The Role of Elasticity in the Anomalous Swelling of Polymer Thin Films in Density Fluctuating Supercritical Fluids

Advances in sample environments for neutron scattering for colloid and interface science

This review describes recent advances in sample environments across the full complement of applicable neutron scattering techniques to colloid and interface science. Temperature, pressure, flow, tensile testing, ultrasound, chemical reactions, IR/visible/UV light, confinement, humidity and electric and magnetic field application, as well as tandem X-ray methods, are all addressed. Consideration for material choices in sample environments and data acquisition methods are also covered as well as discussion of current and potential future use of machine learning and artificial intelligence.

REFLEX: a program for the analysis of specular X-ray and neutron reflectivity data

The use of X-ray and neutron reflectivity has been generalized worldwide for scientists who want to determine specific physical properties (such as electron-density profile, scattering-length density, roughness and thickness) of films less than 200 nm thick deposited on a substrate. This paper describes a freeware program named REFLEX, which is a standalone program dedicated to the simulation and analysis of X-ray and neutron reflectivity from multilayers. This program was first written two decades ago and has been constantly improved since, but never published until now. The latest version of REFLEX covers generalized types of calculation of reflectivity curves including both neutron and X-ray reflectivity. In the case of X-rays, the program can deal with both s and p polarization, which is quite important in the soft X-ray region where the two polarizations can yield different results. Neutron reflectivity is calculated within the framework of non-spin-polarized neutrons. REFLEX has also been designed to include any type of fluid (such as supercritical CO2) on top of the analysed film and includes corrections of the footprint effect for analysis on an absolute scale.

Novel Effects of Compressed CO2 Molecules on Structural Ordering and Charge Transport in Conjugated Poly(3-hexylthiophene) Thin Films

We report the effects of compressed CO₂ molecules as a novel plasticization agent for poly(3-hexylthiophene) (P3HT)-conjugated polymer thin films. In situ neutron reflectivity experiments demonstrated the excess sorption of CO₂ molecules in the P3HT thin films (about 40 nm in thickness) at low pressure (P = 8.2 MPa) under the isothermal condition of T = 36 °C, which is far below the polymer bulk melting point. The results proved that these CO₂ molecules accelerated the crystallization process of the polymer on the basis of ex situ grazing incidence X-ray diffraction measurements after drying the films via rapid depressurization to atmospheric pressure: both the out-of-plane lamellar ordering of the backbone chains and the intraplane π-π stacking of the side chains were significantly improved, when compared with those in the control P3HT films subjected to conventional thermal annealing (at T = 170 °C). Electrical measurements elucidated that the CO₂-annealed P3HT thin films exhibited enhanced charge carrier mobility along with decreased background charge carrier concentration and trap density compared with those in the thermally annealed counterpart. This is attributed to the CO₂-induced increase in polymer chain mobility that can drive the detrapping of molecular oxygen and healing of conformational defects in the polymer thin film. Given the universality of the excess sorption of CO₂ regardless of the type of polymers, the present findings suggest that CO₂ annealing near the critical point can be useful as a robust processing strategy for improving the structural and electrical characteristics of other semiconducting conjugated polymers and related systems such as polymer:fullerene bulk heterojunction films.

Anomalous long-range repulsion between silica surfaces induced by density inhomogeneities in supercritical ethanol

Anomalous long-range repulsion, extending over several micrometres, emerged between silica surfaces around the ridge of density fluctuations in supercritical ethanol at temperatures and pressures near the gas/liquid critical point (T(c) = 241 °C, P(c) = 6.14 MPa). Analysis shows that augmentation of ethanol density around silica surfaces in the presence of density fluctuations facilitates dissociation of silanol groups, leading to long-range electrostatic repulsion in the nonpolar medium.

Melt crystallization/dewetting of ultrathin PEO films via carbon dioxide annealing: the effects of polymer adsorbed layers

The effects of CO2 annealing on the melting and subsequent melt crystallization processes of spin-cast poly(ethylene oxide) (PEO) ultrathin films (20-100 nm in thickness) prepared on Si substrates were investigated. By using in situ neutron reflectivity, we found that all the PEO thin films show melting at a pressure as low as P = 2.9 MPa and at T = 48 °C which is below the bulk melting temperature (Tm). The films were then subjected to quick depressurization to atmospheric pressure, resulting in the non-equilibrium swollen state, and the melt crystallization (and/or dewetting) process was carried out in air via subsequent annealing at given temperatures below Tm. Detailed structural characterization using grazing incidence X-ray diffraction, atomic force microscopy, and polarized optical microscopy revealed two unique aspects of the CO2-treated PEO films: (i) a flat-on lamellar orientation, where the molecular chains stand normal to the film surface, is formed within the entire film regardless of the original film thickness and the annealing temperature; and (ii) the dewetting kinetics for the 20 nm thick film is much slower than that for the thicker films. The key to these phenomena is the formation of irreversibly adsorbed layers on the substrates during the CO2 annealing: the limited plasticization effect of CO2 at the polymer-substrate interface promotes polymer adsorption rather than melting. Here we explain the mechanisms of the melt crystallization and dewetting processes where the adsorbed layers play vital roles.

