Neutron Reflectometry Experiments Research Articles

Neutron Reflectivity Study on the Postpolymerization Modification of Poly(2-hydroxyethyl methacrylate) Brushes

Postpolymerization modification reactions are widely employed to prepare functional polymer brushes. Relatively little is known, however, about the distribution of functional groups in such postmodified brushes. Using neutron reflectometry and UV–vis spectroscopy as principal tools, this article investigates the p-nitrophenyl chloroformate (NPC)-mediated postpolymerization modification of poly(2-hydroxyethyl methacrylate) (PHEMA) brushes, prepared via surface-initiated atom transfer radical polymerization, with D-10 leucine and D-3 serine. The neutron reflectometry experiments indicate that the postpolymerization modification depends both on brush thickness and grafting density. Whereas for dense brushes, postpolymerization modification with D-10 leucine is limited to the top ∼200 Å of the brush, independently of the brush thickness, the extent of postmodification can be significantly enhanced by decreasing the grafting density of the brush or by using the more hydrophilic and sterically less demanding D-3 serine, which reflects the ability of this amino acid to more readily penetrate the brush. UV–vis experiments revealed that the NPC activation is also nonuniform, but brush thickness and grafting density dependent, which adds to brush thickness and density and the nature of the amino acid as another of a complex set of variables that determine the final distribution of functional groups in postmodified brushes.

Voltage-Induced Swelling and Deswelling of Weak Polybase Brushes

We have investigated a novel method of remotely switching the conformation of a weak polybase brush using an applied voltage. Surface-grafted polyelectrolyte brushes exhibit rich responsive behavior and show great promise as "smart surfaces", but existing switching methods involve physically or chemically changing the solution in contact with the brush. In this study, high grafting density poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes were grown from silicon surfaces using atom transfer radical polymerization. Optical ellipsometry and neutron reflectivity were used to measure changes in the profiles of the brushes in response to DC voltages applied between the brush substrate and a parallel electrode some distance away in the surrounding liquid (water or D(2)O). Positive voltages were shown to cause swelling, while negative voltages in some cases caused deswelling. Neutron reflectometry experiments were carried out on the INTER reflectometer (ISIS, Rutherford Appleton Laboratory, UK) allowing time-resolved measurements of polymer brush structure. The PDMAEMA brushes were shown to have a polymer volume fraction profile described by a Gaussian-terminated parabola both in the equilibrium and in the partially swollen states. At very high positive voltages (in this study, positive bias means positive voltage to the brush-bearing substrate), the brush chains were shown to be stretched to an extent comparable to their contour length, before being physically removed from the interface. Voltage-induced swelling was shown to exhibit a wider range of brush swelling states in comparison to pH switching, with the additional advantages that the stimulus is remotely controlled and may be fully automated.

Neural Stem Cell Spreading on Lipid Based Artificial Cell Surfaces, Characterized by Combined X-ray and Neutron Reflectometry.

We developed a bioadhesive coating based on a synthetic peptide-conjugate (AK-cyclo[RGDfC]) which contains multiples of the arginyl-glycyl-aspartic acid (RGD) amino acid sequence. Biotinylated AK-cyclo[RGDfC] is bound to a supported lipid bilayer via a streptavidin interlayer. Layering, hydration and packing of the coating is quantified by X-ray and neutron reflectometry experiments. AK-cyclo[RGDfC] binds to the streptavidin interlayer in a stretched-out on edge configuration. The highly packed configuration with only 12% water content maximizes the number of accessible adhesion sites. Enhanced cell spreading of neural stem cells was observed for AK-cyclo[RGDfC] functionalized bilayers. Due to the large variety of surfaces which can be coated by physisorption of lipid bilayers, this approach is of general interest for the fabrication of biocompatible surfaces.

Time-odd correlation in a neutron reflectometry experiment

We find that neutron transmission of magnetic systems with the noncollinear magnetization contains a time-odd correlation. The neutron reflection from such a system violates detailed balance. Time-odd correlation is shown to violate T-invariance even in the presence of an irreversibly produced by losses and described by the imaginary part in the neutron-matter interaction.

Magnetic field dependence of the magnetization of a 29 nm thick AuFe spin glass film

We performed polarized neutron reflectometry (PNR) experiments on a 29 nm thick Au93Fe7 film. The measurements were done in a temperature range from 300 K to 2 K in a magnetic field up to 6 T applied in the sample's plane, at 2 K we took PNR data up to 11 T. The magnetization as determined by PNR can be described with a Brillouin function from 295 K down to 50 K assuming the magnetic moment of isolated Fe atoms, i.e. 4 |μB per Fe atom. Below 50 K the onset of the spin-glass freezing was observed as a strong reduction of the magnetic moment compared to the isolated Fe atom.

