Depth-resolved grazing-incidence time-of-flight neutron scattering from a solid–liquid interface

Understanding near-surface polymer dynamics by a combination of grazing-incidence neutron scattering and virtual experiments

Neutron spin-echo spectroscopy is a unique experimental method for the investigation of polymer dynamics. The combination of neutron spin-echo spectroscopy with grazing-incidence geometry (GINSES) opens the possibility to probe the dynamics of soft-matter materials in the vicinity of the solid substrate in the time range up to 100 ns. However, the usage of the GINSES technique has some peculiarities and, due to the novelty of the method and complexity of the scattering geometry, difficulties in further data analysis occur. The current work discusses how virtual experiments within the distorted-wave Born approximation using the BornAgain software can improve GINSES data treatment and aid the understanding of polymer dynamics in the vicinity of the solid surface. With two examples, poly(N-isopropyl acrylamide) brushes and poly(ethylene glycol) microgels on Si surfaces, the simulation as well as the application of the simulation to the GINSES data analysis are presented. The approach allowed a deeper insight to be gained of the background effect and scattering contribution of different layers.

Time Resolved Polarised Grazing Incidence Neutron Scattering from Composite Materials

Neutron scattering experiments are a unique tool in material science due to their sensitivity to light elements and magnetic induction. However, for kinetic studies the low brilliance at existing sources poses challenges. In the case of periodic excitations these challenges can be overcome by binning the scattering signal according to the excitation state of the sample. To advance into this direction we have performed polarised and time resolved grazing incidence neutron scattering measurements on an aqueous solution of the polymer F127 mixed with magnetic nano-particles. Magnetic nano-composites like this provide magnetically tuneable properties of the polymer crystal as well as magnetic meta-crystals. Even though the grazing incidence small angle scattering and polarised signals are too weak to be evaluated at this stage we demonstrate that such experiments are feasible. Moreover, we show that the intensity of the 111 Bragg peak of the fcc micellar crystal depends on the actual shear rate, with the signal being maximised when the shear rate is lowest (and vice-versa).

Grazing incidence scattering

Reflectometry experiments probe the scattering length density along the normal of interfaces by analysing the specularly scattered intensity. Lateral fluctuations result in intensity scattered away from the specular condition. In this paper the principles and peculiarities of grazing incidence scattering experiments are explained. One specific example, the self assembly of polymer micelles close to interfaces, is taken as a show case in order to introduce the scattering geometry and accessible length scales. The basic idea of the distorted wave Born approximation is lined out and some scientific examples are summarized.

Grazing-incidence small-angle neutron scattering from structures below an interface

Changes of scattering are observed as the grazing angle of incidence of an incoming beam increases and probes different depths in samples. A model has been developed to describe the observed intensity in grazing-incidence small-angle neutron scattering (GISANS) experiments. This includes the significant effects of instrument resolution, the sample transmission, which depends on both absorption and scattering, and the sample structure. The calculations are tested with self-organized structures of two colloidal samples with different size particles that were measured on two different instruments. The model allows calculations for various instruments with defined resolution and can be used to design future improved experiments. The possibilities and limits of GISANS for different studies are discussed using the model calculations.

The Search for Nanobubbles by Using Specular and Off-Specular Neutron Reflectometry

We apply specular and off-specular neutron reflection at the hydrophobic silicon/water interface to check for evidence of nanoscopic air bubbles whose presence is claimed after an ad hoc procedure of solvent exchange. Nanobubbles and/or a depletion layer at the hydrophobic/water interface have long been discussed and generated a plethora of controversial scientific results. By combining neutron reflectometry (NR), off-specular reflectometry (OSS), and grazing incidence small angle neutron scattering (GISANS), we studied the interface between hydrophobized silicon and heavy water before and after saturation with nitrogen gas. Our specular reflectometry results can be interpreted by assuming a submolecular sized depletion layer and the off-specular measurements show no change with nitrogen super saturated water. This picture is consistent with the assumption that, following the solvent exchange, no additional nanobubbles are introduced at significant concentrations (if present at all). Furthermore, we discuss the results in terms of the maximum surface coverage of nanobubbles that could be present on the hydrophobic surface compatibly with the sensitivity limit of these techniques.

Towards neutron scattering experiments with sub-millisecond time resolution

Neutron scattering techniques offer several unique opportunities in materials research. However, most neutron scattering experiments suffer from the limited flux available at current facilities. This limitation becomes even more severe if time-resolved or kinetic experiments are performed. A new method has been developed which overcomes these limitations when a reversible process is studied, without any compromise on resolution or beam intensity. It is demonstrated that, by recording in absolute time the neutron detector events linked to an excitation, information can be resolved on sub-millisecond timescales. Specifically, the concept of the method is demonstrated by neutron reflectivity measurements in time-of-flight mode at the Liquids Reflectometer located at the Spallation Neutron Source, Oak Ridge National Laboratory, Tennessee, USA, combined within siturheometry. The opportunities and limitations of this new technique are evaluated by investigations of a micellar polymer solution offering excellent scattering contrast combined with high sensitivity to shear.