Electron-Nuclear Multiple Resonance on Stable and Transient Radicals

The physical principles underlying steady-state electron-nuclear double resonance (ENDOR) and electron-nuclear-nuclear triple resonance (TRIPLE) experiments on radicals in solution are discussed. The gain in spectral resolution achievable by these techniques, as compared with standard ESR, allows hyperfine coupling constants to be measured even for large and low-symmetry radicals. ENDOR/TRIPLE is applied to cation radicals related to bacterial photosynthesis. The measured hyperfine couplings are interpreted by means of the all-valence electron MO method RHF-INDO/SP. Information about the spatial structure of chromophores in the reaction centers of photosynthetic bacteria, Rb. sphaeroides and Rps. viridis, is obtained from an analysis of the spin density distribution and from minimization of the total energy. The results are compared with recently available structural data of the primary donor obtained from X-ray diffraction experiments. The spin density distribution in the primary donor cation radical P⁺₉₆₀, which consists of two bacteriochlorophyll b molecules, is discussed in terms of symmetry and perturbations from the protein environment.     

Study of miscibility enhancement in poly(styrene‐co‐4‐vinylphenol)/poly(ethylene oxide) blends by the spin labelling method

The electron spin resonance (ESR) spectra of end‐group spin labelled poly(ethylene oxide) (SLPEO) using 2,2,6,6‐tetramethyl‐piperdine‐1‐oxyl nitroxide and its blends with poly(styrene‐co‐4‐vinylphenol) (STVPhs) of different hydroxyl contents were recorded over a wide temperature range. For a blend of SLPEO and pure polystyrene (PS), the ESR spectrum was composed of a single motion component, indicating that PS was immiscible with PEO. For blends composed of SLPEO and different‐hydroxyl‐content STVPhs, two spectral components with different motion rates were observed over a certain temperature range. The difference between the motion rates should be attributed to micro‐heterogeneity in the blends, with the faster rate corresponding to a nitroxide radical motion trapped in the PEO‐rich domain and the slower rate corresponding to a nitroxide radical motion trapped in the STVPh‐rich domain. Variations in the values of a number of the ESR parameters (T_a, T_d and T_50G) and the apparent activation energy (E_a) with hydroxyl content in the blends indicated that the miscibility of the blends increased with increasing hydrogen‐bonding density due to specific interactions between the hydroxyl groups in STVPh and the ether oxygens in PEO. Copyright © 2004 Society of Chemical Industry     

The study of poly(styrene‐co‐p‐(hexafluoro‐2‐hydroxylisopropyl)‐α‐methylstyrene)/poly(ethylene oxide) blends

Different hydroxyl content poly(styrene‐co‐p‐(hexafluoro‐2‐hydroxylisopropyl)‐α‐methylstyene) [PS(OH)‐X] copolymers were synthesized and blends with 2,2,6,6‐tetramrthyl‐piperdine‐1‐oxyl end spin‐labeled PEO [SLPEO] were prepared. The miscibility behavior of all the blends was predicted by comparing the critical miscible polymer–polymer interaction parameter (χ_crit) with the polymer–polymer interaction parameter (χ). The micro heterogeneity, chain motion, and hydrogen bonding interaction of the blends were investigated by the ESR spin label method. Two spectral components with different rates of motion were observed in the ESR composite spectra of all the blends, indicating the existence of microheterogeneity at the molecular level. According to the variations of ESR spectral parameters T_a, T_d, ΔT, T_50G and τ_c, with the increasing hydroxyl content in blends, it was shown that the extent of miscibility was progressively enhanced due to the controllable hydrogen bonding interaction between the hydroxyl in PS(OH) and the ether oxygen in PEO. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2312–2317, 2004     

IN SITU visualization of water adsorption in cellulose nanofiber film with micrometer spatial resolution using micro-FTIR imaging

Hygroscopic behavior is an inherent characteristic of nanocellulose film which strongly affects its applications. In order to gain a better understanding of water adsorption, micro-Fourier transform infrared (FTIR) imaging was used to investigate the water adsorption in cellulose nanofiber film with a spatial resolution of 20 um. Four spectral peaks at 2905?cm^?1, 1428?cm^?1, 1371?cm^?1, and 1317?cm^?1 attributed to CH and CH₂ groups were used to generate 2D micro-FTIR images of cellulose distribution, and the most intense peak at 3348?cm^?1 was employed to generate 2D micro-FTIR image of OH group distribution. On this basis, difference 2D micro-FTIR images of OH group distribution at different relative humidity (RH) levels demonstrated the development of adsorbed water distribution in cellulose nanofiber film during the water adsorption process. The study results confirmed that the micro-FTIR imaging was one promising tool for in situ visualization of water adsorption with micron-scale resolution.     

