Pressure dependence of free radical decay in irradiated isotactic polypropylene

Free radicals were generated in isotactic polypropylene by gamma-irradiation. The samples were annealed at pressures between 1 and 8000 atm and temperatures between 60 and 110°. The concentration of free radicals was estimated by the ESR method. The rate constants of free radical decay were determined for various pressures and temperatures. The rate constant of free radical decay decreases with increasing pressure while the activation energy increases. The relationship between the kinetics of molecular motion and the kinetics of free radical decay is discussed.     

Pressure dependence of free radical decay in polyamide VI: An ESR study

The ESR spectrum of polyamide VI has been analysed and the rate constant of free radical decay was determined over a wide range of pressure (1–16,000 atm) and temperature (80–120°). It appeared that the rate constant of decay decreased with increasing pressure. The ESR spectrum consisted of three component spectra, the proportions of which determined the shape of the overall spectrum for various conditions.     

Promoting-mode free formalism for excited state radiationless decay process with Duschinsky rotation effect

In the present work, through the path integral of Gaussian type correlation function, a new formalism based on Fermi-Golden Rule for calculating the rate constant of nonradiative decay process with Duschinsky rotation effect in polyatomic molecules is developed. The advantage of the present path-integral formalism is promoting-mode free. In order to get the rate constant, a "transition rate matrix" needs to be calculated. The rate constant calculated previously is only an approximation of diagonal elements of our "transition rate matrix " . The total rate should be the summation over all the matrix elements.     

Radiation chemistry of polyethylene. XIV. Allyl radical decay kinetics in different types of polyethylene

The decay kinetics of the chain allyl free radical has been studied in the following morphological forms of polyethylene (PE): Marlex bulk film, hydrogenated PE, and extended-chain PE. Coupled with previous work on single-crystalline PE it can be seen that the decay rate is greater the more amorphous the sample. In the Marlex bulk film and hydrogenated PE the decay can be interpreted in terms of a simultaneous fast and slow decay process by means of our Q-function equation, but with rising temperature the decay approximates a single rate process. With extended-chain PE the allyl decay rate does not become appreciable until the melting range is approached. The fraction of allyl radicals decaying by the slow process is 2 to 10 times greater than the fraction of fast decaying radicals. The ratio of the fast decay rate constant to that of the slow rate constant is greater for the bulk Marlex film than for the hydrogenated PE. All ratios decrease with rising temperature. For times up to about 150 min the allyl decay in the extended-chain PE accurately follows a single second-order decay law with a time-independent diffusion controlled reaction rate constant.     

Radiation chemistry of polybutadiene and butadiene-styrene block copolymers. II. Kinetics and mechanisms of free-radical decay reactions

The decay of free radicals produced in polybutadiene, polystyrene, and block copolymers of butadiene and styrene by γ irradiation at 77 K has been studied at ?110°C in the case of polybutadiene and at ?95°C for the other samples. The free-radical decay rate is best interpreted in terms of an equation based on a second-order decay mechanism of a fraction of the free radicals decaying in the presence of other nondecaying free radicals. Hydrogen gas accelerates the free-radical decay. Increase of radiation dose increases the fraction of the radicals that decay, while increase of the fraction of styrene segments decreases the decaying fraction. In pure polybutadiene the higher the cis content, the greater fraction of decaying free radicals, but the second-order decay constant is less in the high-cis-content polybutadiene and is also less at the higher dose, probably owing to the hindrance of the radiation-produced crosslinks on the free-radical decay. The decrease of the second-order constant with increase of dose is also true for all the block copolymers studied.     

Influence of Thermal Radiation on Hot Cluster Decay Rates and Abundances

The influence of radiative cooling on the unimolecular decay rates of free, hot clusters and molecules with unspecified excitation energies is quantified. Two different regimes, dedined by the magnitude of the energy of the photons emitted, are identified and the boundary between them is given. The boundary is determined in terms of the photon emission rate constants and thermal properties of the particles. Also the abundance spectra are calculated for the continuous cooling case, corresponding to small photon energies. The two regimes correspond to continuous cooling and single photon quenching of the unimolecular decay. The radiative effect can be parametrized by a redefinition of the time each individual cluster has available to undergo evaporation, expressed by an effective radiative time constant.     

