Functional groups can be used to modify the equilibrium position and tune the reactivity of one electron oxidised aromatic compounds.

No abstract

Polymer electrolyte fuel cell (PEFC) membranes are subject to radical-induced degradation. Antioxidant strategies for hydrocarbon-based ionomers containing aromatic units can focus on intermediates that are formed upon attack by hydroxyl radicals (HO • ). Among the different intermediates, the cation radical P •+ is the most promising target for repair, for example by cerium(III). For the "repair" reaction of Ce(III) with radicals of a poly(α-methylstyrene sulfonate) oligomer we determined an activation energy of (9 ± 2) kJ mol −1 and a rate constant of 1.6 • 10 8 M −1 s −1 at 80°C by pulse-radiolysis. For the reduction of Ce(IV) by hydrogen peroxide the activation energy was determined by stopped-flow as (30 ± 1) kJ mol −1 with a rate constant of 4.8 • 10 6 M −1 s −1 at 80°C. These parameters are fed into a kinetics model to estimate the efficacy of the cerium (III)/(IV) redox couple as a catalytic repair agent in hydrocarbon-based fuel cell membranes. While cerium can mitigate polymer degradation, repair efficacy depends on the polymer degradation pathway and the nature and lifetime of the intermediates.

No abstract

Oppositely charged, soft, temperature-responsive PNIPAM microgels are used to assemble core–satellite-type colloidal molecules, thereby enabling convenient in situ tuning of size, volume fraction and inter-particle interactions with temperature.

During the operation of aromatic hydrocarbon-based proton exchange membrane fuel cells, formed radical species attack the membrane. The most deleterious radical formed is HO•, both strongly electrophilic and oxidising. Oligomers of α-methylstyrene sulfonates were used as model compounds. We report on the complex reaction cascade following the oxidative attack on aromatic cores bearing proton conductive sulfonate groups. UV-absorption bands of initial oxidation products indicate the formation of radical adducts and aromatic cation radicals. Subsequently, a transformation associated with an absorbance build-up at 580 nm is observed, presumably also related to aromatic cation radicals. Build-up and decay are significantly accelerated at high ionic strength levels that are also typical in fuel cells. Increased ionic strength causes phase separation: dynamic light scattering experiments indicate particle formation that is dependent both on chain length and on ionic strength. Aromatic cation radicals are known strong oxidants. With a presumed redox potential of E°((PAMSS-580nm)•+/PAMSS) ~ 2 V this oxidizing species should react also with mediocre reductants. Here, Mn(II) was oxidised to Mn(III) with rate constants of (5–10) × 106 M-1s-1. Implications for experimental design of kinetics experiments and understanding chemical mechanisms are discussed.

Liquids I Internal rotation I Nuclear magnetic relaxation 'H nuclear magnetic relaxation rates of CH3CD2OD and CD3CH2OD in various mixtures with glycerol-d8 in the temperature range 120< T<300K have been measured. Whereas the log(1/rl)(l/D curve for pure ethanol shows only the simple "elementary form" with one maximum, in agreement with Versmold's observation on the deuteron relaxation the addition of glycerol produces shoulders or second maxima. For comparison i-propanol (CH3)2CDOD and CH3OD were also studied in their mixtures with glycerol-d8. In the former case a shoulder appeared on the log(l/7",)(l/7*) curve, in the latter case no particularities showed up. It turned out that the weight factors derived from these observations could not be correlated with conventional molecular geometry in a satisfactory way. Die kernmagnetischen 'H-Relaxationsraten von CH3CD2OD und CD3CH2OD in verschiedenen Gemischen mit Glycerin-d8 wurden in einem Temperaturbereich von 120 bis 300 K gemessen. Wahrend die log(l/T1)(l/T)-Kurve fur reincs Athanol nur die einfache ..Elementarform" mit einem Maximum zeigt, bewirkt in Ubereinstimmung mit Versmolds Befunden an der Deuteronenrelaxation die Beimischung von Glycerin das Auftreten von Schultern oder zweiten Maxima. Zum Vergleich wurden auch i-Propanol (CH3)2CDOD und CH3OD in ihren Gemischen mit Glycerin-d8 untersucht. Im ersteren Falle erschien gleichfalls eine Schulter an der log(1/7,1)(l/r)-Kurve, im letzteren Falle traten keine Besonderheiten auf. Eszeigtesich, daBdie Gewichtsfaktoren, die aus diesen Messungen erhalten wurden, sich nicht in befriedigender Weise mit den Parametern der iiblich anerkannten Molekiilgeometrie verkniipfen lassen.

Hydrocarbon-based fuel cell membranes are susceptible to radical-induced degradation. Such polymers commonly contain aromatic units, which react rapidly with HO·. Antioxidant strategies therefore need to focus on repairing intermediates formed upon radical attack to mitigate irreversible degradation. We summarize our recent ex situ g-irradiation studies on an aromatic model compound and in situ fuel cell tests of radiation grafted membranes with a polymer-bound crown ether-cerium complex to elucidate its antioxidant mechanism. Cerium is effective in reducing aromatic hydrocarbon membrane degradation and likely follows a catalytic mode of action.