In situ electron spin resonance study of styrene grafting of electron irradiated fluoropolymer films for fuel cell membranes

Three different electron beam irradiated fluoropolymers (ETFE, FEP, and PVDF) as well as their grafting reactions with styrene in different diluents were investigated by means of electron spin resonance (ESR). Depending on the atmosphere during irradiation, ESR spectra of peroxy and alkyl radicals were observed. Radical decay as a function of time and temperature was investigated in the presence and absence of solvent. Grafting levels and number of monomer units per chain were calculated for both types of radicals. Irradiation atmosphere, grafting temperature, and added solvent affect the morphology of the fluoropolymers, in particular the crystalline versus amorphous fractions and their swelling. They thus influence the rate at which the initial radicals at the polymer backbone are reached during the grafting process. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3323–3336, 2006     

Photoreactions of radicals during polyolefin photooxidation

Polypropylene (PP) and polyethylene (PE) peroxy radicals undergo photoreactions, but under commonly encountered photodegradation conditions these reaction rates are much lower than those of conventional radical reactions; for example, for PP peroxy radicals in noon summer sunlight at 25°C their rate of photolysis to alkyl radicals is less than one-tenth of their rate of hydrogen abstraction from the polymer. At lower temperatures( < ?10°C) or when more intense radiation is used, however, peroxy radical photolysis becomes a proportionately more important source of alkyl radicals. In addition, occurrence of photoinduced radical combination is confirmed but is shown to be important only when photolysis generates an alkyl radical sufficiently close to a peroxy radical that termination can occur before oxygen reconverts the alkyl radical to a peroxy radical. This termination mechanism therefore becomes more important for radicals generated at lower temperatures when the average separation of a radical pair is lower.     

Titanium tetra-<Emphasis Type="Italic">tert</Emphasis>-butoxide-<Emphasis Type="Italic">tert</Emphasis>-butyl hydroperoxide oxidizing system: Physicochemical and chemical aspects

The reaction of titanium tetra-tert-butoxide with tert-butyl hydroperoxide (1: 2) (C₆H₆, 20 C) involves the steps of formation of the titanium-containing peroxide (t-BuO)₃TiOOBu-t and peroxytrioxide (t-BuO)₃TiOOOBu-t. The latter decomposes with the release of oxygen, often in the singlet form, and also homolytically with cleavage of both peroxy bonds. The corresponding alkoxy and peroxy radicals were identified by ESR using spin traps. The title system oxidizes organic substrates under mild conditions. Depending on the substrate structure, the active oxidant species can be titanium-containing peroxide, peroxytrioxide, and oxygen generated by the system.     

Aging and degradation of polyolefins. I. Peroxide-initiated oxidations of atactic polypropylene

Efficiencies of polymer radical production by thermal decomposition of di-tert-butylperoxy oxalate (DBPO) have been measured in bulk atactic polypropylene (PP) at 25–55°C; they range from 1 to 26%, depending on [DBPO], temperature, and presence of oxygen. Most of the polymer radicals thus produced disproportionate in the absence of oxygen but form peroxy radicals in its presence. Most of the pairs of peroxy radicals interact by a first-order reaction in the polymer cage. The fraction that escapes gives hydroperoxide in a reaction that is half order in rate of initiation. In interactions of polymer peroxy radicals, in or out of the cage, about one-third give dialkyl peroxides and immediate chain termination, two-thirds give alkoxy radicals. About one-third of the later cleave at 45°C; the rest abstract hydrogen to give hydroxy groups and new polymer and polymer peroxy radicals. The primary peroxy radicals from cleavage account for the rest of the chain termination. Cleavage of alkoxy radicals and crosslinking of PP through dialkyl peroxides nearly compensate. Up to 70% of the oxygen absorbed has been found in hydroperoxides. The formation of these can be completely inhibited, but cage reactions are unaffected by inhibitors. Concentrations of free polymer peroxy radicals have been measured by electron spin resonance and found to be very high, about 10^?3M at 58–63°C. Comparison with results on 2,4-dimethylpentane indicate that rate constants for both chain propagation and termination in the polymer are much smaller than those for the model hydrocarbon but that the ratio, k_p/(2k_t)^½, is about the same.     

