Publications

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This paper presents the development of a sensor to detect the oxidative and radiation induced degradation of polypropylene. Recently we have examined the use of crosslinked assemblies of nanoparticles as a chemiresistor-type sensor for the degradation products. We have developed a simple method that uses a siloxane matrix to fabricate a chemiresistor-type sensor that minimizes the swelling transduction mechanism while optimizing the change in dielectric response. These sensors were exposed with the use of a gas chromatography system to three previously identified polypropylene degradation products including 4-methyl-2-pentanone, acetone, and 2-pentanone. The limits of detection 210 ppb for 4-methy-2-pentanone, 575 ppb for 2-pentanone, and the LoD was unable to be determined for acetone due to incomplete separation from the carbon disulfide carrier. © 2009 Materials Research Society.

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This paper presents the development of a sensor to detect the oxidative and radiation induced degradation of polypropylene. Recently we have examined the use of crosslinked assemblies of nanoparticles as a chemiresistor-type sensor for the degradation products. We have developed a simple method that uses a siloxane matrix to fabricate a chemiresistor-type sensor that minimizes the swelling transduction mechanism while optimizing the change in dielectric response. These sensors were exposed with the use of a gas chromatography system to three previously identified polypropylene degradation products including 4-methyl-2-pentanone, acetone, and 2-pentanone. The limits of detection 210 ppb for 4-methy-2-pentanone, 575 ppb for 2-pentanone, and the LoD was unable to be determined for acetone due to incomplete separation from the carbon disulfide carrier.

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PP samples, in which the three unique carbon atom positions along the chain were selectively labeled with C-13, have been subjected to γ-irradiation in the presence of oxygen, and the resulting organic volatile products analyzed by GC/MS. The isotopic labeling patterns in 33 organic degradation compounds have been assigned by comparison of the four mass spectra for each compound (from unlabeled PP, and from the three labeled PP materials). The volatile products have been "mapped" onto their positions of origin from the PP macromolecule, and insights have been gained into the chemistry through which these compounds must have formed. Most products show high specificity of isotopic labeling, indicating a single dominant reaction pathway. Oxidation chemistry occurred heavily at the C(2) tertiary carbon, with chemistry also at C(1) methylene. Methyl ketones are in abundance, along with alcohols, some aliphatic hydrocarbons, and other compound types. The C(3) methyl carbon remained attached to its original C(2) position in all catenated degradation products, and underwent no chemistry. However, products containing "non-catenated" carbons (i.e., not bonded to any other carbon atom) consisted entirely of a mix of C(3) and C(1). By examination of the labeling patterns, many products could be assigned to two successive chain scission events in close proximity, while others are clearly seen to arise from cleavage, followed by radical-radical recombination reactions. Interestingly, the former products (two chain scissions) are all found to have an odd number of carbon atoms along their chain, while the latter (scission followed by radical-radical reaction) all have an even number of carbons. An explanation of this odd/even phenomenon is provided in terms of the symmetry of the PP macromolecule.

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Making use of polypropylene samples that are selectively labeled with carbon-13 at each of the three unique positions within the repeating unit, we are conducting mass spectral analyses of the volatile organic oxidation products that are produced when the polymer is subjected to elevated temperature in the presence of air. By examination of both the parent and fragmentation ion peaks in the mass spectrum, we are able to identify the positioning of the C-13 labels within the volatile compounds, and thereby map each compound onto its site of origin from within the macromolecular structure of polypropylene. Most of the organic oxidation products are remarkably specific in terms of their genesis from the polymer. The structural results are discussed in terms of the oxidation chemistry of the macromolecule.

HFE-7100 and FC-72 fluorinert are two fluids used during weapon component manufacturing. HFE-7100 is a solvent used in the cleaning of parts, and FC-72 is the blowing agent of a polymeric removable foam. The presence of either FC-72 or HFE-7100 gas in weapon components can provide valuable information as to the stability of the materials. Therefore, gas standards are needed so HFE-7100 and FC-72 gas concentrations can be accurately measured. There is no current established procedure for generating gas standards of either HFE-7100 or FC-72. This report outlines the development of a method to generate gas standards ranging in concentration from 0.1 ppm to 10% by volume. These standards were then run on a Jeol GC-Mate II mass spectrometer and analyzed to produce calibration curves. We present a manifold design that accurately generates gas standards of HFE-7100 and FC-72 and a procedure that allows the amount of each to be determined.