Decay Of Free Radicals Research Articles

Concept of mobile voids in the interpretation of decay of free radicals in solid polymers

An alternative idea of decay of free radicals in solid polymers based upon a concept of voids existing in each polymer with the mobility induced by the motion of respective polymer segments and vice versa has been put forward taking into account formal reaction kinetics. It has been shown that this approach enables the qualitative interpretation of second-order decay, stepwise decay of free radicals initiated by a stepwise increase of temperature as well as the effect of high pressure on the overall rate constant of the decay process.

EPR study of melanin-protein interaction: Photoinduced free radicals and progressive microwave power saturation

The photoinduction of free radicals in synthetic L-dihydroxyphenylalanine ( l-DOPA) melanin in the presence of bovine serum albumin (BSA) was studied by electron paramagnetic resonance (EPR) spectroscopy. By monitoring the signal intensities and progressive microwave power saturation it was shown that l-DOPA melanin in solution behaves as a single macromolecule, interacting with BSA and molecular oxygen. In the absence of oxygen, the EPR signal of l-DOPA melanin was homogeneously broadened; the magnetic interaction with oxygen induced inhomogeneous broadening. In aqueous solution, the presence of BSA decreased the accessibility of oxygen to paramagnetic centres in the melanin. On UV-visible illumination, the presence of BSA modified the rates of formation and decay of photoinduced free radicals, resulting in a net enhancement of the EPR signal compared with that observed in pure l-DOPA melanin.

Fourier transform-EPR studies of photoinduced electron transfer in homogeneous and micellar solutions

Fourier transform (FT) EPR was used to study the pulsed-laser-induced electron transfer from porphyrins to quinones in homogeneous and micellar solutions. By monitoring the EPR signal of the quinone anion radicals as a function of delay time (τd) between laser and microwave pulses, with τd ranging from nanoseconds to 1 millisecond, information was obtained on the kinetics of free radical formation and decay. The time evolution of the signal also gave an insight into the chemically induced dynamic electron polarization (CIDEP) mechanisms that affect signal amplitudes. FT-EPR spectra of electron transfer products generated in micellar solution provide evidence for the generation of long-lived spincorrelated radical pairs.

Non-steady state model for free radical polymerization

The non-steady state theory for the kinetics of free radical polymerization taking no account of gel-effect is reviewed. Considering the facts that the monomer consumption in chain propagation is much higher than that in chain initiation and the rate constant of chain termination is much larger than that of chain propagation or transfer, a few very close approximations are introduced to solve the set of kinetical differential equations of free radical polymerization. The expressions for various molecular parameters, such as the molecular size function, the number- and weight-average degrees of polymerization and the dispersity, are derived. In accord with the non-steady state theory, the curves of free radical decay with reaction time or monomer conversion and the molecular parameters mentioned can be predicted from the reaction conditions. Several numerical examples are given.

Cooperative motion in amorphous polymers: application to the study of free-radical decay in solids

A method for the simulation of cooperative motion in amorphous polymers has been proposed. The influence of this motion on the free-radical decay in solid polymers is studied. The cooperative motion is shown to extend beyond the local regions and lead to diffusion of radicals throughout the volume.

Free-radical decay in a polymer blend PMMA/PEO studied by the EPR method

The EPR method was used for studying decay of free radicals in a blend of poly(methyl methacrylate) (PMMA) with poly(ethylene oxide) (PEO). The influence of the composition of the blend (up to 70 vol% PEO) and of the time of mixing on the transfer of a radical centre was determined. The time of mixing for a blend affects molecular motions as a result of specific interactions between dissimilar PMMA and PEO chains. These molecular motions control the transfer of the radical centre so that free radicals decay more rapidly in the blend mixed for 6 min than in that mixed for 12 min. A slower decrease in the concentration of free radicals in a sample of a PMMA/PEO blend containing 50 vol% PEO compared with samples with 10, 30 and 70 vol% is also observed.

