E.s.r. study of annealing and melting effects on the molecular motion of spin trapped chain-end radicals located on the polyethylene surface

E.s.r. spectra of chain-end spin labels on the surface of polyethylene powder were obtained for high and low density polyethylene samples at various stages of heat treatment process. The subsequent crystalline structures of the samples were examined with wide angle and small angle X-ray diffraction spectra. The effects of annealing on the motion of the labels were dependent on annealing temperature and the type of the samples. Higher temperature annealing decreased the rotational correlation times (τ_c) and their activation energy in high density polyethylene but did not affect those of low density polyethylene. Melting decreases the values of τ_c and ΔE for both high an low density polyethylene samples. The cause of these decreases were attributed to the changing of the sites of the chain-end labels. The motion of the labels appeared to be influenced by the local segmental motion of the polymer main chains.     

Surface modification of polyethylene by electrical discharge treatment and the mechanism of autoadhesion

A reexamination of previous studies concerning the electrical (‘corona’) discharge treatment of polyethylene and the resulting enhancement of autoadhesion has been carried out. X-ray photoelectron spectroscopic data provide new insight into the phenomenon by showing surface oxidation to result from treatment in ‘inert’ gases. Treatment in hydrogen is an exception and results in no autoadhesion enhancement even though energy input into the film is more efficient than in air. Autoadhesion theories based on electret formation are rejected; those based on hydrogen bonding are largely up-held and shown to be more generally applicable than at first imagined.     

Electron spin resonance studies of spin-labelled synthetic polymers: 13. Lithiation of copolymers of chloro-and bromostyrenes

The products of the reaction between n-butyllithium, complexed with tetramethylethylenediamine or THF, and chloro- and bromostyrene copolymers were identified by trapping the lithiated sites with 2-methyl-2-nitrosopropane. From the e.s.r. spectra of the resulting nitroxide-labelled polymers it was concluded that bromostyrene units undergo direct lithium—bromine exchange. Styrene—bromostyrene copolymers are therefore useful precursors to labelled polystyrene. Chlorostyrene units, by contrast, react with butyllithium to yield intermediate benzynes to which another molecule of butyllithium adds. The product contains a mixture of lithiated sites, each carrying a n-butyl group ortho to the lithium atom; the resulting nitroxide-labelled polymers are unsuitable for dynamic studies. The observations on halogenated styrene copolymers were complemented by experiments on small model molecules.     

Rheo-optical studies on the deformation mechanism of semi-crystalline polymers. IX. Dynamic birefringence behaviour of a high-density polyethylene film having row nucleated crystalline texture of c-axis orientation

The dynamic birefringence behaviour of a high-density polyethylene film having a row-nucleated crystalline texture of cylindrites oriented along the machine direction of fabrication was investigated over frequency and temperature ranges covering the α mechanical dispersion of this material. The results are discussed in combination with the dynamic X-ray diffraction behaviour of the material so as to explore the structural origins of the α mechanical dispersion, not only for this particular material, but also for bulk-crystallized polyethylene having a spherulitic texture. Two deformation processes contribute to the α₁ mechanism, corresponding to the lower temperature relaxation process in the α mechanical dispersion; i.e. (i) a lamellar detwisting process involving the rotation of crystal grains within the crystal lamellae or of lamellar segments around the crystal b-axis or the lamellar axis, which predominates in the MD (machine direction) specimen, and (ii) lamellar shearing associated with the rotation of the crystal grains or the lamellar segments around the crystal a-axis, which is accentuated in the TD (transverse direction to fabrication) specimen. An additional deformation process, lamellar bending, is also observed in the MD specimen as being likely elastic in the dynamic response, and its contribution is found to be substantial, not to the α dispersion, but rather to the β dispersion of this material. The α₂ mechanism corresponding to the higher temperature relaxation process in the α mechanical dispersion is observed to be more pronounced in the TD than in the MD specimen. The apparent dynamic crystal lattice compliance shows a definite dispersion during activation of the α₂ mechanism, but a slight dispersion during activation the α₁ mechanism, suggesting that the α₂ mechanism must be related to an intracrystal relaxation process whereas the α₁ mechanism must be associated with an intercrystal relaxation process at their boundaries.     