Spectroscopic properties and hydration of the Cm(III) aqua ion from 10 to 85 °C

The optical absorption, fluorescence excitation, and emission spectra of the Cm(III) aqua ion in 0.001 M perchloric acid were studied in pure H(2)O, pure D(2)O, and in mixtures of H(2)O-D(2)O at temperatures from 10 to 85 °C. The quantum yield of the fluorescence of the Cm(III) aqua ion in pure H(2)O and D(2)O was also measured in this temperature range and the radiative decay rate constant was obtained from these data. The results indicate that, from 10 to 85 °C, the effect of temperature on the absorption, excitation, and emission spectra is very small. By correcting the observed decay rate constant for the radiative rate constant, a set of correlations between the observed fluorescence decay rate constant and the hydration number of Cm(3+) in H(2)O at temperatures from 10 to 85 °C was developed. A weak temperature dependence was observed for the nonradiative decay rate constant for the (6)D'(7/2)-(8)S'(7/2) transition and described by the Arrhenius equation. The activation energy of the nonradiative decay was measured to be 0.9 kJ mol(-1), approximately matching the energy gap between the first and the second (A(1) and A(2)) levels of the metastable (6)D'(7/2) multiplet of the Cm(III) aqua ion. On the basis of these observations, it is postulated that the slight increase in the observed fluorescence decay rate constant as the temperature increases from 10 to 85 °C is due to the effect of thermal population of the A(2) level.     

Cooperative motion in amorphous polymers; application to the study of free-radical migration in solids

A Monte Carlo method has been used for studying the effect of the motion of some submolecular structures on the migration of radical centres and on the free-radical decay and its dependence on density. Motions of crank, crankshaft, kink, and double kink type are considered. A cooperative type of motions is also taken into account. The results show that cooperative motions support diffusion of radical centres and thus also the free-radical decay but, at higher densities, the cooperation of motions is restricted. The density of the system, where the decay of radicals is followed, affects the rate of the decay, its increase causes radical decay retardation. This is in line with the high pressure effect, which also retards the radical decay rate.     

Nonequilibrium capillary electrophoresis of equilibrium mixtures--a single experiment reveals equilibrium and kinetic parameters of protein-DNA interactions

We introduce a novel electrophoretic method, nonequilibrium capillary electrophoresis of equilibrium mixtures (NECEEM), and demonstrate its use for studying protein-DNA interactions. The equilibrium mixture of protein and DNA contains three components: free protein, free DNA, and the protein-DNA complex. A short plug of such a mixture is injected into the capillary, and the three components are separated under nonequilibrium conditions. The resulting electropherograms are composed of characteristic peaks and exponential curves. An easy nonnumerical analysis of a single electropherogram reveals two parameters: the equilibrium binding constant and the monomolecular rate constant of complex decay. The bimolecular rate constant of complex formation can then be calculated as the product of the two experimentally determined constants. NECEEM was applied to study the interaction between single-stranded DNA binding protein and a fluorescently labeled 15-mer oligonucleotide. It allowed us to measure for the first time the rate constant of complex decay for this important protein-DNA pair, k-1 = 0.03 s-1. The value of the equilibrium binding constant, Kb = 3.6 x 10-6 M-1, was in good agreement with those measured by other methods. As low as 10-18 mol of the protein was sufficient for the measurements. Thus, the new method is simple, informative, and highly sensitive. Moreover, it can be equally applied to other noncovalent protein-ligand complexes. These features of NECEEM make this method an indispensable tool in studies of macromolecular interactions. They also emphasize the potential role of NECEEM in the development of extremely sensitive protein assays using nucleotide aptamers.     

Noble-gas broadening of an optical transition of I2 measured by coherent transients. III. Two-pulse photon echo and free induction decay

We report two-pulse photon echo decay and free induction decay measurements of iodine as a function of noble-gas pressure. The non-exponential behavior of the two-pulse photon echo decay which in contrast to the free induction decay shows a t³ dependence, is extensively discussed. A comparison with the results of the three-pulse stimulated echo measurements is made. The results are interpretated in terms of a quantum mechanical transport equation, and analytical expressions are derived for the three-pulse stimulated echo, two-pulse echo and free induction decay.     

Effects of anharmonicities on radiationless transitions in large molecules

A new computational method to account for the effect of anharmonicities on non-radiative decay rates of large molecules in the statistical limit is given by using expansions of the generating functions which have been used successfully in multiphonon processes. This method is free from the ambiguity which, as in previous conventional methods, arises from the factorization which separates the decay rate into a product of an electronic term, a Franck-Condon factor and a density of final states. The rate expression is asymmetric with respect to the sign of the displacement of the accepting modes. Actual calculations are performed for a model molecule which closely resembles benzene. The numerical data for the decay rate exceed the results of the harmonic approximation by about two orders of magnitude.     

