Rana Ashkar

Wetting-Dewetting and Dispersion-Aggregation Transitions Are Distinct for Polymer Grafted Nanoparticles in Chemically Dissimilar Polymer Matrix.

Simulations and experiments are conducted on mixtures containing polymer grafted nanoparticles in a chemically distinct polymer matrix, where the graft and matrix polymers exhibit attractive enthalpic interactions at low temperatures that become progressively repulsive as temperature is increased. Both coarse-grained molecular dynamics simulations, and X-ray scattering and neutron scattering experiments with deuterated polystyrene (dPS) grafted silica and poly(vinyl methyl ether) PVME matrix show that the sharp phase transition from (mixed) dispersed to (demixed) aggregated morphologies due to the increasingly repulsive effective interactions between the blend components is distinct from the continuous wetting-dewetting transition. Strikingly, this is unlike the extensively studied chemically identical graft-matrix composites, where the two transitions have been considered to be synonymous, and is also unlike the free (ungrafted) blends of the same graft and matrix homopolymers, where the wetting-dewetting is a sharp transition coinciding with the macrophase separation.

Dynamical theory calculations of spin-echo resolved grazing-incidence scattering from a diffraction grating

Neutrons scattered or reflected from a diffraction grating are subject to a periodic potential analogous to the potential experienced by electrons within a crystal. Hence, the wavefunction of the neutrons can be expanded in terms of Bloch waves and a dynamical theory can be applied to interpret the scattering phenomenon. In this paper, a dynamical theory is used to calculate the results of neutron spin-echo resolved grazing-incidence scattering (SERGIS) from a silicon diffraction grating with a rectangular profile. The calculations are compared with SERGIS measurements made on the same grating at two neutron sources: a pulsed source and a continuous wave source. In both cases, the spin-echo polarization, studied as a function of the spin-echo length, peaks at integer multiples of the grating period but there are some differences between the two sets of data. The dynamical theory explains the differences and gives a good account of both sets of results.

Thermoresponsive PNIPAM Coatings on Nanostructured Gratings for Cell Alignment and Release

Thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) has been widely used as a surface coating to thermally control the detachment of adsorbed cells without the need for extreme stimuli such as enzyme treatment. Recently, the use of 2D and 3D scaffolds in controlling cell positioning, growth, spreading, and migration has been of a great interest in tissue engineering and cell biology. Here, we use a PNIPAM polymer surface coating atop a nanostructured linear diffraction grating to controllably change the surface topography of 2D linear structures using temperature stimuli. Neutron reflectometry and surface diffraction are utilized to examine the conformity of the polymer coating to the grating surface, its hydration profile, and its evolution in response to temperature variations. The results show that, in the collapsed state, the PNIPAM coating conforms to the grating structures and retains a uniform hydration of 63%. In the swollen state, the polymer expands beyond the grating channels and absorbs up to 87% water. Such properties are particularly desirable for 2D cell growth scaffolds with a built-in nonextreme tissue-release mechanism. Indeed, the current system demonstrates advanced performance in the effective alignment of cultured fibroblast cells and the easy release of the cells upon temperature change.

Lightweight, Flexible, High-Performance Carbon Nanotube Cables Made by Scalable Flow Coating

Coaxial cables for data transmission are ubiquitous in telecommunications, aerospace, automotive, and robotics industries. Yet, the metals used to make commercial cables are unsuitably heavy and stiff. These undesirable traits are particularly problematic in aerospace applications, where weight is at a premium and flexibility is necessary to conform with the distributed layout of electronic components in satellites and aircraft. The cable outer conductor (OC) is usually the heaviest component of modern data cables; therefore, exchanging the conventional metallic OC for lower weight materials with comparable transmission characteristics is highly desirable. Carbon nanotubes (CNTs) have recently been proposed to replace the metal components in coaxial cables; however, signal attenuation was too high in prototypes produced so far. Here, we fabricate the OC of coaxial data cables by directly coating a solution of CNTs in chlorosulfonic acid (CSA) onto the cable inner dielectric. This coating has an electrical conductivity that is approximately 2 orders of magnitude greater than the best CNT OC reported in the literature to date. This high conductivity makes CNT coaxial cables an attractive alternative to commercial cables with a metal (tin-coated copper) OC, providing comparable cable attenuation and mechanical durability with a 97% lower component mass.

