Statistical mechanics of semiflexible ribbon polymers

The statistical mechanics of a ribbon polymer made up of two semiflexible chains is studied using both analytical techniques and simulation. The system is found to have a crossover transition at some finite temperature, from a type of short range order to a fundamentally different sort of short range order. In the high temperature regime, the 2-point correlation functions of the object are identical to worm-like chains, while in the low temperature regime they are different due to a twist structure. The crossover happens when the persistence length of individual strands becomes comparable to the thickness of the ribbon. In the low temperature regime, the ribbon is observed to have a novel ``kink-rod'' structure with a mutual exclusion of twist and bend in contrast to smooth worm-like chain behaviour. This is due to its anisotropic rigidity and corresponds to an {\it infinitely} strong twist-bend coupling. The double-stranded polymer is also studied in a confined geometry. It is shown that when the polymer is restricted in a particular direction to a size less than the bare persistence length of the individual strands, it develops zigzag conformations which are indicated by an oscillatory tangent-tangent correlation function in the direction of confinement. Increasing the separation of the confining plates leads to a crossover to the free behaviour, which takes place at separations close to the bare persistence length. These results are expected to be relevant for experiments which involve complexation of two or more stiff or semiflexible polymers.Comment: 20 pages, 11 figures. PRE (in press

Micellization of Sliding Polymer Surfactants

Following up a recent paper on grafted sliding polymer layers (Macromolecules 2005, 38, 1434-1441), we investigated the influence of the sliding degree of freedom on the self-assembly of sliding polymeric surfactants that can be obtained by complexation of polymers with cyclodextrins. In contrast to the micelles of quenched block copolymer surfactants, the free energy of micelles of sliding surfactants can have two minima: the first corresponding to small micelles with symmetric arm lengths, and the second corresponding to large micelles with asymmetric arm lengths. The relative sizes and concentrations of small and large micelles in the solution depend on the molecular parameters of the system. The appearance of small micelles drastically reduces the kinetic barrier signifying the fast formation of equilibrium micelles.Comment: Submitted to Macromolecule

Domains in Melts of Comb-Coil Diblock Copolymers: Superstrong Segregation Regime

Conditions for the crossover from the strong to the superstrong segregation regime are analyzed for the case of comb-coil diblock copolymers. It is shown that the critical interaction energy between the components required to induce the crossover to the superstrong segregation regime is inversely proportional to mb = 1 + n/m, where n is the degree of polymerization of the side chain and m is the distance between successive grafting points. As a result, the superstrong segregation regime, being rather rare in the case of ordinary block copolymers, has a much better chance to be realized in the case of diblock copolymers with combs grafted to one of the blocks.

Заболеваемость детей сальмонеллезом и уровень резистентности клинических штаммов сальмонелл к антибактериальным препаратам в Санкт-Петербурге

The aim of the study was to study the dynamics of the incidence of salmonellosis children in St. Petersburg and phenotypic resistance of clinical isolates of S. Enteritidis and S. Typhimurium to antibiotics in recent years. Materials and methods. The incidence of salmonellosis children studied according to the report for the first nine months of Rospotrebnadzor in 2013–2014. Incidence of salmonellosis in the structure of bacterial intestinal infections caused by pathogens in children hospitalized in the Department of intestinal infections in 2013–2014, studied according to annual reports. Antibiotic sensitivity was studied 86 Salmonella isolates (S. Enteritidis strain 64 and strain 22 S. Typhimurium), isolated from patients in children 2010–2014. Used the method of serial microdilution broth. Salmonella isolates were divided into sensitive, resistant, intermediate sensitivity to antibiotics. The Results. Analysis of the incidence of salmonellosis children of St. Petersburg has revealed its decline in 2014 (109.2) compared to 2013 (123,9) but relatively long-term average level was an increase in incidence (107,6). In the structure of salmonellosis in children prevailed salmonellosis Group D. In hospitalized children in the structure of bacterial intestinal infections detected Excess of share of salmonellosis in 2014 (36,9±3,4%) compared to 2013 (24,5±2,4%; p <0,01). A reduction in the frequency sensitivity of S. Enteritidis to ampicillin, cefepime, ceftazidime and chloramphenicol. Compared to S. Enteritidis S. Typhimurium isolates were more resistant to ceftazidime and ampicillin, but more sensitive to ciprofloxacin. Conclusion. Morbidity of salmonellosis in recent years characterized by a relatively long-term average increase of the level. In the structure of salmonellosis in children prevailed salmonellosis Group D. There was a reduction of sensitivity S. Enteritidis isolates to cephalosporins new generations, and S. Typhimurium isolates to cephalosporins new generations, ampicillin, chloramphenicol.Цель исследования: изучение динамики заболеваемости сальмонеллезом детей в Санкт-Петербурге и фенотипической антибиотикорезистентности изолятов S. Enteritidis и S. Typhimurium в последние годы. Материалы и методы: заболеваемость сальмонеллезом детей изучали по данным отчета Роспотребнадзора за девять месяцев 2013–2014 гг. Частоту сальмонеллеза в структуре бактериальных кишечных инфекций, вызванных патогенными возбудителями, у детей, госпитализированных в отделение кишечных инфекций НИИДИ в 2013–2014 гг., изучали по данным годовых отчетов. Исследовали антибиотикочувствительность 86 изолятов сальмонелл (64 штамма S. Enteritidis и 22 штамма S. Typhimurium), выделенных от больных детей в 2010– 2014 гг. Использовали метод серийных микроразведений в бульоне, изоляты сальмонелл разделяли на чувствительные, резистентные, с промежуточной чувствительностью к антибиотикам. Результаты: анализ заболеваемости сальмонеллезом детей Санкт-Петербурга выявил ее снижение в 2014 г. (109,2 на 100 тыс. нас.) по сравнению с 2013 г. (123,9 на 100 тыс. нас.), относительно многолетнего уровня отмечалось повышение заболеваемости (107,6 на 100 тыс. нас.). В структуре сальмонеллезов преобладали сальмонеллезы группы D. У госпитализированных детей в структуре бактериальных кишечных инфекций выявлено превышение доли сальмонеллеза в 2014 г. (36,9±3,4%) по сравнению с 2013 г. (24,5±2,4%; p<0,01). Установлено снижение частоты чувствительности изолятов S. Enteritidis к ампициллину, цефепиму, цефтазидиму и хлорамфениколу. По сравнению с S. Enteritidis изоляты S. Typhimurium были более резистентны к цефтазидиму и ампициллину, но более чувствительны к ципрофлоксацину. Заключение: заболеваемость детей сальмонеллезом в последние годы характеризовались нарастанием относительно среднего многолетнего уровня. В структуре сальмонеллезов преобладали сальмонеллезы группы D. Выявлено снижение чувствительности изолятов S. Enteritidis к цефалоспоринам новых генераций, а изолятов S. Typhimurium – к цефалоспоринам новых генераций, ампициллину, хлорамфениколу. 

