摘要:

The ESR spectra of radicals produced in Co60γ-irradiated polystyrene have been studied for doses ranging from 1 to 700 megarads. At least three distinct radical species are observed to occur in this dose range. The observed 100 electron volt yield of radicals for 25 °C radiolysis is 0.102 ± 0.005 and is approximately ¼ of the previously reported value. The observed spectrum of the species produced at low doses has been previously reported and interpreted as the disubstituted benzyl radical. The yield for this radical is estimated to be 0.09 per 100 electron volts. The second species, giving a single broad line at g = 2.002, formed with a yield of 0.012, is the most thermally stable species and may involve a crosslink center. This yield is in good agreement with the additional crosslink yield reported to occur when the polymer is heated above the glass transition temperature before exposure to oxygen following vacuum radiolysis. A third radical species is observed at intermediate doses (> 5 Mrad) and is attributed to the chain radical formed by loss of a beta hydrogen. The observed yield of this radical is < 0.002 per 100 electron volts deposited. All three species are observed to decay isothermally at temperatures greater than about 55 °C with the crosslink species having the greatest stability and the beta radical the least. Exposure to oxygen causes the irreversible bleaching of all three species with the same rate. These data are interpreted in terms of the mechanisms of crosslinking, scission and post-irradiative oxidation of polystyrene.

ISSN:0369-1152    

DOI:10.1080/15421406908082739

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

A theory of radiationless transitions in isolated molecules is presented, considering the decay of a compound molecular state into a quasi continuum of zero order vibronic levels. The theory is then applied to electronic relaxation processes in molecular crystals with an emphasis on the implications of level shifts in solids, and the nature of the decay of metastable exciton states located above the direct threshold to the conduction band.

ISSN:0369-1152    

DOI:10.1080/15421406908082740

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

Delayed fluorescence in single crystals of high purity phenanthrene has been studied. The lifetime of triplet excitons has been obtained as a function of temperature and estimates have been made of the rate constant for triplet-triplet annihilation and of the triplet diffusion constant. The results have been interpreted in terms of impurity trapping of the triplet excitions.

ISSN:0369-1152    

DOI:10.1080/15421406908082741

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

We have found that photoemission of electrons from cesium, sodium, calcium, and magnesium into anthracene crystals can be observed. This implies that the interfacial complex layers are thin enough not to modify the energy distribution of photoexcited electrons passing from the metal to the crystal. The main interfacial property is related to surface states which are able to trap electrons if the energy separation of the Fermi level of the metal from the edge of the lowest conduction band of anthracene is lower than the trapping energy. A maximum trap depth of 1.30 ± 0.2 eV is estimated. The occupation of the surface states therefore depends on the work function of the metal contact. These surface charges modify the threshold energies for photoemission from the metal to the crystal. They can be released optically thus giving rise to a component of the photoinjection current displaying a spectral response curve which is identical with the optical absorption of anthracene mononegative ion. The spectral response curve for injection of photoexcited metal electrons to the anthracene crystal is interpreted as a sum of two contributions: (i) injection into the narrow first conduction band and (ii) injection into a broad second band located 0.55 ± 0.05 eV above the first one.

ISSN:0369-1152    

DOI:10.1080/15421406908082742

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

X band microwave-optical double resonance studies of the metastable triplet states of naphthalene-d8and quinoxalene are presented and the amplitudes and linebreadths of the signals examined. It is noted that the amplitudes of the lines are consistently an order of magnitude or more smaller than expected on the basis of a simple theory; the discrepancy is shown to arise in part from the long triplet spin-lattice relaxation times which prevailed under the conditions of the measurements.Kband microwave optical double resonance results for quinoxalene support this analysis and indicate that the double resonance method can be extended to permit study of the ESR of triplet systems of photochemical interest which are too short-lived to be observable by conventional ESR methods. A successful application of the double resonance method to triplet cyclopentanone is reported.