The electrochemistry of silver in koh at elevated temperatures—III. Potentiostatic study

The electrochemistry of polycrystalline silver in 1 mol kg^?1 KOH at temperatures between 295 K and 478 K has been studied using potentiostatic polarization. At potentials corresponding to silver dissolution and to the early stages of Ag₂O formation, the transient current data at all the temperatures studied exhibit regions that are linearly dependent on the inverse square root of time. This dependence indicates mass transport to be a rate limiting step in these processes. Log (steady-state current) vs potential data show that at all temperatures silver undergoes an active-to-passive transition. However, the potential of the transition shifts to less positive values and the passive current increases with increasing temperature. In the region of formation of surface oxide phases, the transients exhibit maxima due to nucleation and growth phenomena. The effect of temperature on the transients is similar to the effect of overpotential; in both cases, the time taken to reach the maximum current decreases.     

13C and 1H n.m.r. dynamic study of poly(γ-benzyl-l-glutamate) poly(β-benzyl-l-aspartate) and poly(l-alanine) in solution

¹H T₁ relaxation times were measured in solutions of poly (γ-benzyl-l-glutamate), PBLG, and poly(β-benzyl-^l-aspartate), PBLA, and ¹³C T₁ relaxation times, NOE factors and line widths in solutions of the two polypeptides and of poly(l-alanine), PLA, over a broad range of solvent composition (CDCI₃—trifluoroacetic acid) including the helix—coil transition. PBLG was also studied through temperature-induced inverted helix—coil transition. Most relaxation parameters change smoothly over the studied range, and the observed changes correspond to increase of mobility with increasing content of trifluoroacetic acid and with increasing temperature except for PBLG which undergoes an inverted helix—coil transition with temperature. Analysis of experimental data by means of the isotropic model yielded, for the backbone carbons of all three polypeptides, τ_eff ～ 2 × 10^?8 sec in the helix and τ_eff ～ 1 × 10^?9 sec in the coil form. For the side-chain carbons, τ_eff decreases in the range 10^?9 ? 10^?10 sec with increasing distance from the backbone, with τ_eff values consistently lower in the coil form. Results of analysis by means of a model with two correlation times are discussed.     

Numerical stability of the classical and the modified Saul'yev's finite-difference methods

Stability criteria a modified Saul'yev's explicit method for parabolic partial differential equations are derived and compared with those of classical Saul'yev's explicit method and Crank-Nicholson's implicit method. The two Saul'yev methods operated in the averaging mode are found to be unconditionally stable with respect to the heat equation. However, when lower order terms are included in the linear test equation, the modified method is shown to have a greater stability region than the classical method, while the Crank-Nicholson method maintains its stability unconditionally. Results of numerical experiments confirm the validity of the derived criteria.     

A basic study of catalyst aging in the H-coal process

Samples of CoMo/Al₂O₃ catalysts used in an H-coal process demonstration run were studied to determine causes of catalyst deactivation. Physical and surface properties of the aged and regenerated catalysts were examined. Model compounds were used to assess four catalyst activity functions, viz., hydrodesulfurization (HDS), hydrogenation, cracking and hydrodeoxygenation (HDO). Other tests were performed to study the effects of coke and metals separately on the four catalyst activity functions.Catalyst coke content and metals deposits first increased rapidly, then more gradually with exposure time in the process run. Surface area and pure volume markedly decreased with exposure time. Catalyst activities of aged catalysts showed a rapid decline with exposure time. One-day exposure to coal resulted in significant losses in HDS and hydrogenation activities and nearly complete loss in cracking and HDO activities. Although metal deposits caused some permanent catalyst deactivation, coke had a much greater effect. Regenerated catalysts showed less recovery of catalytic activity as processing time increased. These results agreed well with product inspections from the process run.Oxygen chemisorption on aged—regenerated catalysts decreased with catalyst exposure time, indicating a significant loss of active sites. However, ESCA results showed no evidence of extensive sintering of the active MoS₂ phase. Permanent deactivation of the longer-time exposed catalysts can be ascribed, at least partly, to lateral growth of the active molybdenum sulfide phase. In addition, some loss in cobalt promotion occurred early in the process, which may account for the rapid loss in HDS and HDO activity in regenerated catalysts.     

