Small-angle X-ray Scattering Curves Research Articles

Multiple-length-scale small-angle X-ray scattering analysis on maghemite nanocomposites

Small-angle X-ray scattering (SAXS) analysis has been performed on maghemite–poly(4-vinylpyridine) nanocomposites prepared by in situ precipitation from iron–polymer coordination compounds. According to electron microscopy observations, the nanocomposites contain isolated spherical particles with a narrow size distribution, uniformly distributed throughout the polymer matrix. The scattering intensity of nanocomposites has relevant contributions from both the polymer and the nanocomposites, showing features characteristic of multiscale structured systems, namely two power laws and a Guinier regime. The data have been analysed in terms of Beaucage's unified approach and it is found that the maghemite particle size increases with the iron/polymer weight ratio used in the preparation of the nanocomposites. SAXS curves also feature a bump that was analysed as arising from a second particle population or from interactions. Magnetization and transmission electron microscopy results give arguments favouring the latter interpretation. It is found that the maghemite particle sizes vary linearly with the iron weight ratio used in the preparation of the nanocomposites.

Reaction-Induced Microphase Separation in Epoxy Thermosets Containing Poly(ε-caprolactone)-block-poly(n-butyl acrylate) Diblock Copolymer

Reaction-induced microphase separation in epoxy thermosets containing an amphiphilic diblock copolymer was investigated in this work. To this end, the diblock copolymer poly(ε-caprolactone)-block-poly(n-butyl acrylate) (PCL-b-PBA) was synthesized via the combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). The block copolymer was incorporated into epoxy thermosets. Before the curing reaction all the subchains of the diblock copolymer were miscible with the precursors of epoxy resin. After curing, only the PBA blocks were separated out whereas the PCL blocks remained miscible with epoxy resin. Such a reaction-induced microphase separation results in the formation of ordered nanostructures in the thermosets. The nanostructures in the thermosets were investigated by means of field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), small-angle X-ray scattering (SAXS), and dynamic mechanical analysis (DMA). It is found that, depending on the concentration of the diblock copolymer, the thermosets can displayed spherical particles or lamellar nanostructures. The SAXS curves with the multiple scattering maxima suggests that the thermosets possess long-range ordered nanostructures.

Porphyrin Effects on Zwitterionic HPS Micelles as Investigated by Small-Angle X-ray Scattering (SAXS) and Electron Paramagnetic Resonance (EPR)

In this work, small-angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR) studies on the interaction of three anionic mesotetrakis (4-sulfonatophenyl) porphyrins, TPPS4, FeTPPS4, and ZnTPPS4, at concentrations in the 2-10 mM range, with micelles of the zwitterionic surfactant 3-(N-hexadecyl-N,N-dimethylammonium) propane sulfonate (HPS, 30 mM) at pH 4.0 and 9.0 are reported. The SAXS results demonstrate that, upon addition of all species of porphyrins, the HPS micelle of prolate shape reduces its axial ratio from 1.8 +/- 0.2 (in the absence of porphyrin) to 1.5 +/- 0.1. Such an effect is accompanied by a shrinking of the paraffinic shortest semiaxis from 22.5 +/- 0.5 A to 18.0 +/- 0.2 A. This shows that the micellar hydrophobic core is affected by porphyrin incorporation, independent of the type of porphyrin and pH. Concurrently, EPR results demonstrate an increase in the micellar packing as noticed from the increase in motional restriction for both nitroxides. Furthermore, increase of the porphyrin concentration induces the appearance of a repulsive interference function over the SAXS curve of zwitterionic micelles, which is typical of an interaction between surface-charged micelles. Such a finding gives strong evidence that the negatively charged porphyrin molecule must accommodate in the HPS micelle dipole layer close to the inner positive charges (near the hydrophobic core), inducing a surface charge (probably a negative one associated with the HPS sulfonate external groups) in the original zwitterionic (overall neutral) micelle. Such a porphyrin location is favored by both electrostatic and hydrophobic contributions, giving rise to binding constant values that are quite large compared to the binding of cationic drugs to HPS micelles (Caetano, W.; Barbosa, L. R. S.; Itri, R.; Tabak, M. J. Coll. Int. Sci. 2003, 260, 414).

