Atomic Scale Mechanisms Controlling the Oxidation of Polyethylene: A First Principles Study

Understanding the degradation mechanisms of aliphatic polymers by thermal oxidation and radio-oxidation is very important in order to assess their lifetime in a variety of industrial applications. We focus here on polyethylene as a prototypical aliphatic polymer. Kinetic models describing the time evolution of the concentration of chain defects and radicals species in the material identify a relevant step in the formation and subsequent decomposition of transient hydroperoxides species, finally leading to carbonyl defects, in particular ketones. In this paper, we first summarize the most relevant mechanistic paths proposed in the literature for hydroperoxide formation and decomposition and, second, revisit them using first principles calculations based on Density Functional Theory (DFT). Our results partially confirm commonly accepted reaction energies, but also propose alternative, more favourable, reaction paths. We highlight the influence of the environment—crystalline or not—on the outcome of some of the studied chemical reactions. A remarkable result of our calculations is that hydroxyl radicals play an important role in the decomposition of hydroperoxides. Based on our findings, it should be possible to improve the set of equations and parameters used in current kinetic simulations of polyethylene radio-oxidation.

Functionalized Hydroperoxide Formation from the Reaction of Methacrolein-Oxide, an Isoprene-Derived Criegee Intermediate, with Formic Acid: Experiment and Theory

Methacrolein oxide (MACR-oxide) is a four-carbon, resonance-stabilized Criegee intermediate produced from isoprene ozonolysis, yet its reactivity is not well understood. This study identifies the functionalized hydroperoxide species, 1‑hydroperoxy-2-methylallyl formate (HPMAF), generated from the reaction of MACR-oxide with formic acid using multiplexed photoionization mass spectrometry (MPIMS, 298 K = 25 °C, 10 torr = 13.3 hPa). Electronic structure calculations indicate the reaction proceeds via an energetically favorable 1,4-addition mechanism. The formation of HPMAF is observed by the rapid appearance of a fragment ion at m/z 99, consistent with the proposed mechanism and characteristic loss of HO2 upon photoionization of functional hydroperoxides. The identification of HPMAF is confirmed by comparison of the appearance energy of the fragment ion with theoretical predictions of its photoionization threshold. The results are compared to analogous studies on the reaction of formic acid with methyl vinyl ketone oxide (MVK-oxide), the other four-carbon Criegee intermediate in isoprene ozonolysis.

Sugar, vitamin C, and polyphenols in commercial apple beverages and their effects on antioxidant and anti-inflammatory activities in vitro

The concentrations of sugar, vitamin C, and polyphenols, as well as antioxidant and anti-inflammatory capacity of commercial apple beverage products in Singapore, were not known. The concentrations of vitamin C, total polyphenol content, and specific polyphenols, quercetin, and catechin of commercial apple beverages were determined using their respective high-performance liquid chromatography – ultraviolet detection methods. 1,1-diphenyl-2-picrylhydrazyl assay and in vitro studies were conducted to examine the antioxidant and anti-inflammatory capacity. The apple beverages (n=17) exhibited significant antioxidant (DPPH radical scavenging assay, inhibition of cellular F2-isoprostanes and lipid hydroperoxide formation) and anti-inflammatory (inhibition of cellular leukotriene B4 formation and myeloperoxidase activity) capacity, which were found to be associated with vitamin C (10.35±1.18g/ 100g), total polyphenols (9.9±1.2mg GAE/ 100g), catechin (1.60±0.10mg/ 100g), and quercetin (0.46±0.09mg/ 100g) concentrations in the beverages. A separate simulation experiment demonstrated that antioxidant and anti-inflammatory capacity were augmented by increasing vitamin C, total and specific polyphenol concentrations.

Quantitative and Predictive Modelling of Lipid Oxidation in Mayonnaise

Food emulsions with high amounts of unsaturated fats, such as mayonnaise, are prone to lipid oxidation. In the food industry, typically accelerated shelf life tests are applied to assess the oxidative stability of different formulations. Here, the appearance of aldehydes at the so-called onset time, typically weeks, is considered a measure for oxidative stability of food emulsions, such as mayonnaise. To enable earlier assessment of compromised shelf-life, a predictive model for volatile off-flavor generation is developed. The model is based on the formation kinetics of hydroperoxides, which are early oxidation products and precursors of volatile aldehydes, responsible for off-flavor. Under accelerated shelf-life conditions (50 °C), hydroperoxide (LOOH) concentration over time shows a sigmoidal curvature followed by an acceleration phase that occurs at a LOOH-concentration between 38–50 mmol/kg, here interpreted as a critical LOOH concentration (CCLOOH). We hypothesize that the time at which CCLOOH was reached is related to the onset of aldehyde generation and that the characterization of the LOOH-generation curvature could be based on reaction kinetics in the first days. These hypotheses are tested using semi-empirical models to describe the autocatalytic character of hydroperoxide formation in combination with the CCLOOH. The Foubert function is selected as best describing the LOOH-curvature and is hence used to accurately predict onset of aldehyde generation, in most cases within several days of shelf-life. Furthermore, we find that the defining parameters of this model could be used to recognize antioxidant mechanisms at play.

Efficacy of Orange Essential Oil and Citral after Exposure to UV-C Irradiation to Inhibit Penicillium digitatum in Navel Oranges

The effect of UV-C irradiation on antifungal properties of orange essential oil (EO) against Penicillium digitatum in inoculated Navel oranges was examined. The UV-C irradiation of orange EO resulted in a 20% loss of the major constituent, limonene, and the generation of three hydroperoxide oxidation products, (2S,4R)-p-mentha-6,8-diene-2-hydroperoxide,(1S,4R)-p-mentha-2,8-diene-1-hydroperoxide, and (1R,4R)-p-mentha-2,8-diene-1-hydroperoxide. The P. digitatum growth in oranges dipped in non-irradiated orange EO at 1000–4000 µL L−1 was not significantly different to control the fruit. Dipping in UV-C treated orange EO inhibited the growth of P. digitatum with 4000 µL L−1 having the greatest effect. No phytotoxic injury to the rind was observed at any concentration. Citral, as a known antifungal chemical, was included for comparison. The non-irradiated citral (1000 µL L−1) was more effective than irradiated orange EO, but elicited rind phytotoxicity. The irradiated citral was less effective in inhibiting P. digitatum growth with the loss of citral, but not hydroperoxide formation. These results suggest UV-C irradiated orange EO as a potential alternative to synthetic fungicides to inhibit P. digitatum decay. The source of orange EO could be waste flavedo generated by the orange juice processing industry.