Small-angle scattering studies of nafion membranes

The physical structure of Nafion membranes has been investigated by small-angle neutron scattering (SANS) and small-angle x-ray scattering (SAXS). Samples in the acid form may exhibit two scattering peaks. The first, observed by SANS at an angle corresponding to a Bragg spacing of 180 Å, is shown to arise from structures in crystalline regions. A second peak at larger scattering angles is shown to arise from ion-containing regions which may be swollen with water. Salt-form samples made by soaking the acid form in an aqueous salt solution can also exhibit the same two scattering signals. But in amorphous salt-form samples produced by quenching from the melt the first peak is absent. This permits a more accurate study of the second peak by SAXS, which shows that the second scattering component is present as a maximum over a wide range of water contents but is absent in a sample dried at 200°C. The position of the peak shifts to lower scattering angles (or larger spacings) at higher water contents. Possible structural models that might give rise to the maximum are discussed. A calculation of the SAX invariant is made and results are consistent with a phase separation of a large fraction of the water.     

含C60球两亲分子有序聚集体研究

综合评述了C60两亲分子有序聚集体的形成、结构及聚集体演变规律,介绍了含C60球两亲分子有序聚集体结构的冷冻刻蚀电子显微镜和电子显微镜、小角度中子散射、小角度X光散射、激光光散射以及聚集体结构模型的研究结果.     

SAXS and SANS studies of polymer colloids

The analysis of latex particles by small-angle scattering (small-angle X-ray scattering, SAXS; small-angle neutron scattering, SANS) is reviewed. Small-angle scattering techniques give information on the radial structure of the particles as well as on their spatial correlation. Recent progress in instrumentation allows to extend SANS and SAXS to the q-range of light scattering. Moreover, contrast variation employed in SANS and SAXS studies may lead to an unambiguous determination of the radial scattering length density of the particles in situ, i.e. in suspension. Hence, these techniques are highly valuable for a comprehensive analysis of polymer colloids as shown by the examples discussed herein.     

A study of carbon black Corax N330 with small-angle scattering of neutrons and X-rays

Carbon black Corax N330 (hereinafter called CB) is used as a filler in elastomers. The properties of the surface are important for the binding of the elastomer to the carbon black particles. Porod's law requires the intensity to satisfy I(q) approximately q(-alpha) with alpha = 4 for large q. Rieker et al. observed alpha = 3.7 +/- 0.1 for small-angle X-ray scattering (SAXS) data and concluded that the particle surface is fractally rough. Ruland critized this and suggested that the observed deviation is due to fluctuations of the spacing of the graphitic layer planes ("graphenes") which contribute a component I(q)fluc = 1Cflucq(-2) to the intensity component satisfying Porod's law. We studied CB by nitrogen adsorption, high-resolution transmission electron microscopy, synchroton SAXS, and small-angle neutron scattering (SANS). Our SAXS experiments with samples of high transmission (Tr = 0.96) confirmed the form of the scattering curves published by Rieker et al., but the correction for I(q)fluc restored Porod's law. SANS experiments were performed with a sample of low transmission in order to analyze the high q-range for scattering from voids and isolated graphenes. We found I(q) approximately q(-beta) with beta approximately 2 at q > 2.5 nm(-1) and will show that this intensity component requires graphenes consisting of about 12 benzene rings. The contrast matching technique revealed the presence of inaccessible voids. The SANS data for a sample with Tr = 0.363 satisfy Porods law, in contrast to the SAXS data for the high transmission samples. The latter discrepancy is likely due to the lower resolution of the SANS measurements because of wavelength smearing and multiple scattering. A SANS sample with Tr = 0.97 shows a minor deviation from Porod's law only (alpha = 3.9). The original SANS data and the SAXS data corrected for the fluctuation component indicate that the CB surface is essentially smooth.     

