Papers by Dietmar Schwahn
Determination of Helium Densities in Bubbles Using a Contrast Variation Method
Because of the vanishingly small solubility of noble gases in solids, metal-helium “alloys” canno... more Because of the vanishingly small solubility of noble gases in solids, metal-helium “alloys” cannot be made by conventional means. In nuclear devices, however, helium is produced by the interaction of fast particles with the nuclei of fuel or structural materials and can lead to drastic property changes already at low concentrations /1/. An example is given in Fig. 1 showing the decrease in creep life of a stainless steel specimen at 1023 K into which helium has been implanted in different ways. This so-called high temperature helium embrittlement is e.g. regarded as a life-time limiting effect for the first wall of future fusion reactors /2,3/. Similar effects have been observed or are expected in fast breeder reactors, spallation neutron sources and tritium storage materials. Typical helium productions rates in these devices are given in table 1 together with the introduction rates attainable by α-particle implantation which is often used for basic studies of helium effects.
Polymer Gels and Networks, 1996
Pressure dependence of the Flory-Huggins interaction parameter for poly(N-isopropylacrylamide)/wa... more Pressure dependence of the Flory-Huggins interaction parameter for poly(N-isopropylacrylamide)/water gel is obtained by measuring the swelling ratio of the gel in equilibrium under hydrostatic pressure up to about 100 MPa and by employing a mean field theory based on the Flory-type of the Gibbs free energy formula. The interaction parameter strongly depends on the volume fraction of a gel network and also on solvent pressure. In the swollen phase equilibrium of a gel under pressure, both the enthalpy and entropy increase with pressure, the second virial term decreases inversely and the interaction parameter is an increasing function of pressure. It can be seen that hydrostatic compression strengthens the hydrophobic interaction between polymer segments because of a loosening hydrogen-bond structure near the monomer unit.
Zeitschrift f�r Physik B Condensed Matter, 1995
Isothermal convection in ferrofluids has been induced by a gradient in particle concentration ant... more Isothermal convection in ferrofluids has been induced by a gradient in particle concentration antiparallel to a magnetic field gradient. The deviation of local particle concentration from its equilibrium value produced by the convective motion of the whole fluid gives rise to a corresponding spatial variation of magnetization. This variation has been observed by magnetic neutron scattering in good agreement with expectations based on flow measurements with an anemometric method.
Zeitschrift f�r Physik B Condensed Matter, 1988
The energy shift due to the inelastic interaction of the neutron with the time dependent magnetic... more The energy shift due to the inelastic interaction of the neutron with the time dependent magnetic fields of a gradient radio-frequency spin flipper was studied. A nondispersive perfect crystal spectrometer was used to measure extremly small energy changes of AE 0.2 peV. The gradient of the magnetic field causes a broadband action of the spin flipper which facilitates its use for new active neutron optical components.
Energy & Fuels, 2005
The commercial grades of ethylene/vinyl acetate (EVA) co-polymers have found application as "pour... more The commercial grades of ethylene/vinyl acetate (EVA) co-polymers have found application as "pour point" depressants in refined fuels. This study focuses on their behavior as additives to crude oils, where the intent is to reduce the yield stress of the gels that can form when the oil exits the reservoir. The model crude oils consisted of 4 wt % wax in decane. At EVA dosage levels of ∼200 ppm, the reduction in yield stress is 3 orders of magnitude for the C 36 wax, whereas the reduction is 1 order of magnitude for C 32 and only 3-fold for the C 28 wax. This decrease in efficiency with decreasing wax carbon number indicates that the EVA materials would not provide an adequate reduction in yield stress to ensure against gelation in pipeline transport. Neutron scattering studies, as a function of temperature, of the self-assembly of the EVA co-polymers show dramatically different aggregated structures in decane. The EVA with the lowest ethylene content shows scattering that increases with a power-law exponent of ∼1.6. This scattering behavior is typical for weakly aggregating polymer gels. In contrast, the EVA with the higher ethylene content shows a transition from surface scattering (found for strongly segregated objects) to a plateau whose height is dependent on temperature. Micrographs of the wax crystal morphology indicate that the ethylene-poor EVA alters the wax crystal habit at higher concentrations more effectively than does its higher-ethylene-content counterpart, whereas the latter EVA grade seems to form more wax crystals at low concentrations.
