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

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.

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.

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.

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.

Energy Decomposition Analysis of the Adhesive Interaction between an Epoxy Resin Layer and a Silica Surface

We investigate the adhesive interaction energy (ΔE_int) between an epoxy resin and a silica surface using pair interaction energy decomposition analysis (PIEDA), which decomposes ΔE_int into four components: electrostatic (ΔE_es), exchange repulsion (ΔE_ex), charge-transfer (ΔE_ct), and dispersion (ΔE_disp) energies based on quantum chemistry. Our previous study with PIEDA showed that synergistic effects of ΔE_es and ΔE_disp are critical at the interface between an epoxy resin fragment and a hydrophilic surface. The present study is designed to show in detail that the synergistic effects are significant at the interface between an epoxy layer model consisting of 20 epoxy monomers and a hydrophilic silica surface. The ratio of the dispersion energies to the total interaction energies of the layer model shows good agreement with experimental values, that is, the dispersion ratio of the work of adhesion (W_ad). The 20 epoxy molecules in the layer model are investigated individually to closely correlate the four decomposed energies with their structural features. Our energy-decomposition analyses show that H-bonding and OH-π interactions play important roles at the interface between an epoxy resin and a silica surface. PIEDA calculations for the epoxy layer model also show that the region 3.6 Å from the silica surface accounts for more than 99% of the total interaction energies.

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.

Theoretical and Experimental Investigation on the Moisture Sorption Kinetics of a PVA/LiCl Composite Membrane in a Dynamic Humidity Environment

The PVA/LiCl composite membrane is an important material with a selective permeation to moisture. In this study, the sorption kinetics characteristics of the PVA/LiCl composite membrane are investigated in a dynamic humidity environment. The sorption kinetics curves of the PVA/LiCl composite membranes with different LiCl contents were measured in the range from 10 to 90% relative humidity, and the kinetics characteristics of the sorption components of the total moisture uptake were analyzed with a triple-mode sorption model. The results show that the process of pooling adsorption occurs only when the amount of Henry absorption exceeds a threshold. With the increase in LiCl content, the threshold gradually decreases, indicating that introducing LiCl in the membrane mainly affects the process of pooling adsorption. In the desorption part of the kinetics curve, the amounts of Henry absorption and pooling adsorption gradually decrease, while the moisture adsorbed by the hydrophilic hydroxyls, i.e., the amount of Langmuir adsorption, is still largely retained in the membrane, resulting in the water-retaining performance in a low-humidity environment. It is noteworthy that in a high-humidity environment, the amounts of Henry absorption and Langmuir adsorption are almost evenly distributed in the film, while the amount of pooling adsorption near the surface is significantly higher than that at the center. The reason is that with the increase in humidity, the rate of pooling adsorption near the surface increases significantly, resulting in the majority of the moisture diffusing into the dense membrane clusters near the surface. The established theoretical method and model parameters can be adopted to predict the moisture sorption and desorption processes of the PVA/LiCl composite membrane in a dynamic humidity environment.