Scattering Length Density Profile Research Articles

Complementary neutron and x-ray reflectivity studies of “near-frictionless” carbon films

“Near-frictionless carbon,” a form of amorphous hydrogenated diamond-like-carbon developed at Argonne National Laboratory, has been investigated by neutron and x-ray reflectivity under ambient conditions, in vacuum, and at elevated temperature. A series of films was made with different ratios of hydrogen to carbon in the plasma; the deuterated counterparts were also synthesized. The details of scattering length density profiles, thickness, and surface and interfacial roughness of the films were obtained from neutron and x-ray reflectivity profiles. This allowed us to calculate the mass density and hydrogen composition of the bulk carbon film, leading to insight into the frictional properties. From the analysis we found that the magnitude of the scattering length density of the top surface layer was greatly affected by the experimental atmosphere in which the films were contained although the trend with hydrogen in the plasma remained the same. All samples were fitted with a three-layer model in addition to the substrate.

Characterization of protein resistant, grafted methacrylate polymer layers bearing oligo(ethylene glycol) and phosphorylcholine side chains by neutron reflectometry

Neutron reflectometry was used to investigate the structures of end-tethered protein resistant polymer layers based on poly(oligo(ethylene glycol) methyl ether methacrylate) [poly(OEGMA)] and poly(2-methacryloyloxyethyl phosphorylcholine) [poly(MPC)]. Layers having different graft densities were studied in both the dry and wet states. A stretched parabolic model was used to fit the neutron data, resulting in a one-dimensional scattering length density profile of the polymer volume fraction normal to the film. Measured in D(2)O, the cutoff thicknesses of OEGMA and MPC layers at high graft density (0.39 chainsnm(2) for OEGMA and 0.30 chainsnm(2) for MPC) and a chain length of 200 repeat units were 450 and 470 A, respectively, close to their contour length of 500 A, suggesting that the grafts become highly hydrated when exposed to water. It was also found that at similar graft density and chain length, the volume fraction profiles of poly(OEGMA) and poly(MPC) layers are similar, in line with the authors' previous results showing that these surfaces have similar protein resistance [W. Feng et al., BioInterphases 1, 50 (2006)]. The possible correlation of protein resistance to water content as indicated by the average number of water molecules per ethylene oxide (N(w,EO)) or phosphorylcholine (N(w,PC)) moiety was investigated. N(w,EO) and N(w,PC), estimated from the volume fraction data, increased with decreasing graft density, and when compared to the reported number of water molecules in the hydration layers of EO and PC residues, led to the conclusion that water content slightly greater than the water of hydration resulted in protein resistant surfaces, whereas water content either less than or greatly in excess of the water of hydration resulted in layers of reduced protein resistance.

Neutron Diffraction Study of Pseudomonas aeruginosa Lipopolysaccharide Bilayers

Lipopolysaccharides (LPSs) are a major class of macromolecules populating the surface of Gram-negative bacteria. They contribute significantly to the bacterium's surface properties and play a crucial role in regulating the permeability of its outer membrane. Here, we report on neutron diffraction studies performed on aligned, self-assembled bilayers of LPS isolated from Pseudomonas aeruginosa PAO1. This LPS system is comprised of a mixture of rough and smooth A-band and B-band LPS, similar to that naturally found in P. aeruginosa. Temperature scans were conducted at various levels of hydration, and the phases adopted by LPS, along with their corresponding transition temperatures, have been identified. Because of LPS's chemical heterogeneity, the gel-to-liquid-crystalline transition was continuous and not abrupt as commonly observed in single-component phospholipid systems. From the construction of one-dimensional scattering length density profiles, we find that water penetrates into the hydrocarbon region up to and including the center of liquid-crystalline LPS bilayers. This permeability to water also extends to bilayers in the continuous phase transition region and could have far-reaching implications as to how small molecules penetrate the outer membrane of Gram-negative bacteria.

On the kinematic approximation to specular reflectivity data

The kinematic approximation to specular reflectivity is very useful for understanding neutron and X-ray scattering observations, because it provides a simple analytical formula that relates the scattering-length density profile of a layered surface or interface, β(z), to the data, R (qz ). Although its derivation from the wave equation, or the optical transfer model, for large qz , is straightforward, the origin of the qz -factor in the denominator is less obvious when the problem is viewed in the limit of Fraunhofer diffraction from the outset. This paper, which is principally educational, aims to bring clarity to the issue.