Spin echo modulated small-angle neutron scattering using superconducting magnetic Wollaston prisms

The spin echo modulated small-angle neutron scattering technique has been implemented using two superconducting magnetic Wollaston prisms at a reactor neutron source. The density autocorrelation function measured for a test sample of colloidal silica in a suspension agrees with that obtained previously by other neutron scattering methods on an identically prepared sample. The reported apparatus has a number of advantages over competing technologies: it should allow larger length scales (up to several micrometres) to be probed; it has very small parasitic neutron scattering and attenuation; the magnetic fields within the device are highly uniform; and the neutron spin transport across the device boundaries is very efficient. To understand quantitatively the results of the reported experiment and to guide future instrument development, Monte Carlo simulations are presented, in which the evolution of the neutron polarization through the apparatus is based on magnetic field integrals obtained from finite-element simulations of the various magnetic components. The Monte Carlo simulations indicate that the polarization losses observed in the experiments are a result of instrumental artifacts that can be easily corrected in future experiments.

Spin-Echo Resolved Grazing Incidence Scattering (SERGIS) at Pulsed and CW Neutron Sources

We have used SERGIS to probe the surface structure of a silicon diffraction grating of period 140 nm. Experiments were performed at: the Los Alamos Neutron Science Center (LANSCE) pulsed neutron source and the National Institute of Standards and Technology (NIST) continuous wave (CW) reactor neutron source. Although both sets of data show peaks of the spin echo polarization at integer multiples of the grating period, as expected, the results differ in detail. We have developed a dynamical theory, based on a Bloch wave expansion, to describe neutron diffraction from a grating. The theory explains the differences between the two sets of data without any adjustable parameters.

A new approach for probing matter in periodic nanoconfinements using neutron scattering

The efficacy of spin-echo small-angle neutron scattering (SESANS) combined with an exact dynamical theory (DT) model in resolving the arrangement of spherical colloidal particles in planar confinements, such as the channels of a rectangular diffraction grating, is reported. SESANS data obtained with a suspension of charge-stabilized 180 nm silica particles in contact with a silicon diffraction grating, with ∼650 nm-wide channels, show clear deviations from the signal expected from a homogenous distribution of the suspension. DT fits to the data indicate that the colloidal particles are almost twice as concentrated in the channels as they are in the neighboring bulk suspension, consistent with a structure in which the particles are arranged in close-packed sheets parallel to the walls of the confining channels.

Dynamical theory: Application to spin-echo resolved grazing incidence scattering from periodic structures

Neutron spin-echo resolved grazing incidence scattering (SERGIS) measurements performed on a silicon diffraction grating with a rectangular profile were shown in our previous publications to be well explained by dynamical theory calculations. The theory is based on a Bloch wave expansion of the neutron wavefunction in the periodic layer of the grating, which includes all multiple scattering within that layer. Calculations show that the spin-echo polarization should be very sensitive to the scattering geometry (i.e., incident angle, sample alignment and beam divergence) and the sample specifications (i.e., grating period, groove depth). To test these predictions, SERGIS measurements have been performed on a set of gratings with different specifications in various scattering geometries. In all cases, simulations based on the dynamical theory, with all the parameters set to their known values, are in good agreement with the collected data.

