212Polmeros: Cincia e Tecnologia, vol. 13, n 4, p. 212-217, 2003A R T I G OT C N I C OC I E N T F I C OAutor para Correspondncia: D. R. Paul, Department of Chemical Engineering and Texas Materials Institute, The University of Texas at Austin, Austin TX 78712, USA. E-mail: drpche.utexas.eduIntroductionPolymer-layered silicate nanocomposites formed from the organically modified clay mineral montmorillonite and related materials have attracted a great deal of technological and scientific interest1-3. These composites offer the promise of greatly improved properties over those of the matrix polymer owing to the nanoscale reinforcement and constraints of the polymer caused by dispersing the 1 nm thick, high aspect ratio aluminosilicate layers. The property enhancements that could be readily anticipated and some that were not obvious have driven the excitement about nanocomposites. However, commercialization of these materials has been limited by the lack of knowledge about formation of nanocomposites using practical and economical processes and by some other unsolved problems. Numerous benefits of nanocomposites formed from a variety of matrix polymers have been described. The property improvements can generally be divided into the following areas: mechanical properties, heat resistance, dimensional stability, barrier and flame retardation. The potential property improvements usually depend on the degree of delamination and dispersion, which as shown here depends on a combination of the proper chemical treatment and optimized processing. These effects are illustrated here using recent results obtained from nanocomposites based on a nylon 6 matrix.Organoclays and terminologyThe aluminosilicate layers of montmorillonite, MMT, have a net negative charge and in the native state the counter ion is typically sodium as shown by the structural schematic in Figure 14. To make these platelets more compatible with polymers, the sodium ions are exchanged with alkyl ammonium ions to produce an organoclay. Figure 2 showsFormation and Properties of Nylon 6 NanocompositesT. D. Fornes, D. R. Paul Department of Chemical Engineering and Texas Materials Institute, The University of Texas at AustinAbstract: Sodium montmorillonite clay consists of platelets, one nanometer thick with large lateral dimensions, which can be used to achieve efficient reinforcement of polymer matrices. Formation of these nanocomposites requires modifying the clay with an appropriate organic surface treatment and optimized processing. Some of these techniques and the resulting property improvements (modulus, thermal expansion, heat distortion temperature, etc.) are reviewed here. It is shown that shear stress exerted on stacks of clay platelets play an important role in the mechanism of exfoliation. The modulus enhancement observed is of the order predicted by composite theories; however, the clay particles clearly affect the crystalline morphology of the polymer phase which may have an additional effect on some composite properties.Keywords: Nanocomposites, nylon 6, organoclay, melt processing.a sampling of the organic amine structures used to make the organoclays and a simple short-hand nomenclature system for identifying these molecules. The ammonium ion usually has one or two alkyl tails formed from naturally occurring oils, e.g., C = cocoa (C12), T = tallow (C18), R = rapeseed (C22), etc. These tails have some level of unsaturation (double bonds) in the native form but may be hydrogenated (H). Other groups on the nitrogen may be hydrogen, methyl (M), hydroxyethyl (HE), benzyl (BZ), etc. The X-ray d-spacing between MMT platelets increases as the size of organic cation increases as illustrated in Figure 35. These organoclays were provided for this research by Southern Clay Products of Gonzales, Texas. Unlike the more common clay minerals used as fillers for plastics, such as talc and mica, MMT can be delaminated and dispersed into individual layers of only 10 thickness. Unseparated MMT layers, after introduction into the polymer, are often referred to as tactoids6. The term intercalation describes the case where a small amount of polymer moves into the gallery spacing between the clay platelets, but causes less than 20 30 separation between the platelets. Exfoliation or delamination occurs when polymer furtherFigure 1. Structure of sodium montmorillonite.213Polmeros: Cincia e Tecnologia, vol. 13, n 4, p. 212-217, 2003Fornes, T. D.; Paul D. R. - Formation and properties of nylon 6 nanocompositesseparates the clay platelets, e.g., by 80 100 or more. A well-delaminated and dispersed nanocomposite as defined in this paper, consists of delaminated platelets distributed homogeneously in the polymer. These terms are shown schematically in Figure 4.ProcessingVaia and Giannelis7, 8 have shown that, with appropriate thermodynamic interactions, polymers can spontaneously intercalate the galleries of organoclays; however, it appears