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.

In an effort to shed additional light on the chemical mechanisms underlying the radiation–oxidation of polypropylene (PP), we are using samples having selective 13C isotopic labeling at the three unique sites within the macromolecular structure. After radiation exposure, we applied GC/MS, solid-state 13C NMR, and FTIR to evaluate the applicability of each technique in identifying the molecular labeling of the oxidation products, with the goal of determining the site of origin of the products with respect to the macromolecule. Using GC/MS, we have identified the position of origin of CO2 and CO from the polymer. Most of the CO2 (60%) and CO (>90%) come from the C(1) (methylene) position of PP, with (30%) of the CO2 originating from the C(3) (methyl) position, and 10% coming from the C(2) (tertiary) position. By GC/MS we have also identified the labeling patterns in four volatile oxidation products (acetone, methylisobutylketone, isobutane, and methyl acetate), and have used this information to map each compound onto the macromolecular framework. Using NMR we have quantified the time-dependent formation of solid-phase degradation products occurring from post-irradiation aging of PP samples held for 28 months at room temperature in air. Most of the solid oxidation products occur at the C(2) (tertiary) site; the predominant species, C(2) peroxides, increase linearly during the first 2 years, after which they plateau at a relatively high concentration.

Polypropylene samples, in which the three different carbon atoms along the chain were selectively labeled with carbon-13, were subjected to radiation under inert and air atmospheres, and to post-irradiation exposure in air at various temperatures. By using solid-state 13C NMR measurements at room temperature, we have been able to identify and quantify the oxidation products. The isotopic labeling provides insight into chemical reaction mechanisms, since oxidation products can be traced back to their positions of origin on the macromolecule. The major products include peroxides and alcohols, both formed at tertiary carbon sites along the chain. Other products include methyl ketones, acids, esters, peresters, and hemiketals formed from reaction at the tertiary carbon, together with in-chain ketones and esters from reaction at the secondary chain carbon. No evidence is found of products arising from reactions at the methyl side chain. Significant temperature-dependent differences are apparent; for example much higher yields of chain-end methyl ketones, which are the indicator product of chain scission, are generated for both elevated temperature irradiation and for post-irradiation treatment at elevated temperatures. Time-dependent plots of yields of the various oxidation products have been obtained under a wide range of conditions, including the post-irradiation oxidation of a sample at room temperature in air that has been monitored for 2 years. Radiation-oxidation products of polypropylene are contrasted to products measured for 13C-labeled polyethylene in an earlier investigation: the peroxides formed in irradiated polypropylene are remarkably longer lived, the non-peroxidic products are significantly different, and the overall ratios of oxidation products in polypropylene change relatively little as a function of the extent of oxidation.

Using a set of three isotactic polypropylene samples that had been individually labeled with carbon-13 at each of the three positions in the monomer unit, we conducted experiments to determine the position of origin of carbon monoxide and carbon dioxide that arise during thermal oxidation of this polymer. By GC–mass-spectral analysis, we find that 2/3 of the CO2 derives from the C(1) [methylene] carbon and the remaining 1/3 comes from the C(2) [tertiary] carbon, with none coming from the C(3) [methyl group] carbon. The CO also comes mainly from the C(1) [methylene] carbon (≥80%). This is in contrast to the solid-phase oxidation products, which have been found (by C-13 NMR on these same labeled PP materials) to originate predominantly (80–85%) from oxidation at the C(2) [tertiary] carbon. These results can be understood in terms of the free-radical reactions that underlie the polypropylene oxidation chemistry.

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.