Fourier Transform EPR Measurement of Homogeneous Electron Transfer Rates

Fourier-Transform Electron Paramagnetic Resonance has measured the rates of homogeneous electron transfer reactions involving duroquinone radical anions. The radicals are generated by laser photolysis of duroquinone in methanol solution containing 10% triethylamine. The duroquinone concentration was varied over a factor of 1000. The rate constant for electron transfer between the radical anion and the neutral duroquinone is 1.5×10⁸M⁻¹s⁻¹ and the intrinsic spin relaxation rates, T⁻¹₁ and T⁻¹₂ are 0.32 MHz and 0.44 MHz, respectively. Two-Dimensional magnetization transfer spectra show that the reaction is a homogeneous electron transfer reaction. The electron transfer rates are measured by two novel pulse sequences designed for more efficient data acquisition in samples with no unresolved inhomogeneous broadening. These two sequences result in a more rapid, accurate determination of the second-order chemical rate constant since the second-order rate constant is obtained directly and is not derived from a single measurement of a pseudo-first-order rate. The transient duroquinone radical anions studied here appear to have the same T₁, T₂ and electron transfer rates as stable duroquinone radical anions in alkaline solutions.     

CFD Modeling of a Bubble Column Reactor Carrying out a Consecutive A → B → C Reaction

In this paper, we develop a CFD model for describing a bubble column reactor for carrying out a consecutive first‐order reaction sequence A → B → C. Three reactor configurations, all operating in the homogeneous bubbly regime, were investigated: (I) column diameter D_T = 0.1 m, column height H_T = 1.1 m, (II) D_T = 0.1 m, H_T = 2 m, and (III) D_T = 1 m, H_T = 5 m. Eulerian simulations were carried out for superficial gas velocities U_G in the range of 0.005–0.04 m/s, assuming cylindrical axisymmetry. Additionally, for configurations I and III fully three‐dimensional transient simulations were carried out for checking the assumption of cylindrical axisymmetry. For the 0.1 m diameter column (configuration I), 2‐D axisymmetric and 3‐D transient simulations yield nearly the same results for gas holdup ?_G, centerline liquid velocity V_L(0), conversion of A, χ_A, and selectivity to B, S_B. In sharp contrast, for the 1 m diameter column (configuration III), there are significant differences in the CFD predictions of ?_G, V_L(0), χ_A, and S_B using 2‐D and 3‐D simulations; the 2‐D strategies tend to exaggerate V_L(0), and underpredict ?_G, χ_A, and S_B. The transient 3‐D simulation results appear to be more realistic. The CFD simulation results for χ_A and S_B are also compared with a simple analytic model, often employed in practice, in which the gas phase is assumed to be in plug flow and the liquid phase is well mixed. For the smaller diameter columns (configurations I and II) the CFD simulation results for χ_A are in excellent agreement with the analytic model, but for the larger diameter column the analytic model is somewhat optimistic. There are two reasons for this deviation. Firstly, the gas phase is not in perfect plug flow and secondly, the liquid phase is not perfectly mixed. The computational results obtained in this paper demonstrate the power of CFD for predicting the performance of bubble column reactors. Of particular use is the ability of CFD to describe scale effects.     

1H-NMR imaging study of molecular mobility in carbon black-filled,TBBS–sulfur-vulcanized cis-polyisoprene

The distributions of the T₂ relaxation times in carbon black filled, TBBS-Sulfur vulcanized cis-polyisoprene were were studied using ¹H NMRI spin–echo experiments. It has been reported that more than two T₂ relaxation times are observed in carbon black-filled rubbers, reflecting the existence of the hard regions adjacent to the crosslinks or filler particle and soft regions distant from such rigid components. Our current concern is how the amount and distribution of the T₂ times are affected by the filler incorporation in the rubber compounds. A decrease in the T₂ relaxation times with an increasae in carbon black content is observed. The average T₂ time, 〈T₂〉, drops from 11.38 ms with no carbon black to 10.05 ms with 15 phr carbon black. The 〈T₂〉 further decreases when the black loading level is increased form 15 to 30 phr and 30 to 50 phr, but the magnitude of the changes in the 〈T₂〉s are not as large as in the initial loading (0 to 15 phr). The observations of the 〈T₂〉s suggest inhomogeneities are induced in the network structure by the black incorporation. The distribution of the T₂ relaxation times becomes narrower as the black loading level increases. There are at least four factors governing the intensity of the images in the swollen, filled rubber vulcanizates as well as the NMR parameters (T₁, T₂, T_R, and T_E: (1) ¹H spin density, (2) inhomogeneity of the network structure, (3) degree of swelling in the sampling solvent, and (4) displacement effect of the carbon black. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1385–1390, 1998     

