Pseudomorphic amorphization of three-dimensional superlattices through morphological transformation of nanocrystal building blocks.

Nanocrystal (NC) superlattices (SLs) have been widely studied as a new class of functional mesoscopic materials with collective physical properties. The arrangement of NCs in SLs governs the collective properties of SLs, and thus investigations of phenomena that can change the assembly of NC constituents are important. In this study, we investigated the dynamic evolution of NC arrangements in three-dimensional (3D) SLs, specifically the morphological transformation of NC constituents during the direct liquid-phase synthesis of 3D NC SLs. Electron microscopy and synchrotron-based in situ small angle X-ray scattering experiments revealed that the transformation of spherical Cu2S NCs in face-centred-cubic 3D NC SLs into anisotropic disk-shaped NCs collapsed the original ordered close-packed structure. The random crystallographic orientation of spherical Cu2S NCs in starting SLs also contributed to the complete disordering of the NC array via random-direction anisotropic growth of NCs. This work demonstrates that an understanding of the anisotropic growth kinetics of NCs in the post-synthesis modulation of NC SLs is important for tuning NC array structures.

Time-resolved AUSAXS at BL28XU at SPring-8.

An anomalous ultra-small-angle X-ray scattering (AUSAXS) system has been constructed at BL28XU at SPring-8 for time-resolved AUSAXS experiments. The path length was extended to 9.1 m and a minimum of q = 0.0069 nm-1 was attained. Scattering profiles at 0.0069 to 0.3 nm-1 were successfully obtained at 17 different X-ray energies in 30 s using the BL28XU optical setup, which enables adjustment of the energy of the incident X-rays quickly without the beam position drifting. Time-resolved measurements were conducted to investigate changes in the structure of zinc compounds in poly(styrene-ran-butadiene) rubber during vulcanization. A change in energy dependence of the scattered intensity with time was found during vulcanization, suggesting the transformation of zinc in the reaction.

A high-temperature and high-pressure cell for in situ visualization of reaction processes by neutron imaging.

This study developed a high-temperature and high-pressure (HTHP) cell for in situ neutron imaging of hydrothermal reactions. The cell's maximum temperature and pressure were 500 °C and 50 MPa, respectively, and its vessel for observing reactions comprised SUS316 stainless steel. Neutron transmission images were obtained to observe the behavior of sub- and supercritical water and the decomposition of two plastics (polypropylene and polyethylene) at HTHP. The images showed that water's density and phase changed with temperature and pressure, affecting neutron transmission (and thus image brightness). The plastics began to melt and change shape at 150-200 °C, and they decomposed at 500 °C and 20 MPa. This study provides a basis for future research using the HTHP cell to examine various reactions such as the decomposition of biomass samples, the reforming of heavy oil, and the synthesis of nano-materials using sub- and supercritical water.

Neutron Reflectometry Analysis of Condensed Water Layer Formation at a Solid Interface of Epoxy Resins Under High Humidity.

Water absorbed by epoxy resins from a humid atmosphere considerably influences their structure and properties. Examining the effects of absorbed water on epoxy resins at their interfaces with solid substrates is crucial because of their adhesive applications in various fields. The spatial distribution of absorbed water in epoxy resin thin films under high humidity was investigated in this study by neutron reflectometry. Water molecules were found to accumulate at the SiO2/epoxy resin interface after exposure at a relative humidity of 85% for 8 h. The formation of an ∼1-nm-thick condensed water layer was observed, and the thickness of this layer varied with curing conditions of epoxy systems. Furthermore, water accumulation at the interface was noted to be affected by high-temperature and high-humidity environments. The formation of the condensed water layer is presumed to be related to the features of the polymer layer near the interface. The construction of the interface layer of epoxy resin would be affected by the interface constraint effect on the cross-linked polymer chain during the curing reaction. This study provides essential information for understanding the factors influencing the accumulation of water at the interface in epoxy resins. In practical applications, the process of improving the construction of epoxy resins near the interface would be a reasonable solution to resist water accumulation in the interface.

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.

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.

Kinetics of the interfacial curing reaction for an epoxy-amine mixture.

A better understanding of the chemical reaction between epoxy and amine compounds at a solid interface is crucial for the design and fabrication of materials with appropriate adhesive strength. Here, we examined the curing reaction kinetics of epoxy phenol novolac and 4,4'-diaminodiphenyl sulfone at the outermost interface using sum-frequency generation spectroscopy, and X-ray and neutron reflectivity in conjunction with a full atomistic molecular dynamics simulation. The reaction rate constant was much larger at the quartz interface than in the bulk. While the apparent activation energy at the quartz interface obtained from an Arrhenius plot was almost identical to the bulk value, the frequency factor at the quartz interface was greater than that in the bulk. These results could be explained in terms of the densification and orientation of reactants at the interface, facilitating the encounter of the reactants present.