Effect of pH on kinetics of the decay of the radicals formed during photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline in aqueous and micellar solutions

Kinetics of the decay of the transient radicals formed from 2,2,4,6-tetramethyl-1,2-dihydroquinoline (TMQ) in aqueous and micellar solutions of sodium dodecyl sulfate were studied by flash photolysis as a function of pH. In aqueous and micellar solutions of TMQ the mechanism of the decay of the transient species and the reaction products are different from those in homogeneous organic solutions. The decay of the transient radicals follows first-order kinetics in the entire range of pH under consideration in both aqueous and micellar solutions. In aqueous solutions at pH 9–12, the decay rate constant decreases from 25.3 to 3.7 s⁻¹. In micellar solutions at different pH, different types of micellar catalysis were observed. At pH 1, the rate constant in a micellar solution is slightly lower than that in an aqueous solution. At pH 3–11, the decay rate constant increases (positive micellar catalysis). The apparent rate constant depends linearly on the concentration of TMQ in micelles. The rate constant for the reaction of the transient radical cation with TMQ was determined (200 L mol⁻¹ s⁻¹). At pH>13, the decay rate constant in micelar solutions is lower than that in aqueous solutions (negative micellar catalysis). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 703–708, April, 1997.     

Role of desorption kinetics in determining marangoni flows generated by using electrochemical methods and redox-active surfactants

We report quantitative measurements of Marangoni flows generated at the surfaces of aqueous solutions by using water-soluble redox-active surfactants in combination with electrochemical methods. These measurements are interpreted within the framework of a simple model that is based on lubrication theory and the proposition that the kinetics of the desorption of redox-active surfactants from the surfaces of aqueous solutions plays a central role in determining the strength of the Marangoni flow. The model predicts that the leading edge velocity of the Marangoni flow will decay exponentially with time and that the rate constant for the decay of the velocity can yield an estimate of the surfactant desorption rate constant. Good agreement between theory and experiments was found. By interpreting experimental measurements of electrochemically generated Marangoni flows within the framework of the model, we conclude that the desorption rate constant of the redox-active surfactant Fc(CH(2))(11)-N(+)(CH(3))(3)Br(-), where Fc is ferrocene, is 0.07 s(-)(1). We also conclude that the ionic strength of the aqueous solution has little effect on the desorption rate constant of the ferrocenyl surfactant.     

Radiation chemistry of linear low-density polyethylene. II. Kinetics of alkyl and allyl free-radical decay reactions

Quenched and annealed samples of linear low-density polyethylene (LLDPE) were γ irradiated in vacuo at 77 K; the kinetics of the alkyl free-radical decay reactions were studied at room temperature, and of the allyl free-radical reactions at 60, 70, and 80°C. The ESR signals saturate at a slightly higher microwave power in the LLDPE than in high-density polyethylene (HDPE), and the alkyl radicals start decaying at a lower temperature in the LLDPE than in the HDPE. As in the HDPE the decay of the alkyl free radicals at room temperature in the LLDPE follows the kinetic equation for two simultaneous first-order reactions with the fraction of the faster-decaying component being slightly greater in the quenched than in the annealed samples. In the case of the allyl free radicals the decay at 60°C follows the equation based on one fraction of the radicals decaying according to second-order kinetics in the presence of other nondecaying radicals. At higher temperatures the data are best understood in terms of a second-order rate equation with a continuously variable time-dependent rate constant as suggested by Hamill and Funabashi.     

An apparent angle dependence for the nonradiative deactivation of excited triplet states of sterically constrained, binuclear ruthenium(II) bis(2,2':6',2' '-terpyridine) complexes

The photophysical properties are reported for a series of binuclear ruthenium(II) bis(2,2':6',2"-terpyridine) complexes built around a geometrically constrained, biphenyl-based bridge. The luminescence quantum yield and lifetime increase progressively with decreasing temperature, but the derived rate constant for nonradiative decay of the lowest-energy triplet state depends on the length of a tethering strap attached at the 2,2'-positions of the biphenyl unit. Since the length of the strap determines the dihedral angle for the central C-C bond, the rate of nonradiative decay shows a pronounced dependence on angle. The minimum rate of nonradiative decay occurs when the dihedral angle is 90 degrees, but there is a maximum in the rate when the dihedral angle is about 45 degrees. This effect does not appear to be related to the extent of electron delocalization at the triplet level but can be explained in terms of variable coupling with a low-frequency vibrational mode associated with the strapped biphenyl unit.     

