Dispersions of nanoparticles (NPs) in a polymer matrix are a key element to set various properties of solution-cast polymer nanocomposite films. While the dispersion state of NPs in nanocomposite films has been extensively studied over the past decades, the structural development during drying and the relation of liquid and solid structure still remains poorly understood. In the present work, we study how NP dispersions develop during drying in polymer nanocomposite films, particularly focusing on the role of particle polymer interaction in the structural development. Aqueous dispersions of nanosilica and poly(vinyl alcohol) are used as model NP/polymer mixtures, where the particle polymer interaction can systemically be varied via the pH. A novel vertical small-angle X-ray scattering technique enables us to directly observe the development of the NP's dispersion state during drying. At a high pH 10, where silica particles have a poor affinity to PVA, SAXS intensity evolution shows phase separation during drying, resulting in the formation of dense aggregates of bare particles in the solid film. On the other hand, at a low pH 3, where silica particles have a good affinity with PVA, the SAXS data indicate a gradual densification of the NPs while maintaining a constant interparticle distance, which is originating from adsorbed PVA. The resulting solid film after drying exhibits an improved dispersion of NPs. The evaluation of the interparticle interaction suggests that the adsorbed polymer plays generally a key role in the uniform distribution of NPs in solid films, as it sterically stabilizes NPs over short ranges during all drying stages whereas depletion attraction dominates at longer ranges.

We employ x-ray scattering on sheared colloidal suspensions and mode coupling theory to study structure factor distortions of glass-forming systems under shear. We find a transition from quadrupolar elastic distortion at small strains to quadrupolar and hexadecupolar modes in the stationary state. The latter are interpreted as signatures of plastic rearrangements in homogeneous, thermalized systems. From their transient evolution with strain, we identify characteristic strain and length-scale values where these plastic rearrangements dominate. This characteristic strain coincides with the maximum of the shear stress versus strain curve, indicating the proliferation of plastic flow. The hexadecupolar modes dominate at the wavevector of the principal peak of the equilibrium structure factor that is related to the cage-effect in mode coupling theory. We hence identify the structural signature of plastic flow of glasses.

Structural distortion and relaxation are central to any liquid flow. Their full understanding requires simultaneous probing of the mechanical as well as structural and dynamical response. We provide the first full dynamical measurement of the transient structure using combined coherent X-ray scattering and rheology on electrostatically interacting colloidal fluids. We find a stress overshoot during the start-up of shear which is due to the strong anisotropic overstretching and compression of nearest-neighbor distances. The rheological response is reflected in uncorrelated entropy-driven intensity fluctuations. While the structural distortion under steady shear is well described by Smoluchowski theory, we find an increase of the particle dynamics beyond the trivial contribution of flow. After the cessation of shear, the full fluid microstructure and dynamics are restored, both on the structural relaxation timescale. We thus find unique structure-dynamics relations in liquid flow, responsible for the macroscopic rheological behavior of the system.

Glasses acquire their solid-like properties by cooling from the supercooled liquid via a continuous transition known as the glass transition. Recent research on soft glasses indicates that besides temperature, another route to liquify glasses is by application of stress that drives relaxation and flow. Here, we show that unlike the continuous glass transition, the failure of glasses to applied stress occurs by a sharp symmetry change that reminds of first-order equilibrium transitions. Using simultaneous x-ray scattering during the oscillatory rheology of a colloidal glass, we identify a sharp symmetry change from anisotropic solid to isotropic liquid structure at the crossing of the storage and loss moduli. Concomitantly, intensity fluctuations sharply acquire Gaussian distributions characteristic of liquids. Our observations and theoretical framework identify mechanical failure as a sharp atomic affine-to-nonaffine transition, providing a new conceptual paradigm of the oscillatory yielding of this technologically important class of materials and offering new perspectives on the glass transition.

Glasses acquire their solid-like properties by cooling from the supercooled liquid via a continuous transition known as the glass transition. Recent research on soft glasses indicates that besides temperature, another route to liquify glasses is by application of stress that forces relaxation and flow. Here we provide experimental evidence that the stress-induced onset of flow of glasses occurs via a sharp first order-like transition. Using simultaneous x-ray scattering during the oscillatory rheology of a colloidal glass, we identify a sharp symmetry change from anisotropic solid to isotropic liquid structure at the transition from the linear to the nonlinear regime. Concomitantly, intensity fluctuations sharply acquire liquid distributions. These observations identify the yielding of glasses to increasing stress as sharp affine-to-nonaffine transition, providing a new conceptual paradigm of the yielding of this technologically important class of materials, and offering new perspectives on the glass transition.

