Small sample in-vivo neutron activation analysis using californium sources

This work describes an in vivo neutron activation analysis facility for small samples, such as rats or human hand, using two 100 g²⁵²Cf neutron sources. The irradiation area is a cylindrical space, of 12 cm diameter and about 15 cm length, with fairly uniform neutron flux distribution. Experimental data on the reproducibility, effects of volume and other conditions for in vivo measurements are given. Comparative atomic absorption data on calcium measurements on rats are reported. The facility is now used for animal experiments as well as human hand irradiations in clinical investigations involving calcium metabolism and bone diseases.     

The effect of 4-vinylcyclohexene on the degradation of polystyrene and on the polymerisation of styrene

It has previously been postulated that the inhibition of polystyrene degradation observed in the initial stages of the isothermal degradation of polystyrene-polybutadiene blends is partly due to reaction of polystyryl radicals with 4-vinylcyclohexene (4VCH) evolved from the polybutadiene. This hypothesis has been tested by examining the effect of small amounts of 4VCH on polystyrene degradation and on the free radical polymerisation of styrene. The results provide support for the hypothesis: there is an induction period in the degradation and the polymerisation is retarded. From molecular weight measurements on polymers made in the presence of various amounts of 4VCH, a chain transfer constant in styrene polymerisation at 60°C of 117 × 10^?5 has been calculated.     

Degradation of polymer mixtures. VI. Blends of poly(vinyl chloride) with polystyrene

The degradation of films containing both PS and PVC has been examined by TVA and TG. Stabilization of both polymers, more notably PS, is observed, but the degradation products are the same as when the polymers are degraded alone. Molecular weight measurements indicate a more rapid decrease in the molecular weight of PS when PVC is present. The possibility of grafting or other processes leading to chlorine incorporation in PS has been excluded by the results of experiments using ³⁶Cl-labeled PVC. The mechanisms of possible interactions between the degrading polymers are discussed. Processes involving reaction of chlorine radicals with PS at lower temperatures and reaction of PS radicals with the residue of PVC dehydrochlorination or its decomposition products at higher temperatures appear probable.     

The thermal degradation of copolymers of vinyl acetate with methyl methacrylate and other monomers

The thermal degradation of copolymers of vinyl acetate with methyl methacrylate, styrene and ethylene has been investigated using thermal volatilization analysis and thermogravimetry, together with analysis of volatile and involatile degradation products. All three copolymer systems show some of the features characteristic of the homopolymers of the monomers concerned. There is evidence, however, for an intramolecular lactonization process in VA—MMA copolymers, involving reaction of adjacent VA and MMA units with elimination of methyl acetate. This reaction occurs less readily than the analogous process in vinyl chloride—MMA copolymers. Mechanisms of the various degradation reactions are discussed.     

Preparation and degradation of copolymers of methyl methacrylate with alkali metal methacrylates—IV. The relationship of product yields to sequence distribution

Experimental yields of methanol and methyl methacrylate (MMA), produced in degradation to 500° of copolymers of MMA with lithium, sodium and potassium methacrylates (KMA) respectively, have been compared with the amounts expected on the basis of composition and sequence distribution. Rates of formation of these products under isothermal conditions have also been measured and activation energies for MMA formation at various compositions in the KMA-MMA copolymer series have been evaluated. The activation energy changes from 35 ± 5 kcal mole^?1 in PMMA to 52 ± 5 kcal mole^?1 in a 75 mole% KMA copolymer, indicating increasing difficulty in depolymerization to MMA as the MMA sequences become shorter. The primary route to methanol is by a cyclization involving adjacent ester and salt units in the chain, giving anhydride rings and metal methoxide as the initial products. Methanol yields and the positions of the maxima in the yield vs copolymer composition curves, however, are found to be inconsistent with those predicted from sequence distribution calculations. It is argued that water retained by the copolymers plays a key part in the reaction scheme by converting the metal methoxide to methanol and hydroxide; the latter then causes conversion of ester groups to salt units, so permitting further cyclization in copolymers initially rich in MMA. Mechanisms are discussed in detail.     