Evolution of self-assembled silica-tetrapropylammonium nanoparticles at elevated temperatures

The time evolution of silica nanoparticles in solutions of tetrapropylammonium (TPA) has been studied using a combination of small-angle scattering, conductivity, and pH measurements to provide the first comprehensive analysis of nanoparticle structural and compositional changes at elevated temperatures. We have found that silica-TPA nanoparticles subjected to hydrothermal treatment (70-90 degrees C) grow via an Ostwald ripening mechanism with growth rates that depend on both pH and temperature. Small-angle X-ray (SAXS) and neutron (SANS) scattering confirm that the core-shell structure of the particles, initially present at room temperature, is maintained during heating, but an evolution toward sphericity is evidenced especially at high values of pH. SAXS absolute intensity calculations were utilized to calculate the changes in nanoparticle composition and concentration over time. These changes along with the conductivity and pH measurements and SANS contrast matching studies indicate that, upon heating, TPA becomes embedded in the core of nanoparticles giving rise to more zeolitic-looking nanomaterials.     

Interaction of blockcopolymer micelles as probed by small-angle neutron and by small-angle X-ray scattering

 The analysis of the interaction of micelles formed by a blockcopolymer is given by means of small-angle X-ray (SAXS) and small-angle neutron scattering (SANS). The blockcopolymer consists of poly(styrene) and poly(ethylene oxide) (molecular weight of each block: 1000 g/mol) and forms well-defined micelles (weight-association number: 400, weight-average diameter: 15.4 nm) in water. The internal structure has been studied previously (Macromolecules 29:4006 (1996)) by SAXS. There it has been shown that the micelles are spherical objects. The structure factor S(q) as a function of the scattering vector q (q=(4π/λ) sin (θ/2); λ: wavelength of the radiation in the medium; θ: scattering angle) can be extracted from both sets of small-angle scattering data (SANS: q≤0.4 nm^-1; SAXS: q≤0.6 nm^-1). It is shown that particle interaction in the present system can be described by assuming soft interaction which is modeled by a square-step potential. Received: 12 May 1997 Accepted: 9 July 1997     

Structure and interactions of block copolymer micelles of Brij 700 studied by combining small-angle X-ray and neutron scattering

Spherical micelles of the diblock copolymer/surfactant Brij 700 (C(18)EO(100)) in water (D(2)O) solution have been investigated by small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS). SAXS and SANS experiments are combined to obtain complementary information from the two different contrast conditions of the two techniques. Solutions in a concentration range from 0.25 to 10 wt % and at temperatures from 10 to 80 degrees C have been investigated. The data have been analyzed on absolute scale using a model based on Monte Carlo simulations, where the micelles have a spherical homogeneous core with a graded interface surrounded by a corona of self-avoiding, semiflexible interacting chains. SANS and SAXS data were fitted simultaneously, which allows one to obtain extensive quantitative information on the structure and profile of the core and corona, the chain interactions, and the concentration effects. The model describes the scattering data very well, when part of the EO chains are taken as a "background"contribution belonging to the solvent. The effect of this becomes non-negligible at polymer concentrations as low as 2 wt %, where overlap of the micellar coronas sets in. The results from the analysis on the micellar structure, interchain interactions, and structure factor effects are all consistent with a decrease in solvent quality of water for the PEO block as the theta temperature of PEO is approached.     

Formation and structure of self-assembled silica nanoparticles in basic solutions of organic and inorganic cations

The phase behavior of silica solutions containing organic and inorganic cations was studied at room temperature using conductivity, pH, and small-angle scattering experiments. A critical aggregation concentration (cac) was observed at approximately 1:1 ratio of SiO(2)/OH(-) for all cation solutions from conductivity and pH studies. From this cac, a phase diagram of the system was developed with three distinct phase regions in pseudoequilibrium: a monomer/oligomer region (I), a monomer/oligomer/nanoparticle region (II), and a gel region (III). Small-angle X-ray and neutron scattering (SAXS and SANS) on solutions of region II formed with tetrapropylammonium hydroxide (TPAOH) revealed that the nanoparticles have a core-shell structure. Structure analysis of the SAXS and SANS data was best fit by a core-shell oblate ellipsoid model. A polydisperse set of core-shell spheres also fit the data well although with lower agreement factors. Similar nanoparticle morphologies were found in solutions of TMAOH, CsOH, and NaOH.     