Absorption of light and heavy water vapours in polyelectrolyte multilayer films

We studied the swelling and the uptake of water (H 2O or D 2O) vapours in polyelectrolyte (PE) multilayer (PEM) samples deposited on solid support (Si wafers) as a function of the isotope nature of the vapour and the charge of the last polymer layer. The samples were prepared with deuterated poly(sodium 4-styrenesulfonate) (dPSS) and poly(allylamine hydrochloride) (PAH). Two types of samples were studied. The sample with a structure Si/PEI/(dPSS/PAH) 6/dPSS was negatively charged. A positively charged sample was PAH terminated and had the structure Si/PEI/(dPSS/PAH) 6. The film thickness and scattering length density were estimated from neutron reflectometry (NR) experiments and the results were complemented with in-situ QCM measurements. We demonstrate that the swelling of PEM in H 2O and D 2O vapours is similar. However, the amount of adsorbed D 2O is around 10% more than the adsorbed H 2O. Such isotope effect correlates well with the rough estimation that the isotope effect usually scales with the difference in the mass density of the different isotope forms of the substances. For precise analysis of the NR data we assumed existence of empty voids in the structure of the PEM. These voids might be filled with “condensed” water when the samples are exposed to water vapors. We show that the layers we studied consist of up to 25% of such voids. We showed that the amount of sorbed water depends on the nature of the last layer which builds the PEM thus confirming the “odd-even effect” already shown in the literature.

Asymmetric lipid bilayer sandwiched in polyelectrolyte multilayer films through layer-by-layer assembly

Lipid bilayer of 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE) was formed on the surface of a linearly growing polyanion/polycation polyelectrolyte multilayer (PEM) film prepared by the layer-by-layer (LbL) technique. The PEM films were prepared with two strong polyelectrolytes—poly(sodium 4-styrenesulfonate) (PSS) as the polyanion and poly-(diallyldimethylammonium chloride) (PDADMAC) as the polycation. The DMPE was always deposited onto the PEM cushion terminated with positively charged PDADMAC. The surface charge of the DMPE-coated film depends on the pH of the buffer solution. Formation of PEM could be continued on the top of the lipid bilayer by tuning the headgroup charge of the DMPE. An asymmetric structure of polycation/lipid/polyanion “sandwich” was created in the case where anionic PSS was used as the first layer of the subsequent PEM. In this system, the DMPE lipid membrane was constructed as a charge asymmetric barrier sandwiched in the interior of the PEM films. The formation of PEM architectures incorporated with an asymmetric DMPE bilayer was confirmed by ζ potential, UV-vis absorption, quartz crystal microbalance (QCM) and neutron reflectometry experiments.

Are sprayed LbL-films stratified? A first assessment of the nanostructure of spray-assembled multilayers by neutron reflectometry

We describe first neutron reflectometry experiments on spray-assembled polyelectrolyte multilayer films containing regularly spaced layers of perdeuterated poly(styrene sulfonate). From samples containing either only non-deuterated layers or containing a maximum of deuterated layers we obtain all model parameters (thicknesses, scattering length densities and roughnesses) required for calculating the reflectivities of multilayer films with a regular superlattice structure. For two different superlattice films we show that we obtain an excellent match between experimental data and calculated reflectivities without any free fit parameter. Our data clearly demonstrate that spray-assembled polyelectrolyte multilayers are indeed stratified although some structural parameters differ enormously from films prepared by classic dipping. Our present experiments confirm a substantial difference in thickness and suggest a reduced interfacial roughness in spray-assembled films. Nous décrivons les premières expériences de réflectométrie de neutrons sur des films multicouches de polyélectrolytes assemblés par pulvérisation contenant des couches régulièrement espacées de poly(styrène sulfonate) perdeutéré. A partir d’échantillons contenant soit seulement des couches non-deutérées soit un maximum de couches deutérées, nous avons obtenu tous les paramètres modèles (épaisseurs, densités de longueur de diffusion et rugosités) nécessaires pour le calcul de la reflectivité des films multicouches avec une structure super-maille régulière. Pour deux films de super-maille différente, nous avons montré que l'on peut obtenir une excellente concordance entre les données expérimentales et les réflectivités calculées sans aucun paramètre d'ajustement libre. Nos données montrent clairement que les multicouches de polyélectrolytes assemblées par pulvérisation sont effectivement stratifiées même si certains paramètres diffèrent énormément des films préparés par trempage. Nos expériences confirment une différence substantielle en épaisseur et suggèrent une rugosité interfaciale réduite dans les films assemblés par pulvérisation.

