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Justino L.L.G.,University of Coimbra | Ramos M.L.,University of Coimbra | Knaapila M.,Institute for Energy Technology of Norway | Marques A.T.,University of Coimbra | And 9 more authors.
Macromolecules | Year: 2011

Interactions between polymer chains of poly(9,9-dioctylfluorene-2,7-diyl) (PF8) have been studied in toluene solution over a wide concentration range using multinuclear NMR spectral and relaxation measurements with both the fully protonated and alkyl chain deuterated polymers, small angle neutron scattering (SANS), together with theoretical calculations using DFT and semiempirical methodologies. Full assignment of the 1H and 13C chemical shifts in the NMR spectra of isolated chains of PF8 has been made using DFT, and are in good agreement with spectra in chloroform and in toluene solutions. Somewhat different behavior is seen in toluene solution, where, upon increasing polymer concentration, broadening of the alkyl chain resonances is seen, consistent with interactions between the side chains. Similar behavior is seen with the 2H resonance of PF8-d34. In both cases, line-narrowing and restoration of the structured alkyl chain resonances is seen upon studying the spectra of concentrated solutions using the magic angle spinning (MAS) technique, in agreement with the attribution of the broadening to interchain interactions between the octyl groups. Support for this interpretation comes from 1H and 13C spin - lattice relaxation time measurements, which also show differences in group dynamics along the alkyl chains. Semiempirical theoretical calculations, using PM3 and PM6 Hamiltonians, add further support to the importance of interactions between the alkyl groups of separate PF8 chains. SANS measurements on PF8 in toluene solutions from very dilute to concentrated solutions, extended to ultrasmall q ranges, provide further insight. Three concentration ranges can be identified. In dilute solutions, the results suggest that PF8 is present as fully dissolved polymer coils. Upon increasing polymer concentration, an intermediate region is observed, in which there are indications of transient contacts between the polymers, which the NMR results suggest involves side chain interactions. As a consequence of interactions between the chains, gel formation occurs. On the basis of these and previous results, some general considerations are presented upon the solubility and aggregation behavior of PF8, including indications of how interactions between alkyl chains may be important in the stabilization of the so-called β-phase of PF8. © 2010 American Chemical Society. Source

Thakur N.,Bhabha Atomic Research Center | Yusuf S.M.,Bhabha Atomic Research Center | Paulose P.L.,Tata Institute of Fundamental Research | Keller L.,Laboratory for Neutron Scattering
Journal of Applied Physics | Year: 2012

We have controlled the nature of magnetic ordering in Rb xBa yMn 3-(x 2y)/2Fe(CN) 6zH 2O prussian blue analogues by alkaline earth metal cation (Ba 2) substitution and application of an external magnetic field. Structural analysis, by employing Rietveld refinement of X-ray and neutron diffraction patterns, reveals a tetragonal crystal structure for all these compounds. The presence of Fe 3-C N-Mn 2 chains in the structure, and absence of both cyanide flipping (Mn 2-C N-Fe 3) and charge transfer from Mn 2 to Fe 3 (Fe 2-C N-Mn 3) have been confirmed by infrared and Mössbauer studies. The analysis of the low temperature neutron powder diffraction data reveals a long-range antiferromagnetic ordering in these compounds. The magnetic structure is layered, consisting of an antiparallel stacking of ferromagnetic sheets along the crystallographic c-direction. Within each sheet in the ab plane, the ordered moments of Mn 2 5.04(1) B for (x 0.84, y 0) and 4.99(7) B for (x 0.19, y 0.3) sample at 1.5 K and Fe 3 0.98(3) B for (x 0.84, y 0) and 0.99(3) B for (x 0.19, y 0.3) sample at 1.5 K are aligned ferromagnetically along the c-axis. Interestingly, dc magnetization study indicates that on application of an external magnetic field, an antiferromagnetic to ferrimagnetic-like phase transition occurs below ∼5 K. This phase transition becomes more and more prominent with increasing magnetic field as well as for higher Ba substitution. The role of a number of factors (such as ionic radius of Ba 2 ion and change in net ligand strength around Mn due to chemical insertion of Ba 2 ions) on the affecting nature of magnetic ordering has been discussed. The observed magnetism in these compounds is seen in light of the orbital model of superexchange interaction between magnetic ions (Mn 2 and Fe 3) mediated by cyanide ligands. © 2012 American Institute of Physics. Source

