Stability of Nanopeptides: Structure and Molecular Exchange of Self-assembled Peptide Fibers.

Often nanostructures formed by self-assembly of small molecules based on hydrophobic interactions are rather unstable, causing morphological changes or even dissolution when exposed to changes in aqueous media. In contrast, peptides offer precise control of the nanostructure through a range of molecular interactions where physical stability can be engineered in and, to a certain extent, decoupled from size via rational design. Here, we investigate a family of peptides that form beta-sheet nanofibers and demonstrate a remarkable physical stability even after attachment of poly(ethylene glycol).

Manipulating Phospholipid Vesicles at the Nanoscale: A Transformation from Unilamellar to Multilamellar by an -Alkyl-poly(ethylene oxide).

We investigated the influence of an -alkyl-PEO polymer on the structure and dynamics of phospholipid vesicles. Multilayer formation and about a 9% increase in the size in vesicles were observed by cryogenic transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), and small-angle neutron/X-ray scattering (SANS/SAXS). The results indicate a change in the lamellar structure of the vesicles by a partial disruption caused by polymer chains, which seems to correlate with about a 30% reduction in bending rigidity per unit bilayer, as revealed by neutron spin echo (NSE) spectroscopy.

Spherical Micelles with Nonspherical Cores: Effect of Chain Packing on the Micellar Shape.

Self-assembly of amphiphilic polymers into micelles is an archetypical example of a "self-confined" system due to the formation of micellar cores with dimensions of a few nanometers. In this work, we investigate the chain packing and resulting shape of C -PEO micelles with semicrystalline cores using small/wide-angle X-ray scattering (SAXS/WAXS), contrast-variation small-angle neutron scattering (SANS), and nuclear magnetic resonance spectroscopy (NMR). Interestingly, the -alkyl chains adopt a rotator-like conformation and pack into prolate ellipses (axial ratio ϵ ≈ 0.

How Detergents Dissolve Polymeric Micelles: Kinetic Pathways of Hybrid Micelle Formation in SDS and Block Copolymer Mixtures.

Mixtures of amphiphilic polymers and surfactants are used in a wide range of applications, e.g., pharmaceuticals, detergents, cosmetics, and drug delivery systems.

Tale of Two Tails: Molecular Exchange Kinetics of Telechelic Polymer Micelles.

Telechelic polymers contain two chain ends that are able to promote self-assembly into "flowerlike" or interconnected micellar structures. Here, we investigate the molecular exchange kinetics of such micelles using time-resolved small-angle neutron scattering. We show that the activation energies of monofunctional and telechelic chain exchange are identical.

Structure and thermodynamics of mixed polymeric micelles with crystalline cores: tuning properties via co-assembly.

We investigate micelles formed by mixtures of n-alkyl-poly(ethylene oxide) block copolymers, Cn-PEO, with different alkyl block lengths in aqueous solution. This model system has previously been used to shed light on the interplay between exchange kinetics and crystallinity in self-assembling systems [König et al., Phys.

Cooperativity during Melting and Molecular Exchange in Micelles with Crystalline Cores.

Molecular exchange processes are important equilibration and transport mechanisms in both synthetic and biological self-assembled systems such as micelles, vesicles, and membranes. Still, these processes are not entirely understood, in particular the effect of crystallinity and the interplay between cooperative melting processes and chain exchange. Here we focus on a set of simple polymer micelles formed by binary mixtures of poly(ethylene oxide)-mono-n-alkyl-ethers (C_{n}-PEO5) which allows the melting point to be tuned over a wide range.

Description of poly(ethylenepropylene) confined in nanopores by a modified Rouse model.

A recent model for unentangled polymer chains in confinement [M. Dolgushev and M. Krutyeva, Macromol.

Telechelic Polymer Hydrogels: Relation between the Microscopic Dynamics and Macroscopic Viscoelastic Response.

Telechelic polymers, that is, hydrophilic polymers with hydrophobic end-groups, spontaneously form hydrogels consisting of interconnected micelles. Here we investigate the relation between the microscopic dynamics determining the connectivity, that is, the lifetime of the physical bonds and the resulting rheological properties. This is achieved by quantitatively relating the chain exchange kinetics measured by time-resolved small-angle neutron scattering (TR-SANS) and the mechanical response obtained from linear oscillatory shear measurements.