Microemulsion droplets decorated by Brij700 block copolymer: phase behavior and structural investigation by SAXS and SANS

The phase behavior and structure of a four-component microemulsion system forming droplets with an oil core surrounded by the non-ionic C12E5 surfactant in water and "decorated" by long PEO chains using the block copolymer/surfactant Brij 700 has been studied. The surfactant-to-oil volume ratio, the coverage density of the droplets with decorating molecules, and the temperature were varied. For a surfactant-to-oil volume ratio of 2, the solutions form isotropic and clear solutions at room temperature, and the addition of Brij molecules stabilize the micelles: the transition to an opaque phase is shifted to higher temperatures as the surface coverage increases. At a surfactant-to-oil ratio of 1, the isotropic microemulsion phase is confined to a very narrow range of temperature, which location is shifted to increasing temperature, as the amount of Brij at the surface of the droplet is increased. For large surface coverages, the lower emulsification boundary varies roughly linearly with the surface coverage. The structure of the droplet phase was investigated by small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS). For a surfactant-to-oil ratio of 2, the SANS data revealed a transition from rodlike to spherical particles when Brij molecules are added to the system, which induces a larger curvature of the surfactant film. For a surfactant-to-oil ratio of 1, the droplets are nearly spherical at all surface coverages. The intermicellar interactions effects become increasingly more pronounced as Brij is added, due to the introduction of the highly swollen corona. A quantitative analysis of some of the SAXS data was done using an advanced model based on Monte Carlo simulations. It demonstrates the strong chain-chain interactions within the corona and confirms the increased interparticle interactions, as the coverage density is increased.     

Applications of small-angle X-ray scattering/small-angle neutron scattering and cryogenic transmission electron microscopy to understand self-assembly of surfactants

The self-assembling structures and dynamics of surfactants determine most of their macroscopic physicochemical properties and performances. Herein, we review recent work on the self-assembly of surfactants by small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) in conjunction with cryogenic transmission electron microscopy (Cryo-TEM) from the perspective of researchers having only limited theoretical knowledge of these techniques but expert in surfactants. Emphasis is placed on the structural analysis of typical surfactant aggregates over a wide range of size scales from nanometers up to microns, including spherical and rod-like micelles, wormlike micelles, vesicles, liquid crystals and coacervates, by combining different numerical approaches to the treatment of small-angle scattering data with the direct Cryo-TEM imaging method. Furthermore, the complementarity between SAXS and SANS, and between the scattering techniques and Cryo-TEM, that is, specific contributions of these techniques, is also covered.     

Orientation of platelets in multilayered nanocomposite polymer films

The orientation of platelets in micro-meter-thick polymer-clay nanocomposite films was investigated with small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and wide-angle X-ray diffraction (WAXD). The films with various clay contents (15–60% by mass fraction) were prepared by a layer-by-layer approach from polymer-clay solutions that led to the formation of a high degree of orientation in both polymer and clay platelets. Shear-induced orientation of polymer-clay solutions is compared with the orientation of polymer-clay films. SANS, SAXS, and WAXD, with beam configurations in and perpendicular to the spread direction of the film, were used to determine the structure and orientation of platelets. In all films, the clay platelets oriented preferentially in the plane of the film. The observed differences in semidilute solutions, with clay surface normal parallel to the vorticity direction, versus bulk films and with clay surface normal parallel to the shear gradient direction at clay mass fractions of 40 and 60%, were attributed to the collapses of clay platelet during the drying process. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3237–3248, 2003     

Stacking structure of concentrated shear ordered dispersions by two scattering methods