High-pressure cell for neutron reflectometry of supercritical and subcritical fluids at solid interfaces

A new high-pressure cell design for use in neutron reflectometry (NR) for pressures up to 50 MPa and a temperature range of 300-473 K is described. The cell design guides the neutron beam through the working crystal without passing through additional windows or the bulk fluid, which provides for a high neutron transmission, low scattering background, and low beam distortion. The o-ring seal is suitable for a wide range of subcritical and supercritical fluids and ensures high chemical and pressure stability. Wafers with a diameter of 5.08 cm (2 in.) and 5 mm or 10 mm thickness can be used with the cells, depending on the required pressure and momentum transfer range. The fluid volume in the sample cell is very small at about 0.1 ml, which minimizes scattering background and stored energy. The cell design and pressure setup for measurements with supercritical fluids are described. NR data are shown for silicon/silicon oxide and quartz wafers measured against air and subsequently within the high-pressure cell to demonstrate the neutron characteristics of the high-pressure cell. Neutron reflectivity data for supercritical CO(2) in contact with quartz and Si/SiO(2) wafers are also shown.

High pressure cell for neutron reflectivity measurements up to 2500 bar

The design of a high pressure (HP) cell for neutron reflectivity experiments is described. The cell can be used to study solid-liquid interfaces under pressures up to 2500 bar (250 MPa). The sample interface is based on a thick silicon block with an area of about 14 cm(2). This area is in contact with the sample solution which has a volume of only 6 cm(3). The sample solution is separated from the pressure transmitting medium, water, by a thin flexible polymer membrane. In addition, the HP cell can be temperature-controlled by a water bath in the range 5-75°C. By using an aluminum alloy as window material, the assembled HP cell provides a neutron transmission as high as 41%. The maximum angle of incidence that can be used in reflectivity experiments is 7.5°. The large accessible pressure range and the low required volume of the sample solution make this HP cell highly suitable for studying pressure-induced structural changes of interfacial proteins, supported lipid membranes, and, in general, biomolecular systems that are available in small quantities, only. To illustrate the performance of the HP cell, we present neutron reflectivity data of a protein adsorbate under high pressure and a lipid film which undergoes several phase transitions upon pressurization.

Pressure cell for investigations of solid-liquid interfaces by neutron reflectivity

We describe an apparatus for measuring scattering length density and structure of molecular layers at planar solid-liquid interfaces under high hydrostatic pressure conditions. The device is designed for in situ characterizations utilizing neutron reflectometry in the pressure range 0.1-100 MPa at temperatures between 5 and 60 °C. The pressure cell is constructed such that stratified molecular layers on crystalline substrates of silicon, quartz, or sapphire with a surface area of 28 cm(2) can be investigated against noncorrosive liquid phases. The large substrate surface area enables reflectivity to be measured down to 10(-5) (without background correction) and thus facilitates determination of the scattering length density profile across the interface as a function of applied load. Our current interest is on the stability of oligolamellar lipid coatings on silicon surfaces against aqueous phases as a function of applied hydrostatic pressure and temperature but the device can also be employed to probe the structure of any other solid-liquid interface.

Hydrate formation at the methane/water interface on the molecular scale

We report the nucleation process of methane hydrate on the molecular scale. A stationary planar interface separating methane gas and liquid water was studied by using in situ neutron reflectivity. We found that the angstrom-scale surface roughening is triggered as soon as the water phase contacts methane gas under the hydrate forming conditions. In addition, it was found that the microscopic surface structure remains unchanged until a macroscopic hydrate film is developed at the interface. We therefore postulate that the angstrom-scale surface roughening is attributed to the formation of microscopic hydrate "embryos" in a "dynamic equilibrium" manner.

Contact angle of water on polystyrene thin films: effects of CO(2) environment and film thickness

We examine the contact angle of water droplets on polystyrene (PS) thin films of varying thicknesses supported by silicon wafers under both air and pressurized carbon dioxide (CO2) environments. At 23 degrees C, the contact angle is found to increase upon increasing CO2 pressure in the vapor regime and then levels off in the liquid CO2 regime. A macroscopic model based on Young's equation and the geometric-mean method for interfacial tensions, and long-range van der Waals interactions, correctly predicts the trends and the magnitude of the contact angle dependence on pressure, although deviations occur at high CO2 activities. The contact angle was also found to depend on film thickness, h, when h was comparable to or smaller than 50 nm. Specifically, the contact angle decreases with decreasing PS film thickness. This behavior could be accounted for with the use of a model that incorporates the effects of film thickness, CO2 pressure, and the long-range van der Waals potential.

