Neutron Reflectivity Study on the Adsorption Layer of Polyethylene Grown on Si Substrate.

To investigate the structure of the interface between polyethylene films and substrates, the neutron reflectivity (NR) of deuterated polyethylene (dPE) thin films deposited on Si substrates was measured, demonstrating water accumulation at the interface, even under ambient conditions. After leaching the thermally annealed dPE films in hot p-xylene, NR measurements were conducted on the layers remaining on the substrate, clearly revealing that the adsorption layer of dPE grew during annealing and consisted of two layers, an inner adsorption layer and an outer adsorption layer, as previously proposed for amorphous polymers. The inner adsorption layer was approximately 3.7 nm thick with a density comparable to that of the bulk. The outer adsorption layer was several nanometers thick and appeared to grow insufficiently on top of the inner adsorption layer under the annealing conditions examined in this study. This study clarifying the growth of the adsorption layer of polyethylene at the interface with an inorganic substrate is useful for improving the performance of polymer/inorganic filler nanocomposites due to the wide utility of crystalline polyolefins as polymer matrix materials in nanocomposites.

Difference in structural changes of surfactant aggregates near solid surface under shear flow versus those in the bulk.

In water, the nonionic surfactant pentaethylene glycol monododecyl ether (C12E5) forms multi-lamellar vesicles upon application of shear, attributed to buckling instability of the surfactant layers. In the standard setup for applying shear, a pair of solid substrates is moved in opposite directions, and a non-slip condition at the solid surface is assumed. Based on theoretical predictions, the effective viscosity of the fluid surrounding the membrane is modified in this process, and this confinement may affect membrane fluctuation. However, only a few studies have analyzed the structural changes near the substrate. From this viewpoint, the structural changes in surfactant aggregates near a solid substrate under the application of shear were investigated herein using neutron reflectometry (NR). By increasing the shear rate, shear thickening at a lower shear rate and shear thinning at a higher shear rate were observed, similar to that in the bulk. However, a discontinuous change in the lamellar structure accompanying the condensation of the surfactant was observed in the NR experiments. This study presents the first experimental evidence indicating that the ramping speed of shear rates governs the shear-induced structuring of surfactant aggregates near the surface.

Neutron reflectivity study on the nanostructure of PMMA chains near substrate interfaces based on contrast variation accompanied with small molecule sorption.

In the case of poly(methyl methacrylate) (PMMA) thin films on a Si substrate, thermal annealing induces the formation of a layer of PMMA chains tightly adsorbed near the substrate interface, and the strongly adsorbed PMMA remains on the substrate, even after washing with toluene (hereinafter called adsorbed sample). Neutron reflectometry revealed that the concerned structure consists of three layers: an inner layer (tightly bound on the substrate), a middle layer (bulk-like), and an outer layer (surface) in the adsorbed sample. When an adsorbed sample was exposed to toluene vapor, it became clear that, between the solid adsorption layer (which does not swell) and bulk-like swollen layer, there was a "buffer layer" that could sorb more toluene molecules than the bulk-like layer. This buffer layer was found not only in the adsorbed sample but also in the standard spin-cast PMMA thin films on the substrate. When the polymer chains were firmly adsorbed and immobilized on the Si substrate, the freedom of the possible structure right next to the tightly bound layer was reduced, which restricted the relaxation of the conformation of the polymer chain strongly. The "buffer layer" was manifested by the sorption of toluene with different scattering length density contrasts.

Hydrophobicity of the Pentafluorosulfanyl Group in Side Chains of Polymethacrylates by Evaluation with Surface Free Energy and Neutron Reflectivity.

A hydrophobic surface or coating is required for surface protection, anti-fouling, adhesion, and other applications. For the achievements of hydrophobic properties, fluorine-based coatings, such as the introduction of trifluoromethyl or difluoromethylene groups, are conventionally employed. Recent developments in synthetic chemistry have indicated other organic fluoroalkyl groups that are suitable for achieving a more hydrophobic surface. In this study, we focused on the hydrophobic properties of the pentafluorosulfanyl (-SF5) group. We synthesized polymethacrylates with -SF5 groups or other functional groups (-CF3, -CH3, and -H) in their side chains and evaluated their hydrophobicity based on contact angles of water and ethylene glycol and the affinities of their films to water through neutron reflectivity measurements to demonstrate the superior hydrophobic properties of the -SF5 group. The water contact angle on the polymethacrylate film with -SF5 groups was larger, which suggested that the surface free energy was lower than that of the other polymethacrylate thin films with pendant side chains of -CF3, -CH3, and -H. In addition, the fitting analyses of the neutron reflectivity profiles of the thin polymer films in contact with air and water revealed the lowest affinity between water and the surface of polymethacrylate films with -SF5 groups among the films of the synthesized polymers. Thus, we demonstrated the potential of pentafluorosulfanyl groups as advanced hydrophobic groups.