Electron spin resonance studies on graft copolymerization of gaseous styrene onto preirradiated polypropylene. I. Preirradiation in the presence of oxygen

Graft copolymerization of gaseous styrene was carried out onto polypropylene preirradiated in the presence of oxygen at ?78°C and at room temperature, respectively. The origin of the graft initiation activities of these polymers was investigated by means of electron spin resonance (ESR) of trapped radicals. More grafting was found for the polymers irradiated at ?78°C than for those irradiated at room temperature. The difference of grafting between polymers irradiated at ?78°C and those irradiated at room temperature was not explained by the total amounts of trapped radicals, and it was found that all radicals are not effective in the grafting reaction. ESR measurements showed that there exist two kinds of peroxy radicals, one has more effective ability of abstracting hydrogen atoms from the surrounding polymer chains to form carbon radicals, and another is less effective at the temperature of grafting reaction (40°C). Although the samples irradiated at ?78°C contain the both types of radicals, those irradiated at room temperature do not contain the former type of radicals. It was shown that the carbon radicals produced by such a hydrogen abstraction reaction are actually the effective centers in the grafting reaction of polymers irradiated in the presence of oxygen.     

ESR study for ion beam induced phenomena in poly (tetrafluoroethylene-co-hexafluoropropylene) (FEP)

Trapped radicals induced in poly (tetrafluoroethylene-co-hexafluoropropylene) (FEP) were observed by X-band electron spin resonance (ESR) spectroscopy at room temperature (RT) under atmospheric field after an irradiation with various kinds of high energy ion beams (6 MeV/u). The irradiation was carried out to a stacked FEP films under vacuum (<4E?4 Pa) at RT with various fluences from 1.0×10⁹ to 1.0×10¹¹ ions/cm². All ESR spectra indicated an existence of peroxy radicals in each of the FEP films without any relation to a kind of ion and a penetration depth. Obtained depth profiles of radical concentrations induced with each ion beam almost correspond to those of stopping power. The trapped radical concentrations were strongly dependent on stopping power. It was found that G-value of trapped radicals by ion beam irradiation was decreased with increasing a stopping power, and was less than the case of gamma-rays irradiation.     

On the reactivity of cellulose free radicals in graft copolymerization reactions

The formation and behavior of photo-and mechanoinduced free radicals in cellulose were studied by ESR spectroscopy and the capability of these free radicals to initiate graft copolymerization reactions was demonstrated. Although an 11-line ESR signal was detected from cellulose irradiated with ultraviolet (UV) light, a higher-intensity ESR signal with a five-line pattern was detected from a sample mechanically milled at 77 K. The decay of photoinduced free radicals when heated took place monotonously, whereas mechanoradicals exhibited an anomalous behavior with an increased signal intensity at 150 K before decaying at a higher temperature. Mechanoradicals have been found to react more efficiently and rapidly with oxygen and methyl methacrylate (MMA) than photoinduced free radicals. The peroxy mechanoradicals, however, were mobile and decayed more rapidly than the peroxy photoinduced radicals. Simultaneous graft copolymerizations of MMA to cellulose demonstrated that mechano-and photoinduced free radicals are capable of initiating grafting reactions, but a higher degree of grafting efficiency was obtained from cellulose treated mechanically.     

ESR STUDY OF PLASMA-TREATED POLYTETRAFLUOROETHYLENE FILMS

The plasma treatment of polytetrafluoroethylene (PTFE) films was carried out in a capacitively coupled reactor with external electrodes. The free radicals generated in the process of treatment were detected by ESR techniques. The ESR spectra tended to indicate that the free radicals of the pLasma-treated PTFE film sample were turned into peroxy radicals on exposure to air. The extrema separation (W) of the ESR spectrum of the peroxy radical increased with the lowering temperature and underwent a sudden change within the temperature range of 170 to 190K. The ESR spectrum observed at 77K was quite different from that observed at room temperature. Finally, the effects of treatment time, input power and system pressure on radical concentration of the treated samples were studied. The attenuation of the peroxy radical at room temperature was also investigated.     