Proton spin–lattice and spin–spin relaxation times in isotactic polypropylene. II. Effects of stretching ratio and temperature

Proton spin–lattice, T₁, and spin–spin, T₂, relaxation times of uniaxially stretched polypropylene film were measured at 40°C using a wide line pulse spectrometer operating at 19.8 MHz. T_1l, the longer T₁, increases almost linearly with increasing stretching ratio, and T_2a, T₂ of the amorphous region, decreases gradually as the stretching ratio is increased. These results can be interpreted in terms of the increased constraints to molecular motion in the amorphous region. The fraction of the rigid protons in the sample, F_c, increases with increasing stretching ratio, while the crystallinity calculated from the density, X_d, does not change largely. The difference between F_c and X_d, therefore, increases as the stretching ratio is increased. This indicates that the physical structure of the highly stretched sample is far from the ideal two-phase model. The influence of the stretching temperature was also investigated. There are only slight increases in T_1l and in F_c for the samples stretched in a temperature range from 80°C to 150°C, whereas the considerable increase in T_2a occurs. The most notable change introduced at a high temperature stretching is the increase in the chain mobility in the amorphous region.     

Pulsed n.m.r. of a SBS ‘macroscopic single crystal’

Pulsed n.m.r. T₁, T₂ and T_1? results are reported for a macroscopic single crystal of styrene—butadiene—styrene copolymer, recorded as a function of the angle between the extrusion axis of the sample and the magnetic field direction. The data confirm the absence of molecular orientation in either component of the copolymer. T₁ and T_1? results indicate a coupling via spin diffusion, between the polystyrene and polybutadiene regions. These spin diffusion effects are analysed in terms of a simple model, presented earlier, which demonstrates a stronger spin diffusion coupling in the rotating frame than in the laboratory frame.     

1H NMR Imaging Study of Molecular Mobility in Silica-Filled,TBBS-Sulfur Vulcanized Cis-Polyisoprene

The interactions of silica in zinc-activated, sulfur-vulcanized cis-1.4 polyisoprene were characterized at the 75% cure state using ¹H NMR imaging spectroscopy. Variables examined included silica loading, mixing conditions, and presence of additives, including a coupling agent and polyethylene glycol. Rheometer curves indicated a decrease in cure rate and cure state as silica was increased. ¹H NMR imaging showed an increase in the T ₂ relaxation times, and a decrease in the proton spin density N(H) as the filler load increases. Mixing conditions did not affect the cure rate, cure state, or the average T ₂ relaxation time; however, the distribution of relaxation times broadened with poor mixing. The presence of a coupling agent increased the cure rate and cure state, as well as decreased the T ₂ relaxation times as compared with samples with the same silica level, but without coupling agent. Polyethylene glycol (PEG) had slightly higher average T ₂ relaxation times, and a slightly broader distribution as compared with the sample without PEG added.     

Synthesis and characterization of polyurethane–chitosan interpenetrating polymer networks

A polyurethane–chitosan (PU–CH) coating was synthesized from castor-oil-based PU prepolymer and highly deacetylated and depolymerized chitosan. The films cast with the coating were used for the characterization. X-ray photoelectron spectroscopy, a surface-sensitive technique, indicated the chemical bonding between the chitosan and PU prepolymer as well as the enrichment of chitosan on the surface of the film PU–CH. Electron spin resonance (ESR) spectroscopy using the nitroxyl radical 4-hydroxy-2,2,6,6-tetramethyl piperidine-1-oxyl (4-hydroxy-TEMPO) as a reporter group was used to study the chain mobility in the film PU–CH. It was observed that T_50G of the probe and the first glass transition temperature (T_g1) of the film PU–CH were 10 and 18°C higher than those in the PU film, respectively, and the activation energy (27.0 kJ mol⁻¹) of tumbling for the probe covalently bonded with PU–CH was 12.8 kJ mol⁻¹ higher than that of the probe with the film PU. It suggests that the molecular motion in the PU–CH was restricted by grafted and crosslinked interpenetrating polymer networks (IPNs). The results of the differential thermal analysis and thermogravimetric analysis proved that the thermostability of the film PU–CH was significantly higher than that of the film PU, and the T_g1 value is in good agreement with that calculated from ESR. It could be concluded that the IPNs resulted from the chitosan grafting and crosslinking with PU exist in the film PU–CH. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1321–1329, 1998     

Permeable domains of segmented polyurethanes studied with paramagnetic spin probe

