Jörg Stellbrink

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 ...

Starlike micelles with starlike interactions: a quantitative evaluation of structure factors and phase diagram.

Starlike PEP-PEO block copolymer micelles offer the possibility to investigate the phase behavior and interactions of regular star polymers (ultrasoft colloids). Micellar functionality f can be smoothly varied by changing solvent composition (interfacial tension). Structure factors obtained by small-angle neutron-scattering can be quantitatively described in terms of an effective potential for star polymers. The experimental phase diagram reproduces to a high level of accuracy the predicted liquid-solid transition. Whereas for intermediate f a bcc phase is observed, for high f the formation of a fcc phase is preempted by glass formation.

Shear induced structures of soft colloids: Rheo-SANS experiments on kinetically frozen PEP–PEO diblock copolymer micelles

We investigated the effect of external steady shear on dilute to concentrated solutions of PEP-PEO diblock copolymer micelles (soft colloids). The degree of softness in terms of particle interactions (intermolecular softness) and deformability of the individual particle (intramolecular softness) was varied by changing the ratio between hydrophobic and hydrophilic blocks from symmetric (1:1, hard sphere-like) to very asymmetric (1:20, star-like). We performed in situ rheology and small angle neutron scattering experiments (Rheo-SANS) to relate macroscopic flow properties to microscopic structural changes. The rheology data qualitatively show the same behavior for both types of micelles: (i) a divergence of the zero shear viscosity η 0 at a critical concentration Φ c approximately following a Vogel-Fulcher-Tammann law and (ii) close to this liquid-solid transition a shear rate dependent viscosity which can be described by the Carreau function with an asymptotic power law η(γ) ∼ γ -0.4 starting at a critical shear rate γ c . Rheo-SANS experiments in the liquid phase close to Φ c were extended into the strong shear thinning region for both types of micelles at Φ/Φ c ≈ 0.8 and γ red = γ/γ c ≈ 10. In our Rheo-SANS data we observe a rather controversial influence of external shear on the structural properties of the two different micellar systems. With increasing shear rate the symmetric, hard sphere-like micelles show a decreasing structure factor S(Q) but a shear rate independent interparticle distance. The asymmetric, star-like micelles show an increase in S(Q) and an increase of the interparticle distance, both in the flow and vorticity direction. This unexpected behavior can be rationalized by a shear induced elongation and tilt of the star-like micelles along the flow direction as predicted by recent MD simulations (Ripoll et al 2006 Phys. Rev. Lett. 96 188302).

Neither Gaussian chains nor hard spheres - star polymers seen as ultrasoft colloids

In dense solution high functionality star polymers show ordering phenomena which give rise to a well-pronounced peak in the static structure factor, S exp(Q), observed by small-angle neutron scattering. The concentration dependence of S exp (Q) gives evidence for unusual phase behaviour as predicted by theory. In addition, the dynamics of the star polymer solutions is dominated by an increasing amount of structural arrest with increasing concentration. The mean square displacement obtained from neutron spin-echo spectroscopy is compared to the blob size obtained from dynamic light scattering. Thermal energy enables each star core to perform restricted motion over a spatial extent equal to the blob size of the surrounding dense star polymer solution.

Poly(ethylene-alt-propylene)-poly(ethylene oxide) diblock copolymer micelles: a colloidal model system with tunable softness

Kinetically frozen micelles formed by the amphiphilic diblock copolymer poly(ethylene-alt-propylene)?poly(ethylene oxide)?(PEP?PEO) are proposed as a new model system for soft colloids. In this context soft is used with a twofold meaning: the intraparticle softness, i.e.?the molecular architecture of an individual micelle, as well as the interparticle softness, i.e.?the effective potential for micellar interactions. Both contributions can be precisely adjusted from hard sphere-like to ultrasoft (star-like) by changing the diblock copolymer composition and/or interfacial tension, as shown by small angle neutron scattering in combination with contrast variation techniques. Depending on the degree of softness, PEP?PEO micellar solutions respond variably to the application of external shear fields. In particular, in the star-like regime solutions are already extremely sensitive at low shear rates. Therefore these micelles are an excellent starting point for a comprehensive study on the relation between softness and non-equilibrium phase behaviour in colloidal systems.

KWS-3: The New (Very) Small-Angle Neutron Scattering Instrument Based on Focusing-Mirror Optics

Ultra-small angle (U-SANS) and small angle neutron scattering (SANS) experiments are performed by two different types of instruments to cover a combined Q-range from ≈10−5Å−1 up to ≈1Å−1. Bonse-Hart cameras (double crystal diffractometers) are used for U-SANS experiments, whereas the “standard” SANS experiment is performed using a pinhole camera. In principle, the Q-range of both instrument classes overlaps. Typical U-SANS instruments like S18 (ILL), PCD (NIST), or DKD (FZJ) may reach maximum Q-vectors of ≈5 × 10−3. The disadvantage of these instruments is that they do not allow taking a full area image on a 2D position sensitive detector. On the other hand, the well-known pinhole instrument D11 at Institut Laue-Langevin (France) reaches a minimum Q-vector of 5 × 10−4Å−1 by use of large wavelengths and sample-to-detector distances (≈40m).

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.

Hierarchical structures formed by partially crystalline polymers in solution: from fundamentals to applications - a combined conventional, focusing and ultra-small-angle neutron scattering study

Multilevel aggregates with characteristic sizes covering four orders of magnitude, from 1 nm to 10 µm, are formed upon cooling decane solutions of poly(ethylene-butene) random copolymers (designated as PEB-n, where n is the number of ethyl side branches per 100 backbone C atoms) and wax-containing mixed solutions. The partially crystalline PEB-7.5 copolymers form two distinct morphologies that evolve on a range of length scales. When these polymers are mixed with wax molecules having a crystallization point lower than the polymer aggregation temperature, a hierarchy of morphologies evolves on decreasing the temperature. The multilevel structures were elucidated by combining conventional small-angle neutron scattering, focusing small-angle neutron scattering and ultra-small-angle neutron scattering investigations with microscopy. Contrast-matching analysis of the wax and copolymer components within the common morphologies revealed the wax-crystal modification capacity of the PEB-7.5 copolymers. Since the copolymers limit the growth of wax crystals, they are potential pour-point depressants for the fuel industry.

Chemically defined, ultrasoft PDMS elastomers with selectable elasticity for mechanobiology

Living animal cells are strongly influenced by the mechanical properties of their environment. To model physiological conditions ultrasoft cell culture substrates, in some instances with elasticity (Youngs modulus) of only 1 kPa, are mandatory. Due to their long shelf life PDMS-based elastomers are a popular choice. However, uncertainty about additives in commercial formulations and difficulties to reach very soft materials limit their use. Here, we produced silicone elastomers from few, chemically defined and commercially available substances. Elastomers exhibited elasticities in the range from 1 kPa to 55 kPa. In detail, a high molecular weight (155 kg/mol), vinyl-terminated linear silicone was crosslinked with a multifunctional (f = 51) crosslinker (a copolymer of dimethyl siloxane and hydrosilane) by a platinum catalyst. The following different strategies towards ultrasoft materials were explored: sparse crosslinking, swelling with inert silicone polymers, and, finally, deliberate introduction of dangling ends into the network (inhibition). Rheological experiments with very low frequencies led to precise viscoelastic characterizations. All strategies enabled tuning of stiffness with the lowest stiffness of ~1 kPa reached by inhibition. This system was also most practical to use. Biocompatibility of materials was tested using primary cortical neurons from rats. Even after several days of cultivation no adverse effects were found.