Thermal stability, mechanical properties and water content of bacterial protein layers recrystallized on polyelectrolyte multilayers

The influence of the temperature on the surface topology, layer thickness and density of recrystallized bacterial S-layers from Bacillus sphaericus CCM2177 on polyelectrolyte multilayers in contact with liquid water was investigated. A quartz crystal microbalance with dissipation (QCM-D) was used to monitor the build-up of the polyelectrolyte multilayer and the adsorption of S-layer protein (1600 ng cm-2). The critical temperature (55 °C) at which the S-layer loses its 2-D structure was obtained from atomic force microscopy (AFM) and confirmed by neutron reflectometry (NR) experiments. The process of S-layer denaturation was found to be irreversible. Aggregates of denatured S-proteins resist lower loads than the crystalline nanostructure formed from folded S-proteins. The combination of the QCM-D results with the scattering length density and film thickness (14 nm) obtained from neutron reflectometry studies permitted the estimation of the density of adsorbed S-protein together with the bound water (M = 1.16 g cm-3), the dry protein scattering length density (2.02 × 10-6 Å-2) and the S-protein mass density (1.48 g cm-3). The results confirmed that S-proteins form very loosely packed layers on polyelectrolyte multilayers incorporating a water volume fraction of around 68%.

A polarized neutron reflectometry study of the spin glass freezing in a 29 nm thick AuFe film

We performed polarized neutron reflectometry (PNR) experiments on a 29 nm thick Au 93Fe 7 film in a temperature range from 295 K down to 2 K in a vertical magnetic field up to 6 T. These high-field experiments were performed on the C5 spectrometer in Chalk River, Canada, using a split-pair cryomagnet. The magnetization as determined by PNR can be described with a Brillouin function from 295 K down to 50 K assuming the magnetic moment of isolated Fe atoms, i.e. 4 μ B per Fe atom. Below 50 K the onset of the spin-glass freezing is observed as a strong deviation from this Brillouin type behavior of isolated atoms.

Concerning simultaneous high-resolution energy and momentum transfer measurements in inelastic neutron reflectometry

A possibility to simultaneously achieve a high momentum transfer, Q, and energy, E, resolution, in a single inelastic neutron reflectometry experiment is discussed. Limitations on the excitation parameters, which can be unambiguously determined by th

Determination of graft density of substrate-supported polymer films

Determining the graft density of polymers adsorbed on a substrate from neutron scattering-length density profile can be challenging. A scheme based on integrating a select region of the profile is proposed and applied to results obtained from a recent neutron reflectometry experiment that investigated the physical properties of polyethylene oxide (PEO) chemisorbed onto a gold-coated Si substrate. Reflectivity measurements were performed with the PEO exposed to a H 2O/D 2O mixture contrast matched to the underlying gold. The graft density obtained by the integration technique is 60% higher than the value determined using scaling relations.

Polarized neutron reflectometry in high magnetic fields

A simple method is described to maintain the polarization of a neutron beam on its way through the large magnetic stray fields produced by a vertical field of a cryomagnet with a split-coil geometry. The two key issues are the proper shielding of the neutron spin flippers and an additional radial field component in order to guide the neutron spin through the region of the null point (i.e., point of reversal for the vertical field component). Calculations of the neutron’s spin rotation as well as polarized neutron reflectometry experiments on an ErFe2∕DyFe2 multilayer show the perfect performance of the used setup. The recently commissioned cryomagnet M5 with a maximum vertical field of up to 7.2T in asymmetric mode for polarized neutrons and 9T in symmetric mode for unpolarized neutrons was used on the C5 spectrometer in reflectometry mode, at the NRU reactor in Chalk River, Canada.

Scientific Reviews: Polarized Neutron Reflectometry at IBR-2

At FLNP JINR (Dubna, Russia), the first neutron reflectometry experiments were carried out to measure the magnetic field penetration depth into a superconducting film on the polarized neutron spectrometer SPN at the IBR-2 fast-pulsed reactor of periodic operation by D.A. Korneev and colleagues, in 1988, after the reactor was put in operation [1]. Later on, SPN-aided investigations were conducted on thin films (2), including superconducting ones (3), on magnetic multilayers (4), and on interfaces in the structure Fe/V at room temperature (5) and at temperatures below the superconducting transition temperature of vanadium (6).