Hybrid pixel semiconductor detectors provide high performance through a combination of direct detection, a relatively small pixel size, fast readout and sophisticated signal processing circuitry in each pixel. For X-ray detection above 20 keV, high-Z sensor layers rather than silicon are needed to achieve high quantum efficiency, but many high-Z materials such as GaAs and CdTe often suffer from poor material properties or nonuniformities. Germanium is available in large wafers of extremely high quality, making it an appealing option for high-performance hybrid pixel X-ray detectors, but suitable technologies for finely pixelating and bump-bonding germanium have not previously been available. A finely-pixelated germanium photodiode sensor with a 256 by 256 array of 55 mu m pixels has been produced. The sensor has an n-on-p structure, with 700 mu m thickness. Using a low-temperature indium bump process, this sensor has been bonded to the Medipix3RX photon-counting readout chip. Tests with the LAMBDA readout system have shown that the detector works successfully, with a high bond yield and higher image uniformity than comparable high-Z systems. During cooling, the system is functional around -80 degrees C (with warmer temperatures resulting in excessive leakage current), with -100 degrees C sufficient for good performance.

Yielding is central to the relaxation, flow and fracture of a wide range of soft and molecular glasses, but its microscopic origin remains unclear. Here, we elucidate the yielding of a colloidal glass by using x-ray scattering to monitor the structure factor during the yielding process. We apply a recently introduced combination of small-angle x-ray scattering and rheology to the oscillatory shear, and follow the structure factor during the increasing strain amplitude. Surprisingly, we observe a sharp transition in the orientational ordering of the nearest-neighbor structure upon yielding, in contrast to the smooth variation of the viscoelastic moduli. This transition is accompanied by a sudden change of intensity fluctuations towards Gaussian distributions. We thus identify yielding as a new, dynamically induced transition of the glass in response to the applied shear.

We report a first application of vertical small-angle X-ray scattering to investigate the drying process of a colloidal suspension by overcoming gravity related restrictions. From the observation of the drying behavior of charge-stabilized colloidal silica in situ, we find the solidification of the colloidal particles exhibits an initial ordering, followed by a sudden aggregation when they overcome an electrostatic energy barrier. The aggregation can be driven not only by capillary pressure but also by thermal motion of the particles. The dominating contribution is determined by the magnitude of the energy barrier at the transition, which significantly decreases during drying due to an increased ionic strength.

The early stages of the formation of inorganic aggregates, composed of iron compounds at the solution-air interface, were investigated in situ. The properties of the solution-air interface were changed by using different Langmuir layers. In order to get insight into the evolution of the sample system in situ, the processes were studied by X-ray scattering and spectroscopy techniques. The formation of aggregates was detected under cationic as well as under anionic Langmuir layers. The observed compounds lack long range order which indicates the formation of amorphous structures. This is supported by extended X-ray absorption fine structure measurements showing only minor order in the formed aggregates. (C) 2013 Elsevier B.V. All rights reserved.

Alignment of microphase separated block copolymers (BCP) has been achieved by shear-fields in order to achieve a deeper understanding of the melt-infiltration of BCP into nanopores (30–400nm). BCP’s composed of different perfluorinated sidechains, namely Am-b-Dn-, A100-b-D100- and A140-b-D60- and Am-b-Cn–BCPs and A50-b-C8 and A50-b-C13 have been studied by rheology coupled to in situ-SAXS. BCP’s were prepared by ROMP-methods, generating the BCP’s with low polydispersities and the expected molecular weights. A strong macroscopic orientation of the investigated block copolymer domains A50-b-C13 occurs upon applying oscillatory shear forces at temperatures significantly above the Tg’s using shear amplitudes of 10% and angular frequencies of 10rad/s for an already short period of time like 100s. Coupled Rheology to SAXS-measurements on block copolymer A100-b-D100 at 180°C revealed a similar picture when sheared at 10% shear amplitude and an angular frequency of 10rad/s over a time of 100s. In both BCP’s a slow relaxation of the oriented microphases was observed, which can be explained by the nanophase-separation of the respective fluorinated chains.

