Aerogels are an interesting class of materials that possess many exotic and extreme properties. These properties are developed as the gel network is produced from solution. As the gel develops, it builds a hierarchical structure, possessing architectures at different size scales through molecular and macro-scale interactions. Once the solvent is removed, and the resultant aerogel is produced, the hierarchical nature of the material produces many desirable properties including: extremely high porosities (greater than 90% pore volume), extremely low thermal conductivities (10-30 mW/m-k), very low densities (as low as 0.002 g/cm3), low refractive indices (as low as 1.01), low dielectric constants (between 1.0 and 1.5), high surface areas,[5,6] and the slowest speed of sound through a solid material.
The first chapter of this thesis deals with the structure/property relationships of polymer/clay aerogels interfused with uniformly distributed air bubbles were examined. Through the incorporation of a polyelectrolyte in a montmorillonite (MMT) clay solution, the viscosity was systematically changed by the addition of ions with different charges. The bubbles were achieved via high speed mixing and were stabilized through the use of the surfactant sodium dodecyl sulfate (SDS). As the charge of the ion increased from +1 (Na+ ions) to +2 (Ca2+ ions) to finally +3 (Al3+ ions), the modulus of the resultant aerogels increased. The foamed polymer/clay aerogels showed a reduction in thermal conductivity while retaining similar mechanical properties to unfoamed polymer/clay aerogels. The most promising composition was one which contained 5% MMT clay/5% poly(vinyl alcohol)/0.5% xanthum gum/0.5% SDS/0.2% Al2(SO4)3·6(H2O) possessing a density of 0.083 g/cm3, an average modulus of 3.0 MPa, and a thermal conductivity of 41 mW/m·K.
The second project investigated the feasibility of incorporating ground recycled polyurethane (PU) foam into clay/polymer aerogels. This was demonstrated and a range of compositions were prepared and characterized to determine the effect of variation in the formulations on density and mechanical properties of the resulting materials. The study followed a modified combinatorial approach. Initially, experiments were performed in water using either sodium exchanged montmorillonite or laponite clay, poly(vinyl alcohol) (PVOH) solution as the polymer binder, and the recycled PU foam. Freezing and freeze-drying the aqueous gels produced aerogels, which were characterized through density and mechanical testing, scanning electron microscopy, and thermal gravimetric analysis. The study was expanded by exploring alternative binder chemistries, including the use of an alginate polymer in place of the PVOH, or adding a polyisocyanate as across-linking agent for PVOH. The effect of recycled PU foam content, clay type and level, and binder type and level on mechanical properties of the aerogels were determined.
The goal of the third project was to determine if lignin could be converted into foam-like aerogels using a well-established and environmentally benign freeze drying process. Interest in lignin as a bio-resource has been gaining popularity in recent years, as it is currently viewed by most industries as a waste product that in most cases is simply burned as a fuel source. The use of lignin in a polymer/clay aerogel offers the potential for a high value-added foam-like material potentially usurping the use of traditional petroleum derived foams in some applications. The present work demonstrates that lignin/clay and lignin/alginate aerogel samples can possess compressive moduli as high as 36.0 MPa.
The final project addresses a fundamental material property concern associated with polyimide aerogels. Polyimide aerogels possess low dielectric constants, low thermal conductivities, high porosity, flexibility and low densities with outstanding mechanical properties. However, polyimide aerogels will undergo thermally induced shrinkage at temperatures far below their glass transition temperatures (Tg) or their onset of decomposition temperatures. Attempts to minimize thermal shrinkage were successful when a rigid filler, such as cellulose nanocrystals (CNCs), were introduced into the polyimide backbone. As an alternative to using rigid fillers, it was proposed that the incorporation of bulky, space filling moieties into the polymer backbone would also provide an effective route to reduce thermal shrinkage. An array of 20 polyimide aerogels were synthesized from 3,3’4,4’-biphenyltetracarboxylic dianhydride (BPDA) and 4,4’-oxydianiline (ODA) and in some cases BPDA and a combination of ODA and 9,9’-bis(4-aminophenyl) fluorene (BAPF). The aerogels were cross-linked with 1,3,5-benzenetricarbonyl trichloride (BTC). The polymer concentration, n-value and molar concentration of ODA and BAPF were varied. The resultant aerogels were fully characterized and were subjected to isothermal heating at 150 °C and 200 °C for up to 500 hours. It was observed that the samples containing BAPF possessed the lowest thermal shrinkages. Reductions in thermal shrinkage of around 20% were observed in samples containing the highest molar concentrations of BAPF.
Keywords: Aerogel, High Surface Area, Low Thermal Conductivity, Montmorillonite Clay, Polyvinyl alcohol, Polyurethane Foam, Lignin, Polyimide, Gel, Network, Cross-Linking, Ice Templating, Freeze-Drying, Lyophilization, Supercritical Fluid Extraction