Cyclization and crosslinking of polybutadiene in solution by electron beam irradiation

Effects of electron beam irradiation on liquid polybutadiene in n-hexane solution were studied at ?10°C. With irradiation, crosslinking of the polymer and loss of double bond by cyclization took place at the same time. These reactions were retarded by the addition of DPPH or triethylamine which indicated that both radical and cationic mechanisms contributed to this system.     

Gamma irradiation induced photochemical reactions in V-doped borate glasses

Glasses of the composition XNa₂O · 4Al₂O₃ (96-X) B₂O₃ (mole%) where X = 10, 20, 30 to which 0.03 g V₂O₅ per 100 g glass was added, were prepared by normal melting. Their absorption characteristics together with the corresponding V-free base glasses were determined before and after gamma irradiation. The characteristic spectra of the unirradiated glasses show absorption bands at 315, 470, 560–580, 610–650, 700–870, and 860–1000 nm, indicating the presence of vanadium ions in more than one oxidation state, viz, V⁵⁺, V⁴⁺, and V³⁺. Gamma irradation of V-containing glasses causes the formation of color centers in the glass matrices, with absorption bands at 330, 500, and 610 nm, and photoreduced [V³⁺] and [V²⁺] ions with absorption bands at 350–355 and 530–570 and 520 nm, respectively. Photoreduced [V⁴⁺] may also be formed, giving rise to absorptions at 690–700 and 750–800 nm. The induced vanadium ions are found to absorb at shorter wavelengths than the intrinsic ones. An explanation based on the difference in the field energy of the two states is given.     

Electron-cation recombination in rigid organic glasses: simulation of charge distribution and isothermal luminescence kinetics

Recent results on isothermal luminescence decays, observed from 10^?8 to 10⁴ s after irradiation of a rigid glass at 77 K, are interpreted in terms of tunnelling electron-ion recombination. After 10^?6 s, recombination of isolated pairs is dominant. Electron-ion distance distributions are obtained by computer simulation and used to calculate the corresponding decay kinetics I(t). I(t) ∝ t^?1.0 is found with realistic chosen parameters throughout the whole time range investigated experimentally, so that tunnelling recombination is quantitatively consistent with observation. Experimental study at shorter times (10^?8-10^?6 s) indicates a contribution from recombinations of pairs which are not isolated. A very simple computer simulation of these effects is proposed and shown to agree with experiment.     

Ion and polymer dynamics in polymer electrolytes PPO-LiClO4. I. Insights from NMR line-shape analysis

We investigate ion and polymer dynamics in polymer electrolytes PPO-LiClO4 performing 2H and 7Li NMR line-shape analysis. Comparison of temperature dependent 7Li and 2H NMR spectra gives evidence for a coupling of ion and polymer dynamics. 2H NMR spectra for various salt concentrations reveal a strong slowdown of the polymer segmental motion when the salt content is increased. The 2H NMR line shape further indicates that the segmental motion is governed by dynamical heterogeneities. While the width of the distribution of correlation times G(log tau) is moderate for low and high salt content, an extremely broad distribution exists for an intermediate salt concentration of 15:1 PPO-LiClO4. For the latter composition, a weighted superposition of two spectral components, reflecting the fast and the slow polymer segments of the distribution, describes the 2H NMR line shape over a broad temperature range. Analysis of the temperature dependent relative intensity of both spectral components indicates the existence of a continuous rather than a discontinuous distribution G(log tau). Such continuous distribution is consistent with gradual fluctuations of the local salt concentration and, hence, of the local environments of the polymer segments, whereas it is at variance with the existence of large salt-depleted and salt-rich domains featuring fast and slow polymer dynamics, respectively. Finally, for all studied PPO-LiClO4 mixtures, the 2H NMR line shape strongly depends on the echo delay in the applied echo-pulse sequence, indicating that the structural relaxation of the polymer segments involves successive rotational jumps about small angles gamma < 20 degrees .     

Ion irradiation induced structural and electrical transition in graphene

The relationship between the electrical properties and structure evolution of single layer graphene was studied by gradually introducing the gallium ion irradiation. Raman spectrums show a structural transition from nano-crystalline graphene to amorphous carbon as escalating the degree of disorder of the graphene sample, which is in correspondence with the electrical transition from a Boltzmann diffusion transport to a carrier hopping transport. The results show a controllable method to tune the properties of graphene.     