Effect of Solvent−Polymer Interaction in Swelling Dynamics of Ultrathin Polyacrylamide Films: A Neutron and X-ray Reflectivity Study

The swelling dynamics of the ultrathin polyacrylamide (PAM) spin-coated films in saturated vapor of D2O and H2O were studied using neutron and X-ray reflectivity. A uniform scattering length density (SLD) profile represents the dry PAM films, whereas the SLD profiles corresponding to the swelled films were characterized with a decreasing solvent concentration along the film thickness from top surface to the film/substrate interface. The diffusion mechanism of D2O into the films was found to be a non-Fickian process, as the D2O diffusion coefficient was observed to be decreasing as a function of film thickness. The thickness dependent structural changes in the dry polymer films were suggested from the increased density of thinner films. The diffusion coefficient of polymer chains in the solvent on the contrary was independent of film thickness. The different nature of D2O−PAM interaction (stronger) as compared to H2O−PAM interaction was found to play a crucial role on the diffusion of polymer, where the diffusion coefficient of the chains was an order of magnitude higher in D2O as compared to that in the H2O. A lower value of the excluded-volume parameter in the case of D2O also indicates stronger monomer−solvent interaction.

Fitting a free-form scattering length density profile to reflectivity data using temperature-proportional quenching

A technique for fitting a free-form scattering length density profile to reflectivity data via least-squares minimization is presented. The approach combines aspects of simulated annealing with a parametrized representation of the scattering length density profile. The ability of the algorithm to accurately recover the scattering length density profile from arbitrary initial parameter values is demonstrated for simulated and experimental data.

Structure and Protein Binding Capacity of a Planar PAA Brush

We performed neutron reflectometry (NR) and total internal reflection fluorescence (TIRF) spectroscopy to characterize the structure and the protein binding capacity of a planar poly(acrylic acid) (PAA) brush at different temperatures. A PAA brush was prepared by spin-coating planar quartz or silicon wafers with a thin film of poly(styrene). Then, the diblock copolymer poly(styrene)-poly(acrylic acid) was deposited on these modified wafers using the Langmuir-Schäfer or Langmuir-Blodgett technique. PAA grafting densities of about 0.1 chains per nm2 were obtained. The NR experiments indicate a remarkable swelling of the PAA brush in contact with a buffer solution, when it is heated to 40 degrees C for several hours. The swollen brush structure remains upon cooling back to 20 degrees C suggesting a disentanglement of the initially formed PAA brush by the temporary heating. At pD = 6.7, the protein bovine serum albumin (BSA) with a negative net charge is strongly adsorbed to the swollen PAA brush. From the scattering length density profiles obtained from the NR curves, an almost homogeneous filling of the whole PAA brush space with BSA molecules can be deduced corresponding to an average BSA volume fraction of about 7-10% and an adsorbed protein mass of about 1.4 mg m-2. By analyzing the TIRF experiments, it is found that BSA adsorption is enhanced when increasing the temperature which represents an evidence for an entropic driving force for protein adsorption. However, the mechanism of BSA adsorption at a PAA brush appears to be different at 20 and 40 degrees C.

Chain Localization and Interfacial Thickness in Microphase-Separated Structures of Block Copolymers with Variable Composition Distributions

The effect of composition distribution on microphase-separated structures of AB diblock and BAB triblock copolymers was investigated at the molecular level by measurements of neutron reflectivity. Monodispersed three poly(styrene)-d8-block-poly(2-vinylpyridine) (DP) and three labeled triblock copolymers (PDP) and unlabeled counterparts were synthesized by a living anionic polymerization process designed to yield constant molecular weight with different volume fractions. The selective labeling method employs the blending of copolymers for a segmental distribution study while the labeled polymers are mixed for an interfacial study. Scattering length density profiles were observed by neutron reflectivity measurements for blended thin films produced by spin-coating and successive thermal annealing. It was confirmed that the blended thin films exhibit simple alternating lamellar structures, an observation consistent with the observed structures of bulk specimens. The localization phenomenon of longer and short...

Determination of graft density of substrate-supported polymer films

Determining the graft density of polymers adsorbed on a substrate from neutron scattering-length density profile can be challenging. A scheme based on integrating a select region of the profile is proposed and applied to results obtained from a recent neutron reflectometry experiment that investigated the physical properties of polyethylene oxide (PEO) chemisorbed onto a gold-coated Si substrate. Reflectivity measurements were performed with the PEO exposed to a H 2O/D 2O mixture contrast matched to the underlying gold. The graft density obtained by the integration technique is 60% higher than the value determined using scaling relations.