ISSN:0369-1152    

DOI:10.1080/15421406908082743

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

It has been reported previously that in thetranscinnamic acids and some substituted butadienes the crystal structure determines whether or not photodimerization occurs and the molecular structure of the dimer, if any. Similar results have been obtained also with chalcones. However, in anthracene and its 9-substituted derivatives the results are qualitatively different. Thus anthracene undergoes dimerization, if slowly, even though potentially combining atoms are 6 Å apart. 9-Cyano and 9-chloro-anthracenes, in structures with short axes of 4 Å, react to give head-to-tail dimers, whereas if structural effects were dominant the head-to-head dimers would be formed. It is concluded that in the last three examples, at least, the reaction does not occur in the bulk (i.e., at ordered sites). Two alternatives are suggested and discussed; the first is that reaction occurs in the vapor phase, the second that reaction occurs at defect sites, some of which are present initially, whereas others are produced during the dimerization. The factors which are responsible for the different behaviors of the anthracenes and of the other systems are discussed.

ISSN:0369-1152    

DOI:10.1080/15421406908082744

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

When organic solids are subjected to ionizing radiation free radicals which often persist for long periods of time are generated. It is shown here that the free radicals in irradiated crystalline propionamide,n-butyramide, isobutyramide, and stearamide can react rapidly with oxygen, nitric oxide, and similar gases. Evidence has been obtained from electron spin resonance, X-ray diffraction, gas absorption, and end-product analysis that the gases diffuse into the crystalline lattice of the amides reaching the radical sites and initiating chain reactions which are influenced by the diffusion rate. The layer type crystal structure which is found in these compounds is important in understanding these effects.

ISSN:0369-1152    

DOI:10.1080/15421406908082745

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

The reactions of γ-irradiated propionamide,n-butyramide, isobutyramide, methacrylamide, valeramide, and stearamide with sulfur dioxide have been studied by ESR spectroscopy, gas absorption measurements, and X-ray diffraction. Sulfur dioxide reacts with the radicals originally generated in the irradiated material to give sulphonyl radicals. These propagate in a chain reaction in which further sulfur dioxide is absorbed. In some cases the original radicals can be recovered from the corresponding sulphonyl radicals. The mechanism of the reactions is discussed.

ISSN:0369-1152    

DOI:10.1080/15421406908082746

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

An examination is made of the possibility of studying diffusion occurring concurrently with another rate process of comparable velocity in penetrant-organic solid systems by means of simple experiments. It is shown that a model of diffusion with time-dependent “thermodynamic” permeability and solubility coefficients offers a useful kinetic description of such systems. Attention is drawn to certain advantages of this approach over earlier similar treatments and to the information which can be obtained by comparing uptake and permeation kinetics. These points are illustrated (with the aid of numerical solutions of the relevant equations where necessary) by detailed discussion of two examples of rate processes which can accompany diffusion, namely: (i) penetrant-induced molecular relaxations in glassy organic solids; and (ii) immobilizing irreversible reaction between the penetrant and suitable groups in the solid. The possibility of quantitative characterization of the accompanying rate process under suitable conditions is also illustrated in example (ii). It is, in fact, shown that considerable information can be obtained about the order and magnitude of the rate constant of the reaction, at least in the simplest systems, by methods similar to those of homogeneous reaction kinetics.

ISSN:0369-1152    

DOI:10.1080/15421406908082747

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor

摘要:

By using the polymerization of acrylamide as an example, it is shown that solid state polymerization is in general just a special case of organic solid state reactions. Insofar aa the solid state aspects of the reaction are concerned, the reaction shows the expected behavior of an ordinary solid state reaction except that certain problems are exaggerated because of the large size of the product molecules involved and the large number of possible configurations the molecule may have. The reaction goes by a two phase mechanism with the polymer in general nucleating at a defect site. The product is noncrystalline because of the mismatch in geometry between the product molecule and parent lattice. The fact that the reaction mostly occurs at the interface between monomer and polymer where the reacting molecule cannot easily flip over probably also contributes to the frequent atacticity of the product. Annealing and recrystallization are factors which probably influence the over-all rate of reaction. The large molecular size of tho product and the frequently disordered nature of the product phase in polymerizations allows the use of some rather simple techniques in the study of solid state polymerizations making these good model systems for the study of some solid state problems.

ISSN:0369-1152    

DOI:10.1080/15421406908082748

出版商:Taylor & Francis Group    

年代:1969

数据来源: Taylor