Surface patterns induced by laser irradiation on thin polymer bilayer films

We report on the preparation of wavelike surface patterns with characteristic wavelengths on thin bilayers of poly(methyl methacrylate) on azobenzene liquid crystalline polymer films (LCP/PMMA) by irradiation of a single polarized pulsed laser beam. The formation of such patterns was influenced by the thickness of the upper layer and the laser fluence. We were also able to guide the wavelike pattern to have a specific orientation by placing an elastic polydimethylsiloxane (PDMS) mold on the surface of bilayer film prior to laser irradiation. Moreover, the property of the laser irradiation, that is, the selectivity through mask-projection systems, allowed us fabricating complicated micropatterns for novel microdevices.     

Silicon tetrafluoride visible luminescence induced by high-power CO2 laser irradiation

The visible luminescence induced by a high-power CO₂ laser in SiF₄ occurs within the laser pulse even at pressures as low as 10^?2 Torr. The unirnolecular nature of the process is also supported by pressure-dependence studies of the luminescence intensity and the incubation period. A possible assignment of the transition involving inverse electronic relaxation is discussed.     

Modelling molecular weight changes induced in polydimethylsiloxane by gamma and electron beam irradiation

A commercial linear polydimethylsiloxane (PDMS) was subject to gamma irradiation under vacuum and in air, as well as to accelerated electron beam radiolysis (EB). All irradiation treatments were done at room temperature. The molecular weight changes induced by the radiation processes have been investigated using size exclusion chromatography (SEC) with refraction index (RI) and multi angle laser light scattering (MALLS) detectors to obtain the number and weight average molecular weights of the irradiated samples.

The analysis of the data indicates that crosslinking reactions predominated over scission reactions in all cases. Gamma irradiation under vacuum was the most efficient process within the analyzed dose range, reaching the gel point earlier. Irradiation in the presence of oxygen induces oxidative effects, both in gamma and EB irradiations. A previously developed mathematical model of the irradiation process that accounts for simultaneous scission and crosslinking and allows for both H and Y crosslinks fitted well the measured molecular weight data.     

Kinetics of geminate-ion recombination by electron tunnelling

A phenomenological model is developed for geminate-ion recombination by electron tunnelling. Experimentally observed decay kinetics of recombination luminescence is well described by this model.     

Blue luminescence induced by confinement in self-assembled films

The fabrication and characterization by means of photoluminescence (PL), UV-vis absorption, electro-luminescence (EL) and X-ray reflectivity of multilayer heterostructures consisting of alternate layers of conjugated and non-conjugated polymers have been studied. The heterostructures are prepared by the layer-by-layer self-assembly technique, using two types of polyelectrolytes. The first are precursors of conjugated polymers such as poly(phenylenevinylene) (PPV) and other poly(arylenevinylene) polymers, and the second are non-conjugated polymers such as poly(styrene-4-sulfonate) (SPS), polyacrylic acid (PAA) and poly(allylamine hydrochloride) (PAH). The heterostructures consist of a repeated sequence of bilayers (layer pair) or multilayers, where the conjugated polymer is formed by heat treatment under vacuum. The thickness of each bilayer or multilayer was controlled by changing the non-conjugated polymer layer. Most importantly, we have found that the PL and EL spectral emissions can be ‘tuned’ by a proper ‘design’ of the heterostructure. Particularly, heterostructures in which the bilayer thickness is rather small and the electroluminescent layers are practically in contact show a blue shift upon decreasing the thickness of the assembly for ultrathin assemblies. In contrast, for assemblies where the electroluminescent layers are well separated by one or several non-conjugated layers (polyelectrolyte spacers), the emission is in the blue and independent of the assembly thickness (number of bilayers). We interpret the results as being due to confinement effects. Using this assembly technique, we were able to fabricate light emitting diodes (LEDs) which emit in the blue region.     

The mechanism of hydrogen formation induced by low-energy electron irradiation of hexadecanethiol self-assembled monolayers