Neutron Reflectivity Study on the Suppression of Interfacial Water Accumulation between a Polypropylene Thin Film and Si Substrate Using a Silane-Coupling Agent.

We measured the neutron reflectivity (NR) of isotactic polypropylene (PP) thin films deposited on Si substrates modified by hexamethyldisilazane (HMDS) at the saturated vapor pressure of deuterated water at 25 °C and 60 °C/85% RH to investigate the effect of HMDS on the interfacial water accumulation in PP-based polymer/inorganic filler nanocomposites and metal/resin bonding materials. We found that the amount of water accumulated at the PP/Si interface decreased with increasing immersion time of the Si substrate in a solution of HMDS in hexane prior to PP film deposition. During the immersion of the Si substrate, the HMDS molecules were deposited on the Si substrate as a monolayer without aggregation. Furthermore, the coverage of the HMDS monolayer on the Si substrate increased with increasing immersion time. At 60 ° C and 85% RH, only a slight amount of interfacial water was detected after HMDS treatment for 1200 min. As a result, the maximum concentration of interfacial water was reduced to 0.1 from 0.3, where the latter corresponds to the PP film deposited on the untreated substrate.

Neutron reflectometry-based in situ structural analysis of an aligning agent additive for the alignment of nematic liquid crystals on solid substrates.

Surface aligning agents, such as amphiphilic surfactants, are widely used to control the initial alignment of nematic liquid crystals (NLCs) in liquid crystal displays (LCDs). Generally, these agents are first coated on a substrate prior to NLC introduction. When mixed with NLCs, long alkyl chain amphiphilic agent additives may control the NLC alignment without requiring pretreatment because they may spontaneously form an adsorbed layer at the solid-NLC interface. These self-assembled layers (SALs) appear promising in the effective control of the initial alignment of LCDs. However, direct observation of the adsorbed layer structure in contact with the NLCs is challenging due to probe limitations. Furthermore, the areal densities and alignments of the amphiphiles adsorbed from NLCs at the solid-NLC interface are not previously reported. Herein, the structure of the surface aligning agent n-hexadecyltrimethylammonium-d42 bromide (d-CTAB) was investigated at the silicon-NLC interface using in situ neutron reflectometry (NR), which indicated that the CTAB self-assembled as a monolayer, with its alignment dependent on the amphiphile concentration. At low amphiphile concentrations, the alignment of the SAL and NLCs was parallel to the substrate. With increasing amphiphile concentration, the number of amphiphiles attached to the substrate increased within the framework of the Gibbs monolayer, with the alignment of the amphiphiles and NLCs becoming perpendicular to the substrate. The experimental setup used here is comparable to those of more natural systems, such as those found in the alignment of NLCs in LCDs.

Structure and Mechanical Properties of Polybutadiene Thin Films Bound to Surface-Modified Carbon Interface.

The structure and mechanical properties of polybutadiene (PB) films on bare and surface-modified carbon films were examined. There was an interfacial layer of PB near the carbon layer whose density was higher (lower) than that of the bulk material on the hydrophobic (hydrophilic) carbon surface. To glean information about the structure and mechanical properties of PB at the carbon interface, a residual layer (RL) adhering to the carbon surface, which was considered to be a model of "bound rubber layer", was obtained by rinsing the PB film with toluene. The density and thickness of the RLs were identical to those of the interfacial layer of the PB film. In accordance with the change in the density, normal stress of the RLs evaluated by atomic force microscopy was also dependent on the surface free energy: the RLs on the hydrophobic carbon were hard like glass, whereas those on the hydrophilic carbon were soft like rubber. Similarly, the wear test revealed that the RLs on the hydrophilic carbon could be peeled off by scratching under a certain stress, whereas the RLs on the hydrophobic carbons were resistant to scratching.

Mechanism of Spontaneous Blebbing Motion of an Oil-Water Interface: Elastic Stress Generated by a Lamellar-Lamellar Transition.