Colloid & Polymer Science, 1987
Decomposition of a polymer blend in the miscibility gap has been studied by small angle scatterin... more Decomposition of a polymer blend in the miscibility gap has been studied by small angle scattering of neutrons, using a double crystal diffractometer. It covers a range of 10-s to 10-3 A-1, which connects the region of elastic light scattering and, at the upper end, the region of conventional neutron small angle scattering instruments. The decomposed structure in the miscibility gap can be described by a characteristic length R which increases with time, t 113 between 7 and 25 min. At longer times, the growth follows a more linear power law. This behaviour is consistent with predictions of the domain growth for liquid mixtures and is estimated by using the self diffusion constant, viscosity, and the surface energy of the polymer.
Chemical Physics, 2008
Neutron radiation offers significant advantages for the study of biological molecular structure a... more Neutron radiation offers significant advantages for the study of biological molecular structure and dynamics. A broad and significant effort towards instrumental and methodological development to facilitate biology experiments at neutron sources worldwide is reviewed.
Biophysical Journal, 2010
Clinical studies and animal experiments have shown that the serum protein fetuin-A is a highly ef... more Clinical studies and animal experiments have shown that the serum protein fetuin-A is a highly effective inhibitor of soft tissue calcification. This inhibition mechanism was elucidated on the basis of an in vitro fetuin-A-mineral model system. In a previous study, we found that in a two-stage process~100-nm sized calciprotein particles (CPPs) were formed whose final stage was stabilized by a compact outer fetuin-A monolayer against further growth. Quantitative small-angle neutron scattering data analysis revealed that even at a fetuin-A concentration close to the stability limit, only approximately one-half of the mineral ions and only 5% of the fetuin-A were contained in the CPPs. To uncover the interplay of the remaining supersaturated mineral ion fraction and of the 95% non-CPP fetuin-A, we explored the fetuin-A monomer fraction in solution by contrast variation smallangle neutron scattering. Our results suggest that the mineral ions coalesce to subnanometer-sized clusters, reminiscent of Posner clusters, which are stabilized by fetuin-A monomers. Hence, our experiments revealed a second mechanism of long-term mineral ion stabilization by the fetuin-A that is complementary to the formation of CPPs.
Small-Angle Neutron Scattering Characterization of Polyhydroxyalkanoates and Their BioPEGylated Hybrids in Solution
Biomacromolecules, 2008
Small-angle neutron scattering was used to probe the molecular conformation of various polyhydrox... more Small-angle neutron scattering was used to probe the molecular conformation of various polyhydroxyalkanoates (PHAs) and their bioPEGylated counterparts (PHA- b-PEG). Analysis of neutron scattering profiles of these polymers dissolved in deuterated chloroform at various concentrations from dilute (approximately 0.1% w/v) to semidilute (approximately 7% w/v) showed the two distinct regimes and established overlap concentrations around 4-9 mg mL(-1). Scattering profiles were similar for all polymers investigated; power laws of approximately Q(-1.66) at high Q demonstrated that chloroform behaves as a good solvent for PHAs and suggests that under conditions synonymous with processing the solvated chains were swollen rather than in Gaussian conformation as previously reported. A gradual change to Guinier knees was followed by slopes of Q(-3) suggesting the presence of supramolecular structures at larger length scales. These observations in both the dilute and semidilute concentrations have not been previously reported. Zimm analysis of the data provided gyration radii and absolute molecular weights consistent with trends established using light scattering but showed some variation in their second virial coefficients. While natural-synthetic hybrids of PHA- b-PEG can self-assemble into microporous films, they showed no noticeable differences in chain conformation when in solution, the fabricating medium. This suggests that some form of entropic inducement is required.
