Anisotropic Clay Aerogel Composite Materials
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1188-LL06-07
Anisotropic Clay Aerogel Composite Materials Matthew D. Gawryla, David A. Schiraldi Department of Macromolecular Science and Engineering, Case Western Reserve University, 2100 Adelbert Road, Cleveland, OH 44106
Abstract Clay aerogel composites have been around for over 50 years but still they represent a relatively under studied class of materials. Clay aerogel composites have been made in our labs that have low densities, 0.05-0.1g/cm3, provide good thermal insulation, k ≈ 0.02W/mK, and are created through an environmentally benign process. The mechanical properties of the composites resemble those of typical foamed polymers such as expanded polystyrene and polyurethane, with compressive moduli ranging from 0.5MPa to 40MPa depending on composition. Aqueous solutions of clay and polymer are frozen in cylindrical molds and freezedried to create these foam-like materials. Typically there is no particular orientation to the often layered structure that results, however if frozen in a unidirectional manner, anisotropic materials can be made. In this paper we will discuss the effects of molecular weight on mechanical properties of various composites as well as discussing the orientated layered structure within the anisotropic materials.
Keywords: Aerogel, Anisotropy, Clay, Composite
Introduction Clay aerogels and clay aerogel/polymer composites are relatively new members of a family of low density materials created through ice templating [1-3]. The use of water soluble polymers and sodium montmorillonite (Na+-MMT) can create materials with mechanical properties comparable to typical foamed polymers [4-7]. The process of ice templating proceeds as depicted in Figure 1, with any non-water material being forced between the growing ice crystals. This occurs due to the crystal wanting to retain as pure a structure as possible and excluding all non-water molecules/matter from the crystal. Once the sample is frozen, the water is sublimed away via freeze-drying, leaving only the templated solid material behind. If the solid material interacts with itself, or there is a reinforcing polymer present, the three dimensional structure will usually be self supporting. Gutierrez has shown that under certain processing conditions, the molecular weight of the polymer plays a significant role in the final structure [8].
Figure 1. Ice templating process for randomly nucleated samples - (A) Stable gel/solution (B) Nucleation at the edge of the vial (C) Ice growth toward the center of the vial (D) Frozen material (E) After sublimation A material is described as anisotropic if the properties differ with respect to orientation of the sample. Wood is a very good example of an anisotropic material, having high strength in the in grain direction, and lesser properties in the other two directions [9]. Anisotropy is useful in creating a range of products, from piezoelectrics to energy absorbing panels [10]. Using a vertical freezing method it is possible to create highly anisotropic materials which can be used to determine the mec
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