A quaternary system composed of surfactant, cosurfactant, oil, and water showing spontaneous motion of the oil-water interface under far-from-equilibrium condition is studied in order to understand nanometer-scale structures and their roles in spontaneous motion. The interfacial motion is characterized by the repetitive extension and retraction of spherical protrusions at the interface, i.e, blebbing motion. During the blebbing motion, elastic aggregates are accumulated, which were characterized as surfactant lamellar structures with mean repeat distances d of 25 to 40 nm. Still unclear is the relationship between the structure formation and the dynamics of the interfacial motion. In the present study, we find that a new lamellar structure with d larger than 80 nm is formed at the blebbing oil-water interface, while the resultant elastic aggregates, which are the one reported before, have a lamellar structure with smaller d (25 to 40 nm). Such transition of lamellar structures from the larger d to smaller d is induced by a penetration of surfactants from an aqueous phase into the aggregates. We propose a model in which elastic stress generated by the transition drives the blebbing motion at the interface. The present results explain the link between nanometer-scale transition of lamellar structure and millimeter-scale dynamics at an oil-water interface.

Membrane formation by preferential solvation of ions in mixture of water, 3-methylpyridine, and sodium tetraphenylborate.

The structure and dynamics of a ternary system composed of deuterium oxide (D2O), 3-methylpyridine (3MP), and sodium tetraphenylborate (NaBPh4) are investigated by means of small-angle neutron scattering (SANS) and neutron spin echo (NSE) techniques. In the SANS experiments, a structural phase transition is confirmed between a disordered-phase and an ordered-lamellar-phase upon variation of the composition and/or temperature of the mixture. The characteristic lengths of the structures is on the sub-micrometer scale. A dispersion relation of the structure is measured through NSE experiments, which shows that the relaxation rate follows a cubic relation with momentum transfer. This implies that the dynamics of the system are determined predominantly by membrane fluctuations. The present results indicate that 3MP-rich domains are microscopically separated from bulk water in the presence of NaBPh4, and that the layers behave as membranes. These results are interpreted that preferential solvation of salt in each solvent induces a microphase separation between the solvents, and the periodic structure of 3MP-rich domains is stabilized by the long-range electrostatic interaction arising from Na(+) ions in D2O-rich domains.

Neutron scattering studies on dynamics of lipid membranes.

Neutron scattering methods are powerful tools for the study of the structure and dynamics of lipid bilayers in length scales from sub Å to tens to hundreds nm and the time scales from sub ps to µs. These techniques also are nondestructive and, perhaps most importantly, require no additives to label samples. Because the neutron scattering intensities are very different for hydrogen- and deuterium-containing molecules, one can replace the hydrogen atoms in a molecule with deuterium to prepare on demand neutron scattering contrast without significantly altering the physical properties of the samples. Moreover, recent advances in neutron scattering techniques, membrane dynamics theories, analysis tools, and sample preparation technologies allow researchers to study various aspects of lipid bilayer dynamics. In this review, we focus on the dynamics of individual lipids and collective membrane dynamics as well as the dynamics of hydration water.

Quasi-elastic neutron scattering reveals the relationship between the dynamical behavior of phospholipid headgroups and hydration water.

The dynamics of hydration water (HW) in 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE) was investigated by means of quasi-elastic neutron scattering (QENS) and compared with those observed in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The headgroup dynamics of DMPE was investigated using a mixture of tail-deuterated DMPE and D2O, and the QENS profiles were interpreted as consisting of three modes. The fast mode comprised the rotation of hydrogen atoms in -NH3+ and -CH2- groups in the headgroup of DMPE, the medium-speed mode comprised fluctuations in the entire DMPE molecule, and the slow mode comprised fluctuations in the membrane. These interpretations were confirmed using molecular dynamics (MD) simulations. The HW dynamics analysis was performed on a tail-deuterated DMPE and H2O mixture. The QENS profiles were analyzed in terms of three modes: (1) a slow mode, identified as loosely bound HW in the DMPC membrane; (2) a medium-speed mode similar to free HW in the DMPC membrane; and (3) a fast mode, identified as rotational motion. The relaxation time for the fast mode was approximately six times shorter than that of rotational water in DMPC, consistent with the results of terahertz time-domain spectroscopy. The activation energy of medium-speed HW in DMPE differed from that of free HW in DMPC, suggesting the presence of different hydration states or hydrogen-bonded networks around the phosphocholine and phosphoethanolamine headgroups.

Water Fraction Dependence of the Aggregation Behavior of Hydrophobic Fluorescent Solutes in Water-Tetrahydrofuran.