Electron spin resonance study on chemical crosslinking reaction mechanisms of polyethylene using a chemical agent. V. Comparison with polypropylene and ethylene‐propylene copolymer

We studied the chemical reaction process of polypropylene (PP), ethylene‐propylene copolymer (EPM), and ethylene‐propylene‐diene copolymer (EPDM) crosslinking induced by dicumyl peroxide (DCP) using electron spin resonance (ESR). Free radicals appeared at an elevated temperature of around 120 °C and the behavior and kinetics of the reaction process were observed at 180 °C. The radical species detected in PP were alkyl type radicals, formed by the abstraction of hydrogen atoms from the tertiary carbon of polymer chains. For EPDM containing a diene component, the radicals were trapped at double bonds in this diene component to form allyl radicals. The resolutions of these spectra were extremely clear; hence, isotropic spectra of these polymer radicals were obtained. We measured the ESR at high temperatures and confirmed that the process of crosslinking induced by DCP was a free radical reaction. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3383–3389, 2000     

The free radical species in polyacrylonitrile fibers induced by γ-radiation and their decay behaviors

Free radicals in vacuum, air and oxygen atmospheres were studied using electron spin resonance (ESR). Mainly two types of radicals, namely alkyl radicals and polyimine radicals, are formed in polyacrylonitrile (PAN) fibers after γ-ray irradiation. The G value of the radical formation was calculated to be 2.1 (number of radicals per 100 eV absorbed) in air at room temperature based on the ESR measurements. The radical stability and decay behaviors at room temperature and elevated temperatures were also investigated under different atmospheres. The alkyl radicals were found to be rather stable when stored in vacuum at room temperature, but they decayed via reaction with oxygen when stored in air. The alkyl radicals disappeared completely after a thermal treatment at 110 °C in vacuum, but only 15% of the polyimine radicals decayed; this indicates that polyimine radicals are more stable compared to the alkyl radicals due to their lower mobility.     

Grafting of functional maleimides onto oligo‐ and polyolefins

In this paper the modification of polyolefins, especially polypropylene (PP), with maleimides is discussed. As there are many problems associated with the analysis of the grafting reaction, commercially available squalane and eicosane were used as model compounds for ethylene‐propylene‐copolymer (EPR) and polyethylene (PE). 2,4,6,8‐Tetramethylnonane (TMN) was synthesized as a novel model compound for PP. N‐Phenylmaleimide and maleimido benzoic acid were grafted onto the oligomers in solution using 2,5‐dimethyl‐2,5‐di‐(tert‐butyl‐peroxy)‐3‐hexyne (Luperox 130) as radical initiator. The grafting efficiency was determined by NMR‐, UV‐spectroscopy, gravimetry and gas‐chromatography. The influence of several reaction parameters on homopolymerformation, amount of modified oligomer and grafting efficiency could be elaborated. NMR‐spectroscopy and SEC were used to analyse the nature of the grafts. Grafting yields were almost quantitative using up to more than 10 mol% monomer, but dropped at higher monomer concentrations to about 60%. The amount of modified oligomer could be increased with higher monomer concentrations or reaction temperatures. Grafting of polymers was done by reactive extrusion. Products were analysed by IR‐spectroscopy. Optimal grafting yields could be achieved with 1 wt% monomer and 0.5 wt% peroxide concentration.     