The desorption of molecular hydrogen during low-energy electron irradiation of self-assembled monolayers containing n-alkanethiols has been previously reported, yet to date, there is no consensus as to the mechanism for the formation of this ubiquitous product. In this study, mixed monolayers containing known ratios of perhydrogenated and perdeuterated alkanethiols were chemisorbed to Au(111)/mica substrates and used as targets for low-energy electron irradiation; by measuring the electron-stimulated production of H(2), D(2), and HD as a function of the film composition, we unambiguously show that the desorbing molecular hydrogen is formed via a two-step bimolecular reaction process. The initial electron-molecule scattering event produces a reactive atomic fragment, which then abstracts a hydrogen atom from a nearby molecular site to produce the measured bimolecular yields; the contribution of one-step unimolecular dissociation channels to the overall molecular hydrogen yields is below the approximately 5% detection limit. The dependence of the electron-induced modifications to the film on the incident electron energy suggests that the primary event is dissociative electron attachment, and that the primary reactive fragment is most likely H(-). Quantitative analysis of the product yields shows that while approximately 80% of the molecular hydrogen is formed by this bimolecular mechanism within the film, the remaining 20% is formed from reactive atomic fragments that are ejected from the film and subsequently react with residual H(2)O adsorbed on the chamber walls.     

Strain‐induced crystallization and strength of elastomers. I. cis‐1,4‐polybutadiene

Crosslinked samples of cis‐1,4‐polybutadiene (BR) were crystallized at low temperatures and then slowly melted. From volume changes and differential scanning calorimetry measurements, the degree of crystallization in the unstrained state was estimated to be about 20%, much lower than for natural rubber (NR). Crystallization and melting were followed in stretched samples by corresponding changes in tensile stress. Crystallization was faster at higher strains, and the melting temperature was raised significantly on stretching but less than for NR, and the decrease in stress on crystallizing was smaller. Measurements of tensile strength were made over a wide temperature range and showed a marked drop with heating to temperatures of 40–60 °C, falling to values of only 1–2 MPa. A similar drop in strength occurred in NR vulcanizates at high temperatures and was attributed to failure to crystallize on stretching (A. G. Thomas & J. M. Whittle, Rubber Chem Technol 1970, 43, 222; A. N. Gent, S. Kawahara & J. Zhao, Rubber Chem Technol 1998, 71, 668). At ambient temperatures, where strain‐induced crystallization occurred, the strength of BR samples was only about one‐half of that of similar NR materials. This was attributed to less strain‐induced crystallinity in BR (verified by X‐ray studies), paralleling the lower amount developed at low temperatures. We speculate that the higher density of molecular entanglements in BR than in NR prevents BR from crystallizing to the same degree as NR. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 811–817, 2001     

Laser-diode-excited intense luminescence and green-upconversion in erbium-doped bismuth-germanate-lead glasses

We investigate the spectroscopic properties of the 1.5-microm emission from the (4)I(13/2)-->(4)I(15/2) transition of Er(3+) ions in bismuth-germanate-lead glasses for applications in broadband fiber amplifiers. The emission peak locates at 1532nm with a full width at half-maximum (FWHM) of approximately 65nm. The measured lifetime and the calculated emission cross-section of this transition are 3.3ms and 8.66x10(-21)cm(2), respectively. IR-to-green-upconversion occurs simultaneously upon excitation of the 1.5-microm emission with a commercially available 980nm laser diode. Effects of PbF(2) content on the thermal stability, structure and spectroscopic properties of Er(3+)-doped bismuth-germanate-lead glasses have been examined. We find that the substitution with PbF(2) provides a couple of potentials: shortening the UV cutoff band and decreasing the phonon energy of host glasses. Codoping of Yb(3+) significantly enhances both the green-upconversion and 1.5-microm emission intensity by means of a nonradiative Yb(3+)-->Er(3+) energy transfer. Energy transfer processes and nonradiative phonon-assisted decays could account for the population of the (2)H(11/2) level, which is an emitting level of the green-upconversion of Er(3+). The results indicate the possibility towards the development of bismuth-germanate-lead based glasses as photonics devices.     

Changes in poly-vinylidene fluoride produced by electron irradiation

A study was made of the effects of electron irradiation of poly-vinylidene fluoride, using the DSC, FTIR, and X-ray techniques, as well as determining the percent cross-linking. Changes in crystallinity and melting point of the sample as a function of dose were found, the latter due to competition between cross-linking and crystallinity. The cross-linking was observed to increase with radiation dose. Chemical changes observed include the formation of carbonyl groups and double bonding, associated with the loss of HF.     