The electrochemistry of silver in KOH at elevated temperatures—IV. AC impedance study

The ac impedance properties of silver in 1 mol kg^?1 KOH were studied under potentiostatic conditions over the temperature range 295–478 K. Measurements were obtained over the frequency range 0.5 Hz–5 kHz, and were analyzed in terms of equivalent in terms of equivalent circuits. It was shown that provision for surface roughness is required for a satisfactory explanation of the impedance data. Values for the double layer capacity and the concentration AgO^? ions at the electrode surface could generally then be obtained. At potentials corresponding to the formation of Ag₂O, it is demonstrated that both diffusion in solution and diffusion in the oxide film are operative. At elevated temperatures, the rate of growth of Ag₂O centres is controlled partly by diffusion in the oxide and party by a reaction step at the growing interfaces of the centers. The subsequent formation of Ag₂O₂, at all temperatures in the range studied, is interpreted on the basis of an electrocrystallisation impedance.     

PtSiO2: II. Characterization of the gel and the platinum particles by X-Ray diffraction

Modern techniques for analysis of X-ray diffraction profiles have been applied to several of the platinum-silica gel catalysts described in Part I to determine average platinum crystallite size, percentage exposed, crystallite shape, size distribution, lattice parameter, residual stresses and strains, presence or absence of faulting, and mean-square amplitude of vibration. In addition the surface area of the gel was determined with small-angle scattering (SAS). There is good agreement between the percentage exposed of platinum measured by gas adsorption in Part I and the results reported here, calculated from crystallite sizes, indicating the crystallite size is the true platinum particle size. Crystallites to sizes as small as ≈25 Å have been examined. Furthermore, the crystallites are equiaxed in shape (and definitely not cuboidal) and are strain and defect free, except in the case where the average size is near the pore size of the gel. Size distributions are sharper when the catalyst preparation is by impregnation rather than by ion exchange. The size distributions indicate that there is coalescence of some surface species during preparation rather than coarsening (Ostwald ripening). The meansquare amplitude of vibration of platinum increases by ≈30% as the particles decrease in size from ≈100 to ≈25 Å. There is no change in the lattice parameter greater than ≈0.1%. The gel surface areas determined by SAS are in agreement with those determined in Part I by physisorbtion of nitrogen.     

Theoretical study of the inactivation of phosphatase during spray drying of skim-milk

A theoretical calculation is presented of the drying history of skim-milk droplets under spray drying conditions. From the unsteady state diffusion equation the water concentration distribution is calculated as a function of time; simultaneously the thermal degradation of the enzyme alkaline phosphatase is calculated and integrated over the particle. Effects accounted for are the temperature and concentration dependence of the water diffusion coefficient and of the degradation rate constant, and the shrinkage due to water loss. Droplet geometries considered are the “solid sphere” model (fully liquid droplets) and the “hollow sphere” model (with air bubble in the centre). The effect on final phosphatase retention of the following process variables was investigated: dissolved solids content of the feed, air temperature, particle size, and gas bubble size.     

