Matthew D. Gawryla

Clay aerogel/cellulose whisker nanocomposites: a nanoscale wattle and daub

Aerogels based on either clay or cellulose nanofibers are representatives of an emerging class of structural materials with ultra-low density. Both types of aerogels are made from abundant raw materials and are formed through environmentally friendly freeze-drying processes. Due to the ultra-low-density layered superstructure that results from templating by the ice crystal morphology, the neat aerogels are unfortunately often rather fragile. The present study explores inorganic/organic hybrid aerogels that comprise montmorillonite and cellulose nanofibers isolated from tunicates. Dynamic mechanical testing revealed that, especially at low densities, these materials exhibit compressive strengths that are significantly higher than predicted by simple additive behavior of the properties of the individual components. At first glance, the data seem to suggest the formation of a nanoscale “wattle-and-daub”, in which the two components (mud-like clay and straw-like cellulose whiskers) complement each other. It appears that the main cause for this synergy is the enhanced formation of three dimensional network structures during the freeze-drying process.

Elastic, low density epoxy/clay aerogel composites

Clay aerogels are ultra-low density materials formed by the freeze-drying of aqueous clay gels. While unmodified clay aerogels exhibit generally poor mechanical properties, incorporation of polymers into these structures can greatly increase their strengths and moduli; such polymer/clay aerogel composites have potential for use in a range of structural and insulation applications. Polymer/clay aerogel composites were formed by a new process in which water-soluble thermoset epoxy precursors are reacted within a clay hydrogel, which is then freeze-dried to produce the polymer/clay aerogel composites. The compressive properties of these polymer/clay aerogel composites greatly exceed those of plain clay aerogels; moreover, some of these composites exhibit novel elastomeric behavior, withstanding and recovering large amounts of compressive strain without failure or significant permanent deformation.

Foam-like materials produced from abundant natural resources

Low density polymer/clay aerogel composites were prepared using casein and sodium montmorillonite clay (Na+-MMT). The use of naturally occurring polymers allows for the creation of environmentally friendly/benign materials. Using a freeze drying process, composites were made that have densities in the range of 0.08–0.15 g cm−3 and exhibit properties similar to typical foamed polymers such as expanded polystyrene and rigid polyurethane foams. It was found that a minimum of at least 5wt% casein is required in the initial aqueous mixture in order to create a viable structure. The casein-containing materials created using this freeze drying process are similar to those previously produced with synthetic water soluble polymers such as poly(vinyl alcohol) but exhibit superior mechanical properties when compared to any synthetic polymer/clay aerogel composite tested to date.

pH Tailoring Electrical and Mechanical Behavior of Polymer-Clay-Nanotube Aerogels.

Aerogels are low density (<0.1 g · cm(-3) ), highly porous materials that are especially interesting for insulating applications. Combinations of clay and water-soluble polymers are commonly used to produce aerogels, but these materials are often mechanically weak. Single-walled carbon nanotubes (SWNT) were combined with clay and found to significantly improve mechanical behavior and impart electrical conductivity to these aerogels. Poly(acrylic acid) (PAA) as the matrix polymer provides a means of tailoring the electrical conductivity and mechanical behavior by altering the pH of the aqueous aerogel precursor suspensions prior to freeze drying. An aerogel, made from a pH 9 aqueous suspension containing 0.5 wt.-% PAA, 5 wt.-% clay, and 0.05 wt.-% SWNT, has a compressive modulus of 373 kPa. In the absence of nanotubes, this modulus is reduced to 43 kPa. Reducing suspension pH to 3, prior to freeze drying, also reduces modulus for these aerogels, but electrical conductivity is increased when nanotubes are present. It was found that bundled nanotubes provide better reinforcement for these low-density composites, which may provide some new insight into the use of nanotubes in materials that will be exposed to compressive loading.

Effects of Fiber Reinforcement on Clay Aerogel Composites

Novel, low density structures which combine biologically-based fibers with clay aerogels are produced in an environmentally benign manner using water as solvent, and no additional processing chemicals. Three different reinforcing fibers, silk, soy silk, and hemp, are evaluated in combination with poly(vinyl alcohol) matrix polymer combined with montmorillonite clay. The mechanical properties of the aerogels are demonstrated to increase with reinforcing fiber length, in each case limited by a critical fiber length, beyond which mechanical properties decline due to maldistribution of filler, and disruption of the aerogel structure. Rather than the classical model for reinforced composite properties, the chemical compatibility of reinforcing fibers with the polymer/clay matrix dominated mechanical performance, along with the tendencies of the fibers to kink under compression.

Clay Aerogel Composite Materials

A simple, inexpensive, and environmentally-friendly process for converting mixtures of clays and polymers has been developed. Polymer and clay are combined in water, and the mixtures are freeze dried to produce materials which have bulk densities typically in the range of 0.03 – 0.15 g/cm3. These low density polymer/clay aerogel materials possess good mechanical properties similar to those of traditional polymer foams, can be reinforced with fibers, modified with nanoparticles, biomineralized, or converted into porous ceramics.

Clay Aerogel Supported Palladium Nanoparticles as Catalysts

Highly porous, low density palladium nanoparticle/clay aerogel materials have been produced and demonstrated to possess significant catalytic activity for olefin hydrogenation and isomerization reactions at low/ambient pressures. This technology opens up a new route for the production of catalytic materials.

Poly(Amide-imide) aerogel materials produced via an ice templating process

Low density composites of sodium montmorillonite and poly(amide-imide) polymers have been created using an ice templating method, which serves as an alternative to the often-difficult foaming of high temperature/high performance polymers. The starting polymer was received in the poly(amic acid) form which can be cured using heat, into a water insoluble amide-imide copolymer. The resulting materials have densities in the 0.05 g/cm3 range and have excellent mechanical properties. Using a tertiary amine as a processing aid provides for lower viscosity and allows more concentrated polymer solutions to be used. The concentration of the amine relative to the acid groups on the polymer backbone has been found to cause significant difference in the mechanical properties of the dried materials. The synthesis and characterization of low density versions of two poly(amide-imide) polymers and their composites with sodium montmorillonite clay are discussed in the present work.