Salt-induced protein resistance of polyelectrolyte brushes studied using fluorescence correlation spectroscopy and neutron reflectometry

We used two-photon excitation fluorescence correlation spectroscopy (FCS) and neutron reflectometry to study in situ the effect of salt concentration on the degree of protein binding to polyelectrolyte brushes. The binding of bovine serum albumin (BSA) to poly(acrylic acid) (PAA) brushes was characterized at neutral pH values where both the protein and the brushes carry a negative charge. Spherical PAA brush particles were used in the FCS experiments, whereas a planar PAA brush served as protein substrate in the neutron reflectometry experiments. It has been found that BSA binds strongly to both the spherical and the planar PAA brushes under electrostatic repulsion at low ionic strength. The BSA volume fraction profile, as determined from the neutron reflectivities, indicates a deep penetration of the BSA molecules into the PAA brush. However, the analysis of the FCS data reveals that the protein affinity of the spherical PAA brush particles decreases drastically when increasing the concentration of sodium chloride to a few 100 mM. This observation is in line with the measured neutron reflectivities of the planar PAA brush. The reflectivity curve obtained in the absence of protein is virtually overlapping with that measured when the PAA brush is in contact with a BSA solution but containing 500 mM sodium chloride which suggests protein resistance of the planar PAA brush at this elevated salt concentration. The results of this study provide evidence for a new kind of protein-resistant interfaces. Whereas protein binding to the PAA brush is likely to be dominated by the release of counterions, this driving force vanishes as the ionic strength of the solution is raised and protein molecules are repelled from the interface by steric interactions. In a general view, the “switching” of the protein affinity of a PAA brush by varying the ionic strength of the protein solution over a relatively small range may appear to be useful for biotechnological applications.

AUSTRON and its notable new features

The 10 Hz/0.5 MW AUSTRON neutron spallation source is dedicated for high-resolution structural and dynamical investigations of condensed matter and for new fundamental research with neutrons. New magnetic focusing and bunching systems are under development, which create additional intensity-gain factors of 10–20. Clean-room conditions including vibration, temperature and humidity control will improve the standard of neutron optical and neutron reflectometry experiments considerably. Narrow band neutron perfect crystal resonators and storage systems are foreseen for a novel beam tailoring. A dedicated new ultra-cold neutron station will open a new horizon for experiments in the sub-neV region. Recently, the project secured additional support of the Austrian Governmental Board for Research and Technological Development, which recommended the forced continuation of the project, the installation of a professional project group and the cooperation with the European Spallation Source (ESS) project and with the neighboring countries.

Polarization analysis of off-specular neutron scattering from domains and rough interfaces in a FeCoV/TiZr multilayer

Specular neutron reflectivity is commonly used as a powerful tool in determining the scattering length density (SLD) profile perpendicular to the surface of thin films. Information about the lateral fluctuations in the SLD, such as the presence of magnetic domains and rough interfaces between layers, can be obtained by studying the pattern of off-specular scattering. Advancements in neutron-source brilliance, beam-line optics and detector efficiency, as well as advances in the theoretical description of off-specular scattering, have contributed to the increasing importance of off-specular neutron-reflectometry studies. We present specular and off-specular polarized neutron reflectometry experiments on FeCoV/TiZr multilayers, with a detailed description of the orientations of the magnetic moments inside the FeCoV layers and at the FeCoV/TiZr interfaces.

On the use of a multilayer monochromator in neutron reflectometry

In neutron reflectometry experiments, the intensity reflected from the sample under investigation is determined as a function of the wave-vector transfer. One way of doing this is to start with a monochromatic beam and perform the so-called θ/2 θ scans. The monochromatic beam can be produced by Bragg reflection from a single crystal, or from a multilayer system. In this paper, we experimentally investigate the performance of a multilayer monochromator. We demonstrate that the large-wavelength contamination, intrinsically present, may lead to large errors in the obtained reflectivity at large values of the wave-vector transfer.

Polarization analysis of neutron reflectometry on non-collinear magnetic media: polarized neutron reflectometry experiments on a thin cobalt film

In this work we present a full data analysis of polarized neutron reflectometry experiments on a thin magnetic film. A magnetic field was applied perpendicular to the layer magnetization, resulting in non spin-flip and spin-flip reflectivities. For moderate external fields, the gain or loss of Zeeman energy for spin-flipped neutrons results in off-specular reflected spin-flipped beams. The theoretical model used to explain all data simultaneously consists of a bulk cobalt layer, together with interfacial layers with low magnetization values.

Status of the AUSTRON spallation project

The characteristic parameters of the present version of the planned AUSTRON neutron spallation source are: (a) 10 Hz repetition rate at the target, (b) 500 kW average beam power, (c) 1.6 GeV proton energy and (d) strongly dedicated to cold neutron research. The machine can be built based on existing techniques and would provide a performance gain between 3 and 10 compared to existing installations. Progress has been made in the planning of the instrumentation where an improved standard instrumentation and several new systems have been designed and evaluated by an international group of neutron scientists. A clean room area with vibration, temperature and humidity control provides new possibilities for neutron optics, neutron reflectometry and nuclear orientation experiments. A proposed magnetic focusing line with an active energy transfer system indicates the capability to achieve a further intensity gain by a factor of 10. The status of the ongoing internationalization process will be reported as well.