Anodic oxidation of some phenols related to lignin — II. Oxidation of isoeugenol methylether and 4-propylveratrole in acetonitrile

The anodic oxidation of isoeugenol methylether and 4-propylveratrole has been studied in unbuffered acetonitrile solution and in solutions buffered with pyridine and 2,6-lutidine using cyclic voltammetry, steady-state measurements on rotating electrodes, coulometry, esr, uv-spectroscopy, nonlinear galvanostatics and preparative electrolysis. Reaction mechanism, which account for the experimental results, are presented and discussed.     

The electrochemical oxidation of lead in various H2OH2SO4 mixtures—II. Ring-disc electrode study

The rotating ring-disc electrode technique was used to study the generation of soluble Pb(II) species during the anodic oxidation of the lead electrode in various H₂OSO₄ media. The concentration range extended from 0.06 to 10.3 M H₂SO₄. For every concentration it was possible to detect a small cathodic ring current variation which was attributable to the reduction of Pb(II) to Pb. Quantitative measurements of the collection efficiency showed that the small cycle life of the lead-acid battery in 5.6 M H₂SO₄ cannot be ascribed to the dissolution step.     

Electrochemical kinetic study of the Fe3+Fe2+ system in oxalate medium

The overall processes were determined for the reduction of Fe³⁺ to Fe²⁺, in an oxalate medium and at oxalate concentrations of less than 0.5M. The coordination numbers of the oxidized and reduced species were calculated, together with the number of intervening protons that depend on the oxalate concentration and the pH.In addition, the species that reduces at the electrode is determined, together with the intensity equations for each process.     

Electrooxidation of terpenes—I. Synthesis of dihydrocarvone and 1-hydroxyneodihydrocarveol by anodic oxidation of limonene

The oxidation of (+) and (?) limonene, 1, in THF-H₂O (25:1), NaClO₄ on a graphite electrode gives, with a relative stereoselectivity, (+) and (?) dihydrocarvone, 5 and (+) and (?) 1-hydroxyneodihydrocarveol, 7, respectively as the major products. A logical route for this oxidation via a protonated epoxide as intermediate is proposed.     

Rigorous and simplified approach to the modelling of continuous photoreactors

Photochemical reactions where the absorption of radiant energy occurs by the reacting species are considered. In this case the radiant energy balance equation can not be decoupled from the mass balance of the reacting species and a highly non linear integro-differential problem arises.Mathematical difficulties have been usually bypassed by modelling the radiation field on the basis of simplified models of questionable physical reliability. A rigorous modelling of the radiation field has not yet been considered for such a class of photochemical processes.An approach based on a rigorous modelling is therefore presented here, the solution being obtained through a transient approach. Results are compared with those obtained on the basis of simplified models in order to investigate the range of the significant parameters where such simple models, which are less computer-time demanding, can be conveniently used.     

Molecular motion in melt samples of poly(propylene glycol) studied using dielectric and Kerr effect relaxation techniques

Dielectric relaxation times and electro-optical Kerr-effect relaxation times have been measured for melt samples of pure poly(propylene glycol) of nominal molecular weight 1025, 2025 and 4000, over the temperature range 209K–255K. Kerr-effect relaxation curves were analysed in terms of two main components: a ‘fast’ primary process (A), associated with a negative optical birefringence; and a ‘slow’ secondary process (B), associated with a positive optical birefringence. Measurement of dielectric loss at different frequencies for poly(propylene glycol) of molecular weight 2025 and 4000 indicated two main relaxation processes: a ‘fast’ process (the normal α-process), which carried with it most of the dielectric loss; and a ‘slow’ secondary process of much smaller amplitude. The close correspondence between dielectric and Kerr-effect relaxation times, for the primary and secondary relaxation processes, indicated that these techniques are probing effectively different aspects of the same molecular motion. Dielectric and Kerr-effect relaxation times for the secondary process depended greatly on the molecular weight of the polymer and were compared with the predictions of the models for the reptational motions of chains in the bulk polymer, as proposed by De Gennes and by Doi and Edwards.