Na-montmorillonite (Na-MMT) and its ion-exchanged derivatives (K-, Li-, Ca-MMT) were used to make nanocomposites by mixing with a low-T g styrene-acrylic latex, and these materials were used to verify the effects of counterions on nanocomposite morphology and mechanical properties. The monovalent cation clays form exfoliated/intercalated nanocomposites with a more than 10-fold increase in modulus, as compared to the pristine polymer, and approximately 200% increase in tensile strength. In the case of Li nanocomposites the mechanical properties are strongly dependent on the extent of drying, as expected considering that these ions are strongly hydrated. Calcium clay-polymer particle adhesion is very good, as evidenced in transmission electron micrographs, but the extent of exfoliation is less pronounced and the changes in the mechanical properties are accordingly lower than using Na, Li, or K counterions. Analytical electron micrographs show that the counterions are always accumulated in the domains containing both clay and polymer, showing that the compatibility of these two phases, which carry excess negative charges, is achieved thanks to cation bridges at the interfaces, following a model that was previously put forward to explain latex-clay nanocomposite formation and properties.

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Recebido em 15/1/09; aceito em 12/3/09; publicado na web em 2/4/09 NATURAL RUBBER AND NANOCOMPOSITES WITH CLAY. The natural rubber is a strategic material which can not be replaced by synthetic rubber in many technological applications. Brazil is a rubber importer, but new techniques of cultivation, breeding and diversification of producing species can reverse this situation. One of the best ways to add value to this commodity is nanotechnology. The production of nanocomposites is already a reality and shows that the sustainable use of this natural resource can lead to new products and boost the national agribusiness setting labor-qualified in the field.Keywords: natural rubber; nanocomposites; sustainability. Borracha naturalA borracha natural (NR) é um polímero de poli(cis-1,4-isopreno) e apresenta propriedades únicas devido a sua estrutura intrínseca, alta massa molar e presença de outros componentes minoritários como proteínas, carboidratos, lipídios e minerais presentes no látex. Cerca de 2500 plantas produzem látex, mas o látex da Hevea brasiliensis se constitui na única fonte comercial importante de látex de borracha natural.

Summary: Colloidal polymers are highly versatile due to the variety of properties and functions that can be created by changing monomer composition, surfactant, initiator and reaction protocol. Microchemical and morphological observation of the particles and particle aggregates as well as films and monoliths made with them is allowing us to understand the connections between nanosized structural features and macroscopic properties and this is essential for the creation of valuable new polymer materials. Electron spectroscopy imaging (ESI) in the transmission electron microscope (TEM) uses electron energy loss spectroscopy (EELS) to provide a wealth of information on particle constituents and their topological distribution, allowing a number of correlations with the polymer mechanical, thermal, optical and electrical properties. On the other hand, scanning probe microscopy (SPM) techniques allow direct measurement of particle and film properties such as adhesion, stiffness, electrostatic potential as well as rheology information with high spatial resolution, down to 10-20 nm and under many different experimental conditions. Information from the joint use of these techniques is revealing a wealth of nanostructures within colloidal polymers requiring a revision of many preconceived ideas on these materials.