Characterization of nanoporous low-k thin films by small-angle neutron scattering contrast variation

Small-angle neutron scattering contrast variation is applied to characterization of nanoporous low-dielectric constant (low-k) thin films. Films are exposed to saturated solvent vapor in air, whereby the pores fill with liquid by capillary condensation. The pores are filled with mixtures of hydrogen- and deuterium-containing solvents to vary the neutron contrast with the matrix (wall). The composition of the solvent mixture is systematically varied to identify a composition that minimizes the scattered intensity (contrast match point). From the contrast match point composition, film characteristics including matrix density and homogeneity are assessed. Four spin-on low-k materials including a methylsilsesquioxane, an organic polymer, a xerogel, and a hydrogensilsesquioxane are characterized by the new technique. Calculated matrix mass densities are compared to independent density measurements obtained by an established specular X-ray reflectivity technique. We find no evidence of "closed pores", defined here as pores inaccessible to the probe solvent, in any of the materials studied.     

Preparation and determination of the structure of high-performance polymer blends by small-angle neutron scattering

Amorphous blends from a deuterated polyaryletherketone and a polyetherimide have been prepared and investigated by neutron scattering. The used components differ remarkably in their structure and their solution properties. The blends on the other hand are combinatorial mixtures: the scattering quantity ϕ₁ϕ₂/S(q) is independent of the composition of the blends if the polymer chains have the same length.     

The determination of tungsten in steels by neutron inelastic scattering

A simple, rapid and non-destructive method is described for determining tungsten in steels over the concentration range 0.08 to about 3% by mass. The method uses the low energy gamma-rays induced in¹⁸³W by neutrons from the 3 MeV bombardment of beryllium with deuterons. The precision is normally better than 10%.     

On the role of block copolymer additives for calcium carbonate crystallization: small angle neutron scattering investigation by applying contrast variation

The role of the double-hydrophilic block copolymer poly(ethylen glycol)-block-poly(methacrylic acid) (PEG-b-PMAA) on the morphogenesis of calcium carbonate (CaCO3) was studied by applying the contrast variation small angle neutron scattering technique. The morphology and size of CaCO3 crystals is strongly affected by the addition of PEG-b-PMAA. In order to determine the partial scattering functions of the polymer and CaCO3 mineral, we developed both an experimental and theoretical approach with a sophisticated method of their determination from the scattering intensity. Partial scattering functions give detailed information for each component. In particular, the partial scattering function of the polymer, Spp, shows a monotonic slope with Q(-2 to -3) where the scattering vector Q is low (Q < 0.01 Angstrom(-1)), which is a clear evidence that the polymer within the CaCO3 mineral has a mass fractal dimension. The other partial scattering functions reflected the geometry of the CaCO3 particles or the "interaction" of polymer and CaCO3 on a microscopic scale, which leads to a coherent view with Spp.     

Determination of micelle structure and charge by neutron small-angle scattering

We present a general method of determining the structure and charge of globular ionic micelles, using neutron small-angle scattering. The micellar solutions may have any concentration within the micellar phase. The method is based in part on an analytic calculation of the interparticle correlations between monodisperse spherical micelles, and we discuss the theory in some detail to justify its application to polydisperse globular particles. Experimental results are presented for several cationic and anionic micellar systems.     

Specular X-ray reflectivity and small angle neutron scattering for structure determination of ordered mesoporous dielectric films

Specular X-ray reflectivity (SXR) and small-angle neutron scattering (SANS) are used to characterize the structure of a thin film containing cylindrical mesopores. The 3-D structure of the mesoporous film was determined from SANS measurements taken at multiple rotation angles between the incident beam and the film. The film was found to be composed of a randomly packed core and surface layers within which the hollow cylinders were regularly packed and oriented along the surface. The packing of the cylindrical mesopores was not hexagonal but rather rectangular with a conical angle of 55.7 degrees instead of 60 degrees expected for hexagonal packing. The extent of the planar orientation of the cylindrical mesopores within the surface layers was estimated from the width of the Bragg reflection in the SXR result to be about 25 repeating layers at both interfaces. This was further confirmed from cross-section transmission electron microscopy (TEM) results. The SXR results of this film exhibit an anomalous decrease in reflected intensity after each Bragg reflection. This anomaly in SXR can be modeled in a Parratt formulism using a depth profile composed of two characteristic lengths, the repeating distance among layers and the curvature of the density profile of each layer.     