Neutron reflectivity studies on the DNA adsorption on lipid monolayers at the air–liquid interface

In situ neutron reflectivity was used to study the DC-Chol and TC-Chol monolayers at the air–liquid interface in the presence and absence of DNA in the subphase. It was found that the DC-Chol is more effective in adsorbing the DNA than the TC-Chol. It was also found that a compact DNA layer formed beneath the DC-Chol monolayer with a DNA gap spacing around 20 Å and a less compact DNA layer adsorbed to the TC-Chol monolayer with a DNA spacing around 60 Å, as estimated from the determined neutron scattering length density. From the determined neutron scattering length density profiles, the adsorbed DNA somewhat penetrates into the head group region of the charged lipids.

Method of separated form factors for polydisperse vesicles

Use of the Schulz or Gamma distribution in the description of particle sizes facilitates calculation of analytic polydisperse form factors using Laplace transforms, {\cal L}[f(u)]. Here, the Laplace transform approach is combined with the separated form factor (SFF) approximation [Kiselevet al.(2002).Appl. Phys. A,74, S1654–S1656] to obtain expressions for form factors,P(q), for polydisperse spherical vesicles with various forms of membrane scattering length density (SLD) profile. The SFF approximation is tested against exact form factors that have been numerically integrated over the size distribution, and is shown to represent the vesicle form factor accurately for typical vesicle sizes and membrane thicknesses. Finally, various model SLD profiles are used with the SFF approximation to fit experimental small-angle neutron scattering (SANS) curves from extruded unilamellar vesicles.

Co-refinement of multiple-contrast neutron/X-ray reflectivity data usingMOTOFIT

The contrast-variation technique is employed in multiple-contrast neutron/X-ray reflectometry experiments to highlight scattering from different structural components that are present at a surface or interface. The advantage of this technique is that the structural model used to describe the interfacial scattering length density profile must apply to all the contrasts measured. A new reflectivity analysis package,MOTOFIT, which runs in theIGOR Proenvironment (http://www.wavemetrics.com), has been created to aid the simultaneous fitting (with the same structural model) of these multiple-contrast data, using an intuitive graphical user interface, which most co-refinement packages do not possess.MOTOFITuses a slab-model approach with the Abeles matrix method, and extensions for surface roughness to perform non-linear least-squares regression on the experimental reflectivity curves. Other features, such as the ability to create complicated interparameter constraints or analyse reflectivity from multilayers, simulated annealing,etc., makeMOTOFITa powerful reflectometry analysis package

Cholesterol Hydroxyl Group Is Found To Reside in the Center of a Polyunsaturated Lipid Membrane

Cholesterol and saturated lipid species preferentially partition into liquid ordered microdomains, such as lipid rafts, away from unsaturated lipid species for which the sterol has less affinity in the surrounding liquid-disordered membrane. To observe how cholesterol interacts with unsaturated phospholipids, we have determined, from one-dimensional neutron scattering length density profiles, the depth of cholesterol in phosphatidylcholine (PC) bilayers with varying amounts of acyl chain unsaturation. Through the use of [2,2,3,4,4,6-(2)H(6)]-labeled cholesterol, we show that in 1-palmitoyl-2-oleoylphosphatidylcholine (16:0-18:1 PC), 1,2-dioleoylphosphatidylcholine (18:1-18:1 PC), and 1-stearoyl-2-arachidonylphosphatidylcholine (18:0-20:4 PC) bilayers the center of mass of the deuterated sites is approximately 16 A from the bilayer center. This location places the hydroxyl group of the sterol moiety at the hydrophobic/hydrophilic bilayer interface, which is the generally accepted position. In dramatic contrast, for 20:4-20:4 PC membranes the hydroxyl group is found, unequivocally, sequestered in the bilayer center. We attribute the change in location to the high disorder of polyunsaturated fatty acids (PUFA) that is incompatible with close proximity to the steroid moiety in its usual "upright" orientation.

