Nyrkova Irina

Abstract: 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 intermolecular specific SIG-based interactions and demonstrate an effective route for tailoring the shape and size of nanostructures derived from the same building unit.

Abstract: End-growth/evaporation kinetics in living polymer systems with “association-ready” free unimers (no initiator) is considered theoretically. The study is focused on the systems with long chains (typical aggregation number N >> 1) at long times. A closed system of continuous equations is derived and is applied to study the kinetics of the chain length distribution (CLD) following a jump of a parameter (T-jump) inducing a change of the equilibrium mean chain length from N(0) to N. The continuous approach is asymptotically exact for t >> t(1), where t(1) is the dimer dissociation time. It yields a number of essentially new analytical results concerning the CLD kinetics in some representative regimes. In particular, we obtained the asymptotically exact CLD response (for N >> 1) to a weak T-jump (epsilon = N(0)/N – 1

Abstract: We analyze twisted membrane structures of chiral surfactants in the framework of the equilibrium theoretical approach based on a continuous elastic model. It is shown that both the surfactant ribbon width and its twist pitch can be defined by equilibrium factors, including the chiral elastic energy. We show however that the theoretically predicted dependencies of the ribbon geometrical parameters on the enantiomeric excess (ee) obtained for the uni-lamellar liquid membrane model are in partial, but qualitative disagreement with experimental data. The surfactant membrane model is then generalized in two directions: (i) we consider also multi-lamellar ribbons (of many bilayers), (ii) we allowed for the crystalline (L-beta) state of the bilayers. The multi-bilayer crystalline structure of self-assembling twisted ribbons is consistent with recent results of X-ray and other experimental studies on gemini surfactants, and, simultaneously, the theory developed for such ribbons yields the ee-dependencies of the twisted ribbon width and pitch that are in reasonable agreement with the data based on TEM images.

Abstract: Theoretically studying the dynamics of entangled living polymers which can reversibly break and recombine, we find a number of diffusion-controlled regimes of essentially nonexponential stress relaxation. This behavior is in contrast to predictions of the classical theory of Cates and its more recent generalizations. The non-exponential relaxation is due to strong correlations between chain scissions and recombinations: multiple self-recombinations are much more frequent than recombinations with other chains and are resulting in fast release of topological constraints (entanglement tube coarsening). The strong renormalization effect of correlations on the lifetime of a living chain (the time period tau(b) until the chain breaks and one of its fragments recombines with another chain) is also elucidated. We show that the correlation effects defining the dynamics are controlled by the activation energy for chain recombination and by the chain rigidity. Copyright (C) EPLA, 2007.

Abstract: Living polymers are formed by reversible association of primary units (unimers). Generally the chain statistical weight involves a factor sigma < 1 suppressing short chains in comparison with free unimers. Living polymerization is a sharp thermodynamic transition for sigma << 1 which is typically the case. We show that this sharpness has an important effect on the kinetics of living polymerization (one-dimensional association). The kinetic model involves i) the unimer activation step (a transition to an assembly-competent state); ii) the scission/recombination processes providing growth of polymer chains and relaxation of their length distribution. Analyzing the polymerization with no chains but unimers at t = 0, with initial concentration of unimers M greater than or similar to M* (M* is the critical polymerization concentration), we determine the time evolution of the chain length distribution and find that: 1) for M* << M << M*/sigma the kinetics is characterized by 5 distinct time stages demarcated by 4 characteristic times t(1), t(2), t(3) and t*; 2) there are transient regimes (t(1) less than or similar to t less than or similar to t(3)) when the molecular-weight distribution is strongly non-exponential; 3) the chain scissions are negligible at times shorter than t(2). The chain growth is auto-accelerated for t(1) less than or similar to t less than or similar to t(2): the cut-off chain length (= polymerization degree < n >(w) N-1 proportional to t(2) in this regime. 4) For t (2) < t < t (3) the length distribution is characterized by essentially 2 non-linear modes; the shorter cut-off length N-1 is decreasing with time in this regime, while the length scale N-2 of the second mode is increasing. (5) The terminal relaxation time of the polymer length distribution, t*, shows a sharp maximum in the vicinity of M*; the effective exponent partial derivative ln 1/t*/partial derivative ln M is as high as similar to sigma(-1/3) just above M*.

Abstract: 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.

Abstract: Various extensions of Onsager theory based on the second virial approximation are constructed in order to describe nematic P-phases in ribbon polymer solutions. If the average coil shape is anisotropic and the concentration is not too high, the coils can be considered as anisotropic solid objects and an ordinary expansion over the coil concentration can be performed. This first approach can be applied to study pancake nematic ordering. At higher concentrations and when the polymer chains are rather long, the polymer solution can be considered as a solution of short fragments connected into chains with a particular statistics reflecting the polymer structure with two rigidities: a similar virial expansion can be constructed. This last approach allows the consideration of the various symmetries of the nematic phases simultaneously. We also take into account the correlation correction to the mean field result which provides an essential additional angular dependence to the interaction free energy. Scaling arguments are applied in the case where the intra-coil correlations are very strong, or if the coils interact as opaque objects. The methods developed here will be used in further publications to study liquid crystalline ordering in solutions of ribbon chains.

Abstract: We discuss the solution properties of polymers with a highly anisotropic rigidity which bend rather freely in a plane (the plane of main flexibility) and are extremely rigid in the direction perpendicular to this plane. Examples of these polymers are the ladder polymers recently synthesized or living polymers formed by the aggregation of peptide rodlike fragments. These polymers have a much higher out of plane persistence length l(2) than their in-plane persistence length l. In the first paper of this series, we mostly investigate the conformation of single chains in solution (at extremely low concentrations). The conformation of an isolated chain with a highly anisotropic rigidity essentially depends on the dimensionless parameter Ohm = l(2)d(2)l(-3) where d is the chain diameter. For small values of Ohm, (l(2) similar to l), the chain behaves as a standard semiflexible chain with isotropic rigidity. For large values of Ohm, (Ohm greater than or similar to 1), the chain adopts a one-dimensional rodlike conformation at length scales smaller than l, an anisotropic disc-like conformation at intermediate scales (corresponding to a contour length L such that l less than or similar to L less than or similar to l(2)) and a three-dimensional swollen coil conformation at larger length scale. In the intermediate range of Ohm, l(2)/d(2) less than or similar to Ohm less than or similar to 1, the same three regimes are expected but the excluded volume interactions do not play any role in the disc-like regime. At the end of the paper we discuss qualitatively the possible liquid crystalline phases (with nematic or smectic symmetry) which can emerge in these solutions at higher concentration.