Studies on the permeable regions of the dense polyurethane-based membranes were performed by electron spin resonance spectroscopy (ESR) using TEMPO spin probe incorporated into the membrane via diffusion from the vapour phase. The ESR spectra were measured as a function of temperature and microwave power for polyurethanes (PU) varying in the molecular structure and morphology. It was found that the TEMPO spin probe exhibited anisotropic rotation whose anisotropy increased as temperature decreased and was more pronounced for PU with shorter soft segments. The simplified method was used to obtain apparent correlation time τ_c enabling the comparison of the polyurethanes studied. This approach was based on the Arrhenius relation of τ_c vs. 1/T determined from motionally narrowed ESR spectra and on the assumption that this behaviour prevails over a broader temperature range at temperatures generally greater than T_g of a given polymer.     

Temperature dependence of molecular motions in the polyurethane-based membranes studied with paramagnetic spin probe

This third paper in this series regarding structure and dynamics of the polyurethane-based membranes studied with TEMPO spin probe presents the results of the ESR measurements performed to characterise mobility of segments in the permeable regions of the membranes. The variations of the spin probe motions with temperature have been analysed for the series of polyurethanes (PU) differing in molecular structure and compared with the results of the DSC studies. Along with T_50 G, the other temperatures, T_n, T_i, T_τ, at which the significant change in dynamics occurs have been determined and correlated with the length of the PU soft and hard segments, and then discussed with regard to the respective relationships of T_g. The results have demonstrated the sensitivity of the ESR method to the segmental motions, which have not been detected by the DSC technique. From the DSC and ESR data the size of the motional chain segment in various PUs has been estimated, which has been found to follow the trend that polymers with higher T_g have bulkier segments. Two unusual observations have been made, concerning the deviation from the Arrhenius relation at high temperature for some PUs, and the increased mobility of the TEMPO molecules in some poly(urethane-urea)s after their thermal treatment. These results have been interpreted so far either in terms of the possible translational diffusion of the TEMPO molecules for those PUs showing lesser amount of the hard segments, or in terms of the increased free volume resulting from the temperature induced structural changes within the permeable regions of poly(urethane-urea)s.     

Magnetic properties of Co3O4 polycrystal powder

The results of investigations of magnetic properties of Co₃O₄ polycrystals in powder morphology with average crystallite size of 10 µm are presented. The temperature dependence of the magnetic susceptibility, measured in a wide temperature interval (1.5≤T≤400 K) using SQUID magnetometer, as well as electron spin resonance (ESR) spectra measured at X-band and Q-band frequency ranges have been studied. Antiferromagnetism with a Néel temperature T_N at about 39 K was observed from the analysis of evolution of molar magnetic susceptibility and ESR intensity as a function of temperature. ESR parameters namely g-factor, intensity (I_ESR), g-value, and linewidth ΔH for powder Co₃O₄ have been obtained. Some deviations from the expected values of some magnetic properties were determined and discussed.     

Electrochemical behaviour and electrochemical polymerization of fluoro‐substituted anilines

The electrochemical behaviour of three fluoro‐substituted aniline monomers, 2‐fluoroaniline (2FAN), 3‐fluoroaniline (3FAN) and 4‐fluoroaniline (4FAN), was investigated in aqueous acidic and organic media by means of cyclic voltammetry (CV) studies. Constant potential electrolysis (CPE) of the monomers in acetonitrile–water mixture (1:1 by volume) using NaClO₄ as supporting electrolyte yielded soluble polymers. The mechanism of electrochemical polymerization was investigated using in situ electron spin resonance (ESR) and in situ UV–VIS spectroscopic techniques for one of the monomers (4FAN). Both CV and in situ UV–VIS measurements indicated that the polymers obtained are in the emeraldine base form. In situ ESR studies indicated that electrochemical polymerization involves a radical‐cation as an intermediate. Characterization of polymer products have been carried out using FTIR and NMR spectroscopic techniques, and thermal behaviour was studied using differential scanning calorimetry (DSC). It was found that conductivity can be imparted to as‐synthesized polyfluoroanilines via iodine doping. © 2002 Society of Chemical Industry     

Proton spin-lattice and spin-spin relaxation times in annealed poly(ethylene terephthalate)

Proton spin-lattice, T₁, and spin-spin, T₂, relaxation times of poly(ethylene terephthalate) film annealed at various temperatures were measured using a broad line pulse spectrometer. The value of T₁ is closely related to the crystallinity of the sample and only one T₁ was observed for each sample, indicating that the spin diffusion is effectively operating. Even in the amorphous sample there are some nuclear spins, the motion of which is strongly restricted.     