Multilamellarity, structure and hydration of extruded POPC vesicles by SANS

The small-angle neutron scattering (SANS) data of 12 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) dispersions at low lipid concentration (1 mg per 100-mg heavy water) prepared by 5, 9 and 29 extrusions through filters of pores with 50, 100, 200 and 400 nm diameter are presented. They were analyzed within a theory that permits the determination of both structural and hydration parameters of the bilayers as well as the portions of multilamellar vesicles in dispersions with negligible long-range order between the vesicles. The scattering length density profile across the bilayers is approximated by assuming a central hydrocarbon core surrounded by a water-accessible coat. It is modeled by two different forms of functions. In the boat model, the scattering length density of the coat changes linearly from core to water, whereas in the strip model it is constant across the water-accessible coat. It was found that the boat model reflects the reality better than the strip model. The decrease of the multilamellar vesicle portions, either with increasing the number of extrusions at same filter size and with decreasing the filter size, was characterized quantitatively.

Hygrothermal Degradation of (3-Glycidoxypropyl)trimethoxysilane Films Studied by Neutron and X-ray Reflectivity and Attenuated Total Reflection Infrared Spectroscopy

Thin films of organosilanes have great technological importance in the areas of adhesion promotion, durability, and corrosion resistance. However, it is well-known that water can degrade organosilane films, particularly at elevated temperatures. In this work, X-ray and neutron reflectivity (XR and NR) were combined with attenuated total reflection infrared (ATR-IR) spectroscopy to study the chemical and structural changes within thin films of (3-glycidoxypropyl)trimethoxysilane (GPS) after exposure for various periods of time to air saturated with either D2O or H2O at 80 degrees C. For NR and XR, ultrathin (approximately 100 A) films were prepared by spin-coating. Both D2O and H2O provide neutron scattering contrast with GPS. Variations in the neutron scattering length density (SLD) profiles (a function of mass density and atomic composition) with conditioning time were measured after drying the samples out and also swelled with H2O or D2O vapor at room temperature. For samples that were dried out prior to measurement, little or no change was observed for H2O conditioning up to 3.5 days, but large changes were observed after 30 days of conditioning. The range of conditioning time for this structural change was narrowed to between 4 and 10 days with XR. The SLD profiles indicated that the top portion of the GPS film was transformed into a thick low-density layer after conditioning, but the bottom portion showed little structural change. A previous NR study of as-prepared GPS films involving swelling with deuterated nitrobenzene showed that the central portion of the film has much lower cross-link density than the region nearest the substrate. The present data show that the central portion also swells to a much greater extent with water and hydrolyzes more rapidly. The chemical degradation mechanism was identified by IR as hydrolysis of siloxane bonds. For ATR-IR, GPS films were prepared by dip-coating, which resulted in a greater and more variable thickness than for the spin-coated samples. The IR spectra revealed an increase in vicinal silanol generation over the first 3 days of conditioning followed by geminal silanol generation. Thus, the structural change detected by NR and XR roughly coincided with the onset of geminal silanol generation. Finally, little change in the reflectivity data was observed for films conditioned with D2O at 80 degrees C for 1 month. This indicates that hydrolysis of Si-O-Si is much slower with D2O than with H2O.

Phase determination in spin-polarized neutron specular reflectometry by using a magnetic substrate

The scattering length density profile for a thin film structure can be determined uniquely if both the modulus and phase of the reflection coefficient is known. Here, we describe a method for recovering the phase information which utilize a magnetic substrate and based on polarization analysis of the reflected beam. The method is derived in the formalism of transfer matrix so it is applicable for any unknown real scattering length density of nonmagnetic films (i.e. in the case where there is no effective absorption).

Structural studies of amphiphiles adsorbed at liquid–liquid interfaces using neutron reflectometry

We report the application and refinement of a recently developed method for structural studies at a liquid liquid interface using neutron reflectometry. The technique involves the entrapment of a thin oil layer between a silicon substrate and an aqueous subphase. The thin oil film is prepared by spin-coating an oil film on to an oleophilically treated silicon substrate. During the reflectivity measurement the sample is maintained in a horizontal position, and the angle of incidence of the neutron beam is varied using a supermirror. Attenuation of neutron reflectivity at the lowest angle of incidence is used to determine the oil-layer thickness. We report information regarding the structure at the interface between hexadecane and a 0.1% w/v aqueous solution of the triblock copolymer Pluronic L64 with EO13PO30EO13 (EO = ethylene oxide; PO = propylene oxide) and the interface between hexadecane and a 3.7 mmol dm(-3) (approximately critical micelle concentration) aqueous solution of the cationic surfactant tetradecyltrimethylammonium bromide (C14TAB). For the C14TAB system, the reflectivity data unambiguously reveal the presence of a region highly concentrated in C14TAB on the oil side of the interface. For the Pluronic L64 system, the data suggest that the polymer adsorbs at the interface occupying both oil and water sides of the interface. Model scattering length density profiles that capture these features are presented and further models that better fit the data are discussed.