Crystallization of bisphenol-A polycarbonate induced by organic salts: Chemical modification of the polymer. I. Model reactions

The chemical modifications induced in diphenyl carbonate (DPC) by sodium arylcarboxylates between 200 and 250°C were studied to model the behavior of bisphenol-A polycarbonate – salt systems. Reaction between the salt and DPC produces sodium phenoxide, the phenyl arylcarboxylate corresponding to the salt, and carbon dioxide. The two latter compounds probably result from the decarboxylation of an unstable intermediate compound, viz., a mixed carboxylic carbonic anhydride. CO₂ and sodium phenoxide act as catalysts transforming DPC into phenyl salicylate via the formation of a small amount of sodium salicylate. Electrophilic acylation of sodium phenoxide by DPC is another possible but minor source of phenyl salicylate. Above 250°C, phenyl salicylate becomes unstable and pyrolyzes into o-phenoxybenzoic acid, which is immedicately esterified in the presence of DPC into phenyl o-phenoxybenzoate. In DPC + sodium o-chlorobenzoate systems, reaction between phenyl o-chlorobenzoate and sodium phenoxide is another source of phenyl o-phenoxybenzoate.     

Ion exchange in dimethylsulfoxide media. I

Summary Distribution ratios between Dowex 50 W-X 8 resin and DMSO-H₂O-HCl systems have been determined for Mg, Ca, Sr, Ba, Ga, In, Sn(II) and Pb(II) as a function of both DMSO content and HCl-molarity. At high DMSO concentrations and high HCl concentrations theD values for all metal ions were not much different from those in corresponding aqueous systems. However, at lower and intermediate HCl concentrations (0.10 to 0.60M) D values differed greatly from those in aqueous systems, especially in the range from 20 to 50 mole% DMSO. Maxima in the logD vs. mole% DMSO plots at constant HCl concentrations were obtained for all cations except Sn(II) and Pb(II). Separation procedures for various pairs and combinations of cations seem feasible in DMSO-media [e. g., Ga/In and Sn(II)/Pb(II)] and the DMSO-water system is suggested as a very promising novel solvent medium for ion exchange separations.

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Zusammenfassung Für Mg, Ca, Sr, Ba, Ga, In, Sn(II) und Pb(II) wurden die Verteilungsverhältnisse zwischen Dowex 50 W-X 8 und Dimethylsulfoxid—Wasser—Salzsäure in Abhängigkeit vom DMSO-Gehalt und der Salzsäuremolarität bestimmt. Bei hohen DMSO-Konzentrationen und hohen HCl-Konzentrationen unterscheiden sich die Verteilungskoeffizienten (D) aller Metallionen nicht sehr von denen in entsprechenden wäßrigen Systemen. Aber bei niederen und mäßigen HCl-Konzentrationen (0,10-bis 0,60-m) istD sehr verschieden gegenüber wäßrigen Systemen, besonders bei Anwesenheit von 20 bis 50 Mol-% DMSO. Trägt man logD gegen Mol-% DMSO bei konstanter HCl-Konzentration auf, so erhält man für alle Metalle außer Sn(II) und Pb(II) Maxima. Trennungsverfahren für verschiedene Ionenpaare und -gemische scheinen in DMSO-hältigem Plasma durchführbar zu sein, z. B. Ga/In und Sn(II)/Pb(II). Wäßrige DMSO-Systeme sind neue, vielversprechende Lösungsmittel für Trennungen mit Hilfe von Ionenaustauschern.

     

Ion dynamics in mixed alkali cadmium fluoride glasses

We have investigated ion dynamics in cadmium fluoride nonoxide glasses, in the frequency range from 10 Hz to 2 MHz and in the temperature range from room temperature to just below the glass transition temperature. We have established the existence of mixed alkali effect for these glasses from different aspects of relaxation dynamics. We have observed lower dimensionality of the conduction pathways in mixed alkali cadmium fluoride glasses compared to that in the single alkali glasses. Although, both n and beta represent the interaction between the charge carriers, we have not found any theoretical correlation between them. Using different scaling approaches we have shown that the relaxation dynamics in mixed alkali cadmium fluoride glasses is independent of temperatures.     

Immobilization of chiral ligands on polymer fibers by electron beam induced grafting and applications in enantioselective catalysis

[reaction: see text] Styrenic TADDOL and L-prolinol-derived monomers were immobilized on polyethylene fibers by electron beam induced preirradiation grafting using styrene as comonomer. The polymer-supported chiral ligands were utilized as catalysts in the asymmetric addition of diethylzinc to benzaldehyde. Fiber-bound titanium TADDOLate gave a quantitative conversion of benzaldehyde to 1-phenylpropan-1-ol in a 97:3 S/R enantiomeric ratio. The catalyst was successfully regenerated and employed in subsequent reactions with retention of high enantioselectivities.