Investigation of Interfacial Water Accumulation between Polypropylene Thin Film and Si Substrate by Neutron Reflectivity.

We performed neutron reflectivity (NR) measurements of isotactic polypropylene (PP) thin films deposited on a Si substrate at the saturated vapor pressure of deuterated water to investigate interfacial water accumulation between the PP and metal surfaces in PP-based polymer/inorganic filler nanocomposites and metal/resin bonding materials. The PP thin films prepared on a Si substrate by a spin-coating technique were adequate as a model system for the PP/metal interface in these materials. A water-rich layer with a maximum water concentration of 0.5, which was considerably higher than those reported in previous studies of organic/inorganic interfaces, was observed within a width of approximately 3 nm at the interface under saturated vapor conditions. This could be attributed to the weak interaction between the PP thin film and the Si substrate. The pathway of moisture transport to the interfacial region was along the interface rather than through the PP film because the hydrophobic PP thin film does not entirely swell with water vapor.

In Situ Neutron Reflectometry Analysis of Interfacial Structure Formation between Phenolic Resin and Silica during Curing.

The structural formation mechanism of phenolic resin-silica interfaces was investigated in situ by neutron reflectometry during curing. There was a 4 nm thick novolac resin adsorption layer on the silica surface before curing. The curing reaction of the novolac resin with hexamethylenetetramine (HMTA) increased the coherent neutron scattering length density of the resin due to the cure shrinkage accompanied by the volatilization of ammonia, which is a byproduct of HMTA decomposition. As curing proceeded at 180 °C, the thickness of the bulk layer increased despite the cure shrinkage, and the thickness of the interfacial layer decreased from 4 to 1 nm. This is attributed to the diffusion of decomposed HMTA fragments generated in the bulk layer into the interfacial novolac adsorption layer during diffusion throughout the bulk layer, incorporating the upper part of the interfacial layer reacting with the fragment into the bulk layer. On the other hand, the fragments could not diffuse into the tightly bound immobile segments of novolac resin in direct contact with the silica surface, retaining the 1-2 nm thick interfacial layer in the cured resin. This structural formation mechanism caused interfacial cross-link inhomogeneity in the cured resin on the silica surface.

Layered Structure in the Crystalline Adsorption Layer and the Leaching Process of Poly(vinyl alcohol) Revealed by Neutron Reflectivity.

We investigated the structure of the crystalline adsorption layer of poly(vinyl alcohol) (PVA) in hot water by neutron reflectivity in two cases: when the adsorption layer is exposed on the substrate by leaching the upper bulk layer and when it is deeply embedded between a relatively thick PVA film and substrate. In both cases, the PVA adsorption layer consists of three layers on the Si substrate. The bottom layer, consisting of amorphous chains that are strongly constrained on the substrate, is not swollen even in hot water at 90 °C. The middle layer, consisting of amorphous chains that are much more mobile compared with those in the bottom layer, has no freedom to assume a crystalline form. Only the molecular chains in the top layer are crystallizable in the adsorption layer, leading to a heterogeneous layered structure in the film thickness direction. This layered structure is attributed to the crystallizable chains of PVA during the formation of the adsorption layer driven by hydrogen bonding. However, the structure and dynamics in the adsorption layer may differ in both cases because the molecular chains in the vicinity of the surface seem to be affected by surface effects even in the adsorption layer.

Neutron Reflectivity on the Mobile Surface and Immobile Interfacial Layers in the Poly(vinyl acetate) Adsorption Layer on a Si Substrate with Deuterated Toluene Vapor-Induced Swelling.

We investigated the polymer chain dynamics in a 2-3 nm thick poly(vinyl acetate) (PVAc) adsorption layer on a Si substrate with a native oxide layer via neutron reflectometry combined with toluene vapor-induced swelling. We can investigate the polymer chain dynamics difference in the film thickness direction by the difference in the degree of swelling of the polymer layers detected by neutron reflectometry. The mobility of the polymer chains depends on the distance from the substrate. The results elucidated that the interfacial layer with a thickness of approximately 1 nm did not swell at all with toluene vapor, which is a solvent for PVAc. Meanwhile, the surface layer excessively swells with toluene vapor compared to the bulk. This indicates that the polymer chain within the interfacial region is immobilized by the substrate through hydrogen-bonding interaction, but in the surface region, the surface effect overcomes this interfacial interaction. We concluded that the polymer chains in the adsorption layer are either strongly constrained to the substrate, owing to hydrogen bonding, or more mobile than the bulk, owing to the surface effect.

Gas-flow humidity control system for neutron reflectivity measurements.

