Control of moisture at buried polymer/alumina interfaces through substrate surface modification

Moisture absorption in poly(4-tert-butoxycarbonyloxystyrene) (PBOCSt) films supported on Al(2)O(3) sputter coated silicon wafers is measured using neutron and X-ray reflectivity. Accumulation of water at the interface during moisture exposure results in an apparent film-thickness-dependent swelling for ultrathin PBOCSt films. The swelling of a film on Al(2)O(3) is less than the swelling of a film of the same thickness on SiO(x) for films thinner than 20 nm. This is due to comparatively less moisture accumulation at the Al(2)O(3)/PBOCSt interface. A simple, zero adjustable parameter model consisting of a fixed water-rich layer at the interface and bulk swelling through the remainder of the film describes the thickness-dependent swelling quantitatively. The influence of four different Al(2)O(3) surface treatments on the moisture distribution within PBOCSt films was examined: bare Al(2)O(3), tert-butylphosphonic acid, phenylphosphonic acid, and n-octyltrichlorosilane. Both the phenyl and the octyl surface treatments reduce the accumulation of water at the polymer/substrate interface. The tert-butyl treatment does not reduce the interfacial water concentration, presumably due to insufficient surface coverage.     

On the origins of sudden adhesion loss at a critical relative humidity: examination of bulk and interfacial contributions

The origins for abrupt adhesion loss at a critical relative humidity (RH) for polymeric adhesives bonded to inorganic surfaces have been explored using a model poly(methyl methacrylate) (PMMA) film on glass. The interfacial and bulk water concentrations within the polymer film as a function of D 2O partial pressure were quantified using neutron reflectivity. Adhesion strength of these PMMA/SiO 2 interfaces under the same conditions was quantified using a shaft loaded blister test. A drop in adhesion strength was observed at a critical RH, and at this same RH, a discontinuity in the bulk moisture concentration occurred. The moisture concentration near the interface was higher than that in the bulk PMMA, and at the critical RH, the breadth of the interfacial water concentration distribution as a function of distance from the SiO 2/PMMA interface increased dramatically. We propose a mechanism for loss of adhesion at a critical RH based upon the interplay between bulk swelling induced stress and weakening of the interfacial bond by moisture accumulation at the PMMA/SiO 2 interface.     

Reversible activation of diblock copolymer monolayers at the interface by pH modulation, 2: Membrane interactions at the solid/liquid interface

A monolayer of the pH-responsive poly[2-(dimethylamino)ethyl methacrylate-block-methyl methacrylate] diblock copolymer [PDMAEMA-PMMA] was transferred from the air/water interface to a silicon substrate for evaluation as a tunable interlayer between biological material and solid substrates. Specular neutron reflectivity experiments revealed that the weak polyelectrolyte PDMAEMA chains at the solid/liquid interface can be reversibly activated by pH modulation. The thickness, scattering length density, and surface roughness of the polymer film can be systematically controlled by pH titration. As a simple model of plasma membranes, a lipid bilayer was deposited onto the polymer film. The membrane-substrate interaction was characterized by neutron reflectivity experiments, demonstrating that the membrane-substrate distance could be reversibly regulated by pH titration. These results confirm the potential of stimuli-responsive polymers for precise control of cell-surface interactions.     

Phase behavior in thin films of cylinder-forming ABA block copolymers: experiments

We experimentally establish a phase diagram of thin films of concentrated solutions of a cylinder forming polystyrene-block-polybutadiene-block-polystyrene triblock copolymer in chloroform. During annealing the film forms islands and holes with energetically favored values of film thickness. The thin film structure depends on the local thickness of the film and the polymer concentration. Typically, at a thickness close to a favored film thickness parallel orientation of cylinders is observed, while perpendicular orientation is formed at an intermediate film thickness. At high polymer concentration the cylindrical microdomains reconstruct to a perforated lamella structure. Deviations from the bulk structure, such as the perforated lamella and a wetting layer are stabilized in films thinner than approximately 1.5 domain spacings.     

Density profile of spin cast polymethylmethacrylate thin films

The density profiles of polymethylmethacrylate (PMMA) thin films on silicon (111) single crystal wafers were investigated via neutron reflectivity measurements. Films were prepared by spin casting PMMA onto silicon wafers from o-xylene solution followed by annealing under vacuum at 90°C for 5 h. A ～45 Å thick layer at the free polymer surface was observed in the as-prepared samples that has a density about half the value of bulk PMMA. After heating above 110°C, this diffuse layer disappeared and the thin film density profile was transformed to one with a sharp free polymer surface. This transition was found to be irreversible. © 1994 John Wiley & Sons, Inc.     

