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 $E/k_BT=15$ over nearly three orders of magnitude in monomer density $\phi$, 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 $\alpha_d=\delta_d=1/(1+\gamma) \approx 0.46$ and $\alpha_s = 1/2 [1+(\gamma-1)/(\nu d -1)] \approx 0.6, \delta_s=1/2$ 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 $\gamma \approx 1.165 \pm 0.01$. ....... Finite-size effects are discussed in detail.Comment: 15 pages, 14 figures, LATE

Enhancing the work of the Islington Integrated Gangs Team: A pilot study on the response to serious youth violence in Islington

This report is the result of research conducted by the Centre for City Criminology at City, University of London, in partnership with Islington’s Integrated Gangs Team (IGT) and the Metropolitan Police Service (MPS). The research was co-funded by MPS and the School of Arts and Social Sciences, City, University of London. Following a collaborative research event in October 2017, City Criminologists were commissioned to carry out a small-scale research project to capture the work of the IGT and to make recommendations regarding its operations, coherence, effectiveness and sustainability. The research team conducted semi-structured interviews over several months with 23 practitioners across the services that constitute the IGT. This report presents the findings and recommendations

Strain rate effects in the mechanical response of polymer anchored carbon nanotube foams

Super-compressible foam-like carbon nanotube films have been reported to exhibit highly nonlinear viscoelastic behaviour in compression similar to soft tissue. Their unique combination of light weight and exceptional electrical, thermal and mechanical properties have helped identify them as viable building blocks for more complex nanosystems and as stand-alone structures for a variety of different applications. In the as-grown state, their mechanical performance is limited by the weak adhesion between the tubes, controlled by the van der Waals forces, and the substrate allowing the forests to split easily and to have low resistance in shear. Under axial compression loading carbon nanotubes have demonstrated bending, buckling8 and fracture9 (or a combination of the above) depending on the loading conditions and on the number of loading cycles. In this work, we partially anchor dense vertically aligned foam-like forests of carbon nanotubes on a thin, flexible polymer layer to provide structural stability, and report the mechanical response of such systems as a function of the strain rate. We test the sample under quasi-static indentation loading and under impact loading and report a variable nonlinear response and different elastic recovery with varying strain rates. A Bauschinger-like effect is observed at very low strain rates while buckling and the formation of permanent defects in the tube structure is reported at very high strain rates. Using high-resolution transmission microscopyComment: 19 Pages, 4 Figure

Catastrophic vs Gradual Collapse of Thin-Walled Nanocrystalline Ni Hollow Cylinders As Building Blocks of Microlattice Structures

Lightweight yet stiff and strong lattice structures are attractive for various engineering applications, such as cores of sandwich shells and components designed for impact mitigation. Recent breakthroughs in manufacturing enable efficient fabrication of hierarchically architected microlattices, with dimensional control spanning seven orders of magnitude in length scale. These materials have the potential to exploit desirable nanoscale-size effects in a macroscopic structure, as long as their mechanical behavior at each appropriate scale – nano, micro, and macro levels – is properly understood. In this letter, we report the nanomechanical response of individual microlattice members. We show that hollow nanocrystalline Ni cylinders differing only in wall thicknesses, 500 and 150 nm, exhibit strikingly different collapse modes: the 500 nm sample collapses in a brittle manner, via a single strain burst, while the 150 nm sample shows a gradual collapse, via a series of small and discrete strain bursts. Further, compressive strength in 150 nm sample is 99.2% lower than predicted by shell buckling theory, likely due to localized buckling and fracture events observed during in situ compression experiments. We attribute this difference to the size-induced transition in deformation behavior, unique to nanoscale, and discuss it in the framework of “size effects” in crystalline strength

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

Polymer lattices as mechanically tunable 3-dimensional photonic crystals operating in the infrared

Broadly tunable photonic crystals in the near- to mid-infrared region could find use in spectroscopy, non-invasive medical diagnosis, chemical and biological sensing, and military applications, but so far have not been widely realized. We report the fabrication and characterization of three-dimensional tunable photonic crystals composed of polymer nanolattices with an octahedron unit-cell geometry. These photonic crystals exhibit a strong peak in reflection in the mid-infrared that shifts substantially and reversibly with application of compressive uniaxial strain. A strain of ∼40% results in a 2.2 μm wavelength shift in the pseudo-stop band, from 7.3 μm for the as-fabricated nanolattice to 5.1 μm when strained. We found a linear relationship between the overall compressive strain in the photonic crystal and the resulting stopband shift, with a ∼50 nm blueshift in the reflection peak position per percent increase in strain. These results suggest that architected nanolattices can serve as efficient three-dimensional mechanically tunable photonic crystals, providing a foundation for new opto-mechanical components and devices across infrared and possibly visible frequencies

Gaussians versus back-to-back exponentials: a numerical study

The underlying magnetic field distribution in many samples studied by the mu R technique is asymmetric. Despite this, quite often fit functions assuming symmetric (Gaussian) distributions are used. Here, a back-to-back exponential function, which can be made asymmetric with fit parameters, is studied numerically alongside a Gaussian function to see how well each fits symmetric and asymmetric simulated data. Both fit symmetric data well, but the back-to-back exponential is found to be superior for fitting asymmetric data

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

The changing UK careers landscape : tidal waves, turbulence and transformation

This article explores how the UK careers landscape in each of the four home nations is changing in response to neo-liberal policies. In this context, careers services are increasingly under pressure to demonstrate their added value, impact and returns on investment. As fiscal arrangements tighten and governments state their preferences and priorities for national careers services, differing strategic responses are beginning to emerge. A quasi-market, experimental approach is now the dominant discourse in England, in contrast to differing and complementary arrangements in Northern Ireland, Scotland and Wales. The article suggests that insofar as these developments are transforming national careers services, they are also creating significant challenges which require new forms of policy imagery and imagination for high-impact, all-age careers services