The relation between shear-mixing time in the melt, polymer crystal size, and electrical conductivity is studied for high density polyethylene carbon nanotubes nanocomposites with the help of in-situ rheo-dielectric and rheo-small angle synchrotron X-ray scattering (SAXS) techniques. Results show that the memory of crystal structure obtained after melt-mixing is not easily removed by re-melting and annealing. The conductivity of composites shear-mixed for 20 min cannot reach that of 10 min samples, even after additional quiescent annealing for 6000 s at 190 degrees C. The rheo-SAXS data further indicate that the temperature needed to melt all the crystals is higher for the longer time shear-mixed composites. These results all suggest that with long processing time, larger crystals are nucleated and grow on the nanotubes, which reduce electrical conductivity, presumably because they prevent electrical contact between the nanotubes. (C) 2012 Elsevier Ltd. All rights reserved.

Submitted for the MAR12 Meeting of The American Physical Society X-ray investigation of colloidal glasses under shear DMITRY DENISOV, TRIET DANG, Van der Waals-Zeeman Institute, University of Amsterdam, Netherlands, BERND STRUTH, Deutsches Elektronen-Synchrotron, Hamburg, Germany, PETER SCHALL, Van der Waals-Zeeman Institute, University of Amsterdam, Netherlands — Understanding glassification or dynamical arrest is one of the grand challenges of material science and is a topic of great current interest. It is a central observation in soft matter systems as well as glass forming molecular systems that – with increasing density or decreasing temperature – the motion of the particles or molecules slows down and eventually becomes arrested. Understanding this dynamical arrest as well as relaxations in the arrested state are fundamental problems, which to a large degree remain unanswered. We use a novel combination of rheological measurement and small angle x-ray scattering (at the synchrotron DESY in Hamburg) to study structure factor of dense suspensions under shear and during relaxation. The suspensions consist of silica particles 50nm in diameter. We observe clear changes of inter particles distances and configurations due to the different shear rates. Together with future dynamic x-ray measurements we aim to develop a universal scale-bridging understanding of dynamic arrest. Dmitry Denisov Van der Waals-Zeeman Institute, University of Amsterdam, Netherlands Date submitted: 13 Nov 2011 Electronic form version 1.4

There is growing interest in the design of molecules that undergo predictable self-assembly. Bioinspired oligomers with well-defined conformational propensities are attractive from this perspective, since they can be constructed from diverse building blocks, and self-assembly can be directed by the identities and sequence of the subunits. Here we describe the structure of monolayers formed at the air-water interface by amphiphilic α/β-peptides with 1:1 alternation of α- and β-amino acid residues along the backbone. Two of the α/β-peptides, one a dianion and the other a dication, were used to determine differences between self-assemblies of the net negatively and positively charged oligomers. Two additional α/β-peptides, both zwitterionic, were designed to favor assembly in a 1:1 molar ratio mixture with parallel orientation of neighboring strands. Monolayers formed by these α/β-peptides at the air-water interface were characterized by surface pressure-area isotherms, grazing incidence X-ray diffraction (GIXD), atomic force microscopy and ATR-FTIR. GIXD data indicate that the α/β-peptide assemblies exhibited diffraction features similar to those of β-sheet-forming α-peptides. The diffraction data allowed the construction of a detailed model of an antiparallel α/β-peptide sheet with a unique pleated structure. One of the α/β-peptide assemblies displayed high stability, unparalleled among previously studied assemblies of α-peptides. ATR-FTIR data suggest that the 1:1 mixture of zwitterionic α/β-peptides assembled in a parallel arrangement resembling that of a typical parallel β-sheet secondary structure formed by α-peptides. This study establishes guidelines for design of amphiphilic α/β-peptides that assemble in a predictable manner at an air-water interface, with control of interstrand orientation through manipulation of Coulombic interactions along the backbone.

New stable states of liquid crystal 8CB could be induced by nonlinear shear conditions and observed by a newly developed rheology/X-ray scattering setup using synchrotron X-ray radiation. Nonlinear,oscillatory shear created a distorted sixth order orientational structure. Even when oscillatory shear is switched off, the induced structure remains stable and can be removed only by heating the system into the isotropic state. We assume the structure to be stabilized by defects that pin the new 6-fold phase.

The formation of a layer of hydrophobic magnetite (Fe(3)O(4)) nanoparticles stabilized by lauric acid is analyzed by in situ X-ray reflectivity measurements. The data analysis shows that the nanoparticles partially disperse their hydrophobic coating. Consequently, a Langmuir layer was formed by lauric acid molecules that can be compressed into an untilted condensed phase. A majority of the nanoparticles are attached to the Langmuir film integrating lauric acid residue on their surface into the Langmuir film. Hence, the particles at the liquid-gas interface can be identified as so-called Janus beads, which are amphiphilic solids having two sides with different functionality.