Structural Basis of Cellulosome Efficiency Explored by Small Angle X-ray Scattering

Cellulose, the main structural component of plant cell walls, is the most abundant carbohydrate polymer in nature. To break down plant cell walls, anaerobic microorganisms have evolved a large extracellular enzyme complex termed cellulosome. This megadalton catalytic machinery organizes an enzymatic assembly, tenaciously bound to a scaffolding protein via specialized intermodular "cohesin-dockerin" interactions that serve to enhance synergistic activity among the different catalytic subunits. Here, we report the solution structure properties of cellulosome-like assemblies analyzed by small angle x-ray scattering and molecular dynamics. The atomic models, generated by our strategy for the free chimeric scaffoldin and for binary and ternary complexes, reveal the existence of various conformations due to intrinsic structural flexibility with no, or only coincidental, inter-cohesin interactions. These results provide primary evidence concerning the mechanisms by which these protein assemblies attain their remarkable synergy. The data suggest that the motional freedom of the scaffoldin allows precise positioning of the complexed enzymes according to the topography of the substrate, whereas short-scale motions permitted by residual flexibility of the enzyme linkers allow "fine-tuning" of individual catalytic domains.

On the Supermolecular Structure of Fullerene C60 Solutions

Small angle X‐ray scattering (SAXS) and wide angle X‐ray scattering (WAXS) techniques were used for investigation of fullerene C60 solutions in toluene and p‐xylene. On all SAXS curves, intensity decreases to some constant value of IC with increase of scattering angle. The value of IC depends on concentration non‐monotonically: it first slightly increases, then drops sharply to some minimal value, and then increases again. A qualitative explanation of such dependence is offered. It is supposed that the presence of fullerene C60 in solution suppresses thermal fluctuations of density in the solvent. In combination with the X‐ray data the results obtained for solutions of fullerene C60 by various others techniques (calorimetry, densimetry, etc.) are discussed. Possible models of a supermolecular structure of fullerene C60 solutions in good solvents are considered.

Size and shape of lead–organic associations

The relevance of mobile organic matter for the transport of Pb through soils has been shown through several studies. Up until this study, no investigation has been published on whether the adsorption of Pb alters the size and/or shape, and consequently the mobility, of organic substances. We hypothesize that the addition of Pb induces the formation of organic precipitates by intermolecular cross-linking. We extracted organic matter from forest floor samples and determined the size and shape of the organic colloids via small angle X-ray scattering (SAXS) and their zeta potential. Measurements were conducted at three Pb concentrations (0, 0.05 and 0.10 mmol l −1) and at varying pH values (2.5, 4, 7, and 8). Under neutral and alkaline conditions, we observed geometrically weakly defined colloids possessing a maximal dimension of 33 nm. Either decreasing the pH value or increasing the Pb concentration induced the formation of organic matter rods with a corresponding decrease in the maximal dimension to 24 nm. Zeta potential measurements indicate that this decrease is due to a reduction of the repulsion between functional groups of the natural polyelectrolytes. SAXS curves indicate the formation of polymolecular Pb–organic agglomerates at pH 4 and 0.1 mM Pb, which exhibit a high colloidal stability. We conclude from our results that Pb is not merely passively co-transported by organic matter in soils but that it may also accelerate its mobile sorbents through the compaction of organic molecules or by the formation of highly mobile organic colloids.

Small-angle X-ray scattering and electron paramagnetic resonance study of the interaction of bovine serum albumin with ionic surfactants.

Small-angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR) techniques have been used to monitor the interaction of bovine serum albumin (BSA) with ionic surfactants such as anionic sodium dodecyl sulfate (SDS), zwitterionic N-hexadecyl-N,N-dimethyl-3-ammonium-1-propane sulfonate (HPS), and cationic cethyltrimethylammonium chloride (CTAC) at pH 7.0. The SAXS results have shown that in the presence of 5 mM SDS and HPS the radius of gyration (Rg) almost does not change as compared to the BSA free-surfactant solution; its value is ca. 30 Angstroms. In the presence of 5 mM CTAC the SAXS data indicate the presence of a particle with a Rg of at least 63 Angstroms, suggesting that in this case, a kind of protein aggregation takes place. In the presence of SDS and HPS surfactants at concentrations above 10 mM, a characteristic broad peak in the region of 0.12-0.18 Angstroms(-1) indicates the presence of micelle-like aggregates in solution. The SAXS curves are consistent with the "pearl necklace" model, where micelle-like aggregates are randomly distributed around the polypeptide chain. EPR results using 5-DSA and 16-DSA spin labels show that in the presence of BSA the EPR spectra are composed of two label populations, one contacting the protein and a second one due to label localization in the micelles. Evidence is also obtained for a competition of the surfactants with the spin labels for the high-affinity binding sites of the stearic acid spin labels as monitored by changes in the fractions of the two label populations as the surfactant concentration is increased. The effect of SDS seems to be stronger in the sense that increased SDS concentration leads to a complete transfer of spin labels from close protein contact sites to micelles, while for HPS, a significant immobilization of probe apparently remains even at higher surfactant concentrations. These two techniques are quite useful since SAXS monitors the overall properties of the scattering particle, while EPR gives information on the dynamics inside this particle and associated with label localization and motion.

