Stephen M. King

SANS at pulsed neutron sources : Present and future prospects

Small-angle diffraction with a pulsed neutron source, using time-of-flight analysis to separate neutrons of different wavelengths, offers a very wide simultaneous Q range coupled to good Q resolution. Data reduction to allow for wavelength-dependent effects may be achieved as a matter of routine. The cold neutron flux available from accelerator-based neutron sources does not yet fully match that of the most intense reactor sources. Simulations show that the performance of proposed future instrumentation would be largely complementary to that of the best fixed-wavelength instruments.

Poly(NIPAM) microgel particle de-swelling: a light scattering and small-angle neutron scattering study

Small-angle neutron scattering (SANS) has been used to investigate structural changes during the de-swelling of poly(N-isopropylacrylamide) [poly(NIPAM)] microgel particles induced by temperature variation and the addition of free polymer [poly(ethylene oxide)]. The extent of particle de-swelling was characterized by photon correlation spectroscopy (PCS). Thermally-induced de-swelling of poly(NIPAM)/deuterated water dispersions occurred on increasing the temperature in the region of the lower critical solution temperature (LCST). The latter was found to be 34°C, which is 2°C higher than the value reported for poly(NIPAM) particles dispersed in water. The SANS data exhibit a Porod form (Q−4) of scattering in regard to the size of the colloidal particles. However, in the swollen state, the scattering measured at temperatures less than the LCST, also has a contribution from poly(NIPAM) chains in a solution-like environment (Ornstein–Zernike scattering). The trends observed in the SANS data as a function of temperature were simulated using a model comprised of Porod and Ornstein–Zernike scattering terms. The SANS data confirm earlier PCS measurements showing that addition of free polymer induces particle de-swelling. The SANS data obtained using added free polymer are the first examples of their type to be reported.

Self-Assembly of Peptide Nanotubes in an Organic Solvent

The self-assembly of a modified fragment of the amyloid beta peptide, based on sequence Abeta(16-20), KLVFF, extended to give AAKLVFF is studied in methanol. Self-assembly into peptide nanotubes is observed, as confirmed by electron microscopy and small-angle X-ray scattering. The secondary structure of the peptide is probed by FTIR and circular dichroism, and UV/visible spectroscopy provides evidence for the important role of aromatic interactions between phenylalanine residues in driving beta-sheet self-assembly. The beta-sheets wrap helically to form the nanotubes, the nanotube wall comprising four wrapped beta-sheets. At higher concentration, the peptide nanotubes form a nematic phase that exhibits spontaneous flow alignment as observed by small-angle neutron scattering.

Small Angle Neutron Scattering Using Sans2d

The Loq small angle neutron scattering (SANS) instrument at ISIS target station one (TS1) has proved an extremely popular and immensely productive SANS facility over the last 20 years. Loq is routinely operated with a 25 Hz disc chopper removing alternate neutron pulses from the 50 Hz source to provide a usable wavelength range of 2.2 to 10 Å. Scattered neutrons are recorded on a 0.64 m square 3He multiwire proportional counter (Ordela 2661N) fixed at ∼ 4 m from the sample, with a total flight path of 15 m from the decoupled liquid hydrogen moderator [1].

Tuneable mechanical properties in low molecular weight gels

The mechanical properties of gels are critical to the final targeted applications. Depending on the application, different properties may be required. Here, we show that the mechanical strength and ability to recover of gels formed using a low molecular weight gelator can be controlled by two independent factors (i) the volume fraction of co-solvent (in this case DMSO) in the system and (ii) the temperature cycle used. These differences correlate with the large scale structure of the network that is formed from the self-assembled fibres. This opens up the potential to prepare gels with very different properties at the same final conditions, allowing the effect of microstructure to be probed.

Spontaneous symmetry breaking: formation of Janus micelles

We describe the preparation and solution properties of Janus micelles, i.e., non-centrosymmetric nanoparticles with compartmentalized shells, viaco-assembly of two fully water-soluble block copolymers. They consist of a mixed core of poly(N-methyl-2-vinyl pyridinium iodide) (P2MVP) and poly(acrylic acid) (PAA), and a shell segregated into two sides, consisting of poly(ethylene oxide) (PEO) or poly(acryl amide) PAAm. These Janus particles form spontaneously and reversibly, i.e., association, dissocation, and reassociation can be carefully controlled via parameters, such as polymer mixing fraction, solution pH, and ionic strength. Dynamic (polarized and depolarized) and static light scattering, cryogenic transmission electron microscopy, small angle neutron and X-ray scattering, and two-dimensional nuclear magnetic resonance spectroscopy are used to monitor the micelle formation and to characterize the micellar structure. The Janus particles were found to be ellipsoidal, with a cigar-like overall shape and a disc-like core. This peculiar morphology is driven by the delicate interplay between two opposing forces: an attraction between the oppositely charged core blocks and a subtle repulsion between the water-soluble, neutral corona blocks.