Molecular Exchange Kinetics of Micelles: Corona Chain Length Dependence.

The rate of molecular exchange in diblock copolymer micelles is strongly dependent on the chain length of the core-forming insoluble block. Less is known about the influence of the soluble block forming the micellar corona. In this study we present a time-resolved small angle neutron scattering (TR-SANS) study exploring systematically the effect of corona chain length on the dynamics of chain exchange.

Tuning the instrument resolution using chopper and time of flight at the small-angle neutron scattering diffractometer KWS-2.

Following demand from the user community regarding the possibility of improving the experimental resolution, the dedicated high-intensity/extended -range SANS diffractometer KWS-2 of the Jülich Centre for Neutron Science at the Heinz Maier-Leibnitz Center in Garching was equipped with a double-disc chopper with a variable opening slit window and time-of-flight (TOF) data acquisition option. The chopper used in concert with a dedicated high-intensity velocity selector enables the tuning at will of the wavelength resolution Δλ/λ within a broad range, from 20% (standard) down to 2%, in a convenient and safe manner following pre-planned or spontaneous decisions during the experiment. The new working mode is described in detail, and its efficiency is demonstrated on several standard samples with known properties and on a completely new crystallizable copolymer system, which were investigated using both the conventional (static) and TOF modes.

Validity of the Stokes-Einstein Relation in Soft Colloids up to the Glass Transition.

We investigate the dynamics of kinetically frozen block copolymer micelles of different softness across a wide range of particle concentrations, from the fluid to the onset of glassy behavior, through a combination of rheology, dynamic light scattering, and pulsed field gradient NMR spectroscopy. We additionally perform Brownian dynamics simulations based on an ultrasoft coarse-grained potential, which are found to be in quantitative agreement with experiments, capturing even the very details of dynamic structure factors S(Q,t) on approaching the glass transition. We provide evidence that for these systems the Stokes-Einstein relation holds up to the glass transition; given that it is violated for dense suspensions of hard colloids, our findings suggest that its validity is an intriguing signature of ultrasoft interactions.

Effect of Core Crystallization and Conformational Entropy on the Molecular Exchange Kinetics of Polymeric Micelles.

Here we systematically study the equilibrium molecular exchange kinetics of a series of amphiphilic -alkyl-poly(ethylene oxide) (C-PEO) micelles containing partly crystallized cores. Using differential scanning calorimetry (DSC), we determined the melting transition and extracted the enthalpy of fusion, Δ, of the -alkyl chains inside the micellar core. Molecular exchange kinetics was measured below the melting point using a time-resolved small-angle neutron scattering technique (TR-SANS) based on mixing deuterated and proteated but otherwise identical micelles.

Studying the concentration dependence of the aggregation number of a micellar model system by SANS.

We present a small-angle neutron scattering (SANS) structural characterization of n-alkyl-PEO polymer micelles in aqueous solution with special focus on the dependence of the micellar aggregation number on increasing concentration. The single micellar properties in the dilute region up to the overlap concentration ϕ* are determined by exploiting the well characterized unimer exchange kinetics of the model system in a freezing and diluting experiment. The micellar solutions are brought to thermodynamic equilibrium at high temperatures, where unimer exchange is fast, and are then cooled to low temperatures and diluted to concentrations in the limit of infinite dilution.

Nanoscopic confinement through self-assembly: crystallization within micellar cores exhibits simple Gibbs-Thomson behavior.

It is well known that liquids confined to small nanoscopic pores and droplets exhibit thermal behavior very different from bulk samples. Less is known about liquids spontaneously confined through self-assembly into micellar structures. Here we demonstrate, using a very well-defined n-alkyl-poly(ethylene oxide) polymer system with a tunable structure, that n-alkane(s) forming 2-3 nm small micellar cores are affected considerably by confinement in the form of melting point depressions.

Surfactant or block copolymer micelles? Structural properties of a series of well-defined n-alkyl-PEO micelles in water studied by SANS.