Nanometer-Scale Immobilization of Polysaccharides on Hydrophobic Polymer Plates in Supercritical Fluoroform/Water Emulsions

Hydrophilic polysaccharides such as dextran and hyaluronan were immobilized on a hydrophobic polystyrene (PSt) plate by a nanometer-scale surface penetration method in the emulsion of aqueous solutions in supercritical fluoroform (scCHF3). Since a supercritical fluid has high diffusiveness, water emulsions of polysaccharides can penetrate into the polymer surface. Dextran was surface-penetrated by two different methods: (1) the penetration of sucrose as a glucose donor and then the enzymatic polymerization to dextran near the surface catalyzed by dextransucrase, and (2) the direct penetration of dextran polymer into the PSt plate. The contact angle for water of the dextran-penetrated PSt plate was decreased to 78 degrees from 95 degrees of the untreated plate. The surface coverage and the penetration depth of polysaccharides could be obtained to be 10-30% and 10-20 nm, respectively, by X-ray photoelectron spectroscopy. These values could be controlled by the pressure of scCHF3. The transparency of the PSt dish did not change after the dextran penetration. Dextrans on the PSt plate could be elongated enzymatically by dextransucrase in the presence of sucrose as a glucose donor, and be detected by the enzyme-linked biotin-avidin assay. When anionic hyaluronan was surface-penetrated on the PSt plate instead of the neutral dextran, the plate showed the specific adhesion for human T-cells having hyaluronan receptors.

Supercritical fluid processing of polymer thin films: an X-ray study of molecular-level porosity

This paper reviews our recent experimental results that address the effects of solvent density inhomogeneities in supercritical carbon dioxide (scCO(2)) on polymer thin film processing. The key phenomenon is excess sorption of CO(2) molecules into polymer thin films even when the bulk miscibility with CO(2) is very poor. We have found that the amount of the excess sorption is attributed to the large density fluctuations in scCO(2) near the critical point. Further, taking advantage of the vitrification process of polymer chains through quick evaporation of CO(2), we can preserve the "expanded" structures as they are. The resultant films have large degree of molecular-level porosity that is very useful in producing coatings with low dielectric constants, enhanced adhesion, and metallization properties. These characteristics can be achieved in an environmentally "green" manner, without organic solvents, and are not specific to any class of polymers.

Anomalous sorption of supercritical fluids on polymer thin films

Unusual sorption has been reported in thin polymer films exposed to near-critical CO2. When the supercritical fluid approaches the critical point, the film appears to thicken, but it is not clear whether the film swells or there is an adsorption layer on the film surface. A combination of the gradient theory of inhomogeneous systems and the Sanchez-Lacombe equation of state has been used to investigate this phenomenon. It is shown analytically that surface adsorption on an attractive surface is proportional to the compressibility of the fluid. We have also investigated numerically the sorption of supercritical CO2 on poly(dimethylsiloxane) and polyisobutylene, and supercritical 1,1-difluoroethane on polystyrene. By calculating the Gibbs adsorption and adsorption layer thickness of the supercritical fluids, we found in all cases (different substrates, different supercritical fluids) that maximum adsorption occurs when the supercritical fluid is near its compressibility maximum.

Effect of density fluctuating supercritical carbon dioxide on polymer interfaces

We investigated an effect of CO2 sorption on the compatibility of immiscible polystyrene (PS) and polybutadiene (PB) bilayers by using in situ neutron reflectivity. By labeling either polymer with deuterium, we found that the excess CO2 molecules were adsorbed to both top PS and bottom PB layers when the bilayers were exposed to CO2 at the narrow T and P regime near the critical point of pure CO2. Furthermore, we clarified that this excess sorption of CO2 molecules increased the interfacial width between the layers up to 100 angstroms even near room temperature, while the interfacial width without CO2 exposure has been reported to be at most 40 A even at the highest temperature (T congruent with 175 degrees C).

Pore size engineering in mesoporous silicas using supercritical CO2

In this paper we investigate the use of supercritical carbon dioxide (sc-CO(2)) for synthesizing calcined mesoporous silicas with tunable pore sizes, wall thickness, and d spacings. Small angle neutron scattering was used to probe the controlled swelling of the triblock copolymer surfactant templating agents, P123 (PEO(20)PPO(69)PEO(20)), P85 (PEO(26)PPO(39)PEO(26)), and F127 (PEO(106)PPO(70)PEO(106)), as a function of CO(2) pressure. The transition from the liquid crystal phase to the calcined mesoporous silicas, formed upon condensation and drying, was also studied in detail. Powder X-ray diffraction, transmission electron microscopy, and nitrogen adsorption techniques were used to establish pore diameters, silica wall widths, and the hexagonal packing of the pores within the calcined silicas. Using a direct templating method, the diameters of mesopores and the spacing between the pores could be tuned with a high level of precision. The swelling process was observed to have no detrimental effects on the quality of silica formed, a distinct advantage over conventional swelling techniques, and all of the silicas synthesized in this study were highly ordered over distances of at least 2000 A.