Biointerphases, 2007
Structure-property relationships of sheared carbon black suspensions determined by simultaneous r... more Structure-property relationships of sheared carbon black suspensions determined by simultaneous rheological and neutron scattering measurements
Complex phase behavior near the Lifshitz line in a ternary polymer blend
Applied Physics A: Materials Science & Processing, 2002
ABSTRACT With small-angle neutron scattering (SANS) we measured thermal composition fluctuations ... more ABSTRACT With small-angle neutron scattering (SANS) we measured thermal composition fluctuations in a homopolymer blend, PB/PS, of critical composition mixed with different amounts of the symmetric diblockcopolymer PB-PS (PB and PS being polybutadiene (1,4) and polystyrene, respectively) in the disordered state along the isopleths. The diblockcopolymer acts as a surfactant enhancing the compatibility of both homopolymers up to the Lifshitz line and simultaneously changes the critical university class from 3D-Ising to isotropic Lifshitz critical behavior. Above &#75&#440.3 diblock content the samples show diblock-like behavior with a first-order ordering transition. The intermediate regime between &#75&#440.075 and 0.3 can be characterized by a micro-emulsion phase. The temperature dependence of the Lifshitz line, the disorder line and the critical line is non-monotonous.
Applied Physics A: Materials Science & Processing, 2002
The crystallization of diluted solutions of C 24 H 50 tetracosane and C 36 H 74 hexatriacontane i... more The crystallization of diluted solutions of C 24 H 50 tetracosane and C 36 H 74 hexatriacontane in decane was studied in the presence of PEB-7 poly(ethylene-butene) random copolymers by SANS. The measurements were performed in the temperature regimes between 80 • C and minus 30 • C to observe single coil and aggregation behavior. The structure of paraffin molecules and the copolymer was distinguished by contrast matching. The aggregated structure of the two paraffin molecules is different. The aggregation of tetracosane is controlled by the copolymer, while, on the other hand, the hexatriacontane molecules control the aggregation mechanism; a periodic structure of two-dimensional platelets is formed by tetracosane and the PEB-7, while well-defined two-dimensional and large three-dimensional objects are observed in the hexatriacontane solution.
Applied Physics A: Materials Science & Processing, 2002
By employing neutron spin echo and small-angle neutron scattering, we determined the structure fa... more By employing neutron spin echo and small-angle neutron scattering, we determined the structure factor of the frozen concentration fluctuations on nano-length scales in a swollen poly(N-isopropyl acrylamide) gel. The frozen contribution, showing a plateau at the low scattering wavenumber q (< 0.02 Å −1) and a rapid decrease close to q −4 at high q (> 0.02 Å −1), is intimately related to the abnormal butterfly scattering pattern appearing at low q under deformation.
Phase behavior of binary polybutadiene copolymer mixtures as an example of weakly interacting polymers
Applied Physics A: Materials Science & Processing, 2002
ABSTRACT
Applied Physics A: Materials Science & Processing, 2002
In order to clarify the self-assembling mechanisms in complex fluids involving amphiphiles, we ha... more In order to clarify the self-assembling mechanisms in complex fluids involving amphiphiles, we have investigated dynamic features of amphiphilic membranes and droplets at high temperature and at high pressure in a ternary microemulsion, consisting of AOT, water, and n-decane. A high-pressure cell for neutron spin echo (NSE) experiments has been improved, and the static and dynamic features of droplets are observed in detail by means of small angle neutron scattering and NSE. It is found that the size fluctuation and the diffusion of droplets are enhanced by increasing temperature, while they are suppressed by increasing pressure.
Scaling law of the Ginzburg number in polymer blends
Concentration fluctuations in aqueous solutions of PEO, PPO, and self-assembling PEO-PPO-PEO. Effect of hydrostatic pressure
Pressure effects in polymer blends and diblock copolymers studied by small-angle neutron scattering
Mean-field to 3d-ising crossover in polymer blends. Critical exponents and discussion of the Ginzburg criterium
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Papers by Dietmar Schwahn