The effect of zinc bromide on the thermal degradation of poly(methyl methacrylate): Part 1—Thermal analysis studies and general nature of the interaction

The degradation behaviour of several different blends of poly(methyl methacrylate) (PMMA) and zinc bromide, under programmed heating to 500°C, has been studied using thermal volatilisation analysis and spectroscopic investigation of the volatile degradation products. The samples were in the form of films cast from a common solution of the components in acetone; these films are found to be transparent, indicating compatibility of PMMA and ZnBr₂. From studies of the visible spectra of cobalt bromide, PMMA and blends of PMMA with CoBr₂, it has been argued that complex formation occurs between the polymer and the transition metal halides: structures are suggested.When degraded alone, PMMA gives only monomer as the degradation product. In the blends with ZnBr₂ (or with CoBr₂), the polymer becomes considerably less stable and the pattern of degradation becomes very complex, with a range of volatile products, of which methyl bromide, carbon dioxide and methanol are the major constituents; carbon monoxide and methane are also formed. It is proposed that complex formation facilitates the release of methyl bromide as the first stage of breakdown, with the formation of zinc methacrylate units in the polymer chain; depolymerisation is prevented or severely inhibited, depending on the amount of ZnBr₂ present.     

Ribosomal synthesis of unnatural peptides

Combinatorial libraries of non-biological polymers and drug-like peptides could in principle be synthesized from unnatural amino acids by exploiting the broad substrate specificity of the ribosome. The ribosomal synthesis of such libraries would allow rare functional molecules to be identified using technologies developed for the in vitro selection of peptides and proteins. Here, we use a reconstituted E. coli translation system to simultaneously re-assign 35 of the 61 sense codons to 12 unnatural amino acid analogues. This reprogrammed genetic code was used to direct the synthesis of a single peptide containing 10 different unnatural amino acids. This system is compatible with mRNA-display, enabling the synthesis of unnatural peptide libraries of 10(14) unique members for the in vitro selection of functional unnatural molecules. We also show that the chemical space sampled by these libraries can be expanded using mutant aminoacyl-tRNA synthetases for the incorporation of additional unnatural amino acids or by the specific posttranslational chemical derivitization of reactive groups with small molecules. This system represents a first step toward a platform for the synthesis by enzymatic tRNA aminoacylation and ribosomal translation of cyclic peptides comprised of unnatural amino acids that are similar to the nonribosomal peptides.     

Far infrared surface plasmon propagation

New experimental evidence is presented which shows that, contrary to an earlier report, far-infrared surface plasmons at metal-air interfaces propagate long distances (tens of centimeters or longer).     

The Rate Problem of a Thermally Regenerable Ion-Exchange System

Earlier equilibrium studies have established the thermal dependence of the equilibrium between salt solution and a mixed bed of weakly basic and weakly acidic ion-exchange resins. High resin utilization can be achieved if the resin properties and equilibrium conditions are optimized; the equilibrium characteristics of polyacrylic acid and polyvinylbenzyldiethylamine resins are quite suited for the practical desalination of brackish waters.

However, the adsorption rates exhibited by normal-sized resin beads of this type are much too slow for satisfactory operation of the process because of the low concentration of protons available for transfer between the resins. It is shown that increasing the porosity of the resins improves amine resin kinetics 10-fold and carboxylic acid resin kinetics 6-fold. Nevertheless such improvements are still inadequate for practical purposes, and it is concluded that for satisfactory rates to be achieved systems having much shorter diffusion paths are necessary.

Two further approaches to the rate problem are discussed, both involving the synthesis of novel resin systems. A mixed bed of microbeads (10-20 p) reacts at acceptable rates but presents mechanical problems; the magnetic flocculation of finely divided magnetic resins is reported as one possible solution to this problem. Another avenue is the synthesis of normal-sized beads of the amphoteric and snake-cage variety. Resins of this type that exchange at suitable rates are described.