Influence of the structure of soft and stiff chain fragments on properties of segmented polyurethanes. I. Phase morphology

Model segmented polyurethanes (SPUs) prepared from either oxypropylene glycol oligomer or butylene adipate glycol oligomer, both of molar mass 2 kg/mol (soft fragments, SFT), and three different diisocyanates (all-trans 4,4'-dicyclohexylmethane diisocyanate, t, t-HMDI-1.0; HMDI with 20% of trans isomers, t, t-HMDI-0.2; and 4, 4'-diphenylmethane diisocyanate, MDI) (stiff fragments, STF) were characterized by specific heat capacity measurements in the temperature interval 140-540 K, and by wide-angle and small-angle X-ray scattering at room temperature. Limited miscibility of SFT and STF chain components resulted in incomplete separation into a regular three-dimensional macrolattice of STF-rich microdomains and SFT-rich microphases. The composition of STF-rich microdomains was estimated by fitting the softening temperatures to the Couchman's equation, whereas the relative contents of SFT-rich and STF-rich microphases were assessed by comparing the specific heat capacity change at the glass transition temperatures to corresponding additive values. The overall degree of microphase separation, as well as the mean macrolattice spacings between STF microdomains decreased in the order, MDI > t, t-HMDI-1.0 ? t, t-HMDI-0.2. The conformation of STF fragments within the STF-rich microdomains changed from nearly extended (for MDI) through slightly contracted (for t, t-HMDI-1.0) to strongly contracted (for t, t-HMDI-0.2).     

Amino acid composition of protein termini are biased in different manners

An exhaustive statistical analysis of the amino acid sequencesat the carboxyl (C) and amino (N) termini of proteins and ofcoding nucleic acid sequences at the 5' side of the stop codonswas undertaken. At the N ends, Met and Ala residues are over-representedat the first (+1) position whereas at positions 2 and 5 Thris preferred. These peculiarities at N-termini are most probablyrelated to the mechanism of initiation of translation (for Met)and to the mechanisms governing the life-span of proteins viaregulation of their degradation (for Ala and Thr). We assumethat the C-terminal bias facilitates fixation of the C endson the protein globule by a preference for charged and Cys residues.The terminal biases, a novel feature of protein structure, haveto be taken into account when molecular evolution, three-dimensionalstructure, initiation and termination of translation, proteinfolding and life-span are concerned. In addition, the bias ofprotein termini composition is an important feature which shouldbe considered in protein engineering experiments.     

Kinetic study of photoinitiated frontal polymerization

The frontal polymerization of a monomer exposed continuously to UV radiation in the presence of a photobleachable initiator has been studied on the basis of the rate equations. The time dependence of photoinitiator concentration profile and that of monomer conversion within the irradiated sample have been calculated, assuming steady‐state conditions. It is thus possible to visualize how fast the polymerization profile is moving within the sample and how effectively the polymerization of a monomer layer located at a given distance from the surface proceeds upon exposure to UV radiation. © 2001 Society of Chemical Industry     

Changes of Thermodynamic and Structural Properties of Wrinkled Pea Starches (Z‐301 and Paramazent varieties) During Biosynthesis

Starches isolated at different stages of maturation (milky, waxy, complete maturation) of wrinkled peas were investigated by differential scanning calorimetry (DSC) and X‐ray diffraction (XRD). It was shown that the maturation of wrinkled peas is accompanied by changes in the structural and thermodynamic properties of starches. The melting process of milky starches could be approximated by means of a “two‐state” model. The melting process of the waxy and completely maturated starches was described as the melting of a mixture of low and high temperature populations of double‐helical type crystallites, denoted as B‐ and B*‐types, as well as the melting of V_h ‐type crystallites. The relative amounts of the three structures were determined by deconvolution of the calorimetric peaks. The values of the melting cooperative units ( ϑ ) and the thickness of the crystalline lamellae (pitch heights) for starches were determined using mathematical models describing the melting processes. The values of the ϑ for milky, waxy, and completely maturated starches were calculated as 18 and 29 anhydroglucose residues, respectively. The thickness of the crystalline lamellae of B‐type crystals in the milky, waxy, and completely maturated starches were calculated. Structural changes in starch granules during maturation of wrinkled pea are discussed.     