Bovine serum albumin (BSA) plays a role in the size of SDS micelle-like aggregates at the saturation binding: the ionic strength effect

The influence of ionic strength on the complexes formed by natural bovine serum albumin (BSA), pH 5.4 (near the isoelectric point), and sodium dodecyl sulfate (SDS) in aqueous buffered (sodium acetate) solution was investigated by using surface tension, fluorescence and small angle X-ray scattering (SAXS) techniques. Ionic strength was varied by changing sodium acetate buffer concentration from 0.020 to 0.5 M. Surface tension revealed that SDS:BSA saturation binding occurs at psp = 42 +/- 2 mM, independent of the solution ionic strength. Further, SAXS curves are consistent with the necklace and bead model, where micelle-like aggregates are randomly distributed along the partial unfolded protein. Micelle-like aggregates grow from small spheres at 10 mM SDS to small ellipsoids (upsilon = 1.3 , ratio between the largest and the shortest axes) near psp, in good agreement with micellar aggregation numbers obtained by fluorescence, independent of salt concentration. Protein-bound micelles stop growing above psp and further SDS addition induces free-micelle formation.

X-ray structure investigations of winter wheat membrane systems. I. Influence of phytohormones on phospholipid orientation in non- and embryogenic cells

Changes in surface structure of liposomes in aqueous solutions obtained from phospholipids of plasmalemma of winter wheat cells were analysed by small-angle X-ray scattering (SAXS) method. Winter wheat cells were obtained from immature inflorescences and cultured in Murashige and Skoog medium supplemented with auxin (non-embryogenic calli, NE) or without phytohormones (embryogenic calli, E). Measurements were performed in the fully hydrated liposomal dispersions in pure water:methanol (100:1 v/v) buffer or in buffer supplemented by auxins (IAA and its chemical analogue melatonine) or cytokinins (kinetin and its analogue mercaptopurine). Both types of liposomes (NE and E) were different in the hydrophilic and hydrophobic phase. The higher amount of PE in comparison to PC as well as the higher content of unsaturated fatty acids, especially 18:2 and 18:3, were observed in E membranes. The SAXS curves from NE and E liposomes indicate the possibility of bilayer formation by these phospholipids. Changes in distance distribution functions derived for NE and E phospholipid liposomes could be explained by changes of bilayer thickness. All investigated hormones were affected only E liposomes and did not have an influence on NE liposomes. These changes of the bilayer size of E membranes after phytohormone treatment were in the range of those detected for NE membranes. This was observed especially for IAA and its chemical analogue. Kinetin influenced bilayer thickness in a lesser degree and inconsiderable differences were noticed for its chemical analogue—mercaptopurine. Thus we suggest that removal of auxins from winter wheat culture media is necessary to maintain structure of membranes characteristic for embryogenic cells.

Particle Size Distribution of Nanospheres by Monte Carlo Fitting of Small Angle X-Ray Scattering Curves

A method for recovering the size distribution of spherical particles from small angle scattering data by using a Monte Carlo interference function fitting algorithm is presented. The method is based on the direct simulation of the small angle scattering data upon the assumption of non-interacting hard sphere ensembles (“dilute” solution approximation). The algorithm for retrieving the particle size distribution does not require any additional parameters apart from the input of the scattering data. The fitting strategy necessarily implies positive particle size distributions, while preserving the advantage of the indirect transformation method for data desmearing. Furthermore, the present approach does not use any regularisation procedures of the best fit solution and favours smooth particle size distributions. The Monte Carlo procedure has been tested against several simulated cases with various types of mono- and bi-modal size distributions and different noise levels. In the special case of non-interacting spheres, the Monte Carlo fitting algorithm had the same retrieving ability as the well assessed indirect transformation, structure interference and maximum entropy methods. Finally, the algorithm was applied to retrieve the distribution of spherical nanopowders produced by gas-to-particle conversion both as free powder and as reinforcing second-phase agent in polymer nanocomposites.