In this paper, the ordering in concentrated charge stabilized colloidal dispersions is considered. Despite the impressive Bragg reflections obtained for shear ordered dispersions by light (LS), small-angle neutron (SANS), and small-angle X-ray scattering (SAXS), a number of open questions remain. Sheared dispersions are usually ordered in layers. For such systems, two questions arise: (1) What is the structure in a layer? (2) What is the stacking structure perpendicular to the layers? The second question requires a method to determine the structure perpendicular to the layers. Although originally interested only in structural aspects, we were forced to consider different methods. Two methods are treated both applicable to neutron and X-ray scattering from concentrated dispersions. One has been used by physicists and chemists for many years to determine the structure of crystals by sample rotation. In colloid science, we have used it previously in neutron and X-ray scattering. A second method is treated here which can be applied in small-angle scattering from a Couette cell. It gives the scattering intensity in a certain direction without sample rotation. Although very useful with the Couette cell, it cannot be found in any of the well-known references on colloid science. A theoretical explanation and experimental examples obtained by synchrotron X-ray scattering from a Couette cell are given in the paper.     

A novel application of small-angle scattering techniques: Quality assurance testing of virus quantification technology

Small-angle scattering (SAS) techniques, like small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS), were used to measure and thus to validate the accuracy of a novel technology for virus sizing and concentration determination. These studies demonstrate the utility of SAS techniques for use in quality assurance measurements and as novel technology for the physical characterization of viruses.     

Small-angle neutron and x-ray scattering study of swollen nylon-6

Results of swelling and small-angle scattering experiments on samples of nylon-6 swollen with heavy water are discussed on the basis of the lamellar and switchboard models. The small-angle neutron scattering (SANS) intensity is very sensitive to the distribution of water in swollen samples, while the small-angle x-ray scattering (SAXS) data characterize the dry samples. The observed values of the mean-square fluctuation of scattering-density can be explained by a model with assumed inhomogeneous swelling of the amorphous phase.     

Wormlike micelle formation and flow alignment of a pluronic block copolymer in aqueous solution

The self-assembly into wormlike micelles of a poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock copolymer Pluronic P84 in aqueous salt solution (2 M NaCl) has been studied by rheology, small-angle X-ray and neutron scattering (SAXS/SANS), and light scattering. Measurements of the flow curves by controlled stress rheometry indicated phase separation under flow. SAXS on solutions subjected to capillary flow showed alignment of micelles at intermediate shear rates, although loss of alignment was observed for high shear rates. For dilute solutions, SAXS and static light scattering data on unaligned samples could be superposed over three decades in scattering vector, providing unique information on the wormlike micelle structure over several length scales. SANS data provided information on even shorter length scales, in particular, concerning "blob" scattering from the micelle corona. The data could be modeled based on a system of semiflexible self-avoiding cylinders with a circular cross-section, as described by the wormlike chain model with excluded volume interactions. The micelle structure was compared at two temperatures close to the cloud point (47 degrees C). The micellar radius was found not to vary with temperature in this region, although the contour length increased with increasing temperature, whereas the Kuhn length decreased. These variations result in an increase of the low-concentration radius of gyration with increasing temperature. This was consistent with dynamic light scattering results, and, applying theoretical results from the literature, this is in agreement with an increase in endcap energy due to changes in hydration of the poly(ethylene oxide) blocks as the temperature is increased.     

Aggregation of 1-dodecyl-3-methylimidazolium nitrate in water and benzene studied by SANS and 1H NMR

Aggregation of imidazolium-based ionic liquid, C(12)mim(+)NO(3)(-), in both polar solvent of water and nonpolar solvent of benzene was elucidated by electrical conductivity, small-angle neutron scattering (SANS), and (1)H NMR measurements. The electrical conductivities of C(12)mim(+)NO(3)(-)-water solutions at 298 K as a function of ionic liquid concentration showed a break point at 8.4 mmol dm(-3) as a cmc. However, those of C(12)mim(+)NO(3)(-)-benzene solutions drastically increase in accordance with a cubic function of concentration, but without a break point. The SANS profiles of both aqueous and benzene solutions obviously differ from each other. The profiles of the aqueous solutions indicated the formation of polydisperse spherical micelles. Those of the benzene solutions revealed Ornstein-Zernike behavior. Thus, C(12)mim(+)NO(3)(-) forms clusters in the benzene solutions, but the shape of clusters is indefinite. On the basis of the (1)H NMR chemical shifts of the aqueous solutions, the effect of nitrate on the formation of micelles was discussed on a microscopic scale. Furthermore, the interactions among C(12)mim(+), NO(3)(-), and benzene molecules in the benzene solutions were considered according to the (1)H NMR data.     