Short-range order effects in amorphous polycondensates as studied by spin polarized diffuse neutron scattering and simulation

Short-range order effects in amorphous polycondensates, including the technologically important bisphenol-A-polycarbonate, have been investigated by elastic diffuse neutron scattering with spin polarization analysis. Selectively deuterated samples of each polycondensate have been used in order to vary the scattering contrast and thereby emphasize different pair correlations. The technique of spin polarization analysis allowed a reliable separation of the coherent scattering and an intensity calibration on the basis of the incoherent scattering as an internal standard. Thus, (d/d)_coh has been measured directly by this method. The experimental results are compared to calculated cross-sections from computer-generated structures. Simulations have been performed with the amorphous cell method which models the static structure of the amorphous polymer in full chemical detail on the basis of a random coil conformation. The results of the simulations yield a fertily ground for the discussion of the measured cross-sections, though a direct comparison with the experiment is not always satisfactory. The observed discrepancies indicate a still insufficient structural relaxation of the simulated structures.Dedicated to Professor Dr. E. W. Fischer of the occasion of his 65th birthday.     

Study of thallium and bismuth characteristic X-rays,after neutron activation,for their determination

X-rays emitted by thallium and bismuth after neutron activation have been examined for their use in the determination of these elements. Interferences have been assessed. Thallium-204,²⁰⁶Tl and²¹⁰Bi are employed. Detection limits are high but purely instrumental determinations are possible from 50 g per g to fractions of percent levels.     

Structure-property relations in crystalline L-leucine obtained from calorimetry, X-rays, neutron and Raman scattering

We have studied the amino acid L-leucine (LEU) using inelastic neutron scattering, X-rays and neutron diffraction, calorimetry and Raman scattering as a function of temperature, focusing on the relationship between the local dynamics of the NH(3), CH(3), CH(2) and CO(2) moieties and the molecular structure of LEU. Calorimetric and diffraction data evidenced two novel phase transitions at about 150 K (T(1)) and 275 K (T(2)). The dynamical susceptibility function, obtained from the inelastic neutron scattering results, shows a re-distribution of the intensity of the vibrational bands that can be directly correlated with the phase transitions observed at T(1) and T(2), as well as with the already reported phase transition at T(3) = 353 K. Through the analysis of the Raman modes, the new structural arrangement observed below T(1) was related to conformational modifications of the CH and CH(3) groups, while the behavior of the N-H stretching vibration, ν(NH(3)), gave insight into the intermolecular N-H…O interactions. The observation of changes in the translational symmetry in the crystalline lattice, as well as anharmonic dynamics, allows for localized motions in LEU.     

Investigation of molecular orientation distributions by polarized raman scattering and polarized fluorescence

It is assumed that the intensity of the vibrational Raman scattering from each of the structural units (molecular segments or crystallites) in an oriented polymer solid is determined by a symmetric second-rank tensor. The distribution of orientations of the principle axes of the tensors is expanded in a series of generalized spherical harmonics, Z_lmn(θ)e^?imψe^?in?, and it is shown that relations among the coefficients in this expansion can be obtained from suitable intensity measurements using polarized radiation. If the orientation of the tensor axes within the structural unit is known (in the general case, for several Raman lines), then the coefficients M_lmn of a similar expansion for axes fixed in the units can be obtained for l, m, and n even and l ≤ 4. The limitation to even m follows from the assumption that the solid has at least orthotropic statistical symmetry but the limitations on n and l arise from the nature of the Raman effect. Some simple special cases are considered and some orientation-independent intensity sums are derived. It is shown that, with the simplifying assumption usually made, the theory of the polarized fluorescence method for determining molecular orientations is a special case of the theory for the Raman method.     

Simultaneous elemental determination of aluminum and silicon in soils by gamma-ray detection following inelastic neutron scattering

The detection of prompt γ-rays produced by the inelastic scattering of fast monoenergetic neutrons has been used to determine the elemental composition of soil samples. Multigram samples were bombarded with 2.5 MeV neutrons, and concentrations were measured by analyzing the spectrum of emitted γ-rays. Particular emphasis has been placed on the simultaneous determination of aluminum and silicon. Possible extensions of the method to light elements are suggested.     