Structure and thermoelasticity of irradiation grafted nano-inorganic particle filled polypropylene composites in the solid state1

Nanoparticles (mean size around 7 nm) of the standard pyrogenic Aerosil 1380 (Degussa) pregrafted by γ-irradiation with styrene were melt-compounded with the general purpose isotactic polypropylene homopolymer (PP) to prepare nanocomposites with filler volume contents up to 4.68%. Solid-state properties of the nanocomposites were characterized by wide-angle x-ray scattering (WAXS), small-angle x-ray scattering (SAXS), differential scanning calorimetry, and stretching calorimetry. The nearly identical overall patterns, the angular positions of the crystalline reflections on the WAXS diagrams as well as the WAXS degrees of crystallinity of both the neat polymer PP-0 and all nanocomposites suggested that the structure of the crystalline PP lamellae remained unchanged, irrespective of the filler content. However, a well-resolved SAXS reflection seen for PP-0 was not detectable on the SAXS curves of nanocomposites with low filler contents due to the sharp increase of SAXS intensity in the same range of scattering vectors. These results implied a significant increase in structural heterogeneity due to the appearance of new and strongly scattering entities (presumably polymer–nanoparticle interfaces and microvoids) with a broad distribution of sizes. In contrast to the basically composition-invariant WAXS crystallinities for nanocomposites, higher the filler volume content, the calorimetric crystallinities for the polymer matrix tended to increase, while the apparent densities of the polymer matrix decreased. Moreover, the Young's moduli of nanocomposites were considerably in excess of, whereas thermal expansivities, limiting strains for elastic behavior, and breaking strains, were much below the reasonable theoretical predictions. These experimental observations were explained by a model assuming that a nonnegligible portion of PP chains in the melt state would be anchored by each end to the available absorption-active sites of two different neighboring nanoparticles. The restricted chain mobility in these sites should facilitate the crystal nucleation in the undercooled PP melt; hence, the same PP chain might be involved in two nucleation events at the surfaces of two adjacent nanoparticles. Presumably, subsequent crystallization in the undercooled melts of both neat PP and nanocomposites would proceed via the usual growth of chain-folded lamellae (therefore, the WAXS patterns should be similar). However, the tie-chains in the interlamellar space of the neat PP are expected to remain in the relaxed, coiled state, whereas in the latter case, a simultaneous lamellar growth at fixed positions of the same PP chains on adjacent nanoparticles would end up with not only a considerable extension of tie-chains but also with a concomitant fall in the local packing density in the interlamellar space. *Dedicated to Prof. F. J. Baltá Calleja on the occasion of his 65th birthday.

The effect of poly(ethylene glycol) on the activity and structure of glucose-6-phosphate dehydrogenase in solution

The effect of poly(ethylene glycol), PEG, on the enzymatic activity of glucose-6-phosphate dehydrogenase (G-6-PDH) in the oxidation of glucose-6-phosphate (G-6-P), using NADP+ as co-enzyme was investigated. The enzymatic activity was determined by means of spectrophotometry in three different media: pure Tris–HCl buffer, solution of PEG400 (20 wt.%) and of PEG4000 (20 wt.%), both in buffer. Comparing the enzymatic activity values measured in pure buffer with those in the polymer solutions, an increase in the enzymatic activity of 20% was observed in the presence of PEG400 as well as in PEG4000. Calorimetric studies indicated the absence of preferential interactions between G-6-PDH and PEG400 or PEG4000. Nevertheless, the interaction enthalpy, Δ H int, between NADP+ and PEG400 and PEG4000 amounted to −9.3 and −26.7 kJ/mol, respectively. Small angle X-ray scattering (SAXS) measurements were performed in a higher concentration range. Data analysis performed from SAXS curves by means of the intra-particle distance distribution function p( r) and Guinier plots yielded for G-6-PDH in pure buffer and PEG400 solutions radius of gyration, R g, of about 70 Å and in PEG4000 solutions, R g of about 40 Å. The latter has the same dimension as that found in the dimeric crystallographic structure of G-6-PDH, evidencing that G-6-PDH preserves its dimeric configuration in PEG4000 solution. On the contrary, different aggregates of G-6-PDH are formed in the presence of either buffer or PEG400. These findings show that the presence of PEG in solution can exert an effect on the enzyme structure and activity.