Photolysis in aprotic solvents of some alkylcobalt(III) complexes; an ESR and spin-trapping technique study

Photolysis of several alkylcobalt(III) complexes in the visible region λ > 420 nm has been studied by ESR and spin-trapping techniques with nitrosuderene, phenyl-t-butylnitrone (PBN) and 5,5′-dimethylpyrroline-N-oxide (DMPO) and their mixtures. DMPO and PBN give spin adducts of two different radicals; the first is a hydrogen atom coming from the equatorial ligand, and the second is an alkyl-free radical coming from the axial position of the cobalt(III) complexes. This indicates that excitation of the complex is followed by the expulsion of one hydrogen atom and homolytic cleavage of the metalalkyl bond.     

An ESR study of the decay reaction of isotactic polypropylene peroxy radicals in air

The decay of peroxy radicals trapped in irradiated isotactic polypropylene has been studied by ESR in air at various temperatures between 284 and 309 K. All the ESR spectra obtained at the various reaction stages are shown to be composed of two components arising from a mobile fraction and an immobile fraction. Only the mobile peroxy radicals decay; those belonging to the immobile fraction are stable. Various reaction mechanisms are examined in order to explain the experimental results; it is concluded that the decay reaction is controlled by diffusion of peroxy radicals and that the immobile peroxy radicals play no role in the decay reaction. Intermolecular hydrogen abstraction of the peroxy radicals, rather than intramolecular abstraction, is suggested as the rate-determining reaction.     

Radical processes in polymer burning and its retardation. II. An ESR study of flame retardation of polypropylene

Qualitative and quantitative free radical transformations between polypropylene alkyl radicals, oxygen, and brominated flame retardants in the molten and gaseous phases of thermally treated samples are described. By ESR technique it has been proved that, depending on the applied pressure during pyrolysis, highly reactive peroxy radicals or less reactive radicals of the retardant are formed. For flame-retarded samples, compared with polypropylene without additives, the concentration of the primary alkyl radicals transferred from the molten to the gaseous phase is reduced by two orders of magnitude (from 7 × 10¹⁴ to 7 × 10¹² spins/0.02 g), whereas the limiting oxygen index [LOI] is raised from 17–18% to 25–26%. The great variety of physical and chemical processes proceeding in molten polymer in the preflame zone and burning gaseous phase calls for use of different retardant types with a programmed release of Br and HBr with the temperature increase. The chain oxidation of the “fuel,” a product of the endothermic decomposition of polymer, determines the temperature of self-ignition according to the number of initiating alkyl and allyl radicals formed per unit time; the [LOI] index depends more on the length of the kinetic chain of propagation reactions in the stationary process of oxidation at a given pressure.     

Sensitive luminescence techniques to study the early stages of polymer oxidation

This paper describes recent developments in the use of chemiluminescence (CL) and profluorescent nitroxides (PFNs) in probing the “induction period” of polymer oxidation. CL measures the instantaneous rate of reaction of hydroperoxides responsible for initiating degradation and the spreading of oxidation, while PFNs can be used to measure the concentration of alkyl radicals produced in oxidation events and thus provide an integrating sensor for the extent of cumulative damage. The PFN additive acts as an oxidation retarder by competing with oxygen to scavenge the alkyl radicals that generate chain carrying peroxy radicals and so mirrors the performance of hindered amine stabilisers (HAS) in one part of their stabilisation cycle. Using the example of polypropylene (PP) and cis-polyisoprene (PIP) as substrates which can rapidly spread oxidative damage, the factors controlling the reaction of PFNs can be determined from CL and fluorescence as well as infra-red (IR) spectroscopy through the detection of oxidation products as measured by the carbonyl index. Matrix effects on the reactivity are demonstrated using a polyethylene-norbornene copolymer (TOPAS) as carrier for both PIP and the PFN and it is seen that the PFN is a radical scavenger only above T_g of the carrier. When PIP alone is oxidized, the PFN is an integrating sensor for free radical production under ambient conditions for up to twelve months while also stabilizing the polymer. Critically, it is thus able to determine the underlying rate of radical production in the oxidation induction period.     