Noninteracting versus interacting poly(N-isopropylacrylamide)-surfactant mixtures at the air-water interface

Two polymer-surfactant mixtures have been studied at the air-water interface using neutron reflectivity and surface tension techniques. For the noninteracting system poly(N-isopropylacrylamide) (PNIPAM)/octaethyleneglycol mono n-decyl ether (C10E8), the adsorption behavior is competitive and driven purely by surface pressure (pi). When pi(polymer) > pi(surfactant), the surface layer consists of almost pure polymer, and for pi(polymer) < pi(surfactant), the polymer is displaced from the surface by the increasing pressure of the surfactant. Beyond the CMC, the polymer is completely displaced from the surface. For the interacting system PNIPAM/sodium dodecyl sulfate (SDS) where the two species interact strongly in the bulk beyond the critical aggregation concentration (CAC), the surface behavior is more original. Earlier neutron reflectivity studies investigated PNIPAM adsorption behavior where the SDS was contrast-matched to the solvent. In the present study, complementary measurements of SDS adsorption where PNIPAM is contrast-matched to the solvent give a complete view of the surface composition of the mixed system. At a constant polymer concentration, with increasing SDS, three main regimes are obtained. For C(SDS) < CAC, adsorption is governed by simple competition and PNIPAM is predominant at the interface. At intermediate SDS concentration (CAC < C(SDS) < x2, where x2 indicates the predominance of free SDS micelles), interfacial behavior is governed by bulk polymer-surfactant interaction. Adsorbed polymer is displaced from the interface to form PNIPAM-SDS complex in the bulk. SDS adsorption remains weak since most of the SDS molecules are used to form bulk polymer-surfactant aggregates. Further increase in SDS concentration results in continued displacement of PNIPAM and an abrupt increase in SDS adsorption. This is a result of saturation of bulk polymer chain with adsorbed micelles. Interestingly, beyond x2, PNIPAM is not completely displaced from the surface. A mixed PNIPAM-SDS adsorbed layer with enhanced packing of the SDS monolayer is formed.     

Adsorption of beta-hairpin peptides on the surface of water: a neutron reflection study

Neutron reflectivity has been used to determine the thickness and surface coverage of monolayers of two 14-residue beta-hairpin peptides adsorbed at the air/water interface. The peptides differed only in that one was labeled with a fluorophore, while the other was not. The neutron reflection measurements were mainly made in null reflecting water, NRW, containing 8.1% D(2)O. Under this isotopic contrast the water is invisible to neutrons and the specular signal was then only from the peptide layer. At the highest concentration of ca. 4 microg/mL studied, the area per peptide molecule (A) was found to be 230 +/- 10 and 210 +/- 10 A(2) for the peptides with and without a BODIPY-based fluorophore, respectively. The thickness of the peptide layers was about 10 A for a Gaussian distribution. With decreasing bulk peptide concentration, both surface excess and layer thickness showed a steady trend of decrease. While the neutron results clearly indicate structural changes within the peptide monolayers with increasing bulk concentration, the outstanding structural feature is the formation of rather uniform peptide layers, consistent with the structural characteristics typical of beta-strand peptide conformations. These structural features are well supported by the parallel measurements of the adsorbed layers in D(2)O. With this isotopic contrast the neutron reflectivity provides an estimate about the extent of immersion of the peptide layers into water. The results strongly suggest that the 14-mer peptide monolayers were fully afloat on the surface of water, with only the carboxy groups on Glu residues hydrated.     

Nonsolvents cause swelling at the interface with poly(methyl methacrylate) films

Density profiles of a perdeuterated poly(methyl methacrylate) (dPMMA) film spin-coated on a substrate in water, hexane, and methanol, which are "nonsolvents" for dPMMA, were examined along the direction normal to the interface by specular neutron reflectivity (NR). The interfaces of dPMMA with the liquids were diffuse in comparison with the pristine interface with air; the interfacial width with water was thicker than that with hexane. Interestingly, in water, the dPMMA film was composed of a swollen layer and the interior region, which also contained water, in addition to the diffused layer. The interface of dPMMA with hexane was sharper than that with water. Although there were slight indications of a swollen layer for the dPMMA in hexane, the solvent molecules did not penetrate significantly into the film. On the other hand, in methanol, the whole region of the dPMMA film was strikingly swollen. To conserve mass, the swelling of the film by the nonsolvents is accompanied by an increase in the film thickness. The change in the film thickness estimated by NR was in excellent accord with the results of direct observations using atomic force microscopy (AFM). The modulus of dPMMA in the vicinity of the interfaces with liquids was also examined on the basis of force-distance curves measured by AFM. The modulus decreased closer to the outermost region of the film. The extent to which the modulus decreased in the interfacial region was consistent with the amount of liquid sorbed into the film.     

