http://matdl.org/repository/ Materials Digital Library en Fez 1.3 http://blogs.law.harvard.edu/tech/rss http://matdl.org/repository/view/matdl:513 Calculations were performed to assign defect-resonance patterns observed in solid state 13C NMR spectra obtained from the crystalline regions of isotactic polypropylene. The spectral features of interest are associated with stereo, regio, and comonomer-type defects which can typically be found in metallocene-synthesized polymers. The calculations were carried out as follows: A model of the crystalline region of defect-free isotactic polypropylene was constructed from available X-ray data corresponding to the a-lattice. A series of irregularities including ethylene comonomer, stereo-mrrm, regio 2,1-erythro, and butylene comonomer defects were introduced one at a time at various positions in a specific stem occupying a central position in the model crystallite. Low-lying conformations were then obtained from simulated annealing calculations that were initiated from these structures. Finally, quantum mechanical calculations were performed on representative segments of the defect-containing chains excised from the annealed crystallites and the calculated chemical shifts were compared to the observed resonances. The results of the calculations were used as a basis for interpreting the NMR intensities of defect-related resonances in terms of the partitioning of defects and to help establish the conformational structures of the defect-containing stems. Wed, 01 Nov 2006 10:51:53 Nyden, M. R. og Vanderhart, D. L. og Alamo, R. G. http://matdl.org/repository/view/matdl:515 In the pursuit of improved approaches to fire retarding polymers a wide variety of concerns must be addressed, in addition to the flammability issues. For commodity polymers the low cost of these materials requires that the fire retardant (FR) approach also be of low cost. This limits solutions to the problem primarily to additive type approaches. These additives must be low cost and easily processed with the polymer. In addition, any additive must not excessively degrade the other performance properties of the polymer, and it must not create environmental problems in terms of recycling or disposal of the final product. Polymer layered silicate (PLS) nanocomposites are materials that may fulfil the above requirements for a high performance flame retardant. PLS nanocomposites are hybrid organic polymer-inorganic materials with unique properties when compared with conventional filled polymers [1]. For example, the mechanical properties of a nylon-6 layered-silicate nanocomposite, with a silicate mass fraction of only 5%, show excellent improvement over those for the pure nylon 6. The nanocomposite exhibits a 40% higher tensile strength, 68% greater tensile modulus, 60% higher flexural strength, and a 126% increased flexural modulus. The heat distortion temperature (HDT) is increased from 65 °C to 152 °C [2]. In some cases, increased thermal stability, an important property for improving flammability performance, as well as decreased gas permeability, and increased solvent resistance accompany the improved physical properties. We have reported on the flammability properties of delaminated nylon 6 layered silicate nanocomposites and intercalated polymer layered-silicate nanocomposites prepared from polystyrene (PS) and polypropylene-graft-maleic anhydride (PP-g-MA) [3, 4]. Here, we will briefly review these results and report on our initial studies of the flammability of thermoset PLS nanocomposites: intercalated vinyl ester silicate and intercalated epoxy silicate nanocomposites. Thu, 02 Nov 2006 10:07:44 Gilman, J. W. og Kashiwagi, T. og Nyden, M. R. og Brown, J. E. T. og Jackson, C. L. og Lomakin, S. og Giannelis, E. P. og Manias, E.