Thermally induced homolytic scissions of interunitary bonds in a softwood lignin solution: A spin‐trapping study

Unstable chemical species, that is, radicals generated by the thermal treatment of a dimethyl sulfoxide (DMSO) solution of the lignin of a softwood, Yezo spruce (Picea jezoensis Carr.), were studied in detail with an electron spin resonance (ESR) method combined with a spin‐trapping technique. An unstable secondary carbon radical (～CH ·) in the solution was trapped as a stable nitroxide spin adduct [R? (N? O ·)? CH～ (R = tert‐butyl benzene)] when the DMSO solution was heat‐treated in the presence of a spin‐trapping reagent [2,4,6‐tri‐tert‐butylnitrosobenzene (BNB)] at about 40°C. This meant that alkyl phenyl ether bonds (～CH? O‐phenyl), known as interunitary lignin bonds, were homolytically scissioned by the thermal treatment in the lignin solution. A detailed analysis of the ESR spectrum revealed that three kinds of radicals—primary (～CH₂ ·), secondary (～CH ·), and tertiary (～C ·) carbon radicals—were trapped as stable spin adducts at about 60°C, although the phenoxy radical (Ph? O ·) was not trapped by the BNB spin trap as the counter radical of the secondary carbon radical. This suggested that a fairly large steric hindrance existed between the so‐called guaiacoxy radical with a methoxy group in the ortho position and the BNB molecule bearing two butyl groups as bulky moieties in the ortho positions. However, the phenoxy radicals in the lignin solution were stable up to about 60°C. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2136–2141, 2004     

Thermogravimetry of Thermoplastic Polyimide Powders under Four Different Atmospheres

Thermal degradation of a thermoplastic polyimide ( TPI ) fine powder was studied in nitrogen, helium, argon, and air, from room temperature to 790°C by a high‐resolution thermogravimetry (TG) at a variable heating rate in response to changes in the weight‐loss rate of the sample and also by traditional TG. In the three inert atmospheres, the high‐resolution TG found a two‐step degradation process with higher resolution for the TPI , which was hardly ever revealed by a traditional TG for the TPI and other similar polyimides. On the contrary, only a traditional TG in air observed a two‐step degradation process for the TPI . The initial thermal degradation temperature T_d, the temperatures at the maximum weight‐loss rate T_dm1 and T_dm2, the first maximum weight‐loss rate (dα/dT)_m1, as well as the degradation activation energy of the TPI all increase with the variation of testing atmosphere in the following order: in nitrogen < in helium < in argon < in air, but the char yield at 700°C appears to increase in a different order: in air < in helium < in argon < in nitrogen.     

ESR studies of solvent penetration into polymer-blend-like material: Particles formed from poly(methyl methacrylate) and polyisobutylene

Solvent penetration into nonaqueous dispersions (NAD) of poly(methyl methacrylate) (PMMA) sterically stabilized by PIB were studied by ESR spectroscopy. These colloidal particles were exposed to dilute solution of spin probe 3-carbomoyl 2,2,5,5-tetramethyl 3-pyrolin-1-yloxy and the bimodal distribution of ESR spectra of this probe molecules were carefully monitored as a function of time. Fickian type diffusion of spin probes into the particles was observed and a spherical model was employed to obtain the diffusion coefficients (D) in various solvents. D values were found around 10^?15 cm²/s, and interpreted as the parameter, inversely proportional to the apparent viscosity of the environment inside the colloid particle. Maximum amount of diffused solvent molecules (M_∞) into the PMMA particle was found inversely proportional to polymer-solvent interaction through the solubility parameters. The interconnected network of PIB was found to be responsible for the penetration of spin-probe (i.e., solvent) molecules into the glassy PMMA phase and/or PIB-PMMA interface.     

E.s.r. and saturation transfer e.s.r. spectroscopy of spin-probed poly(vinyl acetate): A new spectral transition correlating with glass transition temperature

From the temperature dependence of the rotational frequency of a nitroxide spin probe dispersed in poly(vinyl acetate) the size of the polymer segment involved in the glass to rubber relaxation can be estimated. The calculation is based on a free volume model which relates f, the ratio of the volumes of the spin probe and the polymer segment, to the rotational frequency of the probe. An increase in f with increasing size of the probe is demonstrated. Saturation transfer electron spin resonance spectra of two nitroxide probes are shown to undergo a rapid change with temperature at a temperature T_R which is characteristic of both the probe and its environment. T_R corresponds to a pseudo-isofrequency point and is analogous to, though at a lower characteristic frequency than, the widely-used parameter T_50G. For the systems examined, T_R correlates with the glass to rubber relaxation.