On chlorosulphonation and the electron microscopy of polyethylene

It is shown that chlorosulphonation is a major aid to the electron microscopy of polyethylene for various samples which had mostly been crystallized at high pressures and included at least a proportion of the so-called chain-extended form. It is confirmed that sheets of excess electron density are produced at lamellar surfaces, but also including lateral surfaces. This is due primarily to the incorporation of chlorine and sulphur rather than to added uranium. The time to achieve an overall reaction varies sensitively with morphology, decreasing as the number of diffusion channels increases. Crystallinity is gradually lost, but sufficient crystals remain when a sample has become uniform, and in their initial orientations, for diffraction studies to be possible. The technique has been used to demonstrate that, during melt crystallization, the thickness of one lamella changes in response to altered growth conditions. This is direct confirmation that lamellar thickness is determined by secondary nucleation at the growth front. The tapered profile of a growing lamella previously observed in thick crystals of various polymers has been observed for chain-folded polyethylene lamellae, providing further evidence that this is a general feature of melt growth. © 1977 Chapman and Hall Ltd.

Structure-reactivity correlations in MgAl hydrotalcite catalysts for biodiesel synthesis

A series of [Mg(1−x)Alx(OH)2]x+(CO3)x/n2− hydrotalcite materials with compositions over the range x = 0.25–0.55 have been synthesised using an alkali-free coprecipitation route. All materials exhibit XRD patterns characteristic of the hydrotalcite phase with a steady lattice expansion observed with increasing Mg content. XPS measurements reveal a decrease in both the Al and Mg photoelectron binding energies with Mg incorporation which correlates with the increased intra-layer electron density. All materials are effective catalysts for the liquid phase transesterification of glyceryl tributyrate with methanol for biodiesel production. The rate increases steadily with Mg content, with the Mg rich Mg2.93Al catalyst an order of magnitude more active than MgO, with pure Al2O3 being completely inert. The rate of reaction also correlates with intralayer electron density which can be associated with increased basicity.© 2005 Elsevier B.V. All rights reserved.

Modeling the peptide-T cell receptor interaction by the comparative molecular similarity indices analysis-soft independent modeling of class analogy technique

A set of 38 epitopes and 183 non-epitopes, which bind to alleles of the HLA-A3 supertype, was subjected to a combination of comparative molecular similarity indices analysis (CoMSIA) and soft independent modeling of class analogy (SIMCA). During the process of T cell recognition, T cell receptors (TCR) interact with the central section of the bound nonamer peptide; thus only positions 4−8 were considered in the study. The derived model distinguished 82% of the epitopes and 73% of the non-epitopes after cross-validation in five groups. The overall preference from the model is for polar amino acids with high electron density and the ability to form hydrogen bonds. These so-called “aggressive” amino acids are flanked by small-sized residues, which enable such residues to protrude from the binding cleft and take an active role in TCR-mediated T cell recognition. Combinations of “aggressive” and “passive” amino acids in the middle part of epitopes constitute a putative TCR binding motif

Low density parity check codes: a statistical physics perspective

The modem digital communication systems are made transmission reliable by employing error correction technique for the redundancies. Codes in the low-density parity-check work along the principles of Hamming code, and the parity-check matrix is very sparse, and multiple errors can be corrected. The sparseness of the matrix allows for the decoding process to be carried out by probability propagation methods similar to those employed in Turbo codes. The relation between spin systems in statistical physics and digital error correcting codes is based on the existence of a simple isomorphism between the additive Boolean group and the multiplicative binary group. Shannon proved general results on the natural limits of compression and error-correction by setting up the framework known as information theory. Error-correction codes are based on mapping the original space of words onto a higher dimensional space in such a way that the typical distance between encoded words increases.

Computer-aided-design of saturated magnetic lenses for electron microscopes

In the last few years, there has been considerable interest in using saturated magnetic objective lenses in high resolution electron microscopes. Such lenses, in present commercial electron microscopes, are energized either by conventional or superconducting coils. Very little work, however, has been reported on the use of conventional coils in saturated magnetic electron lenses. The present investigation has been concerned with the design of high flux density saturated objective lenses of both single and double polepiece types which may be energized by conventional coils and in some cases by superconducting coils. Such coils have the advantage of being small and capable of carrying high current densities. The present work has been carried out with the aid of several computer programs based on the finite element method. The effect of the shape and position of the energizing coil on the electron optical parameter has been investigated. Electron optical properties such as chromatic and spherical aberration have been studies in detail for saturated single and double polepiece lenses. Several high flux density coils of different shapes have been investigated. The choice of the most favourable coil shape and position subject to the operational requirements, has been studied in some detail. The focal properties of such optimised lenses have been computed and compared.

Radial-field magnetic electron lenses

This investigation looks critically at conventional magnetic lenses in the light of present-day technology with the aim of advancing electron microscopy in its broadest sense. By optimising the cooling arrangements and heat transfer characteristics of lens windings it was possible to increase substantially the current density in the winding, and achieve a large reduction in the size of conventional magnetic electron lenses. Following investigations into the properties of solenoidal lenses, a new type of lens with only one pole-piece was developed. The focal properties of such lenses, which differ considerably from those.of conventional lenses, have been derived from a combination of mathematical models and experimentally measured axial flux density distributions. These properties can be profitably discussed with reference to "half-lenses". Miniature conventional twin pole-piece lenses and the proposed radial field single pole-piece lenses have been designed and constructed and both types of lenses have been evaluated by constructing miniature electron optical columns. A miniature experimental transmission electron microscope (TEM), a miniature scanning electron microscope (SEM) and a scanning transmission microscope (STEM) have been built. A single pole-piece miniature one million volt projector lens of only lOcm diameter and weighing 2.lkg was designed, built and tested at 1 million volts in a commercial electron microscope. iii. Preliminary experiments indicate that in single pole lenses it is possible to extract secondary electrons from the specimen in spite of the presence of the magnetic field of the probe-forming lens. This may well be relevant for the SEM in which it is desirable to examine a large specimen at a moderately good resolution.