This work investigates the water fraction dependence of the aggregation behavior of hydrophobic solutes in water-tetrahydrofuran (THF) and the elucidation of the role of THF using fluorescence microscopy, dynamic light scattering, neutron and X-ray scattering, and photoluminescence measurements. On the basis of the obtained results, the following model is proposed: hydrophobic molecules are molecularly dispersed in the low-water-content region (10-20 vol %), while they form mesoscopic particles upon increasing the water fraction to ∼30 vol %. This abrupt change is due to the composition fluctuation of the water-THF binary system to form hydrophobic areas in THF, followed by THF-rich droplets where hydrophobic solutes are incorporated and form loose aggregates. Further increasing the water content prompts the desolvation of THF, which decreases the particle size and generates tight aggregates of solute molecules. This model is consistent with the luminescence behavior of the solutes and will be helpful to control the aggregation state of hydrophobic solutes in various applications.

Formation of a multiscale aggregate structure through spontaneous blebbing of an interface.

The motion of an oil-water interface that mimics biological motility was investigated in a Hele-Shaw-like cell where elastic surfactant aggregates were formed at the oil-water interface. With the interfacial motion, millimeter-scale pillar structures composed of the aggregates were formed. The pillars grew downward in the aqueous phase, and the separations between pillars were roughly equal. Small-angle X-ray scattering using a microbeam X-ray revealed that these aggregates had nanometer-scale lamellar structures whose orientation correlated well with their location in the pillar structure. It is suggested that these hierarchical spatial structures are tailored by the spontaneous interfacial motion.

Aggregation of 1-dodecyl-3-methylimidazolium nitrate in water and benzene studied by SANS and 1H NMR.

Aggregation of imidazolium-based ionic liquid, C(12)mim(+)NO(3)(-), in both polar solvent of water and nonpolar solvent of benzene was elucidated by electrical conductivity, small-angle neutron scattering (SANS), and (1)H NMR measurements. The electrical conductivities of C(12)mim(+)NO(3)(-)-water solutions at 298 K as a function of ionic liquid concentration showed a break point at 8.4 mmol dm(-3) as a cmc. However, those of C(12)mim(+)NO(3)(-)-benzene solutions drastically increase in accordance with a cubic function of concentration, but without a break point. The SANS profiles of both aqueous and benzene solutions obviously differ from each other. The profiles of the aqueous solutions indicated the formation of polydisperse spherical micelles. Those of the benzene solutions revealed Ornstein-Zernike behavior. Thus, C(12)mim(+)NO(3)(-) forms clusters in the benzene solutions, but the shape of clusters is indefinite. On the basis of the (1)H NMR chemical shifts of the aqueous solutions, the effect of nitrate on the formation of micelles was discussed on a microscopic scale. Furthermore, the interactions among C(12)mim(+), NO(3)(-), and benzene molecules in the benzene solutions were considered according to the (1)H NMR data.

Gelation effect on the synthesis of high-aspect-ratio gold nanorods.

The growth process of high-aspect-ratio gold nanorods in gelled surfactant solution was studied. As for the application of gold nanorods, the surface plasmon is quite useful, whose absorption depends on their aspect ratio. Hence it is important to synthesize gold nanorods with favorable aspect ratio in high yield. For shorter nanorods (aspect ratio < -10), the synthesis and the growth mechanism have been studied well. For the longer nanorods (aspect ratio > -30), however, the growth mechanism has not yet been understood well, although it has been known that the high-aspect-ratio gold nanorods could be synthesized in high yield in gelled surfactant solution. In this paper, we studied the relationship between the growth process of high-aspect-ratio gold nanorods and the gelation of surfactant growth-solution. Small angle X-ray scattering (SAXS) revealed the microscopic feature of gelation as the structural transition of self-assembly of surfactant molecules from micellar to lamellar. These results will be helpful for better understanding on the growth mechanism of high-aspect-ratio gold nanorods.

Experimental Evidence of Slow Mode Water in the Vicinity of Poly(ethylene oxide) at Physiological Temperature.

In some synthetic polymers used for medical applications, hydration water in the vicinity of the polymer chains is known to play an important role in biocompatibility and is referred to as intermediate water. The crystallization of water below 0 °C observed during thermal analysis has been considered as evidence of the presence of intermediate water. However, the origin and physicochemical properties of intermediate water have not yet been elucidated. In this study, as a typical biocompatible polymer, poly(ethylene oxide) and its hydration water were investigated with the use of terahertz time-domain spectroscopy and quasi-elastic neutron scattering. The obtained results prove the existence of a significant amount of mobile water that interacts with the polymer chains even when the water content is low at physiological temperatures.