Fabrication of metal nanoparticle monolayers on amphiphilic poly(amido amine) dendrimer Langmuir films

A newly designed 1.5th generation poly(amido amine) dendrimer with an azacrown core, hexylene spacers, and octyl terminals was spread on gold nanoparticle (Au-NP) suspension. The surface pressure-area isothermal curves indicated that the molecular area of dendrimer on Au-NP suspension was significantly smaller than that on water, indicating the formation of dendrimer/Au-NP composites. The dendrimer Langmuir films on the Au-NP suspension were transferred to copper grids at various surface pressures and observed by transmission electron microscopy. The transferred films consisted of a fractal-like network of nanoparticles at low surface pressure and of a defect-rich monolayer of nanoparticles at high surface pressure. From these results, it was suggested that the dendrimers bind Au-NPs, and dendrimer/Au-NP composites formed networks or monolayers at the interface. From the intensity decrease of the Au plasmon band of Au-NP suspension after the formation of composite, it was estimated that some (approximately 14) dendrimer molecules bind to one Au-NP. Furthermore, neutron reflectivity at the air/suspension interface and X-ray reflectivity of the film transferred on a silicon substrate revealed that the dendrimer molecules are localized on the upper-half surface of Au-NP. Metal affinity of azacrown, flexibility of hexylene spacer, and amphiphilicity of dendrimer with octyl terminals played important roles for the formation of dendrimer/Au-NP hybrid films. The present investigation proposed a new method to fabricate the self-assembled functional polymer/nanoparticle hybrid film.     

Thin film order of symmetric diblock copolymers

The ordering in thin films of symmetric diblock copolymers of polystyrene and poly(methyl methacrylate) has been investigated by neutron reflectivity as a function of film thickness and temperature. The order-disorder transition in the thin films was found to lose its first order character in that the transition occurs in a continuous manner without the correlation length becoming infinite. In addition, a transition from a partially to fully ordered state was observed which was fully reversible. This transition depended in a power law manner on the film thickness and extrapolates to the bulk order-disorder transition temperature for thick films.     

Collapse transition in thin films of poly(methoxydiethylenglycol acrylate)

The thermal behavior of poly(methoxydiethylenglycol acrylate) (PMDEGA) is studied in thin hydrogel films on solid supports and is compared with the behavior in aqueous solution. The PMDEGA hydrogel film thickness is varied from 2 to 422?nm. Initially, these films are homogenous, as measured with optical microscopy, atomic force microscopy, X-ray reflectivity, and grazing-incidence small-angle X-ray scattering (GISAXS). However, they tend to de-wet when stored under ambient conditions. Along the surface normal, no long-ranged correlations between substrate and film surface are detected with GISAXS, due to the high mobility of the polymer at room temperature. The swelling of the hydrogel films as a function of the water vapor pressure and the temperature are probed for saturated water vapor pressures between 2,380 and 3,170?Pa. While the swelling capability is found to increase with water vapor pressure, swelling in dependence on the temperature revealed a collapse phase transition of a lower critical solution temperature type. The transition temperature decreases from 40.6?°C to 36.6?°C with increasing film thickness, but is independent of the thickness for very thin films below a thickness of 40?nm. The observed transition temperature range compares well with the cloud points observed in dilute (0.1?wt.%) and semi-dilute (5?wt.%) solution which decrease from 45?°C to 39?°C with increasing concentration.     

Formation of n-alkane layers at the vapor/water interface

We present a study on the initial wetting behaviors of two low molecular weight alkanes, heptane and octane, at the vapor/water interface using both neutron and X-ray reflectometry. Combined X-ray and neutron reflectivity studies data showed that a uniform film, which has never been reported, was formed continuously at 25 degrees C. As the adsorptive deposition continued, each adsorbed film was saturated at a specific equilibrium thickness: 48 and 36 A for deuterated heptane and octane, respectively, and 21 A for hydrogenated octane. The thickness of the adsorbed layer measured by neutron reflectivity is in agreement with that measured using X-ray reflectivity. Our observations of continuous and saturated adsorption behaviors are analyzed qualitatively using a kinetic adsorption model.     