Rapid Large-Scale Assembly and Pattern Transfer of One-Dimensional Gold Nanorod Superstructures

The utility of gold nanorods for plasmonic applications largely depends on the relative orientation and proximity of the nanorods. Though side-by-side or chainlike nanorod morphologies have been previously demonstrated, a simple reliable method to obtain high-yield oriented gold nanorod assemblies remains a significant challenge. We present a facile, scalable approach which exploits meniscus drag, evaporative self-assembly, and van der Waals interactions to precisely position and orient gold nanorods over macroscopic areas of 1D nanostructured substrates. By adjusting the ratio of the nanorod diameter to the width of the nanochannels, we demonstrate the formation of two highly desired translationally ordered nanorod patterns. We further demonstrate a method to transfer the aligned nanorods into a polymer matrix which exhibits anisotropic optical properties, allowing for rapid fabrication and deployment of flexible optical and electronic materials in future nanoscale devices.

Comparison of dynamical theory and phase-object approximation for neutron scattering from periodic structures

Dynamical theory (DT) calculations have been successfully developed toexplainneutronspin-echoresolvedgrazing-incidencescatteringfromdiffractiongratings. The theory, without any adjustable parameters, has been shown inprevious publications to accurately reproduce the sensitivity of the spin-echopolarization signal to sample specifications and scattering geometry. The phase-object approximation (POA), which is computationally less demanding than theDT, has also been used to analyze neutron spin-echo polarization data obtainedfrom diffraction gratings. In this paper, POA and DT calculations are comparedfor neutron scattering from various diffraction gratings in different geometricalsettings. POA gives a good description of the data for transmission cases, wherethe neutron beam is incident at large angles to the average grating surface.However, for the grazing-incidence reflection cases that were studied, the POAdoes not fit the data using the independently determined dimensions of themeasured gratings. On the other hand, the good agreement between dynamicaltheory and the data from gratings with known profiles paves the way for its useto extract profile information from periodic samples with unknown structures.1. IntroductionSince the early use of scattering techniques to study crystalproperties, many theories have been developed to explain thescattering data. These theories simulate scattering based onthe characteristics of the scattering potential as well as thoseof the scattered radiation. By fitting a theoretical model to themeasured scattering, unknown properties of the sample areextracted. For perfect crystals, a two-beam dynamical theorywas proposed by Bethe (1928) to describe dynamical scat-tering effects of electrons. However, this theory is fairlycomplicated and when more than two diffracted beams had tobe considered the calculations were tedious, especially withthelimitedcomputationalpoweravailablethen.Moreover,formost samples (e.g. crystals with defects) it is not easy todescribe the individual scattering events accurately. Hence,alternative scattering theories involving further approxima-tions were proposed to make calculation schemes easier toevaluate. The challenge has always been to find an approx-imation thatboth reasonably modelsthe scattered wave and isreadily computed.In 1957, Cowley & Moodie (1957) proposed a new‘physical-optics’ approach to interpret the diffraction ofelectrons by thin crystals. The authors assumed that, when anelectron traverses a region with a varying electrostaticpotential, the electron wave at the exit suffers an amplitudemodification (due to inelastic scattering) and a phase modifi-cation (due to elastic scattering) relative to the incident wave.In the limit where the scattering is perfectly elastic, the effectis due solely to a ‘phase-object’. Later, Jap & Glaeser (1978)used a Feynman path formulation to explain the scattering ofhigh-energy electrons from thin specimens with an evensimpler approximation, in which the accumulated phase of anelectron is calculated along a single classical path parallel tothe incident beam direction. This straight-line path approx-imation is known as the phase-object approximation (POA).A higher-order POA, which involves zigzag as well as straight-line paths, has been also derived (Jap & Glaeser, 1978) andwas found, in the case of forward scattering, to be theapproximation of Cowley and Moodie.The POA has been recently applied to small-angle neutronscattering from micrometre-scale silicon diffraction gratings(de Haan et al., 2007). The theoretical approximation allowsgood fits to the experimental data in the case of transmissionbut cannot reproduce reflection data without adjustment ofsome parameters (groove depth and/or period of the grating)(Plomp, 2009; Plomp et al., 2007). This poses a question