Capillary Thinning of Viscoelastic Threads of Unentangled Polymer Solutions.

In this paper, we theoretically consider the process of the capillary thinning of a polymer fluid thread bridging two large immobile droplets in the regime of highly stretched polymer chains. We first derive a new relation between the pressure p and the flow velocity v in unentangled polymer solutions, which is called the anti-Bernoulli law: it shows that p is higher where v is faster. Using this equation, it is shown that the flow field is asymptotically irrotational, in particular, in the thread/droplet transition zones (in the case, the negligible solvent viscosity and inertial effects). On this basis, we predict the free surface profile and the thread thinning law for the FENE-P model of polymer dynamics. The predictions are compared with recent theoretical results and some experimental data on capillary thinning.

Adsorption of Wormlike Chains onto Partially Permeable Membranes.

Reversible adsorption of a single stiff wormlike macromolecule to flat membranes with various permeabilities is considered theoretically. It is shown that the adsorbed layer microstructure is significantly different from either a flexible chain or a stiff chain adsorption at a solid surface. Close to the critical point, the adsorbing wormlike chain forms a strongly anisotropic proximal layer near the membrane in addition to a nearly isotropic distal layer. The proximal layer is characterized by the algebraic monomer concentration profile, c(x)∝x-ß, due to the self-similar distribution of aligned polymer loops. For a perfectly penetrable membrane, ß=1 which is different from ß=4/3 obtained for semiflexible chain adsorption at a solid surface. Moreover, we establish that the critical exponent for a partially permeable membrane depends on its properties (porosity w) and propose an asymptotically exact theory (based on the generalized Edwards equation) predicting this dependence, ß=ß(w). We also develop a scaling theory elucidating, in particular, an intricate competition of loops and tails in both proximal and distal sublayers.

Homodyne dynamic light scattering in supramolecular polymer solutions: anomalous oscillations in intensity correlation function.

Dilute solutions of electronically active molecules capable of irradiation-driven supramolecular self-assembly are studied by dynamic light scattering. We detect unusual well-defined oscillations in the long time range of the homodyne intensity correlation function for all solutions that were irradiated with white light prior to the measurements. The oscillation effect is attributed to the local laser-induced heating of the samples due to strongly enhanced absorption manifested by the supramolecular filaments. It is found that the oscillation frequency depends on the irradiation time, solution concentration, and the incident laser power, but is independent of the scattering angle. These observations are explained with a semi-quantitative theory relating the oscillation effect to thermo-gravitational convection flows generated by laser beam. The results suggest that the presence of such homodyne oscillations could be a sensitive probe for aggregation in many complex systems.

Peptide strand length controls the energetics of self-assembly and morphology of ß-sheet fibrils.

Self-assembling peptides can be used as versatile, natural, and multifunctional building blocks to produce a variety of well-defined nanostructures, materials and devices for applications in medicine and nanotechnology. Here, we concentrate on the 1D self-assembly of de novo designed Px-2 peptide ß-strands into anti-parallel ß-sheet tapes and higher order aggregates. We study six members of the Px-2 family, ranging from 3 amino acids (aa) to 13 aa in length, using a range of complementary experimental techniques, computer simulation and theoretical statistical mechanics. The critical concentration for self-assembly (c*) is found to increase systematically with decreasing peptide length. The shortest peptide found to self-assemble into soluble ß-tapes in water is a 5 amino acid residue peptide. These investigations help decipher the role of the peptide length in controlling self-assembly, aggregate morphology, and material properties. By extracting free energies from these data using a statistical mechanical analysis and combining the results with computer simulations at the atomistic level, we can extract the entropy of association for individual ß-strands.

Solvent-controlled reversible switching between adsorbed self-assembled nanoribbons and nanotubes.