Evaluation of isotopic boron (11B) for the fabrication of low activation Mg11B2 superconductor for next generation fusion magnets

In this study, we analyze the properties of boron isotope (¹¹B)-rich powders from three different sources, that is, American, Cambridge, and Pavezyum, to fabricate the bulk Mg¹¹B₂ superconductors and evaluate their superconducting properties. While ¹¹B-rich powder is an essential precursor to fabricate Mg¹¹B₂ superconductors for fusion magnet applications, the properties of the ¹¹B powder turned out to be critical to determine the quality of the final superconducting product. Therefore, appropriate control of processing conditions is needed to comply with the requirements of the nuclear fusion application. Analysis of the B isotope ratio by accelerator mass spectroscopy and neutron transmission revealed that all three types of powder are enriched with ¹¹B to better than 99 at % quality. In addition, Pavezyum's ¹¹B shows the lowest crystallinity and smallest crystalline domain size as evidenced by the high-resolution X-ray diffractometer and scanning electron microscopy. The chemical states of the boron isotope investigated with near edge X-ray absorption fine structure spectroscopy and X-ray photoemission spectroscopy also reveals that Pavezyum boron has amorphous structure. Mg¹¹B₂ bulks and multi-filamentary (12-filament) wires have been manufactured, sintered at different temperatures and characterized via the transport critical current density. The wire with Pavezyum ¹¹B shows three times higher current carrying capacity at a particular magnetic field compared to the wire using Cambridge ¹¹B and hence, Pavezyum ¹¹B boron has the potential for manufacturing fusion grade Mg¹¹B₂ based magnets. The results of this study demonstrated that Boron powders with higher purity, smaller grain size and lower crystallinity are critical for improving the superconducting and electronic properties of Mg¹¹B₂ samples fabricated from the powder. Thus, the low-neutron-activation Mg¹¹B₂ is possibly an affordable and technically viable candidate to replace NbTi superconductors in the low field poloidal field and correction coils for the next-generation fusion reactors.     

3D-printed polymer packing structures: Uniformity of morphology and mechanical properties via microprocessing conditions

Three-dimensional (3D) printing is an attractive approach to fabricate highly porous extremely lightweight structures for architecture antivibrational packaging. We report 3D printing processing of model packaging structures using biodegradable poly(lactic acid) (PLA) as a source material, with acrylonitrile butadiene styrene (ABS) utilized as a common 3D printing source material as a traditional benchmarked material. The effects of printing temperature, speed, and layer morphology on the layer-by-layer 3D-printed structures and their mechanical properties were considered. Three different characteristic morphologies were identified based on printing temperature; the microscopic surface roughness was dependent on the printing speed and layer height. We demonstrate that the mechanical performances and surface properties of 3D-printed PLA structures could be improved by optimization of printing conditions. Specifically, we evaluate that these PLA-based 3D structures printed exhibited better surface qualities and enhanced mechanical performance than traditional ABS-based structures. Results showed that the PLA-based 3D structures possessed the favorable mechanical performance with 34% higher Young's modulus and 23% higher tensile strength in comparison to the ABS-based 3D structures. This study provides guidelines for achieving high-quality 3D-printed lightweight structures, including smooth surfaces and durable mechanical properties, and serves as a framework to create biodegradable 3D-printed parts for human use.     

Sodium Nitroprusside-Induced Activation of Vascular Smooth Muscle BK Channels Is Mediated by PKG Rather Than by a Direct Interaction with NO

Nitric oxide (NO) is a powerful vasodilator in different vascular beds and NO-donors are widely used in clinical practice. Early data suggested that NO and NO-donors activate vascular smooth muscle high-conductance, calcium-activated potassium channels (BK channels). There exist two hypotheses explaining the effect of NO and NO-donors on BK channels—one stating that protein kinase G (PKG) mediates the effect of NO, and the other one stating that NO acts directly on the channel. Thus, the degree of contribution of PKG to the NO-induced activation of the BK channel is still not completely clear. This study tested the hypothesis that the sodium nitroprusside (SNP)-induced activation of vascular smooth muscle BK channels is fully mediated by PKG. This hypothesis was investigated using the patch-clamp technique and freshly isolated smooth muscle cells from rat tail artery. In whole-cell experiments, SNP considerably increased the outward current compared with the addition of the bath solution. SNP did not alter the current in the presence of iberiotoxin, the specific blocker of BK channels, during co-application with hydroxocobalamin, an NO-scavenger, and in the presence of Rp-8-Br-PET-cGMPS, the specific PKG-inhibitor. In inside-out patches, the activity of BK channels was increased by SNP, SNAP, and DEA-NO. However, these effects did not differ from the effect of the application of drug-free bath solution. Furthermore, a similar increase in single BK channel activity was induced by Rp-8-Br-PET-cGMPS, Rp-8-Br-PET-cGMPS together with SNP, hydroxocobalamin, and hydroxocobalamin together with SNP or DEA-NO. Finally, the activity of excised BK channels did not change in the absence of any application but was considerably increased by PKG compared with the addition of drug-free bath solution. These results suggest that NO released from NO-donors stimulates the BK current only through activation of PKG.     