Application of polarized neutron in determination of the phase in neutron reflectometry

In order to uniquely determine the scattering length density profile from measured reflectivity of specularly reflected neutrons from a film, the phase of complex reflection coefficient is needed. It has been shown that the phase can be determined by using three reference layers of finite thicknesses. However, this can be done by using one magnetic layer which can have three different scattering length densities corresponding to up, down, and non-magnetization. Moreover, in this case, polarization of reflected beam can be used in determination of the phase. In this paper it is shown that by using the transfer matrix method, it is possible to determine the complex reflection coefficient for any unknown real scattering length density profiles in situations in which a magnetic reference layer is mediated between the substrate and unknown sample. This method is based upon relations between the polarization of the reflected beam and the transfer matrix elements for the unknown sample. Thus, in this manner, only by polarization measurements we can find the reflection coefficient of any unknown sample. However, to avoid facing multiple solutions, we suggest measuring three quantities, at least one of which must be reflectivity and the others must contain polarization of the reflected beam normal to the magnetization of the magnetic layer and parallel to the sample.

Adsorption from alkane+perfluoroalkane mixtures at fluorophobic and fluorophilic surfaces. II. Crossover from critical adsorption to complete wetting

Using neutron reflectometry, adsorption from an equimolar mixture of hexane + perfluorohexane to a fluorophobic, octadecyl-coated, silicon substrate has been investigated as a function of temperature in the one-phase region upon approach to liquid-liquid coexistence. The composition of the investigated mixture, x(F) = 0.50, is well removed from the critical composition of x(F) = 0.36, where x(F) is the perfluorohexane mole fraction. To aid the modeling, mixtures with three different neutron refractive index contrasts have been used: namely, mixtures of C(6)H(14) + C(6)F(14) (H-F), C(6)D(14) + C(6)F(14) (D-F), and a mixture of C(6)H(14) + C(6)D(14) + C(6)F(14) which has been adjusted to have the same refractive index as silicon (CMSi). For all three contrasts, the principal features of the composition profile normal to the interface follow similar trends as the temperature T is reduced towards T(0), the coexistence temperature. These features consist of: (i) a hexane-rich primary adsorption layer appended to the octadecyl coupled layer. This primary layer is 22 +/- 5 A thick and becomes increasingly enriched in hexane as T(0) is approached. (ii) A tail that decays exponentially towards the bulk composition with a characteristic decay length zeta. As T(0) is approached, zeta increases. The scattering length density profiles have been converted to volume fraction profiles and the surface excess of hexane Gamma has been determined as a function of temperature for all three contrasts. As T(0) is approached Gamma increases, and its behavior can be represented using the scaling law Gamma approximately |T - T(0)|(-m). The resulting values of m are 0.71 +/- 0.09, 0.68 +/- 0.04, and 0.68 +/- 0.06 for the D-F, H-F, and CMSi contrasts, respectively. The behavior of Gamma with temperature does not adhere to the Gamma approximately |T - T(0)|(-1/3) law expected for complete wetting in systems with van der Waals interactions nor does it correspond to Gamma approximately |T - T(c)|(-0.305) expected for critical adsorption. The magnitude of the exponent m indicates that the adsorption resides in the crossover region between critical adsorption and complete wetting.

Characterization of polybutadiene-poly(ethyleneoxide) aggregates in aqueous solution: A light-scattering and small-angle neutron-scattering study

For diblockcopolymers of polybutadiene-poly(ethyleneoxide) (PB-PEO) type, water is a selective solvent. In dependence of the length of both blocks and the block length ratio, these polymers form a multitude of self-assembled structures in solution. In the present work scattering methods are used to investigate the water-soluble polymer PB125-PEO155 . It is found to form long rodlike micelles, which are characterized with respect to the aggregate length L , the cross sectional radius RCS , the radial scattering length density profile sigma r, and the radial aggregation number Nrad. Model-independent as well as model-based approaches are used for the scattering data analysis. From dynamic light scattering (DLS) and static light scattering (SLS) experiments the weight averaged length Lw of these stiff elongated aggregates is determined to Lw=350 nm . Small angle neutron scattering (SANS) reveals a cross sectional radius of RCS=17 nm and in combination with results from the SLS the radial aggregation number is found to be Nrad=70 .