We have demonstrated that solutions of 3,5-bis-(5-hexylcarbamoylpentyloxy)-benzoic acid decyl ester (BHPB-10) can form metastable nanostructures on solid substrates and in the bulk. BHPB-10 is an achiral molecule involving several distinct, strongly interacting groups (SIGs), one aromatic-ester ring and two amide groups per molecule. Specific solvents affect the interactions between particular SIGs, thus promoting various nano-structures: lamellae, nanoribbons, helical ribbons, or nanotubes. In cyclohexane, a solvent allowing for both inter-amide hydrogen bonds and mutual attraction of rings, the formation of nanotubes with a diameter of 28 ± 5 nm was observed in the bulk and on surfaces. By contrast, in cyclohexanone, which suppresses inter-amide hydrogen bonds, flat nanoribbons with a specific width of 12 ± 4 nm were formed on solid substrates after drying. By annealing in cyclohexane vapor, we followed the process of switching surface structures from nanoribbons to nanotubes and observed helical ribbons as the precursor of nanotubes. We also turned nanotubes back into nanoribbons by adding cyclohexanone, thus demonstrating reversible switching along the route: tubes → lamellae → flat ribbons → helical ribbons → tubes. We propose models explaining the observed nanostructures and their transformations, including the origin of spontaneous chirality of the helical ribbons. Our findings on the self-assembly in the achiral BHPB-10 solutions provide insight into the influence of complementary inter-molecular specific SIG-based interactions and demonstrate an effective route for tailoring the shape and size of nanostructures derived from the same building unit.

Anisotropic Self-Assembly of Supramolecular Polymers and Plasmonic Nanoparticles at the Liquid-Liquid Interface.

The study of supramolecular polymers in the bulk, in diluted solution, and at the solid-liquid interface has recently become a major topic of interest, going from fundamental aspects to applications in materials science. However, examples of supramolecular polymers at the liquid-liquid interface are mostly unexplored. Here, we describe the supramolecular polymerization of triarylamine molecules and their light-triggered organization at a chloroform-water interface. The resulting interfacial nematic layer of these 1D supramolecular polymers is further used as a template for the precise alignment of spherical gold nanoparticles coming from the water phase. These hybrid thin films are spontaneously formed in a single process, without chemical prefunctionalization of the metallic nanoparticles, and their ordering is improved by centrifugation. The resulting polymer chains and strings of nanoparticles can be co-aligned with high anisotropy over very large distances. By using a combination of experimental and theoretical investigations, we decipher the full sequence of this oriented self-assembly process. In such a highly anisotropic configuration, electron energy loss spectroscopy reveals that the self-assembled nanoparticles behave as plasmonic waveguides.

Supramolecular self-assembly and radical kinetics in conducting self-replicating nanowires.

By using a combination of experimental and theoretical tools, we elucidate unique physical characteristics of supramolecular triarylamine nanowires (STANWs), their packed structure, as well as the entire kinetics of the associated radical-controlled supramolecular polymerization process. AFM, small-angle X-ray scattering, and all-atomic computer modeling reveal the two-columnar "snowflake" internal structure of the fibers involving the π-stacking of triarylamines with alternating handedness. The polymerization process and the kinetics of triarylammonium radicals formation and decay are studied by UV-vis spectroscopy, nuclear magnetic resonance and electronic paramagnetic resonance. We fully describe these experimental data with theoretical models demonstrating that the supramolecular self-assembly starts by the production of radicals that are required for nucleation of double-columnar fibrils followed by their growth in double-strand filaments. We also elucidate nontrivial kinetics of this self-assembly process revealing sigmoid time dependency and complex self-replicating behavior. The hierarchical approach and other ideas proposed here provide a general tool to study kinetics in a large number of self-assembling fibrillar systems.

On the theory of aggregation and micellization: PEO-PVP copolymer in water.

We develop a theoretical approach to micellization of the PEO-PVP block-copolymer in water. This copolymer is a weak polyelectrolyte due to protonation of VP blocks. The theory accounts for non-linear ion screening, and predicts strong position dependence of both ion concentration and the effective Debye length. We consider both the case when the local Debye length is small compared to the core radius and the case when it is large. We found that the effective (local) pH is not uniform even inside one micellar core, hence non-uniform protonation of the core with higher charge density near the surface. In many cases the core charge is concentrated in a relatively thin surface layer. Considering statistical weights of non-equilibrium micelles and their continuous evolution we show that kinetics of both formation and dissociation of typical block-copolymer or surfactant micelles can be extremely slow. Thus micelle formation at the genuine (equilibrium) critical micelle concentration (c.m.c.) is totally suppressed (involves astronomical time scales) if the micelles are big enough. An 'apparent' critical micelle concentration (c.m.c.*) is introduced to account for this effect. The apparent c.m.c.* could be much higher than the genuine equilibrium c.m.c., i.e. a significant hysteresis is inherent in these systems. We also determine the ranges of meta-stability of micelles depending on the experimental time-scales.