Surface roughness of sputter-deposited gold films: a combined x-ray technique and AFM study

We applied grazing incidence x-ray specular and non-specular reflectivity (GIXR) and small-angle x-ray scattering (SAXS) to study the surface roughness of gold films on quartz glass substrates. Two model systems were prepared: one series of Au films was sputtered in Ar at 3×10−3mbar (Au1 films) and one in residual air at 3×10−1 mbar (Au2). The combined x-ray experiments showed that the Au1 films are compact and smooth, with-mean-square (rms) roughness values too low to be quantified by means of atomic force microscopy (AFM) or SAXS. The surfaces of film and substrate exhibit a strong cross correlation that is gradually lost with increasing film thickness. The surface height–height correlation functions are well described by a self-affine model. In contrast, as directly proved via AFM, the Au2 films consist of islands that are coarsening with increasing film thickness. The film surface showed no cross-correlation with the substrate surface at all. The SAXS curves from the Au2 films were attributed to a volume scattering effect, by interpreting the films in terms of a two-dimensional array of hard hemispheres. Copyright © 1999 John Wiley & Sons, Ltd.

Physical properties and structure of organic-inorganic hybrid materials produced by sol-gel process

Two approaches of the sol-gel process to prepare organic-inorganic hybrids are reviewed. One method is simple and involves mixing an organic polymer with a metal alkoxide such as tetraethoxysilane (TEOS). During the sol-gel process the inorganic mineral is deposited in the organic polymer matrix forming hydrogen bonding between organic phase and inorganic phase. In the small angle X-ray scattering (SAXS) profiles of such hybrids, the scattering intensity increases with increasing amount of inorganic mineral incorporated into the organic matrix, but no peak occurs. The dynamic modulus of such hybrids increases with increasing TEOS content and is accompanied by a decrease in tan δ peak intensity. No significant change in the position of the tan δ peak occurs, because the molecular motions of the organic polymer are not restricted by the deposited silica component. Hydrogen bonded hybrid systems of hydroxypropyl cellulose (HPC), poly(vinyl alcohol) (PVA), and poly(vinylidene fluoride) (PVDF) are described. Another method is to introduce triethoxy silyl groups into the organic polymer prior to the sol-gel reactions with TEOS. In this review two methods are described. One method involves terminating the ethoxy silanes on both ends of the organic polymer with urethane or urea linkages using isocyanatopropyltriethoxysilane. Another method is to copolymerize the organic monomer with vinyl triethoxysilane. The SAXS profiles of covalently bonded hybrids are much different than those for hydrogen bonded hybrids. A large sharp peak occurs in the SAXS curves and the peak height increases with increasing TEOS content. This is perhaps due to micro phase separation between silica-rich domains and the organic polymer matrix. The dynamic modulus increases with increasing TEOS content and the tan δ peak height decreases. The tan δ peak position shifts towards higher temperatures due to the molecular motions of the organic polymer being restricted by the silica rich domains. Highly functional hybrids derived from the sol-gel process are being developed for practical applications such as coatings, contact lenses and non-linear optical, electrochromic, and electroconductive materials.

Analysis of Slit-Smeared Small-Angle Scattering Data from Interacting SystemsviaIndirect-Transformation Methods

The indirect-transformation method in reciprocal space (ITR) is used to obtain the desmeared small-angle X-ray scattering (SAXS) curve of semidilute aqueous solutions of fragmented DNA, which have interparticle interference effects. Systematic errors are observed only for a particular concentration, at very low values of the scattering vector q of the calculated desmeared intensity J desm . Although interparticle interactions already exist for the less concentrated solution (10 g l -1 ), the interference effects fall below the measured q range. In order to know if these effects are in fact negligible on the corresponding SAXS curve, the cross-section distance-distribution function p c (r) is calculated using the indirect-transformation program (ITP) and both the ITR program and ITP are tested to calculate J desm . It is concluded that for the less concentrated solution ITP gives the best solution, allowing determination of the particle form factor. For the higher concentrations, the use of ITR desmearing allows one to obtain the interference function.