Parekh K.,Indian Institute of Technology Gandhinagar | Almasy L.,Laboratory for Neutron Scattering | Lee H.S.,Korea Institute of Geoscience and Mineral Resources | Upadhyay R.V.,Charotar University of Science & Technology
Applied Physics A: Materials Science and Processing | Year: 2012

Zero field cooled dc-magnetization measurements of monodispersed Mn 0.5Zn 0.5Fe 2O 4 nanoparticles dispersed in kerosene exhibit two transitions at low temperatures. These transitions correspond to (i) the superparamagnetic to blocked superparamagnetic and (ii) the blocked superparamagnetic to surface spin-glass like/quantum superparamagnetic state upon lowering the temperature. The existence of a disorder surface is confirmed by recording small-angle neutron scattering data below and above the Curie temperature. Magnetic relaxation analysis shows a plateau at low temperature (below 5 K) with a slight minimum at 3 K, which is a characteristic of the surface spin-glass-like state. This is analyzed considering the energy distribution n(E)1/E. The existence of surface disorder dominates at low temperature and mimics the transition from superparamagnetic to quantum superparamagnetic states. © 2011 Springer-Verlag. Source

Strunz P.,Nuclear Physics Institute of Czech Republic | Petrenec M.,Academy of Sciences of the Czech Republic | Gasser U.,Laboratory for Neutron Scattering | Tobias J.,Academy of Sciences of the Czech Republic | And 2 more authors.
Journal of Alloys and Compounds | Year: 2014

Nickel base superalloy IN738LC has been studied after low-cycle fatigue by Small Angle Neutron Scattering (SANS). Samples subjected to high-temperature low-cycle fatigue were annealed at various temperatures to change the size and the distribution of precipitates. Ex and in situ SANS and TEM studies were performed. It was found that additional precipitates are formed either during slow cooling from high temperatures or after reheating above 570 C. Their size and distribution were evaluated. The precipitates arise regardless the application of the mechanical load. Nevertheless, these small precipitates influence low-cycle fatigue resistance. From the SANS data, it can be also deduced that the equilibrium volume fraction of γ′-precipitates at temperatures from room temperature to 825 C is significantly higher than 45%. The kinetics of formation of small and medium-size γ′ precipitates at 700 and 800 C was determined as well. © 2013 Elsevier Inc. All rights reserved. Source

Knaapila M.,Institute for Energy Technology of Norway | Evans R.C.,Trinity College Dublin | Garamus V.M.,Helmholtz Center Geesthacht | Almasy L.,Laboratory for Neutron Scattering | And 5 more authors.
Langmuir | Year: 2010

We report on the phase transitions, solution structure, and consequent effect on the photophysical properties of poly[3-(6-trimethylammoniumhexyl) thiophene] bromide (P3TMAHT) in aqueous sodium dodecylsulfate (SDS). Polythiophene was mixed with SDS or deuterated SDS to form P3TMAHT(SDS)x complex (x = the molar ratio of surfactant over monomer units) in D 2O and studied by small-angle neutron and X-ray scattering (SANS/SAXS) and optical spectroscopy. At room temperature, P3TMAHT forms charged aggregates with interparticle order. The addition of SDS eliminates the interparticle order and leads to rod-like (x=1/5) or sheet-like polymer-SDS aggregates (x=1/2 to 1) containing rod-like (x=1/5 to 1/2) or sheet-like (x=1/2 to 1) polymer associations. Partial precipitation occurs at the charge compensation point (x = 1). Ellipsoidal particles without interparticle order, reminiscent of SDS micelles modified by separated polymer chains, occur for x=2 to 5. Free SDS micelles dominate for x=20. Structural transitions lead to a concomitant variation in the solution color from red (P3TMAHT) to violet (x=1/5 to 1) to yellow (x > 2). The photoluminescence fingerprint changes progressively from a broad featureless band (x = 0) through the band narrowing and appearance of vibronic structure (x=1/5 to 1) to the return to a blue-shifted broad emission band (x=5). The polymer stiffness reaches a maximum for x=1, which leads to minimization of the Stokes shift (0.08 eV). This work gives fundamental information upon how surfactant complexation can influence both the solution structure and photophysical properties of a water-soluble polythiophene. © 2010 American Chemical Society. Source

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