Dietmar Schwahn

Nucleation and Growth of CaCO3 Mediated by the Egg-White Protein Ovalbumin : A Time-Resolved in situ Study Using Small-Angle Neutron Scattering

Mineralization of calcium carbonate in aqueous solutions starting from its initiation was studied by time-resolved small-angle neutron scattering (SANS). SANS revealed that homogeneous crystallization of CaCO 3 involves an initial formation of thin plate-shaped nuclei which subsequently reassemble to 3-dimensional particles, first of fractal and finally of compact structure. The presence of the egg-white protein ovalbumin leads to a different progression of mineralization through several stages; the first step represents amorphous CaCO 3, whereas the other phases are crystalline. The formation and dissolution of the amorphous phase is accompanied by Ca (2+)-mediated unfolding and cross-linking of about 50 protein monomers showing the characteristic scattering of linear chains with a large statistical segment length. The protein complexes act as nucleation centers for the amorphous phase because of their enrichment by Ca (2+) ions. SANS revealed the sequential formation of CaCO 3 starting from the amorphous phase and the subsequent formation of the crystalline polymorphs vaterite and aragonite. This formation from less dense to more dense polymorphs follows the Ostwald-Volmer rule.

Synthesis and Characterization of Gelatin-Based Magnetic Hydrogels

A simple preparation of thermoreversible gelatin-based ferrogels in water provides a constant structure defined by the crosslinking degree for gelatin contents between 6 and 18 wt%. The possibility of varying magnetite nanoparticle concentration between 20 and 70 wt% is also reported. Simulation studies hint at the suitability of collagen to bind iron and hydroxide ions, suggesting that collagen acts as a nucleation seed to iron hydroxide aggregation, and thus the intergrowth of collagen and magnetite nanoparticles already at the precursor stage. The detailed structure of the individual ferrogel components is characterized by small-angle neutron scattering (SANS) using contrast matching. The magnetite structure characterization is supplemented by small-angle X-ray scattering and microscopy only visualizing magnetite. SANS shows an unchanged gelatin structure of average mesh size larger than the nanoparticles with respect to gel concentration while the magnetite nanoparticles size of around 10 nm seems to be limited by the gel mesh size. Swelling measurements underline that magnetite acts as additional crosslinker and therefore varying the magnetic and mechanical properties of the ferrogels. Overall, the simple and variable synthesis protocol, the cheap and easy accessibility of the components as well as the biocompatibility of the gelatin-based materials suggest them for a number of applications including actuators.

Early state of spinodal decomposition studied with small angle neutron scattering in the blend deuteropolystyrene and polyvinylmethylether: A comparison with the Cahn–Hilliard–Cook theory

The relaxation behavior of a polymer blend, deutero‐polystyrene and polyvinylmethylether (d‐PS/PVME) was studied by investigating the equilibrium structure factor S(Q) and its time dependence S(Q,t) after the blend had been exposed to a rapid temperature step from T0 to Tf. The structure factor S(Q,t) was determined as a function of scattering vector Q by time‐resolved neutron scattering experiments. Two cases were investigated (i) with T0 and Tf both in the homogeneous and (ii) with Tf in the unstable region of the phase diagram. The relaxation was compared with the Cahn–Hilliard–Cook theory. The strongly Q‐dependent (nonlocal) Onsager coefficient was determined from the relaxation time and compared with predictions by Pincus et al. The self‐diffusion constants of the components were obtained for Q→0. Finally, the experiments at Tf led to the critical behavior of the susceptibility S(Q=0) near the spinodal. An Ising‐type critical behavior was observed across the (extrapolated) spinodal.

Investigation of the phase diagram and critical fluctuations of the system polyvenylmethylether and d‐polystyrene with neutron small angle scattering

The system PVME/d‐PS has been investigated by neutron small angle scattering. By extrapolating the critical scattering, the spinodal was determined and used to calculate the Flory–Huggins parameter χ(φ,T) in the frame of mean field theory. Separating the Gibbs potential of mixing ΔG in a Flory–Huggins term χ(φ,T) and a configurational term for the entropy ΔS, it turns out that χ is independent of the molecular weight, and of the weight distribution, i.e., it is a local or ‘‘segmental’’ quantity. The critical scattering was carefully investigated at the critical concentration. At temperatures Tc−T>2.4 K(Tc=141.9 °C), a mean field behavior of the susceptibility S(Q=0) was clearly observed, with a critical exponent γ=1. However, in a region of Tc−2.4 K≤T<Tc, a crossover occurs to a critical exponent γ≂1.26; it resembles the Ising behavior which is well established for normal liquid mixtures. The function S(Q) also describes the characteristic segmental length σ. Both σ and the range parameter of χ, r0, are i...

Concentration fluctuations in polymer gel investigated by neutron scattering: static inhomogeneity in swollen gel.

