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Lower Edge (lower + edge)

Distribution by Scientific Domains

Life Sciences	60%

Selected Abstracts

Cloud-Point Pressure Curves of Ethene/Poly[ethylene- co -((meth)acrylic acid)] Mixtures

MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 4 2003
Michael Buback
Abstract Ethene-methacrylic acid (MAA) and ethene-acrylic acid (AA) copolymers of narrow polydispersity and high chemical homogeneity have been synthesized at acid unit copolymer contents up to 9 mol-% within a continuously operated stirred tank reactor at overall monomer conversions of about 2%. Cloud-point pressures (CPPs) of mixtures of 3 wt.-% copolymer in ethene (E) have been measured in an optical high-pressure cell at pressures and temperatures up to 3,000 bar and 260,°C, respectively. The CPP weakly increases with acid copolymer content up to about 3.5 mol-%. Toward higher acid contents, the CPP is strongly enhanced, in particular at the lower edge of the experimental temperature range at around 200,°C. This increase in CPP is more pronounced for the AA than for the MAA systems. The data suggest that hydrogen-bonding interactions are operative in the pressurized E/poly(E- co -(M)AA) mixtures at temperatures of 260,°C and perhaps even above. E-AA and E-MAA copolymers with acid contents of about 5.6 mol-% have also been completely methyl-esterified to yield the associated methyl esters. The CPPs of the resulting E-methyl acrylate and E-methyl methacrylate copolymers in mixtures with E are significantly below the CPPs of the corresponding E/poly(E- co -(M)AA) systems. Cloud-point pressure vs temperature curves for several E/poly(E- co -methacrylic acid) systems. The subscripts denote copolymer content in mol-%. A cloud-point pressure curve for E/polyethylene (PE) is included for comparison. [source]

A method to optimize the bandwidth of UWB planar monopole antennas

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 1 2006
D. Valderas
Abstract This paper describes a method to optimize a planar hexagonal monopole antenna (PHMA) for ultra-wideband (UWB) applications. In the Smith chart, a design process is followed to improve the broadband matching of a fixed lower-frequency-limit rectangular monopole by a suitable geometry for the lower edge and monopole width. A prototype is simulated, constructed, and tested. As a result of the design, the achieved impedance bandwidth is larger than 1:37 (VSWR < 2). The radiation characteristics are also discussed. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 155,159, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21291 [source]

Reproduction of an early-flowering Mediterranean mountain narrow endemic (Armeria caespitosa) in a contracting mountain island

PLANT BIOLOGY, Issue 4 2009
R. García-Camacho
Abstract Reproduction at population lower edges is important for plant species persistence, especially in populations on contracting high-mountain islands. In this context, the ability of plants to reproduce in different microhabitats seems to be important to guarantee seed production in stressful environments, such as Mediterranean high mountains. We hypothesised that the warmer and drier conditions at the lower edge would be deleterious for the reproduction of Armeria caespitosa, an early-flowering plant. In addition, reproductive plasticity along this mountain gradient may also be microhabitat-dependent. We studied factors affecting the reproductive success of A. caespitosa, an endemic of the Spanish Sistema Central. We considered a complex set of predictors, including phenology, plant size and environmental factors at different scales using generalised estimating equations and generalised linear models. Microhabitat, together with the position in the altitudinal gradient and inter-annual variability affected the reproduction of A. caespitosa. In addition, individuals with longer flowering periods (duration of flowering) had significantly lower fruit set and a higher number of unviable seeds; delayed flowering peaks favoured the production of both viable and unviable fruits. Microhabitat variability over an altitudinal range is relevant for the reproduction of A. caespitosa, and is more important at the lower edge of the altitudinal range, where the species faces the most adverse conditions. In addition, the ability to reproduce in different microhabitats might increase the chances of the species to cope with environmental uncertainties under on-going climate warming. Finally, reproduction of this early-flowering plant is constrained by summer drought, which might shape its reproductive phenology. [source]

Flow-substrate interactions create and mediate leaf litter resource patches in streams

FRESHWATER BIOLOGY, Issue 3 2006
TRENT M. HOOVER
Summary 1. The roles that streambed geometry, channel morphology, and water velocity play in the retention and subsequent breakdown of leaf litter in small streams were examined by conducting a series of field and laboratory experiments. 2. In the first experiment, conditioned red alder (Alnus rubra Bongard) leaves were released individually in three riffles and three pools in a second-order stream. The transport distance of each leaf was measured. Several channel and streambed variables were measured at each leaf settlement location and compared with a similar number of measurements taken at regular intervals along streambed transects (,reference locations'). Channel features (such as water depth) and substrate variables (including stone height, stone height-to-width ratio, and relative protrusion) were the most important factors in leaf retention. 3. In the second experiment, the role of settlement location and reach type in determining the rate of leaf litter breakdown was examined by placing individual conditioned red alder leaves in exposed and sheltered locations (on the upper and lower edges of the upstream face of streambed stones, respectively) in riffle and pool habitats. After 10 days, percent mass remaining of each leaf was measured. Generally, leaves broke down faster in pools than in riffles. However, the role of exposure in breakdown rate differed between reach types (exposed pool > sheltered pool > sheltered riffle > exposed riffle). 4. In the third experiment, the importance of substrate geometry on leaf litter retention was examined by individually releasing artificial leaves upstream of a series of substrate models of varying shape. Substrates with high-angle upstream faces (were vertical or close to vertical), and that had high aspect ratios (were tall relative to their width), retained leaves more effectively. 5. These results show that streambed morphology is an important factor in leaf litter retention and breakdown. Interactions between substrate and flow characteristics lead to the creation of detrital resource patchiness, and may partition leaf litter inputs between riffles and pools in streams at baseflow conditions. [source]