Pressure dependence of free radical decay in γ-irradiated main chain liquid crystalline polymers

A decay of free radicals in γ-irradiated rigid main chain thermotropic polymer BRY 856 was studied. The dependence of the rate constant for free radical decay over the pressure region between 1 MPa and 800 MPa was determined in the temperature range between 80°C and 230°C. Activation volumes of free radical decay were established. A decrease in activation volume at temperatures above the glass transition temperature of the polymer was observed and attributed to rotational structural order along the long axis of the macromolecules.

Photoinduced electron transfer from porphyrins to quinones in micellar systems: an FT-EPR study

The photoinduced electron transfer from zinc tetrakis (4-sulfonatophenyl) porphyrin (ZnTPPS) to duroquinone (DQ) in several micellar solutions was investigated with Fourier transform-electron paramagnetic resonance (FT-EPR). The measurements give information on the formation and decay of free radicals, following pulsed-laser excitation of ZnTPPS, with nanosecond time resolution. It was found that the characteristics and time evolution of the EPR spectra depend strongly on surfactant properties. In particular, long-lived [ZnTPPS 3−… DQ −] spin-correlated radical pairs (lifetime > 1 μs) were observed in solutions of the cationic surfactant cetyltrimethylammonium chloride (CTAC). By contrast, radical pair lifetimes in anionic and neutral micelles are shorter than 100 ns. The experimental data can be rationalized in terms of the Coulombic and hydrophobic interactions that determine the spatial organization and stability of the different species (reactants and products) in the microheterogenous systems.

Volume effects in the decay of free radicals in organic crystals

The decay kinetics of the free radicals produced by γ-irradiation of single crystals of organic dicarboxylic acids is studied at hydrostatic pressures up to 200 MPa. Correlation is established between the reaction's activation parameters ( V∗ and E∗) and the crystals macrocharacteristics—the compressibility and thermal expansion coefficients. A common equation is proposed to describe the variation of the radical decay rate constant with temperature and pressure in malonic, succinic, adipic, glutaric, suberic, and sebacic acids.

Fourier transform-electron paramagnetic resonance study of the photochemical reaction of acetone with 2-propanol

Fourier transform-electron paramagnetic resonance (FT-EPR) was used to study the pulsed-laser induced reduction of acetone with 2-propanol. By monitoring the EPR signal of the acetone ketyl radical as function of delay time (τd) between laser pulse and microwave pulse, with τd ranging from nanoseconds to 100 μs, information was obtained on the kinetics of free radical formation and decay. The time evolution of the signal also gave an insight into the chemically induced dynamic electron polarization (CIDEP) mechanisms that affect signal amplitudes. It was found that the spectra obtained with τd settings ranging from 0 to 400 ns contain a dispersive signal contribution that is due to the presence of spin-correlated radical pairs (SCRP) at the time of the microwave pulse. For acetone(D6) in 2-propanol(D8) the rate constants of formation and decay of the SCRP are found to be 7.5±3.7×106 and ∼5×107 s−1, respectively. The SCRP lifetime in 2-propanol(D8) at room temperature corresponds to what would be expected for diffusion controlled cage escape. The rate constant for ketyl(D7) radical formation is found to be 5.8±0.5×106 s−1. The decay rate constant kdN(0), where N(0) is the maximum radical concentration, is found to be 1.0×105 s−1. The rate constant for radical formation increases to 1.1×107 s−1 for the acetone(H6)/2-propanol(H8) system. The pronounced isotope effect on radical formation is consistent with a transition state involving C–H bond stretching. The FT-EPR spectra show the effects of radical pair CIDEP. However, a detailed analysis of signal growth and decay kinetics shows that there is no triplet mechanism CIDEP contribution. This finding is in agreement with the relative magnitudes of acetone triplet spin lattice relaxation and radical formation rate constants. The results also demonstrate that the rate constant for hydrogen abstraction is triplet sublevel independent.

Influence of pressure on a free radical decay in a rigid main chain thermotropic polymer

Free radical decay has been studied in a gamma-irradiated rigid rod-like liquid crystalline copolymer over wide ranges of pressure (1–900 MPa) and temperature (70–140°). Rate constants and activation volumes for the decay were determined. A remarkable decrease of activation volume at temperatures above T g of the polymer was observed and attributed to rotational structural order along the long axis of the rod-life molecules.

