Advances in cycloaddition polymerizations

Cycloaddition reactions have been employed in polymer synthesis since the mid-nineteen sixties. This critical review will highlight recent notable advances in this field. For example, [2 + 2] cycloaddition reactions have been utilized in numerous polymerizations to enable the construction of strained polymer systems such as poly(2-azetidinone)s that can, in turn, afford polyfunctional beta-amino acid derived polymers. Polymers have also been synthesized successfully via (3 + 2) cycloaddition methods utilizing both thermal and high-pressure conditions. 'Click chemistry'-a process involving the reaction of azides with olefins, has also been adopted to generate linear and hyperbranched polymer architectures in a very efficient manner. [4 + 2] Cycloadditions have also been utilized under thermal and high-pressure conditions to produce rigid polymers such as polyimides and polyphenylenes. These cycloaddition polymerization methods afford polymers with potential for use in high performance polymers applications such as high temperature resistant coatings and polymeric organic light emitting diodes.

Ordering on multiple lengthscales in a series of side group liquid crystal block copolymers containing a cholesteryl-based mesogen

Hierarchical ordering in a side group liquid crystal block copolymer is investigated by differential scanning calorimetry, polarized optical microscopy, small-angle X-ray and neutron scattering (SAXS and SANS) and transmission electron microscopy (TEM). A series of block copolymers with a range of compositions was prepared by atom transfer radical polymerization, comprising a polystyrene block and a poly(methyl methacrylate) block bearing chiral cholesteryl mesogens. Smectic ordering is observed as well as microphase separation of the block copolymer. Lamellar structures were observed for far larger volume fractions than for coil-coil copolymers (up to a volume fraction of liquid crystal block, f(LC) = 0.8). A sample with f(LC) = 0.86 exhibited a hexagonal-packed cylinder morphology, as confirmed by SAXS and TEM. The matrix comprised the liquid crystal block, with the mesogens forming smectic layers. For the liquid crystal homopolymer and samples with high f(LC), a smectic-smectic phase transition was observed below the clearing point. At low temperature, the smectic phase comprises coexisting domains with monolayer S-A,S-1 coexisting with interdigitated S-A,S-d domains. At high temperature a SA,1 phase is observed. This is the only structure observed for samples with lower f(LC). These unprecedented results point to the influence of block copolymer microphase separation on the smectic ordering.

Thermo-responsive poly(methyl methacrylate)-block-poly(N-isopropylacrylamide) block copolymers synthesized by RAFT polymerization: micellization and gelation

RAFT polymerization was used to prepare PMMA-b-PNIPAM copolymers. Two different chain transfer agents, tBDB and MCPDB, were used to mediate the sequential polymerizations. Micellar solutions and gels were prepared from the resulting copolymers in aqueous solution. When heated above T-c of PNIPAM (about 31 degrees C), DLS revealed that PNIPAM coronas collapsed, resulting in aggregation of the original micelles. The micellar gels underwent syneresis above T-c as water was expelled from the ordered gel structure, the lattice periodicity of which was determined by SANS. A large decrease in lattice spacing was observed above T-c. The gel became more viscoelastic at high temperature, as revealed by shear rheometry which showed a large increase in G".

Aerogel-coated metal nanoparticle colloids as novel entities for the synthesis of defined supported metal catalysts

Nanometer metal particles of tailored size (3-5 nm) and composition prepared via inverse microemulsion were encapsulated by ultrathin coatings (<2.5 nm) of inorganic porous aerogels covered with surface -OH groups. These composite materials formed metastable colloids in solvent(s), and the organic surfactant molecules were subsequently removed without leading to aggregation (the ethanolic colloid solution was shown to be stable against flocculation for at least weeks). We demonstrate that the totally inorganic-based composite colloids, after the removal of surfactant, can be anchored to conventional solid supports (gamma-alumina, carbons) upon mixing. Application of a high temperature resulted in the formation of strong covalent linkages between the colloids and the support because of the condensation of surface groups at the interface. Detailed characterizations (X-ray diffraction (XRD), pore analysis, transmission electron microscopy (TEM), CO chemisorption) and catalytic testing (butane combustion) showed that there was no significant metal aggregation from the fine metal particles individually coated with porous aerogel oxide. Most of these metal sites on the coated nanoparticles with and without support are fully accessible by small molecules hence giving extremely active metal catalysts. Thus, the product and technology described may be suitable to synthesize these precursor entities of defined metal sizes (as inks) for wash coat/impregnation applications in catalysis. The advantages of developing inorganic nanocomposite chemical precursors are also discussed.

