A Study of the Activation of Carbon Using Sample Controlled Thermal Analysis

A constant rate method involving the control of the concentration of evolved CO₂ at a constant level was used to study the air activation of pure and copper-doped carbon prepared from sodium carboxymethylcellulose. Whereas under a linear heating regime, both types of carbon reacted suddenly and quickly with O₂, under constant rate conditions this violent reaction was avoided and oxidation proceeded steadily at a lower temperature until complete burn off of the carbon was achieved. The catalytic effect of the copper on carbon gasification was noted with lower reaction temperatures for both linear heating (380°C compared to 500°C) and for the constant rate experiments (320°C compared to 400°C). This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal and mechanical characterization of high performance epoxy systems with extended cure times

Five epoxy resins of different chemistry and functionality were cured with DDS (4,4-diaminodiphenyl sulfone) using 2, 8 and 14 h curecycles. Both Differential Scanning Calorimetry (DSC) and Thermomechanical Analysis (TMA) were used to characterize reaction behavior and cured properties of the resin systems. In addition, static mechanical tests and density measurements were integrated with the thermal characterization methods to correlate resin properties with process time. Flexural three-point bending experiments showed that the resins tended to have higher yield stress and toughness values at extended cure times. The improved mechanical properties could be attributed to the full development of the epoxy molecular structure, in the form of cross-linked networks and molecular rearrangement. These results suggest that extended cure times or high temperature post-curing may be required to obtain the resin's ultimate mechanical properties for high performance composites.The authors would like to thank Dr. Andri Filippov of Shell Development Company for his interest in this work. Financial assistance and material support for this research were provided by Shell Development Company while instrument support was provided by TA Instruments through project support to the Polymeric Composites Laboratory of the University of Washington.     

Chemical origins of permanent set in a peroxide cured filled silicone elastomer - tensile and H NMR analysis

The aging of a commercial filled siloxane polymeric composite in states of high stress and Co-60 γ-radiation exposure has been studied. DC-745 is a commercially available silicone elastomer consisting of dimethyl, methyl-phenyl, and vinyl-methyl siloxane monomers crosslinked with a peroxide vinyl specific curing agent. It is filled with ∼ 30 wt.% mixture of high and low surface area silicas. This filled material is shown to be subject to permanent set if exposed to radiation while under tensile stress. Tensile modulus measurements show that the material becomes marginally softer with combined radiation exposure and tensile strain as compared to material exposed to radiation without tensile strain. In addition, the segmental dynamics as measured by both uniaxial NMR relaxometry and Multiple Quantum NMR methods indicate that the material undergoes radiatively-induced crosslinking in the absence of tensile strain. In the presence of tensile strain, relaxometry and MQ NMR studies show a strain dependent change in the dynamic order parameter and in the number of polymer chains associated with the filler surface. Solvent swelling measurements indicated no dependence on network crosslink density on strain ratio. Variable tau CPMG echo experiments indicate that a fraction of the polymer chains diffuses through areas of strong magnetic field gradients both at the filler-polymer interface and adjacent to micro-voids within the network. The population of the polymer chains influenced by the field gradients was observed to be dependent on the cumulative dose and degree of tensile strain applied during exposure. The relative change in crosslink density from the NMR and solvent swelling data deviates from that predicted from the Tobolsky model, particularly at higher doses. The likely reasons for this deviation are changes in the filler-polymer interface, increasing deviation from Gaussian chain statistics, and/or the formation of increased numbers of elastically ineffective network chains.     

Portable, low-cost NMR with laser-lathe lithography produced microcoils

Nuclear Magnetic Resonance (NMR) is unsurpassed in its ability to non-destructively probe chemical identity. Portable, low-cost NMR sensors would enable on-site identification of potentially hazardous substances, as well as the study of samples in a variety of industrial applications. Recent developments in RF microcoil construction (i.e. coils much smaller than the standard 5mm NMR RF coils), have dramatically increased NMR sensitivity and decreased the limits-of-detection (LOD). We are using advances in laser pantographic microfabrication techniques, unique to LLNL, to produce RF microcoils for field deployable, high sensitivity NMR-based detectors. This same fabrication technique can be used to produce imaging coils for MRI as well as for standard hardware shimming or "ex-situ" shimming of field inhomogeneities typically associated with inexpensive magnets. This paper describes a portable NMR system based on the use of a 2 kg hand-held permanent magnet, laser-fabricated microcoils, and a compact spectrometer. The main limitations for such a system are the low resolution and sensitivity associated with the low field values and quality of small permanent magnets, as well as the lack of large amounts of sample of interest in most cases. The focus of the paper is on the setting up of this system, initial results, sensitivity measurements, discussion of the limitations and future plans. The results, even though preliminary, are promising and provide the foundation for developing a portable, inexpensive NMR system for chemical analysis. Such a system will be ideal for chemical identification of trace substances on site.