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Small Angle Neutron Scattering (small + angle_neutron_scattering)
Selected AbstractsA New View of the Anionic Diene Polymerization MechanismMACROMOLECULAR SYMPOSIA, Issue 1 2004A.Z. Niu Abstract We investigated the anionic polymerization of butadiene in d-heptane solvent using tert -butyl lithium as initiator. Two complementary techniques were used to follow the polymerization processes: 1H NMR and small angle neutron scattering (SANS). The time resolved 1H NMR measurements allowed us to evaluate quantitatively the kinetics of the processes involved. The initiation event commences slowly and then progressively accelerates. This indicates an autocatalytic mechanism. The microstructure of the first monomer units attached is to a high extent 1,2. The disappearance of initiator --- at about 10% monomer conversion --- signals the onset of the normal ,6% vinyl content of the chain. Small angle neutron scattering was used to study the aggregation behavior of the carbon lithium head groups. It is well known that the polar head groups aggregate and form micellar structures. For dienes in non-polar solvents the textbook mechanism assumes the formation of only tetramers during the propagation reaction. By combining 1H NMR and SANS results we were able to determine quantitatively the aggregation number during all stages of the polymerization. Our measurements show the existence of large-scale structures during the initiation period. The initial degree of aggregation of more than 100 living polymer chains diminished as the polymerization progressed. In addition, even larger, giant structures with Nagg >>1000 and Rg , 1000Å were found. [source] Nano-sized ceramics of coated alumina and zirconia analyzed with SANSJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-1 2000U. Keiderling The sintering behaviour of two new types of coated ceramics, made from alumina grains coated with a zirconia shell and from zirconia grains coated with an alumina shell, was analyzed with small angle neutron scattering (SANS). Measurements were performed both for the plain samples, and with contrast variation using D2 O as immersion liquid. The size distribution and the volume fraction of grains and pores were determined from the corrected scattering curves using a direct model fitting, applying two different approaches, a sphere model and a combined sphere/spherical shell model. Results are discussed in context with the macroscopic density of the samples. The sintering behaviour of the two ceramics types was found to be very different. [source] Scattering vector dependence of the small-angle scattering from mixtures of hydrogenated and deuterated organic solventsJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-1 2000Lise Arleth In connection with a contrast variation small angle neutron scattering (SANS) study of water-in-oil microemulsions, mixtures of hydrogenated and deuterated n -decane and similar mixtures of iso-octane have been measured as backgrounds. For the pure deuterated and hydrogenated solvents the spectra were flat but for all the mixtures the large molecular size of the alkanes give rise to a scattering vector dependence in the small angle scattering region. An explanation of this can be given based on the scattering contributions which have been identified for mixtures of water. The intensity is proportional to x(x, 1) where x is the volume fraction of deuterated molecules. Using the analogy to scattering from polymer blends of hydrogenated and deuterated chains, the scattering spectra are analysed using the model for random gaussian coils. For the solvent mixtures investigated in the present study, the radius of gyration was determined to be 3.8 Å for decane and 2.9 Å for iso-octane, independent of the fraction of deuterated molecules. [source] Development of spin-echo small-angle neutron scatteringJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-1 2000W. G. Bouwman A polarised neutron spin echo technique is used to build a novel kind of small angle neutron scattering (SANS) instrument. The basis of this instrument is a symmetric set-up with a spin flipper in the centre, which creates a spin echo, even with a divergent beam. The precession regions on either side of the spin flipper are shaped such as to produce a very sensitive relation between the vertical angle of the neutron path and the total precession angle. Any SANS of a sample placed in the instrument reduces the symmetry of the neutron path and therefore decreases the echo. Magnetised foils define the precession regions by rotating the neutron spin from being parallel to the magnetic field to perpendicular to the field, to start the precession. These foils and the flipper were built and tested. A spin echo SANS signal is measured with the complete set-up . It should be possible with this technique to measure within minutes a full correlation function in samples over distances from 5 to 1000 nm. [source] SANS polarization analysis with nuclear spin-polarized 3HeJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-1 2000T.R. Gentile A neutron spin filter based on transmission through nuclear-spin-polarized 3He gas has been applied to polarization analysis of small angle neutron scattering (SANS). Such spin filters, which are based on the large spin dependence of the absorption of neutrons by 3He, make SANS polarization analysis possible because of their large angular acceptance. In the present experiment, a 3He-based analyzer was employed to separate nuclear scattering into its coherent and spin-incoherent components. Polarized 3He analyzers were prepared by two different optical pumping methods and installed on the NG3 SANS instrument at the NIST Center for Neutron Research (NCNR). Measurements were taken on cellophane tape and silica gel, for which the scattering is almost completely incoherent and coherent, respectively, and on a combined sample. For the combined sample, separation of the coherent part from the incoherent part was successfully demonstrated using polarization analysis. [source] Self-assembling properties of ionic-complementary peptides,JOURNAL OF PEPTIDE SCIENCE, Issue 3 2009Gabriella D'Auria Abstract Self-complementary synthetic peptides, composed by 8 and 16 residues, were analyzed by CD, NMR and small angle neutron scattering (SANS) techniques in order to investigate the relevance of charge and hydrophobic interactions in determining their self-assembling