Small-Angle X-ray Scattering from Aqueous Solutions of Tetra(oxyethylene)-n-octyl Ether

Aqueous solutions of the nonionic surfactant C8E4 with different mass fractions of the amphiphile have been studied by small-angle X-ray scattering (SAXS) with dependence on temperature. The demixing temperatures of the samples were determined by viscosity and ultrasonic speed measurements. The temperature dependence of measures shows discontinuities at demixing of the sample. Below the demixing temperature the viscosity measurements indicate increasing size of the solute particles. The SAXS curves can be well described throughout the whole temperature and composition range with the model of a inhomogeneous sphere with a core radius of about 1 nm. This value is in good agreement with the theoretical length of an all-trans-octyl chain. The combination of these results with the viscosity data supports the model of aggregating spherical micelles.

A small-angle x-ray scattering study of microstructure evolution during sintering of sol-gel-derived porous nanophase titania

The evolution of microstructure as a function of firing temperature in sol-gel derived porous titania xerogels was investigated by small-angle x-ray scattering (SAXS). SAXS curves for xerogels fired below 550 °C exhibit a well-defined structure peak. This peak indicates the presence of a high degree of order in the electron density correlations associated with the interparticle structure factor. Results from electron microscopy and SAXS give a primary particle mean diameter of 50 Å, while scaling analysis of the scattered intensity at large momentum transfer values yields the Porod exponent 4, indicating a sharp transition zone between the solid and void phases. The pore volume fraction of the unfired xerogel is consistent with random close-packing of spheres. The internal surface area decreases almost linearly with increasing firing temperature. Rapid grain growth and pore coarsening begin near 90% of theoretical density, and lead to a breakdown in pore interconnectedness and the development of isolated pores. Observed enhanced sintering properties may be attributed primarily to the large surface-to-mass ratio of the sol-gel particles. The SAXS curves were adequately fit using a bicontinuous phase model developed for late-stage spinodal decomposition structures. Alternatively, the microstructure can be described by a hierarchical close-packing of spheres model. SAXS results are compared with data from gas adsorption and electron microscopy.

Surface Graininess and a New Model of Nonionic Micelle of Octaoxyethylene Alkyl Ether

AbstractSurface graininess and micelle size of n‐dodecyl (octaoxyethylene glycol) monoether C12H25(OCH2CH2)8OH in an isotropic liquid phase (L1 phase) and a hexagonal mesophase (H1 phase) of an aqueous colloid were investigated through light scattering and small angle X‐ray scattering (SAXS). The micelle is almost spherical and the micelle radius remains about 28–30 Å over a wide concentration range from the critical micelle concentration (ca. 4 · 10−5 g/ml) to 0.3 g/ml. The aggregation number is 70–80 and depends slightly on temperature. The specific inner surface Os of the spherical micelle is obtained from the tail portion (large angle region) in the SAXS curve. The value of Os of the micelle in the L1 phase is ca. 0.2 Å−1 and this value is approximately twice as large as that of a smooth sphere. A new micelle model is also proposed.

Small-angle x-ray scattering studies on epoxidized butadiene-styrene block copolymers

Small-angle x-ray scattering (SAXS) and scanning electron microscopy (SEM) investigations for epoxidized butadiene-styrene (BS) block copolymers were performed. For unepoxidized copolymers, the SAXS curve exhibits a maximum which indicates that the copolymer has heterogeneity domain morphology. Using the standard theory for a two-phase system, mean distances between domains, the correlation length lp, and the thickness of the phase boundary were calculated from the SAXS data. It was found that the epoxidation of BS copolymers decreases the ability of the copolymer to separate the individual components. As the content of the epoxide groups increases, the dimensions of the domains decrease until they disappear, the boundary between domains and the matrix becomes less and less definite, and the copolymer composes a homogeneous system. The disappearance of the two-phase structure of the BS copolymers indicates an increase in the compatibility of polystyrene and epoxidized polybutadiene. According to the method of Van Krevelen, the solubility parameters of polystyrene and epoxypolybutadiene were calculated. Small differences between these parameters support the conclusions drawn from the SAXS investigations

Structural study of silica sonogels

Silica sonogels obtained by the hypercritical drying of gels from the hydrolysis of a tetraethoxysilane + water mixture submitted to the action of ultrasounds were studied using small-angle X-ray scattering (SAXS), BET and density measurements. The Guinier regions in SAXS curves are wider and gyration radii are smaller than those of aerogels prepared from alcoholic dilution. These sonogels do not present self-similarity in the accessible scale length of the SAXS measurements. Slopes greater than 4 in the high angle region reveal important electronic density fluctuations on the pore-matrix boundaries, due to their very fine porosity as a consequence of the sonocatalysis.