Chemiluminescence study of polymer oxidative stability

The application of the chemiluminescence technique to the study of polymer oxidative stability is discussed. For the initial stages of oxidation a sigmoidal change in the chemiluminescence intensity with time is justified when the experiment is performed isothermally at high oxygen pressure, whereas at low oxygen pressure and constant heating rate an exponential increase in the chemiluminescence intensity is expected. For advanced stages of oxidation depending on the ratio between the initial and equilibrium levels of hydroperoxides, the intensity of emitted light may either grow or decay until the equilibrium concentration of hydroperoxides is established. Two major parameters of an autocatalytic oxidation (induction time and oxidation rate) can be obtained by carrying out the experiment in an oxygen atmosphere at constant temperature. Alternatively, the extent of oxidation in a certain temperature region can be evaluated when a constant-heating-rate experiment is conducted under nitrogen.     

Förster energy transfer from nonexponentially decaying donors

Necessary modifications to the expression for the F?rster energy transfer rate are discussed when fluorescence decay of the donor in the absence of acceptor is nonexponential. Discrete and continuous models of the nonexponentiality are taken into account. No general solution of the problem is found. It is, however, suggested that in many of the biochemical problems the most appropriate modification of the transfer rate can be that which is based on the assumption of the same constant value of the radiative decay rate for all donor molecules. The effect of the assumed form of the F?rster energy transfer rate on the recovered values of the distance distribution and dynamics parameters of some exemplary bichromophoric systems is examined.     

γ射线辐射医用级超高分子量聚乙烯自由基的演变

采用γ射线对医用级超高分子量聚乙烯(UHMWPE)进行辐照处理, 利用电子自旋共振波谱仪(ESR)研究了辐照诱导自由基的种类及其在氩气和不同氧分压下的衰减行为. 在氩气中, 辐射诱导UHMWPE主要产生烷基自由基和烯丙基自由基, 总的辐射化学产额约为0.48/100 eV. 室温下烷基自由基的稳定性差, 其寿命仅有 1 d左右. 在含氧气氛中, 自由基主要通过氧化反应而衰减, 其衰减速率随氧分压的增加而增加, 半衰期则由1×10⁵ Pa氩气中的224.0 h降至5×10⁵ Pa O₂气中的1.8 h. 根据此结果推算, 室温下陷落在晶区的自由基迁移至微晶表面的速率非常快, 仅需小时量级.     

Excited Donor–Acceptor Complexes in the Polymerization of Vinyl Monomers

Butyl methacrylate was found to affect the composition of radical intermediates formed in the photoreduction of benzophenone with triethylamine. In the presence of the monomer, the yield of free radicals decreased and the yield of complexes of the geminate radical pair increased. This was explained by the formation of excited ternary complexes resulted from the interaction of the excited triplet state of benzophenone with the ground-state complex of butyl methacrylate and triethylamine. The substituent effect in benzophenone on the stability of the radical complex was studied. The reaction rate constant for the decay of the radical complex was correlated with the Hammett ⁰ _c constant that determines the mesomeric effect of the substituent.     

Self-trapping limited exciton diffusion in a monomeric perylene crystal as revealed by femtosecond transient absorption microscopy

Self-trapping and singlet-singlet annihilation of the free excitons in a monomeric (beta) perylene crystal were studied by using femtosecond transient absorption microscopy. The free exciton generated by the photo-excitation of the beta-perylene crystal relaxed to the self-trapped exciton with a rate constant of 7 x 10(10) s(-1). The singlet-singlet annihilation of the free exciton observed under the high excitation density conditions was competed with the self-trapping of the free exciton; we estimated the annihilation rate constant for the free exciton to be 1 x 10(-8) cm(3) s(-1) from the excitation density dependence of the free exciton decay. After self-trapping of the free exciton, no annihilation was observed in the 100 ps time range, suggesting that the diffusion coefficient was reduced drastically by self-trapping. The results show that the major factor limiting the exciton diffusion in the beta-perylene crystal is a relaxation of the free exciton to the self-trapped exciton, and not the lifetime of the exciton. Though the singlet-singlet annihilation rate constants and fluorescence lifetime of the beta-perylene crystal are similar to those of the anthracene crystal, the estimated exciton diffusion length (2 nm) in the beta-perylene crystal is much smaller than that (100 nm) in the anthracene crystal as a result of the exciton self-trapping.