Reducing acrylamide precursors in raw materials derived from wheat and potato

A review of agronomic and genetic approaches as strategies for the mitigation of acrylamide risk in wheat and potato is presented. Acrylamide is formed through the Maillard reaction during high-temperature cooking, such as frying, roasting, or baking, and the main precursors are free asparagine and reducing sugars. In wheat flour, acrylamide formation is determined by asparagine levels and asparagine accumulation increases dramatically in response to sulfur deprivation and, to a much lesser extent, with nitrogen feeding. In potatoes, in which sugar concentrations are much lower, the relationships between acrylamide and its precursors are more complex. Much attention has been focused on reducing the levels of sugars in potatoes as a means of reducing acrylamide risk. However, the level of asparagine as a proportion of the total free amino acid pool has been shown to be a key parameter, indicating that when sugar levels are limiting, competition between asparagine and the other amino acids for participation in the Maillard reaction determines acrylamide formation. Genetic approaches to reducing acrylamide risk include the identification of cultivars; and other germplasm in which free asparagine and/or sugar levels are low and the manipulation of genes involved in sugar and amino acid metabolism and signaling. These approaches are made more difficult by genotype/ environment interactions that can result in a genotype being "good" in one environment but "poor" in another. Another important consideration is the effect that any change could have on flavor in the cooked product. Nevertheless, as both wheat and potato are regarded as of relatively high acrylamide risk compared with, for example, maize and rice, it is essential that changes are achieved that mitigate the problem.

Glutathione and related thiol compounds. I. Glutathione and related thiol compounds in flour

The high pressure liquid chromatography method for determination of glutathione in free and protein-bound forms was re-established and has successfully been developed to measure glutathione related thiol compounds, i.e. L-cysteine, gamma-L-glutamyl-L-cysteine and L-cysteinyl-L-glycine, in both free and protein-bound forms. The natural levels of those compounds in typical strong, weak flours, and flours from 36 wheat varieties grown in the UK were investigated. The total free and protein-bound glutathione compounds found in the 36 UK varieties was 358 +/- 51 and 190 +/- 17 nmol/g, respectively. Multiple correlation analysis did not show a clear-cut relationship between the natural level of glutathione and any related thiol compound in either free or protein-bound forms and flour quality attributes, including rheological properties, baking performance, protein content and SDS sedimentation test values. Therefore, it can be suggested that glutathione and related thiol compounds at natural levels do not lead to significant differences in the rheological properties of dough and the baking performance of flour. (C) 2003 Elsevier Ltd. All rights reserved.

Effect of small, acid-soluble proteins on spore resistance and germination under a combination of pressure and heat treatment

To find the range of pressure required for effective high-pressure inactivation of bacterial spores and to investigate the role of alpha/beta-type small, acid-soluble proteins (SASP) in spores under pressure treatment, mild heat was combined with pressure (room temperature to 65 degrees C and 100 to 500 MPa) and applied to wild-type and SASP-alpha(-/)beta(-) Bacillus subtilis spores. On the one hand, more than 4 log units of wild-type spores were reduced after pressurization at 100 to 500 MPa and 65 degrees C, On the other hand, the number of surviving mutant spores decreased by 2 log units at 100 MPa and by more than 5 log units at 500 MPa. At 500 MPa and 65 degrees C, both wild-type and mutant spore survivor counts were reduced by 5 log units. Interestingly, pressures of 100, 200, and 300 MPa at 65 degrees C inactivated wild-type SASP-alpha(+)/beta(+) spores more than mutant SASP-alpha(-)/beta(-) spores, and this was attributed to less pressure-induced germination in SASP-alpha(-)/beta(-) spores than in wild-type SASP-alpha(+)/beta(+) spores. However, there was no difference in the pressure resistance between SASP-alpha(+)/beta(+) and SASP-alpha(-)/beta(-) spores at 100 MPa and ambient temperature (approximately 22 degrees C) for 30 min. A combination of high pressure and high temperature is very effective for inducing spore germination, and then inactivation of the germinated spore occurs because of the heat treatment. This study showed that alpha/beta-type SASP play a role in spore inactivation by increasing spore germination under 100 to 300 MPa at high temperature.