Scattering structure factor of colloidal gels characterized by static light scattering, small-angle light scattering, and small-angle neutron scattering measurements

The scattering structure factor of a colloidal gel in a q range of 5 orders of magnitude has been determined by combining static light scattering, small-angle light scattering, and neutron scattering measurements. It exhibits simultaneously two types of structure information: a mass fractal scaling within the clusters that constitute the gel and a surface fractal scaling for length scales larger than that of the clusters. Such scattering behavior can be well interpreted by the pair-correlation function proposed in the literature to model an ideal structure constituted of mass fractal objects inside surface fractal objects.     

Water distributions in polystyrene-block-poly[styrene-g-poly(ethylene oxide)] block grafted copolymer system in aqueous solutions revealed by contrast variation small angle neutron scattering study

We develop an experimental approach to analyze the water distribution around a core-shell micelle formed by polystyrene-block-poly[styrene-g-poly(ethylene oxide (PEO)] block copolymers in aqueous media at a fixed polymeric concentration of 10 mg/ml through contrast variation small angle neutron scattering (SANS) study. Through varying the D(2)O/H(2)O ratio, the scattering contributions from the water molecules and the micellar constituent components can be determined. Based on the commonly used core-shell model, a theoretical coherent scattering cross section incorporating the effect of water penetration is developed and used to analyze the SANS I(Q). We have successfully quantified the intramicellar water distribution and found that the overall micellar hydration level increases with the increase in the molecular weight of hydrophilic PEO side chains. Our work presents a practical experimental means for evaluating the intramacromolecular solvent distributions of general soft matter systems.     

Taking snapshots of photoexcited molecules in disordered media by using pulsed synchrotron X-rays

Photoexcited molecules are quintessential reactants in photochemistry. Structures of these photoexcited molecules in disordered media in which a majority of photochemical reactions take place remained elusive for decades owing to a lack of suitable X-ray sources, despite their importance in understanding fundamental aspects in photochemistry. As new pulsed X-ray sources become available, short-lived excited-state molecular structures in disordered media can now be captured by using laser-pulse pump, X-ray pulse-probe techniques of third-generation synchrotron sources with time resolutions of 30-100 ps, as demonstrated by examples in this review. These studies provide unprecedented information on structural origins of molecular properties in the excited states. By using other ultrafast X-ray facilities that will be completed in the near future, time-resolution for the excited-state structure measurements should reach the femtosecond time scales, which will make "molecular movies" of bond breaking or formation, and vibrational relaxation, a reality.     

Crystal structure and hydrogen-bonding system in cellulose Ibeta from synchrotron X-ray and neutron fiber diffraction

The crystal and molecular structure together with the hydrogen-bonding system in cellulose Ibeta has been determined using synchrotron and neutron diffraction data recorded from oriented fibrous samples prepared by aligning cellulose microcrystals from tunicin. These samples diffracted both synchrotron X-rays and neutrons to better than 1A resolution (>300 unique reflections; P2(1)). The X-ray data were used to determine the C and O atom positions. The resulting structure consisted of two parallel chains having slightly different conformations and organized in sheets packed in a "parallel-up" fashion, with all hydroxymethyl groups adopting the tg conformation. The positions of hydrogen atoms involved in hydrogen-bonding were determined from a Fourier-difference analysis using neutron diffraction data collected from hydrogenated and deuterated samples. The hydrogen atoms involved in the intramolecular O3...O5 hydrogen bonds have well-defined positions, whereas those corresponding to O2 and O6 covered a wider volume, indicative of multiple geometry with partial occupation. The observation of this disorder substantiates a recent infrared analysis and indicates that, despite their high crystallinity, crystals of cellulose Ibeta have an inherent disorganization of the intermolecular H-bond network that maintains the cellulose chains in sheets.     

