5,330 research outputs found
Formation and Equilibrium Properties of Living Polymer Brushes
Polydisperse brushes obtained by reversible radical chain polymerization
reaction onto a solid substrate with surface-attached initiators, are studied
by means of an off-lattice Monte Carlo algorithm of living polymers (LP).
Various properties of such brushes, like the average chain length and the
conformational orientation of the polymers, or the force exerted by the brush
on the opposite container wall, reveal power-law dependence on the relevant
parameters. The observed molecular weight distribution (MWD) of the grafted LP
decays much more slowly than the corresponding LP bulk system due to the
gradient of the monomer density within the dense pseudo-brush which favors
longer chains. Both MWD and the density profiles of grafted polymers and chain
ends are well fitted by effective power laws whereby the different exponents
turn out to be mutually self-consistent for a pseudo-brush in the
strong-stretching regime.Comment: 33 pages, 11 figues, J.Chem. Phys. accepted Oct. 199
Dynamical Monte Carlo Study of Equilibrium Polymers : Static Properties
We report results of extensive Dynamical Monte Carlo investigations on
self-assembled Equilibrium Polymers (EP) without loops in good solvent. (This
is thought to provide a good model of giant surfactant micelles.) Using a novel
algorithm we are able to describe efficiently both static and dynamic
properties of systems in which the mean chain length \Lav is effectively
comparable to that of laboratory experiments (up to 5000 monomers, even at high
polymer densities). We sample up to scission energies of over
nearly three orders of magnitude in monomer density , and present a
detailed crossover study ranging from swollen EP chains in the dilute regime up
to dense molten systems. Confirming recent theoretical predictions, the
mean-chain length is found to scale as \Lav \propto \phi^\alpha \exp(\delta
E) where the exponents approach
and in the
dilute and semidilute limits respectively. The chain length distribution is
qualitatively well described in the dilute limit by the Schulz-Zimm
distribution \cN(s)\approx s^{\gamma-1} \exp(-s) where the scaling variable
is s=\gamma L/\Lav. The very large size of these simulations allows also an
accurate determination of the self-avoiding walk susceptibility exponent
. ....... Finite-size effects are discussed in
detail.Comment: 15 pages, 14 figures, LATE
Susceptibility of Chardonnay grapes to sunburn
Research Not
Overwintering and presence of Colletotrichum acutatum (ripe rot) on mummified bunches, dormant wood, developing tissues and mature berries of Vitis vinifera
Colletotrichum acutatum, the casual agent of ripe rot in tropical and sub-tropical grape growing regions of Australia, affects crop yield and wine quality. To elucidate the epidemiology of the fungus, its presence was studied on shoots, pea sized and mature berries, mummified bunches, spurs and canes during two consecutive growing seasons and one winter season. The presence of C. acutatum was analysed by both traditional and real-time PCR methods. A protocol was developed to achieve PCR amplification for fungal DNA extracted from 'difficult' wood samples. C. acutatum was detected on mature berries, mummified berries, the peduncles of mummified berries, winter spurs and canes but not on the remaining analysed tissues. Thus the fungus overwinters on Vitis vinifera plant tissues and the conidia on spurs and dormant wood are likely to be the most important source of primary inoculum for the next growing season.
Effect of temperature on Botrytis cinerea, Colletotrichum acutatum and Greeneria uvicola mixed fungal infection of Vitis vinifera grape berries
Detached Vitis vinifera 'Cabernet Sauvignon' berries (12.5° Bé) were inoculated either singularly or in combination with spore suspensions of Botrytis cinerea, Colletotrichum acutatum and Greeneria uvicola and the degree of disease expression examined at either 20 or 27 °C. Berries were more susceptible to B. cinerea at 20 °C and to G. uvicola at 27 °C but were highly susceptible to C. acutatum at either temperature. In experiments involving inoculation of berries with mixtures of fungal organisms, B. cinerea infection was diminished at 27 °C by either C. acutatum or G. uvicola but only by C. acutatum at 20 °C. G. uvicola infection was diminished by C. acutatum at both temperatures investigated. B. cinerea reduced the level of infection of both C. acutatum and G. uvicola at 20 °C. The findings have implications for seasonal bunch rot management of grapes in relation to predicted changes in global temperature
Higher compressive strengths and the Bauschinger effect in conformally passivated copper nanopillars
Our current understanding of size-dependent strength in nano- and microscale crystals is centered around the idea that the overall strength is determined by the stress required to propagate dislocation sources. The nature and type of these dislocation sources is the subject of extensive debate, however, one commonality amongst these theories is that the ability of the free surface to absorb dislocations is a necessary condition for transition to a source controlled regime. In this work we demonstrate that atomic layer deposition (ALD) of conformal 5–25 nm thick TiO_2/Al_(2)O_3 coatings onto electroplated single crystalline copper pillars with diameters ranging from 75 nm to 1 μm generally inhibits the ability of a dislocation to vanish at the free surface. Uniaxial compression tests reveal increased strength and hardening relative to uncoated pillars at equivalent diameters, as well as a notable recovery of plastic strain during unloading, i.e. the Bauschinger effect. Unlike previous reports, these coated pillars retained the stochastic signature in their stress–strain curves. We explain these observations within the framework of a size-dependent strength theory based on a single arm source model, dislocation theory, and microstructural analysis by transmission electron microscopy
Chiral molecular films as electron polarizers and polarization modulators
Recent experiments on electron scattering through molecular films have shown
that chiral molecules can be efficient sources of polarized electrons even in
the absence of heavy nuclei as source of a strong spin-orbit interaction. We
show that self-assembled monolayers (SAMs) of chiral molecules are strong
electron polarizers due to the high density effect of the monolayers and
explicitly compute the scattering amplitude off a helical molecular model of
carbon atoms. Longitudinal polarization is shown to be the signature of chiral
scattering. For elastic scattering, we find that at least double scattering
events must take place for longitudinal polarization to arise. We predict
energy windows for strong polarization, determined by the energy dependences of
spin-orbit strength and multiple scattering probability. An incoherent
mechanism for polarization amplification is proposed, that increases the
polarization linearly with the number of helix turns, consistent with recent
experiments on DNA SAMs.Comment: 5 Pages, 4 figure
Competition in defense acquisition: Myths and facts
The article of record as published may be found at https://doi.org/10.1080/0743017890840540
- …