Local Dynamics of the Hydration Water and Poly(Methyl Methacrylate) Chains in PMMA Networks.

The dynamic behavior of water molecules and polymer chains in a hydrated poly(methyl methacrylate) (PMMA) matrix containing a small amount of water molecules was investigated. Water molecules have been widely recognized as plasticizers for activating the segmental motion of polymer chains owing to their ability to reduce the glass transition temperature. In this study, combined with judicious hydrogen/deuterium labeling, we conducted quasi-elastic neutron scattering (QENS) experiments on PMMA for its dry and hydrated states. Our results clearly indicate that the dynamics of hydrated polymer chains are accelerated, and that individual water molecules are slower than bulk water. It is therefore suggested that the hydration water affects the local motion of PMMA and activates the local relaxation process known as restricted rotation, which is widely accepted to be generally insensitive to changes in the microenvironment.

Quasi-Elastic Neutron Scattering Studies on Hydration Water in Phospholipid Membranes.

The dynamic behavior of hydration water in phospholipid membranes has been investigated to understand the relationship between water and biological molecules using various experimental techniques. Quasi-elastic neutron scattering (QENS) is an effective method for this purpose because the dynamic behaviors of both water and lipid molecules could be identified by using selective deuteration. In addition, the measurable ranges from the 10-12 to 10-9 s time scale and the 10-11 to 10-8 m length scale are suitable to investigate the slowing down of water molecules due to their interaction with lipid membranes. In this mini-review, QENS experiments on the dynamic behavior of hydration water molecules in neighboring phospholipid membranes are summarized.

Probing the adsorption of nonionic micelles on different-sized nanoparticles by scattering techniques.

The interaction of nanoparticles with surfactants is extensively used in a wide range of applications from enhancing colloidal stability to phase separation processes as well as in the synthesis of noble functional materials. The interaction is highly specific depending on the charged nature of the surfactant. In the case of nonionic surfactants, the micelles adsorb on the surface of nanoparticles. The adsorption of nonionic surfactant C12E10 as a function of surfactant concentration for two different sizes of anionic silica nanoparticles (16 and 27 nm) has been examined using dynamic light scattering (DLS) and small-angle neutron scattering (SANS). SANS measurements have been carried out under different contrast-matched conditions, where nanoparticles, as well as surfactant micelles, have been contrast-matched to the solvent. The adsorption of micelles is determined from the contrast-matched condition of silica nanoparticles with the solvent. SANS data under surfactant contrast-matched condition suggest that there is no modification in the structure and/or interaction of the silica nanoparticles in presence of nonionic micelles. The adsorption of micelles on nanoparticles is found to follow an exponential behavior with respect to the surfactant concentration. These results are consistent with the variation of hydrodynamic size of nanoparticle-surfactant system in DLS. The study on different-sized nanoparticles shows that the lower curvature enhances the packing fraction whereas the loss of surface-to-volume ratio suppresses the fraction of adsorbed micelles with the increase in the nanoparticle size. The adsorption coefficient has higher value for the larger size of the nanoparticles. In the mixed system of two sizes of nanoparticles, no preferential selectivity of micelle adsorption is observed.

Shear-induced liquid-crystalline phase transition behaviour of colloidal solutions of hydroxyapatite nanorod composites.

Liquid-crystalline (LC) bio-inspired materials based on colloidal nanoparticles with anisotropic morphologies such as sheets, plates, rods and fibers were used as functional materials. They show stimuli-responsive behaviour under mechanical force and in electric and magnetic fields. Understanding the effects of external stimuli on the structures of anisotropic colloidal particles is important for the development of highly ordered structures. Recently, we have developed stimuli-responsive hydroxyapatite (HAP)-based colloidal LC nanorods that are environmentally-friendly functional materials. In the present study, the ordering behaviour of HAP nanorod dispersions, which show LC states, has been examined using in situ small-angle neutron scattering and rheological measurements (Rheo-SANS) under shearing force. The structural analyses and dynamic viscosity observations provided detailed information about the effects of shear force on the structural changes of HAP nanorods in D2O dispersion. The present Rheo-SANS measurements unraveled three kinds of main effects of the shear force: the enhancement of interactions between the HAP nanorods, the alignment of HAP nanorods to the shear flow direction, and the formation and disruption of HAP nanorod assemblies. Simultaneous analyses of dynamic viscosity and structural changes revealed that the HAP nanorod dispersions exhibited distinctive rheological properties accompanied by their ordered structural changes.