The electron optical limits of performance of single-polepiece magnetic electron lenses

The thesis is concerned with the electron properties of single-polepiece magnetic electron lenses especially under conditions of extreme polepiece saturation. The electron optical properties are first analysed under conditions of high polepiece permeability. From this analysis, a general idea can be obtained of the important parameters that affect ultimate lens performance. In addition, useful information is obtained concerning the design of improved lenses operating under conditions of extreme polepiece saturation, for example at flux densities of the order of 10 Tesla. It is shown that in a single-polepiece lens , the position and shape of the lens exciting coil plays an important role. In particular, the maximum permissible current density in the windings,rather than the properties of the iron, can set a limit to lens performance. This factor was therefore investigated in some detail. The axial field distribution of a single-polepiece lens, unlike that of a conventional lens, is highly asymmetrical. There are therefore two possible physical arrangements of the lens with respect to the incoming electron beam. In general these two orientations will result in different aberration coefficients. This feature has also been investigated in some detail. Single-pole piece lenses are thus considerably more complicated electron- optically than conventional double polepiece lenses. In particular, the absence of the usual second polepiece causes most of the axial magnetic flux density distribution to lie outside the body of the lens. This can have many advantages in electron microscopy but it creates problems in calculating the magnetic field distribution. In particular, presently available computer programs are liable to be considerably in error when applied to such structures. It was therefore necessary to find independent ways of checking the field calculations. Furthermore, if the polepiece is allowed to saturate, much more calculation is involved since the field distribution becomes a non-linear function of the lens excitation. In searching for optimum lens designs, care was therefore taken to ensure that the coil was placed in the optimum position. If this condition is satisfied there seems to be no theoretical limit to the maximum flux density that can be attained at the polepiece tip. However , under iron saturation condition, some broadening of the axial field distribution will take place, thereby changing the lens aberrations . Extensive calculations were therefore made to find the minimum spherical and chromatic aberration coefficients . The focal properties of such lens designs are presented and compared with the best conventional double-polepiece lenses presently available.

The role of organoclay in promoting co-continuous morphology in high-density poly(ethylene)/poly(amide) 6 blends

The effect of organically modified clay on the morphology, rheology and mechanical properties of high-density polyethylene (HDPE) and polyamide 6 (PA6) blends (HDPE/PA6 = 75/25 parts) is studied. Virgin and filled blends were prepared by melt compounding the constituents using a twin-screw extruder. The influence of the organoclay on the morphology of the hybrid was deeply investigated by means of wide-angle X-ray diffractometry, transmission and scanning electron microscopies and quantitative extraction experiments. It has been found that the organoclay exclusively places inside the more hydrophilic polyamide phase during the melt compounding. The extrusion process promotes the formation of highly elongated and separated organoclay-rich PA6 domains. Despite its low volume fraction, the filled minor phase eventually merges once the extruded pellets are melted again, giving rise to a co-continuous microstructure. Remarkably, such a morphology persists for long time in the melt state. A possible compatibilizing action related to the organoclay has been investigated by comparing the morphology of the hybrid blend with that of a blend compatibilized using an ethylene–acrylic acid (EAA) copolymer as a compatibilizer precursor. The former remains phase separated, indicating that the filler does not promote the enhancement of the interfacial adhesion. The macroscopic properties of the hybrid blend were interpreted in the light of its morphology. The melt state dynamics of the materials were probed by means of linear viscoelastic measurements. Many peculiar rheological features of polymer-layered silicate nanocomposites based on single polymer matrix were detected for the hybrid blend. The results have been interpreted proposing the existence of two distinct populations of dynamical species: HDPE not interacting with the filler, and a slower species, constituted by the organoclay-rich polyamide phase, which slackened dynamics stabilize the morphology in the melt state. In the solid state, both the reinforcement effect of the filler and the co-continuous microstructure promote the enhancement of the tensile modulus. Our results demonstrate that adding nanoparticles to polymer blends allows tailoring the final properties of the hybrid, potentially leading to high-performance materials which combine the advantages of polymer blends and the merits of polymer nanocomposites.

In situ aberration corrected-transmission electron microscopy of magnesium oxide nanocatalysts for biodiesels

Biofuels are promising renewable energy sources and can be derived from vegetable oil feedstocks. Although solid catalysts show great promise in plant oil triglyceride transesterification to biodiesel, the identification of active sites and operating surface nanostructures created during their processing is essential for the development of efficient heterogeneous catalysts. Systematic, direct observations of dynamic MgO nanocatalysts from a magnesium hydroxide-methoxide precursor were performed under controlled calcination conditions using novel in situ aberration corrected-transmission electron microscopy at the 0.1 nm level and quantified with catalytic reactivity and physico-chemical studies. Surface structural modifications and the evolution of extended atomic scale glide defects implicate coplanar anion vacancies in active sites in the transesterification of triglycerides to biodiesel. The linear correlation between surface defect density (and therefore polarisability) and activity affords a simple means to fine tune new, energy efficient nanocatalysts for biofuel synthesis. © 2009 Springer Science+Business Media, LLC.