A layer of polymer chains tethered by one end to a surface is called polymer brush and known to show various unique properties such as prevention of protein adsorption and anti-fouling activity. The surface segregation phenomena of copolymers with surface-active blocks should be useful for preparing such a brush layer in spontaneous process. We reported hydrophilic polymer brushes formed at the interface between water and hydrophobic polymer matrix by the segregation of amphiphilic diblock copolymers blended in the matrix. In this system, while the hydrophilic block with high surface energy avoids air surface, it segregates to cover the interface between hydrophobic PDMS and water. The structures of formed brush layers at D2O/polymer interfaces were analyzed by neutron reflectivity. The brush density was calculated to be as high as 2 chains/nm2, which is surprisingly comparable to the polymer brush fabricated by the “grafting-from” method.
Counter anion-specific hydration state of a cationic polyelectrolyte brush film, and ion-specificity and salt concentration dependence in hydration state of a zwitterionic poly(sulfobetaine) brush film were investigated using neutron reflectivity (NR). The asymmetric chain density profiles in the charged polymer brushes were clearly visualized. The poly(quaternary ammonium cation) brushes with strongly hydrated chloride counter anions produce a more diffuse tail and less swollen bound layer under salt-free deuterium oxide than those with weakly hydrated thiocyanate counter anions. The extent of swelling and the dimensions of swollen chain structure in the poly(sulfobetaine) brushes were dependent on the ion species and salt concentration in the solutions. The anion specificity could be ascribed to ion-specific interactions between the quaternary ammonium cation in sulfobetaine groups and the anions. The insights into the hydration state of the charged polymer brushes described here could be useful to tailor the response of surface-tethered charged polymer films in the presence of various ions.
Solution- and thermal annealing-processed organic/organic interfacial structures in organic light-emitting devices (OLEDs) were investigated by neutron reflectivity measurement. Two types of solution-processed interfaces, polymer hole-transporting layer (HTL)/small molecules light-emitting layer (EML) and small molecules EML and small molecule electron-transporting layer (ETL) interfaces, were investigated. In the both processes, asymmetric diffusion was observed in the polymer HTL/small molecules EML interface, while interdiffusion was observed in the small molecules EML/small molecule ETL interface. This difference was attributed to the difference of the diffusivity between the small molecules and the polymer. Solution-processed OLEDs were fabricated to investigate correlations between the interfacial structures and their characteristics. Interlayer mixing by the solution-processing brought significant impacts on the device characteristics.
A promising novel technique has been developed to visualize the inhomogeneity of buried layers and interfaces in thin films non-destructively. The present technique gives real-space neutron image corresponding to positions in the sample, by combining the image reconstruction method and neutron reflectivity, which is extremely sensitive to the layered structures. It has become possible to obtain neutron reflectivity profile at each local point in the sample, without the use of micro beam.
Tribological properties under lubrication are largely influenced by the existence of adsorbed additive layers, and neutron reflectometry is one of powerful tools for in-situ estimation of the layer thickness and density at the solid-liquid interface. Recent application of quantum beam analyses involving neutron reflectometry and the expectation for the improvement to operand mode in near future are introduced.
In the normal use of neutron reflectometry, samples with a film thickness of tens nm to a few hundred nm are mainly targeted. In the special case of samples with thicker films about microns, analysis using Kiessig fringes with low order appearing on the low-q region are needed. But in many cases of front incidence, total reflection occurs in the same low-q region. Kiessig fringes with lower order, those are required for analysis of thick films, did not appear and it causes that thick films with thickness of microns were not subject to measurement. However, in the measurement by the neutron introduced from the substrate side (back incidence), it is possible to reduce the critical momentum transfer qc caused by the difference in scattering length density between the substrate and the vacuum as the incident medium. It becomes possible to measure Kiessig fringes on low-q side which is necessary for analysis of thick films with thicknesses of few micrometers.
The pressure-induced quantum phase transition in the quantum magnet CsFeCl3 has been studied by means of neutron scattering technique under pressures. Neutron diffraction evidences a pressure-induced magnetic long-range order and identifies a realization of the 120º structure with a magnetic propagation vector kmag = (1/3,1/3,0). The evaluated critical exponent of the order parameter suggests that CsFeCl3 belongs to the universality class of U(1)×Z2 symmetry. Inelastic neutron scattering demonstrates softening of the magnetic excitation by applying pressures. In the ordered phase, an excitation with a minimum energy of 0.6 meV is observed in addition to a gapless excitation. Calculation of the spin spectrum reveals that the excitation having the minimum energy of 0.6 meV corresponds to the non-trivial mixed mode of the transverse and longitudinal fluctuations.
Neutron guides are one of the devices to transport neutron beam for long distance without sacrificing much neutrons, it can supply neutrons to many experimental instruments distributed in a large experimental hall. Also, by using a curved guide, only the neutrons in a required energy range can be transported, and gamma rays and fast neutrons can be effectively eliminated, therefore the signal to background ratio is improved. In addition, a neutron beam can be branched by applying curved guides which include compact multichannel bender using supermirrors. Neutron guides are also used to control the divergence angle and intensity of the neutron beam supplied to the neutron instruments. Typical example is the single crystal diffractometer, SENJU, installed at J-PARC.
Adequate shielding of neutrons and associated gamma-rays is of importance from viewpoints of the radiation safety of researchers and good experimental data taking by reducing the background. This article introduces basics of neutron shielding, physics and suitable materials for neutron and gamma-ray shielding, and an example of conceptual shielding design for the 1-MW spallation neutron source of J-PARC MLF.