We developed a novel humidity control system for neutron reflectivity measurements based on the two-way gas-flow method that can generate up to 85% relative humidity (RH) within a temperature range of 5 °C-85 °C. The system consists of a gas-flow-type humidity generator and a thermostatic sample chamber, each of which can independently control the temperature. The key features include rapid humidity response and long stable operation time. The humidity reaches equilibrium within 2 to 5 min during the humidity change, and the system exhibited acceptable stability over a three-day, nonstop experimental measurement duration, with a precision of ±1% RH at 85 °C and 85% RH. The sample chamber is capable of measuring substrate samples with dimensions of up to 2-in. in diameter and 5-mm in thickness. We demonstrate the reflectivity data measured at a pulsed neutron facility, MLF BL17, in the Japan Proton Accelerator Research Complex. The combined use of this system with neutrons permits in situ, time-resolved studies of the swelling process of polyvinyl alcohol and adhesive materials.

Fine-Structure Analysis of Perhydropolysilazane-Derived Nano Layers in Deep-Buried Condition Using Polarized Neutron Reflectometry.

A large background scattering originating from the sample matrix is a major obstacle for fine-structure analysis of a nanometric layer buried in a bulk material. As polarization analysis can decrease undesired scattering in a neutron reflectivity (NR) profile, we performed NR experiments with polarization analysis on a polypropylene (PP)/perhydropolysilazane-derived SiO2 (PDS)/Si substrate sample, having a deep-buried layer of SiO2 to elucidate the fine structure of the nano-PDS layer. This method offers unique possibilities for increasing the amplitude of the Kiessig fringes in the higher scattering vector (Qz) region of the NR profiles in the sample by decreasing the undesired background scattering. Fitting and Fourier transform analysis results of the NR data indicated that the synthesized PDS layer remained between the PP plate and Si substrate with a thickness of approximately 109 Å. Furthermore, the scattering length density of the PDS layer, obtained from the background subtracted data appeared to be more accurate than that obtained from the raw data. Although the density of the PDS layer was lower than that of natural SiO2, the PDS thin layer had adequate mechanical strength to maintain a uniform PDS layer in the depth-direction under the deep-buried condition.

Detailed Structural Study on the Poly(vinyl alcohol) Adsorption Layers on a Si Substrate with Solvent Vapor-Induced Swelling.

We investigated in detail the structures in the poly(vinyl alcohol) (PVA) adsorption layers on a Si substrate, which remained on the substrate after immersing the relatively thick 30-50 nm films in hot water, by neutron reflectometry under humid conditions. For the PVA with a degree of saponification exceeding 98 mol %, the adsorption layer exhibits a three-layered structure in the thickness direction. The bottom layer is considered to be the so-called inner adsorption layer that is not fully swollen with water vapor. This may be because the polymer chains in the inner adsorption layer are strongly constrained onto the substrate, which inhibits water vapor penetration. The polymer chains in this layer have many contact points to the substrate via the hydrogen bonding between the hydroxyl groups in the polymer chain and the silanol groups on the surface of the Si substrate and consequently exhibit extremely slow dynamics. Therefore, it is inferred that the bottom layer is fully amorphous. Furthermore, we consider the middle layer to be somewhat amorphous because parts of the molecular chains are pinned below the interface between the middle and bottom layers. The molecular chains in the top layer become more mobile and ordered, owing to the large distance from the strongly constrained bottom layer; therefore, they exhibit a much lower degree of swelling compared to the middle amorphous layer. Meanwhile, for the PVA with a much lower degree of saponification, the adsorption layer structure consists of the two-layers. The bottom layer forms the inner adsorption layer that moderately swells with water vapor because the polymer chains have few contact points to the substrate. The molecular chains in the middle layer, therefore, are somewhat crystallizable because of this weak constraint.

Elucidation of a Heterogeneous Layered Structure in the Thickness Direction of Poly(vinyl alcohol) Films with Solvent Vapor-Induced Swelling.

We investigated the swelling behaviors of poly(vinyl alcohol) (PVA) films deposited on Si wafers with water vapor, which is a good solvent for PVA for elucidating structural and dynamical heterogeneities in the film thickness direction. Using deuterated water vapor, structural and dynamical differences in the thickness direction can be detected easily as different degrees of swelling in the thickness direction by neutron reflectivity. Consequently, the PVA film with a degree of saponification exceeding 98 mol % exhibits a three-layered structure in the thickness direction. It is considered that an adsorption layer consisting of molecular chains that are strongly adsorbed onto the solid substrate is formed at the interface with the substrate, which is not swollen with water vapor compared with the bulk-like layer above it. The adsorption layer is considered to exhibit significantly slower dynamics than the bulk. Furthermore, a surface layer that swells excessively compared with the underneath bulk-like layer is found. This excess swelling of the surface layer may be related to a higher mobility of the molecular chains or lower crystallinity at the surface region compared to the underneath bulk-like layer. Meanwhile, for the PVA film with a much lower degree of saponification, a thin layer with a slightly lower degree of swelling than the bulk-like layer above it can be detected at the interface between the film and substrate only under a high humidity condition. This layer is considered to be the adsorption layer composed of molecular chains loosely adsorbed onto the Si substrate.