properties. All the sequences are potentially able to form fibrils and membranes as they share, with the prototype EAK16, a strictly alternating arrangement of polar and nonpolar residues. We find that 16-mer peptides show higher self-assembling propensities than the 8-mer analogs and that the aggregation processes are favored by salts and neutral pH. Peptide hydrophobic character appears as the most relevant factor in determining self-assembling. Solution conformational analysis, diffusion and SANS measurements all together show that the sequences with a higher self-assemble propensity are distributed, in mild conditions, between light and heavy forms. For some of the systems, the light form is mostly constituted by monomers in a random conformation, while the heavy one is constituted by ,-aggregates. In our study we also verified that sequences designed to adopt extended conformation, when dissolved in alcohol-water mixtures, can easily fold in helix structures. In that media, the prototype of the series appears distributed between helical monomers and ,-aggregates. It is worth noticing that the structural conversion from helical monomer to ,-aggregates, mimics ,-amyloid peptide aggregation mechanisms. Copyright © 2008 European Peptide Society and John Wiley & Sons, Ltd. [source] A New View of the Anionic Diene Polymerization MechanismMACROMOLECULAR SYMPOSIA, Issue 1 2004A.Z. Niu Abstract We investigated the anionic polymerization of butadiene in d-heptane solvent using tert -butyl lithium as initiator. Two complementary techniques were used to follow the polymerization processes: 1H NMR and small angle neutron scattering (SANS). The time resolved 1H NMR measurements allowed us to evaluate quantitatively the kinetics of the processes involved. The initiation event commences slowly and then progressively accelerates. This indicates an autocatalytic mechanism. The microstructure of the first monomer units attached is to a high extent 1,2. The disappearance of initiator --- at about 10% monomer conversion --- signals the onset of the normal ,6% vinyl content of the chain. Small angle neutron scattering was used to study the aggregation behavior of the carbon lithium head groups. It is well known that the polar head groups aggregate and form micellar structures. For dienes in non-polar solvents the textbook mechanism assumes the formation of only tetramers during the propagation reaction. By combining 1H NMR and SANS results we were able to determine quantitatively the aggregation number during all stages of the polymerization. Our measurements show the existence of large-scale structures during the initiation period. The initial degree of aggregation of more than 100 living polymer chains diminished as the polymerization progressed. In addition, even larger, giant structures with Nagg >>1000 and Rg , 1000Å were found. [source] Calcium induced volume transition in polyelectrolyte gelsMACROMOLECULAR SYMPOSIA, Issue 1 2003Ferenc Horkay Abstract The osmotic properties and the small angle neutron scattering (SANS) behaviour of fully neutralized sodium polyacrylate gels are investigated in the presence of calcium ions. Analysis of the SANS response displays three characteristic length scales, two of which are of thermodynamic origin, while the third, associated with the frozen-in structural inhomogeneities, is static. The SANS results are consistent with direct osmotic observations which indicate that the thermodynamic properties cannot be adequately described by a single correlation length. The concentration dependence of the osmotic pressure displays a power law behaviour with an exponent that decreases with increasing calcium concentration. [source] Small angle neutron scattering study of the structure and hydration of polyoxyethylene- block -polyoxybutylene in aqueous solutionPOLYMER INTERNATIONAL, Issue 7 2006J Patrick A Fairclough Abstract The micellar structure of a polyoxyethylene- block -polyoxybutylene diblock copolymer (E18B10; E = (OCH2CH2); B = OCH2CH(CH2CH3)) in water was studied by small angle neutron scattering. Results show a polydisperse spherical micelles at temperatures below 45 °C, a possible mixed phase at 45 °C and flexible worm-like micelles at 65 °C. The spherical micelles at 25 and 35 °C have radii of 39 and 40 Å and degrees of hydration of 28% and 26%, respectively. The worm-like micelles have a total length of approximately 1850 Å, with segment lengths of 123 Å, a core radius, Rax = 30 Å and hydration of 23%, at 65 °C. Copyright © 2006 Society of Chemical Industry [source] Self-Assembly of Two-Component Gels: Stoichiometric Control and Component SelectionCHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2009Andrew Abstract Two-component systems capable of self-assembling into soft gel-phase materials are of considerable interest due to their tunability and versatility. This paper investigates two-component gels based on a combination of a L -lysine-based dendron and a rigid diamine spacer (1,4-diaminobenzene or 1,4-diaminocyclohexane). The networked gelator was investigated using thermal measurements, circular dichroism, NMR spectroscopy and small angle neutron scattering (SANS) giving insight into the macroscopic properties, nanostructure and molecular-scale organisation. Surprisingly, all of these techniques confirmed that irrespective of the molar ratio of the components employed, the "solid-like" gel network always consisted of a 1:1 mixture of dendron/diamine. Additionally, the gel network was able to tolerate a significant excess of diamine in the "liquid-like" phase before being disrupted. In the light of this observation, we investigated the ability of the gel network structure to evolve from mixtures of different aromatic diamines present in excess. We found that these two-component gels assembled in a component-selective manner, with the dendron preferentially recognising 1,4 - diaminobenzene (>70,%), when similar competitor diamines (1,2- and 1,3-diaminobenzene) are present. Furthermore, NMR relaxation measurements demonstrated that the gel based on 1,4-diaminobenzene was better able to form a selective ternary complex with pyrene than the gel based on 1,4-diaminocyclohexane, indicative of controlled and selective ,,, interactions within a three-component assembly. As such, the results in this paper demonstrate how component selection processes in two-component gel systems can control hierarchical self-assembly. [source] | |||||||||||||