ADVERTISEMENT RETURN TO ISSUEPREVCommunication to the...Communication to the EditorNEXTInsights into Oxidation Pathways, from Gaseous Products of Polypropylene with Selective Isotopic LabelingSteven M. Thornberg, Robert Bernstein, Daniel M. Mowery, Sara B. Klamo, James M. Hochrein, Jason R. Brown, Dora K. Derzon, and Roger L. CloughView Author Information Sandia National Laboratories, Albuquerque, New Mexico 87185, and Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125 Cite this: Macromolecules 2006, 39, 17, 5592–5594Publication Date (Web):July 20, 2006Publication History Received28 April 2006Revised29 June 2006Published online20 July 2006Published inissue 1 August 2006https://pubs.acs.org/doi/10.1021/ma060945mhttps://doi.org/10.1021/ma060945mrapid-communicationACS PublicationsCopyright © 2006 American Chemical SocietyRequest reuse permissionsArticle Views290Altmetric-Citations12LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Alkyls,Labeling,Organic acids,Oxygen,Thermoresponsive polymers Get e-Alerts

Unstabilized polypropylene (PP) films having selective C-13 isotopic labeling were subjected to thermal aging at 50, 80, and 109 degrees C and to gamma-irradiation at 24 and 80 degrees C. The oxidized films were examined using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Dramatic differences were found in the type and distribution of oxidation products originating from the three carbon atom sites within the PP macromolecule (tertiary carbon, secondary carbon, and methyl side group). Most of the oxidation products that formed on the polymer chain originated through chemical reactions at the PP tertiary carbons. Under all of the aging conditions examined, tertiary peroxides (from the PP tertiary site) were the most abundant functional group produced. Also originating from the PP tertiary carbon were significant amounts of tertiary alcohols, together with several more minor products that included "chain-end" methyl ketones. No significant amount of peroxides or alcohols associated with the PP secondary carbon sites was detected. A substantial yield of carboxylate groups was identified (acids, esters, etc.). The majority of these originated from the PP secondary carbon site, from which other minor products also formed, including in-chain ketones. We found no measurable Yield of oxidation products originating from reaction at the PP methyl group. Remarkably similar distributions of the major oxidation products were obtained for thermal aging at different temperatures, whereas the product distributions obtained for irradiation at the different temperatures exhibited significant differences. Time-dependent concentration plots have been obtained, which show the amounts of the various oxidation products originating at the different PP sites, as a function of the extent of material oxidation.

Accelerated aging of Nylon 6.6 fibers used in parachutes has been conducted by following the tensile strength loss under both thermal–oxidative and 100% relative humidity conditions. Thermal–oxidative studies (air circulating ovens) were performed for time periods of weeks to years at temperatures ranging from 37°C to 138°C. Accelerated aging humidity experiments (100% RH) were performed under both an argon atmosphere to examine the ‘pure’ hydrolysis pathway, and under an oxygen atmosphere (oxygen partial pressure close to that occurring in air) to mimic true aging conditions. As expected the results indicated that degradation caused by humidity is much more important than thermal–oxidative degradation. Surprisingly when both oxygen and humidity were present the rate of degradation was dramatically enhanced relative to humidity aging in the absence of oxygen. This significant and previously unknown phenomena underscores the importance of careful accelerated aging that truly mimics real world storage conditions.

High-temperature oven aging exposures from 110°C to 80°C have been conducted on a commercial chloroprene rubber cable jacketing material where the time-dependent degradation at each temperature is monitored by following the ultimate tensile elongation. By time–temperature superposing the results at the lowest experimental temperature (80°C), empirical shift factors are first derived and then analysed using the conventional Arrhenius approach and finally extrapolated using the derived activation energy Ea of 96kJ/mol in order to make predictions at 25°C. To test the constant Ea assumption underlying the Arrhenius extrapolation, we conducted oxygen consumption measurements at six temperatures ranging from 25°C to 95°C and found evidence for a slow drop in Ea from ∼96kJ/mol above 80°C to an average value of ∼82kJ/mol below 80°C. This curvature predicts a 50% reduction in room temperature lifetime and suggests that certain higher Ea degradation processes become less important as room temperature is approached. Analyses of the production rates for CO2 show that CO2 becomes less important relative to the oxygen consumed as the temperature is lowered, evidence in accord with the above suggestion. Further evidence for a similar drop in Ea (from 89kJ/mol above 70°C to 71kJ/mol below 70°C) comes from comparisons of accelerated aging elongation data taken ∼25 years ago on another chloroprene rubber cable jacketing material with recent elongation results taken on samples that have aged for ∼24 years at ∼24°C.