (2006)High Temperature and Flammability Behavior of Nanocomposites and Polymer Blends Menachem Lewin (Polytechnic University) DMR-0352588