Effect of Preferential Orientation of Lamellae in the Interfacial Region between a Block Copolymer-based Pressure-Sensitive Adhesive and a Solid Substrate on the Peel Strength.

We have investigated the relationship between the peel strength of a block copolymer-based pressure-sensitive adhesive comprising of poly(methyl methacrylate) (PMMA) and poly(n-butyl acrylate) (PnBA) components from the substrate and the microdomain orientations in the interfacial region between the adhesive and the substrate. For the PMMA substrate, the PMMA component in the adhesive with a strong affinity for the substrate is attached to the surface of the substrate during an aging process of the sample at 140 °C. Next, the PMMA layer adjacent to the substrate surface is overlaid with a PnBA layer, which gets covalently connected, resulting in the horizontal alignment of the lamellae in the interfacial region. The peel strength of the adhesive substantially increases during aging at 140 °C, which takes the same time as the completion of the horizontally oriented lamellar structure. However, in the case of the polystyrene (PS) substrate, both the components in the adhesive repel the substrate, leading to the formation of the vertically oriented lamellar structure. As a result, the peel strength of the adhesive with respect to its PS substrate does not entirely increase on aging. It is suggested that the peel strength of the adhesive is highly correlated with the interfacial energy between the adhesive and substrate, which can be estimated from the microdomain orientation in the interfacial region.

Clarification of cross-linkage structure in boric acid doped poly(vinyl alcohol) and its model compound as studied by an organized combination of X-ray single-crystal structure analysis, Raman spectroscopy, and density functional theoretical calculation.

When boric acid (BA) is added to poly(vinyl alcohol) (PVA), a chemical reaction occurs to form the cross-linkages between the amorphous PVA chains. The local structural change caused by this reaction has been clarified concretely from the microscopic level on the basis of the X-ray-analyzed crystal structure, Raman spectra, and ab initio density functional theory using a model compound produced by the reaction between pentanediol (PENT) and boric acid (PENT-BA). The PENT-BA compound was found to take the TT and TG conformations in the methylene segmental parts depending on the stereoregularity of the PENT molecule itself, meso and racemo configurations, respectively. These two conformations give the Raman bands at the different positions. By comparison of the Raman spectra between the PVA-BA and PENT-BA model compounds, the local structures of PVA chains connected to BA molecules have been derived concretely: the syndiotactic PVA parts in the amorphous region form the TG-type ring structure with the 3-coordinate boron atom, where T and G are trans and gauche conformers, respectively. On the other hand, the isotactic PVA part takes the TT conformation when it forms a ring with boron atom. The thus-created rings are hydrogen-bonded to form a dimer, which plays a role as cross-linkage between the neighboring PVA chain segments in the amorphous region.

Perpendicular orientation of sub-10 nm channels in polystyrene-b-poly(4-hydroxyl styrene)/PEG oligomer blend thin films.

Vertically oriented cylinders via the directional coalescence of the spheres embedded in the blend thin films from polystyrene-b-poly(4-hydroxyl styrene) (PS-b-PHS) and PEG induced by solvent annealing were achieved. Removal of PEG water led to the formation of nanochannels throughout the films. The diameter of these channels could be as small as 9 nm which might enhance the applicability of the nano-porous films as size-selective membranes and controllable drug delivery systems for the objects less than 10 nm.

Thermal expansion behavior of ultrathin polymer films supported on silicon substrate.

The thermal expansion behavior of polystyrene (PS) thin films was investigated using x-ray reflectivity, focusing on ultrathin films below 10 nm. It was found that the glass transition temperature T(g) decreases with thickness as reported by many researchers while it is almost independent of thickness and constant at 354 K for films below approximately 10 nm. The thickness dependence of T(g) was well reproduced by a two-layer model consisting of a mobile surface layer with T(g) of 354.5 K and a bulklike layer with T(g) of 373 K ( =bulk T(g) ), suggesting that the so-called immobile dead layer near the substrate is negligible or very thin in this system. This surface T(g) of 354 K was confirmed by the relaxation of surface roughness of as-deposited films at about 354 K. It was also found that the thermal expansivity decreases with thickness in the glassy state as well as in the molten state while the reduction is smaller in the molten state.