Multimerization: Closed or open association scenario?

We address the problem of classification of the type of association (multimerization) in solutions of identical unimers. Although the aggregation is still routinely characterized in terms of either “open association” or “closed association” models, neither of the two models can provide a fair description of many aggregation processes. We demonstrate that a realistic multimerization may show mixed basic features, i.e. simultaneously those typical of the classical open association and those typical of the classical closed association. In particular, we show that a living polymerization may seem to exhibit a critical micelle concentration (CMC), whereas the basic integral characteristics of a “closed” system with rather monodisperse micelles ( (m w/m n $\left.\vphantom{m_w/m_n}\right)_{{mic}}^{}$ = 1.02) may not imply a well-defined CMC. Therefore, a multimerization must be characterized by the detailed micelle size distribution that largely defines the equilibrium and dynamical properties of the system. To this end, we suggest a simple method of deriving the micelle size distribution cm (depending also on the total concentration of amphiphilic molecules) from the concentration dependence of just the mean aggregation number, m w(c). Our results thus invite a reconsideration of the basic methods used for interpretation of experimental data on micellization

Correlation effects in dynamics of living polymers

Theoretically studying the dynamics of entangled living polymers which can reversibly break and recombine, we find a number of diffusion-controlled regimes of essentially non-exponential 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

Non-linear scission/recombination kinetics of living polymerization

Living polymers are formed by reversible association of primary units (unimers). Generally the chain statistical weight involves a factor σ < 1 suppressing short chains in comparison with free unimers. Living polymerization is a sharp thermodynamic transition for σ ≪ 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 ≳ 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 */σ the kinetics is characterized by 5 distinct time stages demarcated by 4 characteristic times t1, t2, t3 and t*; 2) there are transient regimes (t 1 ≲ t ≲ t 3) when the molecular-weight distribution is strongly non-exponential; 3) the chain scissions are negligible at times shorter than t2. The chain growth is auto-accelerated for t 1 ≲ t ≲ t 2 : the cut-off chain length (= polymerization degree 〈n〉w N 1 ∝ 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 N1 is decreasing with time in this regime, while the length scale N2 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 ${\frac{{\partial\ln1/t^{*}}}{{\partial\ln M}}}$ is as high as ∼ σ-1/3 just above M*

Adsorption of semiflexible wormlike polymers to a bar and their double-chain complex formation

International audienceWe theoretically study pairing (double-strand complexation) of semiflexible wormlike chains (WLC) due to their side-to-side attraction. Considering binding of two WLCs of high stiffness we start with the case of infinite stiffness of one chain which is replaced with a straight bar. A combination of the quantitative transfer matrix approach with scaling arguments in terms of trains, loops of different sizes, tails and supertrains allowed us to characterize all the regimes of semiflexible chain adsorption on a bar. In particular, we predict a self-similar monomer concentration profile c(r) ∝ r-10/3 near the bar (at distances r below the chain Kuhn length l) at the critical point for adsorption. Such localized critical profile leads to a sharp adsorption transition. Furthermore, we found that supertrains serve as the basic structural elements in WLC complexes leading to bridging, network formation and condensation of semiflexible polymers in dilute solutions. Polymer collapse (precipitation) and redissolution on increasing attraction strength are predicted in qualitative agreement with experiments on aqueous solutions of DNA and F-actin

The Rheological Behavior of Polymer Solution Threads

International audienceTheoretical results of the authors in the field of the capillary thinning of polymer solution threads are reviewed. The dynamics of threads of both concentrated solutions without entanglements and dilute solutions, where hydrodynamic interactions play an important role, is considered. A molecular approach, in which macromolecules are simulated by a semiflexible chain, is used as a basis. This makes it possible to describe, from common positions, the nonlinear elasticity of solution and interactions taking into account the orientation of the chains. Particular attention is given to thread thinning in the region of the elastic behavior of solution where macromolecules unfold along the axis of stretching. The results of analysis of the capillary stability of a thread and conditions for solvent droplets emergence on its surface are presented, and the dynamics of formation of the hierarchical structure of beads-on-string droplets is viewed. Mechanisms behind the subsequent merging of droplets related to solvent overflow and droplets diffusion along the polymer string are discussed. The polymer string breakup occurs at time scales higher than the Rouse relaxation time of the polymer chain. String hardening and fiber formation may be an alternative