David W. Hatchett

Composites of Intrinsically Conducting Polymers as Sensing Nanomaterials

Composites of intrinsically conducting polymers (ICPs) are materials that utilize conjugated polymers and at least one secondary component that can be inorganic or organic materials or biologically active species. The goal is to produce a new composite material that has distinct properties that were not observed in the individual components. This may include either new or improved chemical properties that can be exploited for chemical or biological sensing. For example, adding carbon nanotubes tends to drastically influence the electrical and thermal conductivity of ICPs. A secondary aspect concerns the stabilization of the secondary component in the polymer matrix. Enhanced optical, electrical, or mechanical properties such as stiffness and strength are common. In some cases, the physical and chemical properties of the secondary component are much different after composite formation. For the purpose of this review we will primarily focus on the ICPs such as polyaniline, polypyrrole, and polythiophene and their derivatives. The resonance-stabilized structure of ICPs allows, for example, incorporation of ions, nanoparticles, or nanowires of metals, metal oxides, carbon, or molecular species such as metallophthalocyanines or biologically active components such as enzymes, antibodies, and antigens. 1 I some cases, the ICP will simply act as a template for the incorporation of the secondary component. In that case, the secondary component will impart the chemical properties required for chemical sensing. In other cases the materials are linked through electrostatic interactions which influence the electronic and physical properties of the materials used to prepare the composite.

Comparison of Chemically and Electrochemically Synthesized Polyaniline Films

The electrochemical growth of thick ({approximately}2 mm) emeraldine, polyaniline (PANI{sup E}) films from solutions containing 2 M HBF{sub 4} and 0.25 M aniline is demonstrated. Electrochemically and chemically prepared PANI{sup E} films, cast from formic acid solutions, are compared. The combination of electrochemical results with Fourier transform infrared spectroscopic data indicates that pure and homogeneous standard material can be reproducibly prepared electrochemically.

Voltammetric measurement of anion adsorption on Ag(111)

Abstract Voltammetric measurement of the oxidative adsorption of 14 anions at Ag(111) is reported. The Ag(111) electrodes are prepared by thermal deposition of Ag onto mica, followed by extensive thermal annealing in vacuum to produce clean, atomically-smooth, (111)-oriented surfaces. These electrodes display unusually well-resolved voltammetric responses towards anion adsorption. Typical cyclic voltammetric responses in aqueous solutions containing halides (F−, Cl−, Br−, I−), hydrosulfide and thiolates (CnH2n+1S−, n=0, 2, 6), and oxyanions (OH−, CO3−2, C2H3O2−, SO4−2, NO3−, H2PO4−, B4O7−2) are presented and discussed in relation to previously published reports documenting anion adsorption at Ag(111).

Note: Experiments in hard x-ray chemistry: In situ production of molecular hydrogen and x-ray induced combustion

We have successfully loaded H(2) into a diamond anvil cell at high pressure using the synchrotron x-ray induced decomposition of NH(3)BH(3). In a second set of studies, radiation-assisted release of O(2) from KCLO(3), H(2) release from NH(3)BH(3), and reaction of these gases in a mixture of the reactants to form liquid water using x-rays at ambient conditions was observed. Similar observations were made using a KCLO(3) and NaBH(4) mixture. Depending on reaction conditions, an explosive or far slower reaction producing water was observed.

FTIR Analysis of Chemical Gradients in Thermally Processed Molded Polyurethane Foam

Thermally processed PU foams are examined as a function of processing temperatures (25, 45, 65, and 85°C) at the side, middle, and center of a simple cylindrical mold. The PU foams show both chemical and morphological differences as a function of the processing temperature and radial position within the mold. Thermal degradation of uretoneimine structures, the emergence of carbodiimide structures, and extent of reaction of isocyanate groups are measured using photoacoustic FTIR spectroscopy. Chemical gradients and morphology differences between the side, middle, and center of the molded foam are observed for all processing temperatures. The data indicate that thermal activation at the center of the mold is important for samples regardless of processing temperature. Furthermore, in spite of thermal processing at temperatures well above the decomposition of uretoneimine structures (40°C), chemical gradients remain within the simple molded foams.

Thermally Induced Changes in the Chemical and Mechanical Properties of Epoxy Foam

The influence of thermal treatment on the chemistry, physical, and mechanical properties of epoxy foam was evaluated. The foam was subjected to repeated thermal cycles (25—95°C), encompassing the temperature regime in which the forward and the retro Diels—Alder reaction occurs. Changes in the chemistry of the foam were evaluated using in situ FTIR spectroscopy during thermal exposure. In addition, the structural siloxane units within the epoxy foam were identified and evaluated using FTIR analysis. Thermal analysis was used to evaluate expansion, degradation, and mass loss during thermal exposure. Finally, the physical and mechanical properties were evaluated to determine how thermal cycling influences the density and modulus of the epoxy foam. Thermal exposure below the temperature required for the breakage of conjugated double bonds via Diels—Alder mechanism increases thermal expansion influencing the structural integrity and packing of siloxane chains. The data indicates that the chemical changes and the thermal expansion of the foam are irreversible. The combination of thermal expansion and the change in chemistry for the system strongly diminished the structural rigidity of the foam lowering the density and modulus of the material.

Effects of Processing Temperature on ReCrete Polyurethane Foam

Research is conducted to determine the effect of processing temperature on some of the physical and mechanical properties of a polyurethane foam called ReCrete. The polyurethane foaming process is manipulated to change the foams density, chemistry, and mechanical properties. There is a 30-min period after ReCrete components are mixed when the materials are still undergoing significant chemical reaction. Researchers manipulate these chemical reactions by changing the environmental temperature during this process. This study investigates the effect of processing temperature on the chemistry and the resulting mechanical properties for a polyurethane foam system molded in aluminum cylinders and boxes. Processing temperature is varied from 25°C to 85°C. Researchers show that the processing temperature has a significant effect on ReCrete chemistry and density. The average density decreases by 19% over this temperature range. The chemistry, in turn, affects the static and dynamic mechanical properties. The axial compressive modulus and strength decrease by 24 and 16%, respectively. The chemistry changes that results from higher processing temperatures produce foam that is less rigid in compression, but tougher and more flexible. The dynamic flexural failure strength increases by 38% when the processing temperature is increased from 25°C to 85°C. Foam processed at 85°C has significantly greater resistance to brittle failure under impact.