Permeation of bioactive constituents from Arnica montana preparations through human skin in-vitro

This study investigated and characterised transdermal permeation of bioactive agents from a topically applied Arnica montana tincture. Permeation experiments conducted over 48 h used polyclimethylsiloxane (silastic) and human epidermal membranes mounted in Franz-type diffusion cells with a methanol-water (50:50 v/v) receptor fluid. A commercially available tincture of A. montana L. derived from dried Spanish flower heads was a donor solution. Further donor solutions prepared from this stock tincture concentrated the tincture constituents 1, 2 and 10 fold and its sesquiterpene lactones 10 fold. Permeants were assayed using a high-performance liquid chromatography method. Five components permeated through silastic membranes providing peaks with relative retention factors to an internal standard (santonin) of 0.28, 1.18, 1.45, 1.98 and 2.76, respectively. No permeant was detected within 12 h of applying the Arnica tincture onto human epidermal membranes. However, after 12 h, the first two of these components were detected. These were,shown by Zimmermann reagent reaction to be sesquiterpene lactones and liquid chromatography/diode array detection/mass spectrometry indicated that these two permeants were 11,13-dihydrohelenalin (DH) analogues (methacrylate and tiglate esters). The same two components were also detected within 3 h of topical application of the 10-fold concentrated tincture and the concentrated sesquiterpene lactone extract.

Investigation of tin-based alternatives for cadmium in optoelectronic thin-films materials

Increasing legislation has steadily been introduced throughout the world to restrict the use of heavy metals, particularly cadmium (Cd) and lead (Pb) in high temperature pigments, ceramics, and optoelectronic material applications. Removal of cadmium from thin-film optical and semiconductor device applications has been hampered by the absence of viable alternatives that exhibit similar properties with stability and durability. We describe a range of tin-based compounds that have been deposited and characterized in terms of their optical and mechanical properties and compare them with existing cadmium-based films that currently find widespread use in the optoelectronic and semiconductor industries. (c) 2008 Optical Society of America.

Scenarios as the basis for assessment of mitigation and adaptation

The possibilities and need for adaptation and mitigation depends on uncertain future developments with respect to socio-economic factors and the climate system. Scenarios are used to explore the impacts of different strategies under uncertainty. In this chapter, some scenarios are presented that are used in the ADAM project for this purpose. One scenario explores developments with no mitigation, and thus with high temperature increase and high reliance on adaptation (leading to 4oC increase by 2100 compared to pre-industrial levels). A second scenario explores an ambitious mitigation strategy (leading to 2oC increase by 2100 compared to pre-industrial levels). In the latter scenario, stringent mitigation strategies effectively reduces the risks of climate change, but based on uncertainties in the climate system a temperature increase of 3oC or more cannot be excluded. The analysis shows that, in many cases, adaptation and mitigation are not trade-offs but supplements. For example, the number of people exposed to increased water resource stress due to climate change can be substantially reduced in the mitigation scenario, but even then adaptation will be required for the remaining large numbers of people exposed to increased stress. Another example is sea level rise, for which adaptation is more cost-effective than mitigation, but mitigation can help reduce damages and the cost of adaptation. For agriculture, finally, only the scenario based on a combination of adaptation and mitigation is able to avoid serious climate change impacts.

Structures and negative thermal expansion properties of the one-dimensional cyanides, CuCN, AgCN and AuCN

The behaviour of the lattice parameters of HTCuCN (high-temperature form), AgCN and AuCN have been investigated as a function of temperature over the temperature range 90–490 K. All materials show one-dimensional negative thermal expansion (NTE) along the ––(M––CN)–– chain direction c (ac(HT-CuCN) ¼32.1 10–6 K1, ac(AgCN)¼23.910–6 K1 and ac(AuCN) ¼9.3106 K1 over the temperature range 90–490 K). The origin of this behaviour has been studied using RMC modelling of Bragg and total neutron diffraction data from AgCN and AuCN at 10 and 300 K. These analyses yield details of the local motions within the chains responsible for NTE. The low-temperature form of CuCN, LT-CuCN, has been studied using single-crystal X-ray diffraction. In this form of CuCN, wavelike distortions of the ––(Cu––CN)–– chains occur in the static structure, which are reminiscent of the motions seen in the RMC modelling of AgCN and AuCN, which are responsible for the NTE behaviour.

Surface science studies of strong metal-oxide interactions on model catalysts

The strong metal support interaction (SMSI) was first described in 1978 by Tauster [1-4]. The effect was observed as a severely negative effect on CO and H2 uptake on the catalyst after high temperature calcination under reducing conditions (heating above ~ 700 K) [1,2]. It also had a negative effect on the reaction rate for reactions, such as alkane hydrogenolysis [5,6]. It appeared that the effect occurred for catalysts comprised of reducible supports which were treated at elevated temperature in reducing conditions [2-4]. A classic support which has manifested this behaviour in many studies is TiO2. Over the years following the first discovery of SMSI it has been recognised that the effect is not always negative – for instance for the CO-H2 reaction for which it appears to have a positive effect [5,6]. Further it was noted that hydrogen reduction was not necessary to observe the effect of CO adsorption suppression, it also occurs by vacuum treatment [7], though it should be noted that vacuum treatment at elevated temperature is, in effect, a reducing environment.