‘Green’ synthesis and optical properties of silver–chitosan complexes and nanocomposites

Silver nanoparticles were synthesized in a chitosan biopolymer by an in situ ‘green’ chemical procedure, using d-glucose as the reducing agent. The reaction intermediates (silver–chitosan complexes) as well as the obtained nanocomposites were investigated using transmission electron microscopy, UV–vis, FTIR and photoluminescence spectroscopy. The theoretical analysis of the UV–vis absorption of the Ag–chitosan complexes suggested that the significant contribution to the complex spectrum arises from clusters of silver containing 4–9 atoms. The absorption spectrum of the nanocomposite exhibited a strong surface plasmon resonance band at 406 nm. The photoluminescence behavior of the pure chitosan, the silver–chitosan complexes and the nanocomposites were discussed in terms of morphology and silver weight content.     

Effects of design features on combustion efficiency and emission performance of a biomass-fuelled fluidized-bed combustor

This paper presents an analysis of some measures leading to intensification of the combustion process in a biomass-fuelled fluidized-bed combustor with a cone-shape bed (or ‘conical FBC’). Two combustors firing rice husks with elevated fuel-ash content were the focus of this study. Compared to the pilot 350-kW_th conical FBC exhibiting combustion efficiency of up to 96%, the newly constructed 400-kW_th combustor included geometrical and design modifications aimed at improving the combustion efficiency and emission performance of the reactor. Differences between the air distributors and Δp–u diagrams (accounting for the total pressure drop across the air distributor and gas–solid fluidized bed) for the two reactors are discussed. Axial temperature and gas concentration (O₂, CO and NO_x) profiles in the combustors were compared for similar operating conditions (excess air and heat release rate per unit cross-sectional area). At excess air of 40–60%, the bed temperature in the advanced conical FBC was substantially, by about 180 °C, higher than that in the pilot combustor, mainly, due to better fuel–air mixing and higher residence time of reactants. The formation and decomposition of CO and NO in the bed region as well as in the freeboard of these two combustors showed quite different trends under similar operating conditions. At excess air of 40–60%, the CO emission from the advanced conical FBC was found to be much (7–8 times) lower than that from the pilot combustor, while the NO_x emissions were represented by almost the same values. High (over 99%) combustion efficiency was achieved when firing rice husk in the advanced 400 kW_th conical FBC for the range of excess air.     

Mechanochemical approach for fabrication of a nano-structured NiO-sensing electrode used in a zirconia-based NO2 sensor

Highly uniform NiO nano-particles with a crystallite size of about 3 nm were obtained by room-temperature ball-milling of the parent Ni(OH)₂, which was derived using a sol-gel method. The obtained nano-structured NiO precursor was then utilized for the fabrication of NiO-sensing electrodes (SEs), which were further examined in the mixed-potential-type YSZ-based planar NO₂ sensor. The obtained results revealed the attractive advantages for the application of mechanochemical approach in regard to achieve high NO₂ sensitivity, NO₂ selectivity and reproducibility. All of the evaluated sensors attached with the nano-structured NiO-SEs, regardless of its sintering temperature, were found to exhibit high NO₂ sensitivity at 800 °C under the wet condition (5 vol.% water vapor). In addition to high NO₂ sensitivity, the sensor attached with 1100 °C-sintered NiO-SE showed highly selective properties towards NO₂. The observed improvement in NO₂-sensing characteristics as well as the attainment of highly reproducible behavior for different sensor devices is discussed based on morphological and electrochemical observations of the studied sensors.