Water-barrier properties of mixed bis[trimethoxysilylpropyl]amine and vinyltriacetoxysilane films

X-ray and neutron reflectivity were employed to elucidate the morphologies of bis[trimethoxysilylpropyl]amine silane (A) and vinyltriacetoxysilane (V) mixed films on Si wafers at different A/V ratios, and the response of these films to saturated water vapor. Due to its insensitivity to chemical composition, X-ray reflectivity was used to assess the film density, whereas neutron reflectivity was used to probe water absorption and chemical change on exposure to water. NMR was employed to determine the reaction mechanism in neat AV mixtures and stoichiometry of the initial reaction. X-ray reflectivity reveals about 30% void volume in the films with the least void volume detected near stoichiometry. Grazing incidence small-angle scattering (GISAXS) shows that the void volume is at the molecular level, with no distinct pores. Neutron reflectivity on D2O-conditioned films shows that silane film is not an effective water barrier with about 30 vol % water being absorbed with only a slight thickness increase. Most water is physically absorbed in the void space with the least amount being absorbed near the stoichiometric A/V ratio. The scattering length density of the films almost returns to the virgin state after re-dry following D2O vapor exposure. The film thickness, however, remains at the water-vapor-conditioned state. The slight increase in scattering length density and irreversible thickness change after re-dry indicate some reaction with water during D2O conditioning. A D-rich layer is also observed at the air side surface in D2O-conditioned films regardless of A/V ratio.     

Swelling behavior of self-assembled monolayers of alkanethiol-terminated poly(ethylene glycol): a neutron reflectometry study

The swelling behavior of alkanethiol-terminated poly(ethylene glycol) with an average molecular weight of 2180 Da (i.e., approximately 45 ethylene glycol, EG, units) in contact with water was investigated by neutron reflectometry as a function of the morphology of the PEG-SH layer. Amorphous films at a low grafting density show significant swelling with an increase of the film thickness from approximately 25 A in the dry state to approximately 70 A in contact with D2O, which corresponds to a total water uptake of approximately 38 mass %. In contrast, quasi-crystalline monolayers exhibit only a small amount of water penetrating into the film (approximately 8 mass %) with a corresponding change of the layer thickness from approximately 110 to approximately 125 A. The water uptake per EG unit corresponds to the literature value of 1.5 for the amorphous layer and to only 0.25 in the case of the quasi-crystalline film.     

Gas permeability in polymer- and surfactant-stabilized bubble films

The gas permeabilities of thin liquid films stabilized by poly(N-isopropylacrylamide) (PNIPAM) and PNIPAM-SDS (sodium dodecyl sulfate) mixtures are studied using the "diminishing bubble" method. The method consists of forming a microbubble on the surface of the polymer solution and measuring the shrinking rates of the bubble and the bubble film as the gas diffuses from the interior to the exterior of the bubble. PNIPAM-stabilized films exhibit variable thicknesses and homogeneities. Interestingly, despite these variable features, the gas permeability of the film is determined principally by the structure of the adsorbed polymer layer that provides an efficient gas barrier with a value of gas permeability coefficient that is comparable to that of an SDS Newton black film. In the presence of SDS, both the film homogeneity and the gas permeability coefficient increase. These changes are related to interactions of PNIPAM with SDS in the solution and at the interface, where coadsorption of the two species forms mixed layers that are stable but that are more porous to gas transfer. The mixed PNIPAM-SDS layers, studied previously for a single water-air interface by neutron reflectivity, are further characterized here in a vertical free-draining film using X-ray reflectivity.     