Colloidal dispersions of octadecyl grafted silica spheres in toluene: a global analysis of small angle neutron scattering contrast variation and concentration dependence measurements

In this paper we report measurements of the form factor and the structure factor of a sterically stabilized colloidal dispersion consisting of silica spheres coated with octadecane in toluene by small angle neutron scattering (SANS). The phase diagram of this system shows the liquid-liquid coexistence line and also a jamming transition at higher concentrations, where the jamming line intersects the coexistence line roughly at the critical point. We have performed SANS experiments at a temperature well above the transition temperature and at various volume fractions phi, spanning from the very dilute regime (phi=0.2%) to the critical concentration (phi=16%) and the highly viscous regime (phi=39.2%). Except for the very dilute regime, we observe a structure factor S(q) in all other cases. We fitted our data over the whole concentration regime using a global fitting routine with a core-shell model for the form factor P(q), taking into account the structure factor, which we describe with the Robertus model for an adhesive polydisperse core-shell particle. At a volume fraction of phi=5% a SANS contrast variation experiment has been performed. From that the product of the volume of the shell and the amount of solvent within the corona of our core-shell particle could be determined. At the most probable shell thickness of 2.3 nm a solvent content of about 50% within the corona was found. Moreover we could conclude that the core is not interpenetrated by solvent molecules. From the contrast variation experiment followed that the structure factor at zero average contrast exhibits a strong q dependence, which is an effect of an inhomogeneous particle in combination with a size distribution.     

Core-shell structure of degradable, thermosensitive polymeric micelles studied by small-angle neutron scattering

The structure of assemblies of block copolymers composed of thermosensitive, biodegradable poly(N-(2-hydroxypropyl) methacrylamide-dilactate) and poly(ethylene glycol) (pHPMAmDL-b-PEG) has been studied by small-angle neutron scattering (SANS). Three amphiphilic copolymers with a fixed PEG of 5 kDa and a partially deuterated pHPMAmDL(d) block of 6700, 10400, or 21200 Da were used to form micelles in aqueous media by heating the polymeric solution from below to above the cloud point temperature (around 10 degrees C) of the thermosensitive block. Simultaneous and quantitative analysis of the scattering cross sections obtained at three different solvent contrasts is expedited using core-shell model, which assumed a homogeneous core of uniform scattering length density. The mean core radius increased from 13 to 18.5 nm with the molecular weight of the pHPMAmDL(d) block, while the thickness of the stabilizing PEG layer was around 8 nm for the three investigated assemblies. In addition, the volume fraction values of the stabilizing PEG chains in the shell are low and decreased from 31% to 14% with increasing the size of pHPMAmDL(d) block which shows that the shell layer of the assemblies is highly hydrated. The corresponding PEG chain grafting densities decreased from 0.22 to 0.11 nm-2 and the distance between PEG chains on the nanoparticles surface increased from 2.4 to 3.4 nm. The pHPMAmDL-b-PEG micelles showed a controlled instability due to hydrolysis of the lactic acid side groups in the thermosensitive block; that is, an increase of the degradation time leads to an increase of the size of the core which becomes less hydrophobic and consequently more hydrated. Neutron experiments supplied accurate information on how the size of the core and the micelle's aggregation number changed with the incubation time. This feature and the initially small size and dense structure in aqueous solution make the polymeric micelles suitable as carriers for hydrophobic drugs.     

Molecular weight determination by the small-angle scattering method without using absolute intensities and contrast

A procedure for the molecular weight determination from small-angle data on a relative scale without knowledge of contrast has been suggested. The contrast may be determined from measurements of scattering curves using at least two concentrations of the dispersed compound. With absolute intensities the method yields the value of excess scattering amplitude. The procedure is applied to the recently published scattering data on bovine serum albumin.     

Physical gels from PVC: molecular structure of pregels and gels by low-angle neutron scattering

In this paper we report on investigations into the short-range molecular structure of pregels (aggregates formed in dilute solutions) and gels as a function of ageing time, temperature and solvent type. In some solvents (diethyl oxalate and bromobenzene) it is found that the structure of the pregels is cylinder-like or ellipsoid-like. The structure is seen to be the same for the gels and the pregels in the investigated range of transfer momentum which leads us to conclude that the gels are probably fiber-like, a statement in agreement with the morphology observed by Yang and Geil. The fiber section is also about the same order of magnitude as that reported by these authors. Also, further evidence is given for the existence of a second type of physical knot, different from those formed by the more syndiotactic sequences. The validity of the molecular structures deduced from these experiments is discussed in the light of macroscopic properties such as compression modulus and swelling.