Gamma-radiolysis of some crown ethers and their analogues. Electron spin resonance specra of free radicals

The method of low temperature ESR spectroscopy was used to study the free radicals generated by γ-irradiation of crown ethers: 12-crown-4 /1/; 15-crown-5 /2/; dicyclohexane-24-crown-8 /3/; and their analogues: tetrahydrofurane /4/ and 1,4-dioxane /5/. ESR spectra of radicals 4 and 5 taken at 77 K represent a simple singlet; ESR spectra of radicals generated from 1, 2 and 3 have a complex, multiplet structure. The kinetics of thermal decay of free radicals 4 in dependence on temperature starting from 103 K was investigated. The radicals 4 decay very fast at 253 K.

Effect of molecular mobility on the decay of free radicals in amorphous polymers: a Monte Carlo study

An algorithm has been formulated for studying the diffusion-controlled reactions in amorphous polymers. A method has been proposed based on molecular mobility and leading to migration of radical centres, their approach and decay. In a volume of dimensions 70 × 100 × 123 A ̊ , using boundary periodic conditions, an amorphous polymer system and free radicals are simulated on a diamond lattice by the Monte Carlo method. Owing to crank, kink and crankshaft motions, some of them approach one another and decay. A change in the concentration of free radicals with time and temperature has been observed by the method described.

Kinetics of bimolecular decay of α-tocopheroxyl free radicals studied by ESR

The kinetics of bimolecular decay of α-tocopheroxyl free radicals (T .) was studied by ESR mainly in ethanol and heptanol solvents. A second-order kinetic law was observed during the whole course of reaction (-d[T .]d t = 2 k[T .] 2) and the following rate constants were determined with good accuracy in the temperature range 281–321 K: ethanol: log(2 k) = 8.2 ± 0.5 — (6.6±0.7 kcal/mol)/(2.3 RT) M −1·s −1; heptanol: log(2 k)=6.1±0.4-(4.3±0.6 kcal/mol)/(2.3 RT) M −1.s −1. The global rate constant clearly increases with solvent polarity.

Kinetics and mechanisms of free radical decay reactions in irradiated solids

A new second order kinetic equation is given which takes into account the fraction of free radicals which cannot react in solids at the observation temperature. This corresponds to the situation when solids irradiated at liquid nitrogen temperature are then heated to a temperature where only some of the free radicals disappear by recombination. This equation is applied to data for isotactic polypropylene, polybutadiene, linear low density polyethylene, etc.

XPS and ESR studies of the photodegradation of polyamidoimide and polyimide in O2, O2 + N2, air, N2, and vacuum atmospheres

The photostability of polyamidoimide (PAI) and polyimide (PI) films has been studied. UV irradiation caused a great change in the composition and structure of the film surface. With irradiation in atmospheres such as air, oxygen, and an oxygen—nitrogen mixture, the Cls spectrum had a distinct structure at high binding energy, and the O/C and N/C ratios were considerably increased. On the other hand, in nitrogen and under vacuum, the O/C and N/C ratios remained almost unchanged during an earlier stage of irradiation, but in the latter stage for the nitrogen atmosphere these ratios decreased to a broad minimum and then increased with an increase of irradiation time. UV irradiation also produced free radicals in the polymer films; PAI gave a much stronger ESR signal intensity than PI. The signal intensity vs. irradiation time curve was strongly influenced by irradiation atmospheres. In atmospheres containing oxygen the curve exhibited a broad maximum during an initial stage of irradiation, while under vacuum and in nitrogen the signal intensity increased greatly in the latter stage. The formation of free radicals during an earlier stage of irradiation in atmospheres containing oxygen was promoted by the oxygen incorporated at the film surface, while the radical production under vacuum was considered to be due to rupturing of bonds in the bulk material. The decay of free radicals in the dark after irradiation occurred at a faster speed in oxygen atmospheres than under vacuum.