The oxidation of hindered amine light stabilizers to nitroxy radicals in solution and in polymers

Cyclohexane and three polyolefins (PE, PP and EPM) have been oxidized by γ-irradiation, and the concentration of nitroxy radicals derived from a typical HALS added either before or after irradiation has been studied by ESR spectroscopy. The results indicate that HALS is rapidly oxidized by peroxy radicals under irradiation and that afterwards it can be oxidized more slowly by hydroperoxides. This last reaction is, however, very limited.     

Photooxidative degradation of cellulose: Reactions of the cellulosic free radicals with oxygen

Photooxidative degradation of cellulose resulted in decreases of degree of polymerization (DP) and α-cellulose content, concurrently producing chromophoric groups; namely, carbonyl, carboxyl, and hydroperoxide groups within the polymer. Electron spin resonance (ESR) studies revealed that cellulosic carbon free radicals readily reacted with oxygen molecules at 143–160 K to produce peroxy radicals, whereas cellulosic oxygen free radicals were inert toward oxygen molecules throughout the photooxygenation reactions. At 77 K it is feasible that only photoexcited oxygen molecules reacted with cellulosic carbon free radicals to produce peroxide radicals. These radicals were themselves stabilized at 273 K by abstraction of hydrogen atoms from cellulose to produce polymer hydroperoxides. Simultaneously, new radical sites, which exhibited three-line ESR spectra, were generated in cellulose.     

High static pressure alters spin trapping rates in solution. Dependence on the structure of nitrone spin traps

Using a competitive spin trapping method, relative spin trapping rates were quantified for various short-lived radicals (methyl, ethyl, and phenyl radicals). High static pressure was applied to the competitive spin-trapping system by employing high-pressure electron spin resonance (ESR) equipment. Under high pressure (490 bar), spin trapping rate constants for alkyl and phenyl radicals increased by 10 to 40%, and the increase was dependent on the structure of nitrone spin traps. A maximum increase was obtained when tert-butyl(4-pyridinylmethylene)amine N-oxide (4-POBN) was used as a spin trap. Activation volumes (DeltaDeltaV(double dagger)) for the two spin trapping reactions were calculated to be -17-(-9) cm(3) mol(-1) for the 4-POBN system.     

Preparation of rechargeable biocidal polypropylene by reactive extrusion with diallylamino triazine

Biocidal polypropylene (PP) was prepared by using a reactive extrusion process, in which PP was grafted with an N-halamine precursor, 2, 4-diamino-6-diallylamino-1, 3, 5-triazine (NDAM). After chlorination the grated PP was converted to biocidal halamine structures. Effects of monomer and initiator concentration on grafting yield, thermal properties, and biocidal efficacy were studied. The Fourier transformed spectroscopy (FTIR) results and nitrogen analysis confirmed the graft polymerization on PP backbone during the reactive extrusion. The results also indicated that at low monomer concentration, increase in initiator concentration led to increase in PP chain scission and decrease in mixing torque, or polymer chain length. As the monomer concentration increases, grafted monomer content in the products showed a steady increase, indicating more grafting copolymerization in the system. The halogenated products exhibited potent antimicrobial properties against Escherichia coli, and the antimicrobial properties were durable and regenerable.     

Oxidation of catechols and catecholamines by horseradish peroxidase and lactoperoxidase: ESR spin stabilization approach combined with optical methods

Oxidation of catechols and catecholamines by horseradish peroxidase (HRP) and lactoperoxidase (LPO) has been studied by electron spin resonance (ESR) and electronic spectroscopies. The ESR technique has been used as an ESR spin stabilization approach by complexation of o-semiquinone free radicals with Zn^II ions. ESR spectra and parameters of these free radical complexed forms are in good agreement with those obtained previously for complexed and uncomplexed species. The K_m values obtained with the two methods show stereoselective effects towards the chiral substrates l- and d-dopa from HRP and LPO. Furthermore, these enzymes display opposite stereochemical interactions, in agreement with the analogous effects observed on l- and d-tyrosine by electronic and NMR binding studies.