Angle resolved XPS characterization of cationic polyacrylamides

In many applications surfaces are modified using polymer films and the polymers used are often complex copolymers. In biomedical applications it is critical to determine the surface properties of a substrate as it is these that mediate the cellular interactions. The surface structure of copolymer films can only rarely be established from their bulk composition alone. In this study angle resolved XPS was used to build a model of the structure of copolymer films produced on glass substrates from a family of poly(acrylamide) copolymers containing cationic blocks. The thickness of the copolymer films was demonstrated to be dependent on the concentration of the polymer solution and the ratio of non‐cationic to cationic blocks in the copolymer. The data demonstrated that the cationic blocks of the copolymer preferentially segregated to the glass surface and the non‐cationic poly(acrylamide) blocks preferentially segregated to the air–vacuum interface. A low concentration of the cationic functional groups was present throughout the poly(acrylamide) layer and it was suggested that this resulted from a small fraction of the cationic blocks being pulled into the poly(acrylamide) layer at points along the polymer chain where the two blocks are connected. Evidence of a thin surface hydrocarbon contamination layer was also observed. Copyright © 2009 John Wiley & Sons, Ltd.     

Refractometric discrimination of void-space filling and swelling during vapour sorption in polymer films

Thin polymeric films have been deposited as upper cladding layers on a new integrated optical interferometer fabricated from layers of silicon oxynitride on a silicon wafer. The evanescent field of the probing waveguide mode transduces refractive index changes in the polymer layer into the measured phase changes in the device. Real-time measurement of index change and its sign is obtained. Upon exposure to humid air, we record water sorption by films of poly(vinyl pyrrolidone) by a rapid positive index change for void-space filling followed by a slow negative index change for swelling. Sorption of water vapor into a thin film of the viscous liquid polymer polyethylenimine shows only swelling mode behaviour and a simple constitutive model can be applied to give the fractional water occupied volume.     

Characterization of thin polymer‐like films formed by plasma polymerization of methylmethacrylate: A neutron reflectivity study

The microstructure of the plasma‐polymerized methylmethacrylate (ppMMA) films is characterized using neutron reflectivity (NR) as a function of the plasma reaction time or film thickness. Variation in the crosslink density normal to the substrate surface is examined by swelling the film with a solvent, d‐nitrobenzene (dNB). In the presence of dNB, uniform swelling is observed throughout the bulk as well as at the air surface, and silicon oxide interfaces. The results indicate that the MMA film prepared by plasma polymerization (ppMMA) has a uniform crosslink density from air surface to substrate surface. Additionally, the scattering length density of the plasma‐polymerized MMA film (SLD ≈ 0.750 × 10⁻⁶ Å⁻²) is much lower than that of a conventional PMMA film (SLD = 1.177 × 10⁻⁶ Å⁻²). The increase in film thickness following dNB sorption is 7.5% and at least 36% for the ppMMA and PMMA films, respectively. This suggests that the films formed by plasma polymerization are different from conventional polymers in chemical structure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2522–2530, 2004     

Moisture absorption and absorption kinetics in polyelectrolyte films: influence of film thickness

Specular X-ray reflectivity (XR) and quartz crystal microbalance (QCM) measurements were used to determine the absorption of water into thin poly(4-ammonium styrenesulfonic acid) films from saturated vapor at 25 degrees C. The effect of film thickness on the absorption kinetics and overall absorption was investigated in the range of thickness from (3 to 200) nm. The equilibrium swelling of all the films irrespective of film thickness was (0.57+/-0.03) volume fraction. Although the equilibrium absorption is independent ofthickness, the absorption rate substantially decreases for film thickness < 100 nm. For the thinnest film (3 nm), there is a 5 orders of magnitude decrease in the diffusion coefficient for water.     

Effect of water contact on the density distributions of thin supported polymer films investigated by an X-ray reflectivity method

The diffusion processes of water molecules into polymer films (PMMA/PS homopolymers and random copolymers) in contact with liquid water were investigated using gravimetric methods and X-ray reflectivity (XRR) analysis. Methods of water contact and XRR measurement were designed for studying the systems in the nonequilibrium state of diffusion. Gravimetric measurements confirmed the Fickian diffusion behavior of films in contact with water. Vertical density distributions in PMMA and methylmethacrylate-rich copolymer films demonstrate the existence of a water-rich layer at the interface. However, with further absorption of water into the film, the overall density increased throughout the film. The results suggest that the diffusion of water into the polymer film occurs to recover density uniformity with a high concentration of water molecules at the surface. Some XRR data for the PS- and styrene-rich copolymer films could not be fit and converted to a vertical density distribution because of their huge diffusion coefficients. However, the reflectivity curves for these films and the vertical density distribution after sufficient water contact suggested that the surfaces of these films were commonly diffused after water contact. Atomic force microscopy (AFM) analysis demonstrated that the surface roughness of these films actually increased with water content.