Cynthia T. Alviso

Carbon aerogels for electrochemical applications

A major advantage of highly crosslinked, organic aerogels is the ability to transform many of these materials into electrically conductive carbon aerogels. Carbon aerogels have been formed as monoliths, microspheres, irregularly-shaped powders, and thin film composites. In all cases, the carbon aerogels retain their high surface area (400–800 m2/g) and ultrafine cell/pore size (<100 nm). Carbon aerogels are being examined as electrodes for double layer capacitors, pseudocapacitors, and capacitive deionization units. This paper examines the synthesis, structure–property relationships, and performance of carbon aerogel electrodes used in electrochemical applications.

Aerogels derived from multifunctional organic monomers

Traditional inorganic aerogels are mad via the hydrolysis and condensation of metal alkoxides. Recently, we reported the synthesis of organic aerogels based upon the aqueous polycondensation of (1) resorcinol with formaldehyde and (2) melamine with formaldehyde. The former materials can also be pyrolyzed in an inert atmosphere to form vitreous carbon aerogels. In both the inorganic and organic systems, the structure and properties of the dried aerogel are dictated by polymerization conditions. Factors such as pH, reactant ratio, and temperature influence the crosslinking chemistry and growth processes taking place prior to gelation. The ability to tailor the structure and properties of aerogels at the nanometer scale opens up exciting possibilities for these novel materials. This paper addresses the chemistry-structure-property relationships of organic aerogels. 22 refs., 7 figs.

Organic aerogels: microstructural dependence of mechanical properties in compression

Aerogels are a unique class of ultrafine cell size (<1000 A), low-density foams. These materials have continuous porosity and a microstructure composed of interconnected colloidal-like particles or chains with characteristic diameters of 100 A. Traditional aerogels are inorganic, made via the hydrolysis and condensation of metal alkoxides (e.g. tetraisopropoxy titanate). Recently, the authors reported the development of organic aerogels from the sol-gel polymerization of resorcinol with formaldehyde. Because these new aerogels are composed of a highly crosslinked aromatic polymer, they can be pyrolyzed in an inert atmosphere to form vitreous carbon aerogels. This work describes how the microstructure of these organic aerogels can be manipulated and controlled. The microstructural dependence of the compressive mechanical properties of both resorcinol-formaldehyde and carbon aerogels is examined in detail.

Correlation between structure and thermal conductivity of organic aerogels

Abstract Organic aerogels are derived from the sol-gel polymerization of resorcinol with formaldehyde. While these materials are usually produced as monoliths, this paper describes a new method for the production of organic aerogel microsphere powders. Supercritical drying provides highly porous aerogels which have an open-cell structure consisting of interconnected solid particles with typical diameters of 10 nm. The structure is controlled by the sol-gel polymerization conditions. This paper addresses the correlation between structure and thermal conductivity of these novel materials. Thermal conductivity measurements have been performed on both monoliths and powders using a hot-wire device. The measurements under variation of gas pressure as well as spectral infrared transmission measurements allow the determination of the solid, gaseous and radiative thermal conductivity as a function of density and catalyst concentration. The results show that the thermal conductivity components are clearly correlated with the aerogel structure: porosity and connectivity between the particles determine the solid conductivity, while the pore size influences the gaseous conductivity and radiative transport depends on the mass specific infrared absorption of the building units.

New organic aerogels based upon a phenolic-furfural reaction☆

The aqueous polycondensation of (1) resorcinol with formaldehyde and (2) melamine with formaldehyde are two proven synthetic routes for the formation of organic aerogels. Recently, we have discovered a new type of organic aerogel based upon a phenolic-furfural (PF) reaction. This sol-gel polymerization has a major advantage over past approaches since it can be conducted in alcohol (e.g., 1-propanol), thereby eliminating the need for a solvent exchange step prior to supercritical drying from carbon dioxide. The resultant aerogels are dark brown in color and can be converted to a carbonized version upon pyrolysis in an inert atmosphere. BET surface areas of 350--600 m{sup 2}/g have been measured, and transmission electron microscopy reveals an interconnected structure of irregularly-shaped particles or platelets with {approximately}10 nm dimensions. Thermal conductivities as low as 0.015 W/m-K have been recorded for PF aerogels under ambient conditions. This paper describes the chemistry-structure-property relationships of these new materials in detail.

Carbon Aerogels and Xerogels

The aqueous polycondensation of resorcinol with formaldehyde proceeds through a sol-gel transition and results in the formation of highly crosslinked, transparent gels. If the solvent is simply evaporated from the pores of these gels, large capillary forces are exerted and a collapsed structure known as a xerogel is formed. In order to preserve the gel skeleton and minimize shrinkage, the aforementioned solvent or its substitute must be removed under supercritical conditions. The microporous material that results from this operation is known as an aerogel. Because resorcinol-formaldehyde aerogels and xerogels consist of a highly crosslinked aromatic polymer, they can be pyrolyzed in an inert atmosphere to form vitreous carbon monoliths. The resultant porous materials are black in color and no longer transparent, yet they retain the ultrafine cell size (< 50 nm), high surface area (600--800 m[sup 2]/g), and the interconnected particle morphology of their organic precursors. The thermal, acoustic, mechanical, and electrical properties of carbon aerogels/xerogels primarily depend upon polymerization conditions and pyrolysis temperature. In this paper, the chemistry-structure-property relationships of these unique materials will be discussed in detail.