Crystallinity dependent free radical formation and decay in irradiated polyethylene in the presence of oxygen

Evidence is presented that both the detectable amount of irradiation induced free radical formation and the subsequent rate of decay of the radicals in the presence of oxygen is significantly affected by the state of crystallinity of the polyethylene sample. For an identical dose there is a higher allyl radical concentration at room temperature in the sample of more developed crystallinity resulting in more pronounced chemical changes (e.g. carbonyl group formation) on subsequent exposure to air. This is manifested also by visually observable morphology dependent discoloration.

Effect of chemical crosslinking on the decay of macroradicals in crosslinked poly(methyl methacrylate)

Free radicals were generated in crosslinked poly(methyl methacrylate) by γ irradiation with 16 kGy at 195 K. Decay of macroradicals was observed as a function of the concentration of ethylene glycol dimethacrylate used as a crosslinking agent by the ESR method. The rate constants for free radical decay were found to decrease with the increasing content of the crosslinking agent. The overall activation energies determined from the temperature variations of the rate constants were reduced with the increasing amount of ethylene glycol dimethacrylate in the copolymer system. The results are discussed from the point of view of molecular motions dominating in the temperature interval examined and controlling the decay process in crosslinked poly(methyl methacrylate).

Preparation of charge-mosaic membranes by radiation-induced graft copolymerization of PE films with styrene and 4-vinylpyridine: II. Effects of pre-irradiation dosage, atmosphere, storage time and storage temperature of grafting of 4-vinylpyridine onto PE films

The graft copolymerization of 4-vinylpyridine onto LDPE films pre-irradiated with a 1.2 MeV electron beam, was carried out in the vapor phase as the second step in the preparation of charge-mosaic membranes. The effects of pre-irradiation dosage, atmosphere, storage time and storage temperature of PE films on the grafting process were studied by measuring the rate of free radical decay under different conditions. The results of ESR study show that: (1) The free radicals, created in the course of pre-irradiation, decrease stepwise with different decay constants. At first, they decay rapidly, but afterwards they decay more and more slowly. (2) At different temperatures, free radicals have different decay constants but they have almost the same decay characteristics as described above. (3) Sampling pre-irradiated PE films during the fast decay period may cause a pronounced error in the percentage graft, so that the standard deviation is much larger. On the other hand, sampling pre-irradiated PE films during the slow decay period greatly improves the precision of the percentage graft, so the standard deviation is much smaller. (4) For the above reason, the optimum storage time of pre-irradiated PE films at 5°C may be considered to be in the range of 48–120 h after the free radicals have been created. The results of graft copolymerization of pre-irradiated PE films with 4-vinylpyridine in the gaseous phase at different temperatures showed that: (1) At higher grafting temperatures, the grafting rate of 4-vinylpyridine onto PE films is much higher than the swelling rate of PE films by monomer vapor. As the grafted macromolecular chains propagated surround the activated sites very quickly, there is a local thickening effect, and the resulting graft copolymer looks rough. (2) At lower grafting temperatures, the grafting rate of 4-vinylpyridine seems to be comparable with the swelling rate of PE films by monomer vapor; an even and smooth copolymeric film can be obtained.

The electron spin resonance of polycarbonate track detectors

Abstract Polycarbonate films are widely used as solid state track detectors (SSTDs) of radiation, but as yet our knowledge of the microscopic nature of the latent track is limited. The processes of chemical etching and thermal annealing are not fully understood. The lack of stability of track parameters bears on the accuracy of the charge determination of energetic heavy nuclei. We have applied the technique of electron spin resonance, (ESR) in order to seek a correlation between the density of tracks and the growth and decay of free radicals detected by ESR. Samples of polycarbonate were irradiated with thermal neutrons and then etched progressively with NaOH. The ESR signal increased initially and then became weaker as the bulk of the polycarbonate was removed. Other samples were annealed after partial etching. ESR monitoring of this annealing showed that free radicals were recombining, probably due to diffusion processes.