Permeation of Penetrants in Nanocomposites: A Test of the Theory of Composites

University Of Texas At Austin, Austin TX

Investigators

Abstract

The project proposed here will have broad impact on the emerging field of polymer nanocomposites since it will determine whether current theories of composites are applicable or not for predicting performance as the size of the filler particles approach molecular dimensions. Further, this project will provide a vehicle for students at all levels to learn how to answer key questions in new technical areas, where all concepts have to be questioned, using well-structured research. As used here, the term nanocomposites refers to a polymer matrix containing well-dispersed filler particles (spheres, rods, or platelets usually comprised of an inorganic material) where at least one dimension is in the range of one to tens of nanometers. The very high surface area of these particles and small distances between them raises the question of whether the particles may influence the nature or properties of the polymer matrix by interfering with the chain conformation, segmental packing, or the force field in which these segments move. The well-developed mathematical theories for predicting performance (stiffness, permeability, etc.) of conventional composites assure the properties for each phase are the same as if the other phase were not there. We must question whether the local properties of the matrix in nanocomposites are really the same as in the bulk state. We propose to answer this key question by probing these local properties by measuring the permeability of various gas and vapor penetrants in well-prepared and characterized nanocomposites. The absolute permeability of any given penetrant will depend on the detailed morphology of the composite (i.e., the aspect ratio, volume fraction, and arrangement of these particles in the matrix). However, according to composite theory, the ratio of the permeability of one penetrant to that of another should be the same in the composite as in the pure matrix. Thus, systematic and accurate measurement of the permeation of a wide range of penetrants of varying size becomes a powerful tool for exploring the applicability of composite theory to nanocomposites, i.e., whether local properties are perturbed by tiny, high surface area nanoparticles. Further, this work will address issues related to developing nanocomposites for applications as barrier materials and as membranes for separations and for fuel cells. The nanoparticles of interest will include the aluminosilicate platelets of clays (for barrier materials), spherical particles of silica and other materials (for separation membranes), particles that conduct protons (for fuel cells,), etc. The matrices will be either crystalline or glassy polymers of varying chain stiffness and interaction with the nanoparticles. The research will identify the materials issues that determine when the matrix properties are significantly affected by the filler and when they are not.