Here we present an extensive small-angle neutron scattering (SANS) structural characterization of micelles formed by poly(ethylene oxide)-mono-n-alkyl ethers (Cn-PEOx) in dilute aqueous solution. Chemically, Cn-PEOx can be considered as a hybrid between a low-molecular weight surfactant and an amphiphilic block copolymer. The present system, prepared through anionic polymerization techniques, is better defined than other commercially available polymers and allows a very precise and systematic testing of the theoretical predictions from thermodynamical models.

Kinetic Pathway of the Cylinder-to-Sphere Transition in Block Copolymer Micelles Observed in Situ by Time-Resolved Neutron and Synchrotron Scattering.

Here we present an in situ study of the nonequilibrium cylinder-to-sphere morphological transition kinetics on the millisecond range in a model block copolymer micelle system revealing the underlying mechanism and pathways of the process. By employing the stopped-flow mixing technique, the system was rapidly brought (≈100 μs) deep into the instability region, and the kinetics was followed on the time scale of milliseconds using both time-resolved small-angle neutron and X-ray scattering (TR-SANS and TR-SAXS, respectively). Due to the difference in contrast and resolution, SAXS and SANS provide unique complementary information.

Structural observation and kinetic pathway in the formation of polymeric micelles.

The route by which amphiphilic molecules self-assemble into micelles is still not fully understood. In this Letter, we present direct structural information on the birth and growth of block copolymer micelles by means of synchrotron x-ray scattering with millisecond time resolution. Using a quantitative model, we show that the self-assembly process can be viewed as a nucleation and growth type process where the elemental growth mechanism is an exchange of single molecules.

The A-B diblock copolymer as a nonordering external field in a three-component A/B/A-B polymer blend.

Thermal copolymer fluctuations were explored in a three-component blend consisting of a critical (A/B) homopolymer blend and a symmetric A-B diblock copolymer using the technique of neutron small angle scattering. The copolymer has the function of an external nonordering field and thereby determines phase behavior as well as the regimes of 3d-Ising, isotropic Lifshitz, and Brasovskiî critical universality. It was found that the random phase approximation (RPA) does not correctly describe the copolymer structure function because of strong thermal fluctuations.

Logarithmic chain-exchange kinetics of diblock copolymer micelles.

We present a study of equilibrium chain-exchange kinetics of a well-defined model system for starlike polymeric micelles. The results show that the kinetics follows a logarithmic time dependence. The same feature has been confirmed for two other micellar systems.

Composition fluctuations in a homopolymer-diblock copolymer mixture covering the three-dimensional Ising, isotropic Lifshitz, and Brasovskiĭ classes of critical universality.

The phase behavior of a three-component polymer blend consisting of a critical mixture of polybutadiene and polystyrene (PB/PS) with varying amount of a symmetric PB-PS diblock copolymer was explored with small-angle neutron scattering. Our focus were thermal composition fluctuations which we discuss in terms of mean field, three-dimensional Ising, isotropic Lifshitz, and Brasovskiĭ classes of critical universality. Particular attention is spent to the observation of a narrow reentrant two-phase regime and double critical point in the Lifshitz critical regime as well as the Lifshitz line.

Ginzburg number of a homopolymer-diblock copolymer mixture covering the 3D-Ising, isotropic Lifshitz, and Brasovskii classes of critical universality.

The Ginzburg number Gi of deuterated poly(butadiene) (dPB) and poly(styrene) (PS) homopolymer blend of critical composition mixed with a dPB-PS symmetric diblock copolymer was determined from small angle neutron scattering. A 3 orders of magnitude change of Gi was determined between binary polymer blend and diblock copolymer melt. The strongest change of Gi is observed within the isotropic Lifshitz regime of critical universality occurring over a 3% range of diblock concentration and interpolates the corresponding Gi of the 3D-Ising and Brasovskii regimes.

Shear-banding and microstructure of colloids in shear flow.

We shall discuss the following phenomena found in various colloidal systems in shear flow. We recently observed shear-banding in suspensions of fd-virus in a cylindrical shear cell. Small angle light scattering experiments revealed that the shear-banding transition is preceded by a relatively fast process (minutes) of nematic-to-paranematic phase separation during which inhomogeneities on the micrometer length scale are formed.