Structure, rheology and shear alignment of Pluronic block copolymer mixtures

The structure and flow behaviour of binary mixtures of Pluronic block copolymers P85 and P123 is investigated by small-angle scattering, rheometry and mobility tests. Micelle dimensions are probed by dynamic light scattering. The micelle hydrodynamic radius for the 50/50 mixture is larger than that for either P85 or P123 alone, due to the formation of mixed micelles with a higher association number. The phase diagram for 50/50 mixtures contains regions of cubic and hexagonal phases similar to those for the parent homopolymers, however the region of stability of the cubic phase is enhanced at low temperature and concentrations above 40 wt%. This is ascribed to favourable packing of the mixed micelles containing core blocks with two different chain lengths, but similar corona chain lengths. The shear flow alignment of face-centred cubic and hexagonal phases is probed by in situ small-angle X-ray or neutron scattering with simultaneous rheology. The hexagonal phase can be aligned using steady shear in a Couette geometry, however the high modulus cubic phase cannot be aligned well in this way. This requires the application of oscillatory shear or compression.

Isotopic labelling and composition dependence of interaction parameters in polyethylene oxide/polymethyl methacrylate blends

Abstract Mixtures of deuterated polyethylene oxide with hydrogenous polymethyl methacrylate and hydrogenous polyethylene oxide with deuterated polymethyl methacrylate have been investigated using small angle neutron scattering. The range of temperatures used was 423–473 K and the volume fraction of polyethylene oxide in the mixtures varied from ca. 0.1 to ca. 0.27. Values of the effective interaction parameter ( χ eff ) were obtained using the incompressible random phase approximation. Both temperature dependence and composition dependence were noted in the values of χ eff . The temperature dependence observed was compared with that predicted by equation of state theory. A notable feature of the composition dependence of χ eff was the change in behaviour as temperature increased. Changes in the excess volume of mixing have been cited as a possible source of this behaviour, but the currently available data do not allow us to confirm this. Using the relation between χ eff and a composition independent interaction parameter proposed from Monte Carlo simulations, approximate values of the composition independent interaction parameter have been obtained.

PGSE-NMR and SANS Studies of the Interaction of Model Polymer Therapeutics with Mucin

The viscous mucus coating that adheres to the epithelial surfaces of mammalian organs provides protection for the underlying tissues and is an efficient barrier to drug delivery. Pulsed-gradient spin-echo NMR and small-angle neutron scattering have been used to study the aqueous solution interaction of various model polymer therapeutics with mucin, the principle organic component within mucus. Nonionic polymers such as linear and star-branched poly(ethylene oxide) (PEO) and dextrin showed no appreciable interaction with mucin but suffered a moderate retardation in their rate of diffusion through the mucin solution. A strong interaction with mucin was observed for a series of polyamidoamine (PAMAM) dendrimers and hyperbranched poly(ethylene imine) (PEI), which displayed a characteristic pH-dependent profile and led to significant reductions in their rates of diffusion. These observations have implications for the design of optimized polymer therapeutic structures being adopted for the delivery of therapeutic moieties through mucin-rich environments.

Exploring How Organic Matter Controls Structural Transformations In Natural Aquatic Nanocolloidal Dispersions

The response of the dispersion nanostructure of surface river bed sediment to the controlled removal and readdition of natural organic matter (NOM), in the absence and presence of background electrolyte, was examined using the technique of small-angle neutron scattering (SANS). Partial NOM removal induced aggregation of the mineral particles, but more extensive NOM removal restored colloidal stability. When peat humic acid (PHA) was added to a NOM-deficient sediment concentration-related structural transformations were observed: at 255 mg/L PHA aggregation of the nanocolloid was actually enhanced, but at 380 mg/L PHA disaggregation and colloidal stability were promoted. The addition of 2 mM CaCl(2) induced mild aggregation in the native sediment but not in sediments with added PHA, suggesting that the native NOM and the PHA respond differently to changes in ionic strength. A first attempt at using SANS to directly characterize the thickness and coverage of an adsorbed PHA layer in a natural nanocolloid is also presented. The results are discussed in the context of a hierarchical aquatic colloidal nanostructure, and the implications for contemporary studies of the role of dissolved organic carbon (DOC) in sustaining the transport of colloidal iron in upland catchments.