J. Allgaier

Dynamics of bicontinuous microemulsion phases with and without amphiphilic block-copolymers

Neutron Spin Echo and Dynamic Light Scattering techniques are used for an extensive investigation of the bicontinuous phase in water/decane microemulsions. The dynamical behavior of different surfactant systems, decyl polyglycol ether (C10E4), C10E4 mixed with polyethylenepropylene/polyethyleneoxide amphiphilic block-copolymers-(PEPx/PEOy), and sodium-bisethylhexylsulfosuccinate (AOT) is investigated under comparable conditions. At scattering wave numbers q large compared to the inverse of the structure length scale, q0=2π/d, always stretched exponential relaxations ∝e−(Γqt)β with Γq∝q3 are found, as predicted theoretically. The relaxation rate increases almost linearly as function of the bicontinuous structure correlation scale—ξ≃d/2. The apparent bare bending modulus κ determined by fitting theoretical predictions to the experimental high-q data yields values of about 1.3kBT—as inferred from previous small angle neutron scattering (SANS) studies and from other methods. The effect of increasing rigidity ...

An in situ study of the t-butyllithium initiated polymerization of butadiene in d-heptane via small angle neutron scattering and 1H-NMR.

We present a combined 1H-NMR and small angle neutron scattering in situ study of the anionic polymerization of butadiene using t-butyllithium as the initiator. Both initiation and propagation phases were explored. This combined approach allows the structural and kinetic characteristics to be accessed and cross compared. The use of the D22 instrument (ILL Grenoble) permits the attainment of Q approximately equal to 2 x 10(-3) A. This, in turn, led to the identification of coexisting large-scale and smaller aggregates during all stages of the polymerization. The smaller aggregates contain most of the reacted monomers. Their structure changes from high functionality wormlike chains at early stages of the reaction to starlike aggregates where the crossover occurs at a degree of polymerization of approximately equal to 40. The initiation event involved these small, high functionality (approximately equal to 120) aggregates that apparently consisted of cross-associated t-butyllithium with the newly formed allylic-lithium head groups. As the initiation event progressed the initiation rate increased while the functionality of these small aggregates decreased and their size increased. Propagation, in the absence of initiation, was found to have a rate constant that was molecular weight dependent. At approximately 11 kg/mol the measured polymerization rate was found to increase while no further structural changes were seen.

Influence of the amphiphilicity profile of copolymers on the formation of liquid crystalline mesophases in microemulsions

In bicontinuous microemulsions, the increase of the solubilization capacity of a surfactant, e.g., by the addition of amphiphilic block copolymers (“boosting effect”), is normally accompanied by the formation of liquid crystalline mesophases. We studied a new class of amphiphilic polymers as “boosters,” so-called gradient polymers, with a gradual change from the hydrophilic to the hydrophobic part. Phase diagram measurements and static and quasielastic neutron scattering experiments reveal that the polymers cause a stiffening of the amphiphilic film while simultaneously the saddle splay modulus decreases such that the formation of liquid crystalline mesophases, normally present with diblock polymers as “boosters,” is suppressed, while bicontinuous structures are geometrically favored. In addition, these findings are supported by theoretical calculations following the works of Lipowsky. Hence, gradient amphiphilic polymers not only increase the efficiency of surfactants but simultaneously suppress the formation of liquid crystalline phases thereby greatly increasing their application potential.

Polymer dynamics under cylindrical confinement featuring a locally repulsive surface: A quasielastic neutron scattering study

We investigated the effect of intermediate cylindrical confinement with locally repulsive walls on the segmental and entanglement dynamics of a polymer melt by quasielastic neutron scattering. As a reference, the corresponding polymer melt was measured under identical conditions. The locally repulsive confinement was realized by hydrophilic anodic alumina nanopores with a diameter of 20 nm. The end-to-end distance of the hydrophobic infiltrated polyethylene-alt-propylene was close to this diameter. In the case of hard wall repulsion with negligible local attraction, several simulations predicted an acceleration of segmental dynamics close to the wall. Other than in attractive or neutral systems, where the segmental dynamics is slowed down, we found that the segmental dynamics in the nanopores is identical to the local mobility in the bulk. Even under very careful scrutiny, we could not find any acceleration of the surface-near segmental motion. On the larger time scale, the neutron spin-echo experiment showed that the Rouse relaxation was not altered by confinement effects. Also the entanglement dynamics was not affected. Thus at moderate confinement conditions, facilitated by locally repulsive walls, the dynamics remains as in the bulk melt, a result that is not so clear from simulations.

Neutrons reveal molecular mechanisms of medical and chemical products

Neutron News Volume 26 • Number 1 • 2015 33 Nowadays industrial products are marketed after thorough tests such that the applicability is approved. From the economical view this is often suffi cient, but the molecular mechanisms are often only vaguely understood, and sometimes the progress to the next generation of products is impassable due to the lack of information. Neutrons probe the right lengthand time-scales of molecules, and, therefore, open new horizons for understanding molecular mechanisms in more detail than testing a macroscopic formulation series. The resistance that all mammals develop against the enduring application of antibiotics is not completely understood, and a new strategy for the next generation antibiotics needs to be developed. An example of the inhibition of the antibiotic effect of Penicillin was investigated in more detail by making hydrogen atoms visible. Only the natural contrast between hydrogen and deuterium for neutrons enables such experiments. Volatile solvents as cleaning agents are about to be replaced in the industry and home sector due to their hazards of inhaling, skin contact and release to the environment. Such vapors are completely avoided by micro emulsions that are only too expensive due to their naturally high surfactant contents. Neutrons revealed the mechanisms and identifi ed the optimal polymer additives for reducing the non-economic surfactant. The pain killer ibuprofen causes stomach bleeding in long-term applications, and neutrons resolve possible mechanisms for such malfunctions. The three selected examples of neutron scattering studies show that the method is not only academic but opens new pathways for the challenges of the modern society. Mechanism of bacterial antibiotic resistance More and more bacteria are becoming resistant to antibiotics. An example is the hydrolysis of the β-lactam ring in Penicillin and related antibiotics, thus making these drugs useless. It is still not fully understood how the enzyme betalactamase in bacteria carries out this process. Some imNeutrons reveal molecular mechanisms of medical and chemical products

Scientific Reviews: Exploring Microemulsions with Small Angel Neutrons Scattering and Neutron Spin-Echo Spectroscopy

Microemulsions are thermodynamically stable dispersions of two otherwise non-miscible fluids—usually “oil” and “water”—that are mediated by a surfactant. The surfactant molecules have to be amphiphilic, i.e., one end of a molecule is soluble in oil and the other end in water. The surfactant forms an interface layer between oil and water in the microemulsion. The total area of this interface is basically given by the amount of surfactant, and for a given topology of the oil/water regions, it controls their dimensions.