By using small-angle neutron scattering (SANS) and neutron spin echo (NSE), we have quantitatively investigated the static inhomogeneity in poly (N-isopropyl acrylamide) gel (PNIPA) in microscopic length scales of 0.015<q<0.16 A(-1), where q is a wave number of scattered neutrons. NSE revealed that at lower q( congruent with 0.015 A(-1)), the concentration fluctuations in the PNIPA gel decays more slowly as compared to the PNIPA solution without crosslinks. According to our scenario that the slower decay found for the PNIPA gel is due to the static inhomogeneity coexisting in the swollen gel, small-angle scattering S(q) obtained by SANS has been quantitatively decomposed into thermal and static scattering components, respectively, S(th)(q) and S(st)(q). It was further revealed that (i) the q-region where S(st)(q) becomes dominant is closely related to that for the abnormal butterfly scattering under stretching, and (ii) as the temperature increases toward the temperature for volume phase transition, S(st)(q) of a squared Lorentzian shape increases more drastically than S(th)(q) of a Lorentzian shape. These findings were quantitatively understood in the theoretical framework by Panyukov and Rabin [Macromolecules 29, 7960 (1996)] or by Onuki [J. Phys. II. France 2, 45 (1992)], taking into account stress-fluctuation coupling under coexistence of the inherent structural heterogeneity in the real gel. We further found that the static inhomogeneity showing S(st)(q) seems to relate to the necklacelike microstructure, appearing after a shallow quench into the collapsed phase.

Thermal composition fluctuations near the isotropic Lifshitz critical point in a ternary mixture of a homopolymer blend and diblock copolymer

We have studied thermal composition fluctuations of a ternary symmetric homopolymer/diblock copolymer system of PEE/PDMS/PEE-PDMS [PEE and PDMS being poly(ethyl ethylene) and poly(dimethyl siloxane), respectively] in its disordered state with small angle neutron scattering for concentration Φ of diblocks up to 15%. The phase diagram shows three characteristic regimes; (1) below the Lifshitz concentration ΦLL≅9%; (2) in the very near vicinity of the Lifshitz concentration; and (3) above ΦLL. In the regime (1) of low diblock content the maximum neutron intensity is obtained at Q=0 and phase separation into macroscopic large domains is observed at low temperatures. With increasing diblock content the thermal fluctuations indicate a crossover from 3d-Ising to isotropic Lifshitz critical behavior with critical exponents of the susceptibility γ=(1.62±0.01) and correlation length ν=(0.99±0.04) appreciably larger than in the 3d-Ising case. In the structure factor this crossover is accompanied by a strong reductio...

ON THE CROSSOVER FROM ISING TO MEAN-FIELD BEHAVIOR IN COMPATIBLE BINARY-POLYMER BLENDS

Susceptibility data for various binary-polymer blends at critical composition far from and near to the critical point of phase separation are compared with a theoretical expression describing the crossover from critical mean field to a 3d-Ising behaviour close to the critical point. The crossover function used is an explicit solution to first order in the perturbation parameter e = 4 - d based upon a renormalisation group analysis. The results obtained using the crossover function are compared with those obtained using the Flory-Huggins form of the bare free-energy density. The observed differences can be interpreted as being due to an underestimate of the perturbation caused by fluctuations in the Flory-Huggins model. The Ginzburg number Gi calculated from the crossover function can be related to the proposed universal constant c of the Ginzburg criterium which is a measure for the width of the Ising regime.

Aspherical refractive lenses for small-angle neutron scattering

Aspherical neutron lenses are discussed in detail as focusing elements for existing small-angle neutron scattering (SANS) diffractometers. The conceptual design can be obtained by analytical equations, while computer simulations provide the intensity distribution of the primary beam, which is strongly correlated with the instrumental resolution and the minimal momentum transfer Q. For large illuminated sample/lens areas of 50 mm diameter the aspherical lens design is essential, while spherical lenses are limited to ca 20 mm diameter with a 60% spherical aberration for standard SANS experiments. Scattering experiments on different samples proved the applicability and revealed the resolution limits of the two lens types. From theoretical considerations it is proposed that lens cooling can be used to reduce the thermal diffuse scattering.

SANS instruments at the Jülich research reactor FRJ-2

A brief description and characterization is given of the three small-angle neutron scattering instruments (SANS) in the new guide laboratory at the Julich FRJ-2 reactor, namely two 40 m slit-collimation instruments and a very high-resolution double-crystal diffractometer. The combination of the two instruments allows one to cover a range of scattering vectors Q of nearly four orders of magnitude.

Small-angle neutron scattering studies of polybutadiene/polystyrene blends as a function of pressure and microstructure: Comparison of experiment and theory

Small-angle neutron scattering (SANS) experiments have been performed for three polybutadiene/polystyrene (dPB/PS) blends of differing dPB microstructure as a function of pressure and temperature. The experimental effective SANS interaction parameters are analyzed using the mean-field lattice cluster theory (LCT). In order to provide a meaningful comparison with the LCT, contributions from the non-mean-field long-range composition fluctuations are removed from the experimental data by use of a crossover function that describes the transition between near-critical and mean-field behaviors for the extrapolated zero-angle scattering. The theory provides a good description of the overall pressure dependence of the effective interaction parameter and its small dependence on the percentage of 1,2 addition units in the dPB chains.