Elastic properties of crosslinked Resorcinol-Formaldehyde gels and aerogels

Abstract Resorcinol-formaldehyde (RF) gels with different monomer concentrations were characterized before and after supercritical drying from carbon dioxide in order to determine the influence of the drying process on elastic moduli and structure. Wet gel shear modulus, Youngs modulus and Poisson ratio were determined using the beam-bending method. In addition, acoustic shear waves were used to measure the shear modulus of wet gels. The two methods are shown to agree well. During supercritical extraction of the pore liquid the elastic moduli typically increase by a factor of 1.5 to 5 depending on density. Although in general the elastic moduli do not exhibit power-law dependence on density, in the limited density range covered by our experiments scaling exponents of 6.2 for the wet gels and 4.8 for the aerogels are derived. Aging in acetic acid is shown to have no significant impact on gel elastic properties. Wet and dry gels were analyzed for their structural efficiency and the fraction of elastically effective mass increases with density from 8 to 70%.

Thermal Degradation Behavior and Product Speciation in Model Poly(dimethylsiloxane) Networks

The thermal degradation behavior of a series of well defined poly(dimethylsiloxane) (PDMS) model networks has been studied using a combination analytical thermal analysis techniques and multivariate statistical analysis in order to probe the influence of network architecture on degradation chemistry. The aim of this research has been to determine the effect differing network architectures: mono and bimodality, a range of crosslink density, inter-chain molar mass and percentage of free chain ends on the mechanisms of PDMS thermal degradation. A series of model PDMS networks have been formulated using of tin catalyzed condensation cure chemistry and a range of linear precursors to yield a matrix of model network systems. The thermal degradation chemistry of these model networks have been characterized in relation to their structure by means of pyrolysis gas chromatography mass spectrometry (Py-GCMS), thermal gravimetric analysis (TGA) and multivariate statistical analysis. The results clearly demonstrate that the structural architecture of (chemically similar) PDMS networks has a significant impact on the mechanisms of PDMS thermal degradation. Notability, with decreasing inter-crosslink chain length, larger cyclic siloxane species (>D5) become more abundant degradation products and that there is a relationship between inter-chain molar mass, degree of crosslinking and the thermal stability on the mechanisms of degradation. This work effectively demonstrates that quantifiable relationships exist between basic network architectures and the distributions of degradation derived species in PDMS networks.

MQ NMR and SPME analysis of nonlinearity in the degradation of a filled silicone elastomer.

Radiation-induced degradation of polymeric materials occurs through numerous, simultaneous, competing chemical reactions. Although degradation is typically found to be linear in adsorbed dose, some silicone materials exhibit nonlinear dose dependence due to dose-dependent dominant degradation pathways. We have characterized the effects of radiative and thermal degradation on a model filled-PDMS system, Sylgard 184 (commonly used in electronic encapsulation and in biomedical applications), using traditional mechanical testing, NMR spectroscopy, and sample headspace analysis using solid-phase microextraction (SPME) followed by gas chromatography/mass spectrometry (GC/MS). The mechanical data and (1)H spin-echo NMR spectra indicated that radiation exposure leads to predominantly cross-linking over the cumulative dose range studied (0-250 kGy) with a rate roughly linear with dose. (1)H multiple-quantum NMR spectroscopy detected a bimodal distribution in the network structure, as expected from the proposed structure of Sylgard 184. The MQ NMR spectra further indicated that the radiation-induced structural changes were not linear in adsorbed dose and that competing chain scission mechanisms made a greater contribution to the overall degradation process in the range of 50-100 kGy (although cross-linking still dominated). The SPME-GC/MS data were analyzed using principal component analysis (PCA), which identified subtle changes in the distributions of degradation products (the cyclic siloxanes and other components of the material) as a function of age that provide insight into the dominant degradation pathways at low and high adsorbed dose.

Resorcinol-formaldehyde and carbon aerogel microspheres

Aerogels are a unique class of materials possessing an open-cell structure with ultrafine cells/pores (<100nm), high surface area (400--1100 m{sup 2}/g), and a solid matrix composed of interconnected particles, fibers, or platelets with characteristic dimensions of 10nm. Although monolithic aerogels are ideal candidates for many applications (e.g. transparent window insulation), current processing methods have limited their introduction into the commercial marketplace. Our research focuses on the formation of resorcinol-formaldehyde (RF) aerogel microspheres which offer an attractive alternative to monolith production. An inverse emulsion polymerization is used to produce these spherical gel particles which undergo solvent exchange followed by supercritical drying with carbon dioxide. This process yields aerogel microspheres (10--80{mu} diameter) which can be used as loosely packed powders, compression molded into near-net shapes using a polymer binder, or used as additives in conventional foaming operations to produce new aerogel composites with superior thermal properties. The emulsification procedure, thermal characterization, mechanical properties, and potential applications of RF aerogel microspheres will be discussed.