Structural characterization of surfactant aggregates adsorbed in cylindrical silica nanopores

The self-assembly of nonionic surfactants in the cylindrical pores of SBA-15 silica with a pore diameter of 8 nm was studied by small-angle neutron scattering (SANS) at different solvent contrasts. The alkyl ethoxylate surfactants C(10)E(5) and C(12)E(5) exhibit strong aggregative adsorption in the pores as indicated by the sigmoidal shape of the adsorption isotherms. The SANS intensity profiles can be represented by a sum of two terms, one accounting for diffuse scattering from surfactant aggregates in the pores and the other for Bragg scattering from the pore lattice of the silica matrix. The Bragg reflections are analyzed with a form factor model in which the radial density profile of the surfactant in the pore is approximated by a two-step function. Diffuse scattering is represented by a Teubner-Strey-type scattering function which indicates a preferred distance between adsorbed surface aggregates in the pores. Our results suggest that adsorption starts with formation of discrete surface aggregates which increase in number and eventually merge to interconnected patches as the plateau value of the adsorption isotherm is approached. A grossly different behavior, viz. formation of micelles as in solution, is found for the maltoside surfactant C(10)G(2), in agreement with the observed weak adsorption of this surfactant in SBA-15.     

Structure and ordering kinetics of micelles in triblock copolymer solutions in selective solvents

We have used small-angle x-ray scattering (SAXS), and small-angle neutron scattering (SANS) to study the micelle structure of a polystyrene-block-poly(ethene-co-butene)-block-polystyrene triblock copolymer in dilute - semidilute solutions in solvents selective for either the outer styrene block (dioxane) or for the middle block (heptane or tetradecane). Measurements of equilibrium structure factors showed that micelles were formed in both types of selective solvents. In the case of dioxane, the micelles are isolated whereas in the case of heptane or tetradecane, a bridged micellar structure may be formed at higher copolymer concentrations. In both cases we observed an ordered cubic structure of insoluble domains (micellar cores) at high concentrations (> 8 %). The micellar scattering function was fit to the Percus-Yevick interacting hard-sphere model. The temperature dependence of the core radius, the hard-sphere interaction radius and the volume fraction of hard spheres were obtained. We also used synchrotron-based time-resolved SAXS to examine the kinetics of ordering of the micelles on a cubic lattice for many different temperature jumps into the ordered cubic phase starting from the disordered micellar fluid phase in different solvents at different concentrations. The time evolution of the structure changes was determined by fitting the data with Gaussians to describe the structure factor of the ordered Bragg peaks and the Percus-Yevick structure factor was used to describe the micellar fluid. The time dependence of the peak intensities and widths as well as of the micellar parameters will be presented. The results showing the kinetics of the transformation from the fluid to the ordered phase were analyzed using the Mehl-Johnson-Avrami theory of nucleation.     

Small-angle X-ray and neutron scattering studies of the morphology of ionomers

Preliminary small-angle neutron scattering (SANS) studies have been made of different ionomers in the dry state and after saturation with water. Scattering from the dry samples arises from differences in the neutron scattering cross sections of the ionic and nonionic units in the polymer. The SANS technique is complementary to previous small-angle x-ray scattering (SAXS) studies since the SANS contrast differences are generally quite different than those for SAXS. A quantitative comparison is made of SANS and SAXS intensities for a dry cesium salt of an ethylene-methacrylic acid (E-MAA) copolymer. For water-saturated samples the technique of isotopic replacement can be used in conjunction with SANS since saturation can be effected with either H₂O or D₂O. In this case information about the chemical composition of the phases is obtained from an analysis of the intensity ratio I/I. Results are consistent with the presence of a separate phase containing water molecules and ions in a matrix of the nonionic units. A Guinier analysis gives a radius of gyration of 17 Å for a water-saturated cesium salt of an E-MAA copolymer.