Shapes of semiflexible polymer rings

The shape of semiflexible polymer rings is studied over their whole range of flexibility. Investigating the joint distribution of asphericity and the nature of asphericity as well as their respective averages, we find two distinct shape regimes depending on the flexibility of the polymer. For a small perimeter to persistence length the fluctuating rings exhibit only planar, elliptical configurations. At higher flexibilities three-dimensional, crumpled structures arise. Analytic calculations confirm the qualitative behavior of the averaged shape parameters and the elliptical shape in the stiff regime.     

High-efficiency single-mode Raman generation in a liquid-filled photonic bandgap fiber

Single-spatial-mode Raman generation in an ethanol-filled photonic bandgap fiber is demonstrated. Due to the limited bandwidth of the fiber, the generation is limited to the first Stokes order only, allowing high generated power without any visible decrease of the conversion efficiency. The realization of these two key properties opens the way to the realization of optimized compact nonlinear wavelength converters that will accommodate a large variety of usable liquids.     

Fluctuating semiflexible polymer ribbon constrained to a ring

Twist stiffness and an asymmetric bending stiffness of a polymer or a polymer bundle is captured by the elastic ribbon model. We investigate the effects a ring geometry induces to a thermally fluctuating ribbon, finding bend-bend coupling in addition to twist-bend coupling. Furthermore, due to the geometric constraint the polymer's effective bending stiffness increases. A new parameter for experimental investigations of polymer bundles is proposed: the mean square diameter of a ribbonlike ring, which is determined analytically in the semiflexible limit. Monte Carlo simulations are performed which affirm the model's prediction up to high flexibility.     

Acid‐Labile Amphiphilic PEO‐b‐PPO‐b‐PEO Copolymers: Degradable Poloxamer Analogs

Poly ((ethylene oxide)‐b‐(propylene oxide)‐b‐(ethylene oxide)) triblock copolymers commonly known as poloxamers or Pluronics constitute an important class of nonionic, biocompatible surfactants. Here, a method is reported to incorporate two acid‐labile acetal moieties in the backbone of poloxamers to generate acid‐cleavable nonionic surfactants. Poly(propylene oxide) is functionalized by means of an acetate‐protected vinyl ether to introduce acetal units. Three cleavable PEO‐PPO‐PEO triblock copolymers (M_n,total = 6600, 8000, 9150 g·mol⁻¹; M_n,PEO = 2200, 3600, 4750 g·mol⁻¹) have been synthesized using anionic ring‐opening polymerization. The amphiphilic copolymers exhibit narrow molecular weight distributions (Ð = 1.06–1.08). Surface tension measurements reveal surface‐active behavior in aqueous solution comparable to established noncleavable poloxamers. Complete hydrolysis of the labile junctions after acidic treatment is verified by size exclusion chromatography. The block copolymers have been employed as surfactants in a miniemulsion polymerization to generate polystyrene (PS) nanoparticles with mean diameters of ≈200 nm and narrow size distribution, as determined by dynamic light scattering and scanning electron microscopy. Acid‐triggered precipitation facilitates removal of surfactant fragments from the nanoparticles, which simplifies purification and enables nanoparticle precipitation “on demand.”

     

Studies of Mental “Noise”

Successive tests of subjects' performance in reaction time tasks were treated as time series and submitted to spectrum analysis. Rather than revealing the white noise expected by the view that variability is due to random error (equivalent power across frequency), the power spectra revealed colored noise. The slopes of the power spectra did not vary much for tasks differing in memory requirements. Spectrum analysis of time series from the logistic map also showed colored noise in regions on the edge of chaos, showing that the presence of colored noise in cognitive data need not oblige a theoretical account based on a complex, high-dimensional, system.     

A stripe-to-droplet transition driven by conformational transitions in a binary lipid-lipopolymer mixture at the air-water interface

We report the observation of an unusual stripe-droplet transition in precompressed Langmuir monolayers consisting of mixtures of poly(ethylene) glycol (PEG) amphiphiles and phospholipids. This highly reproducible and fully reversible transition occurs at approximately zero surface pressure during expansion (or compression) of the monolayer following initial compression into a two-dimensional solid phase. It is characterized by spontaneous emergence of an extended, disordered stripe-like morphology from an optically homogeneous phase during gradual expansion. These stripe patterns appear as a transient feature and continuously progress, involving gradual coarsening and ultimate transformation into a droplet morphology upon further expansion. Furthermore, varying relative concentrations of the two amphiphiles and utilizing amphiphiles with considerably longer ethylene glycol headgroups reveal that this pattern evolution occurs in narrow concentration regimes, values of which depend on ethylene oxide headgroup size. These morphological transitions are reminiscent of those seen during a passage through a critical point by variations in thermodynamic parameters (e.g., temperature or pressure) as well as those involving spinodal decomposition. While the precise mechanism cannot be ascertained using present experiments alone, our observations can be reconciled in terms of modulations in competing interactions prompted by the pancake-mushroom-brush conformational transitions of the ethylene glycol headgroup. This in turn suggests that the conformational degree of freedom represents an independent order parameter, or a switch, which can induce large-scale structural reorganization in amphiphilic monolayers. Because molecular conformational changes are pervasive in biological membranes, we speculate that such conformational transition-induced pattern evolution might provide a physical mechanism by which membrane processes are amplified.     

Transition-metal-complexed cyclic [3]- and [4]pseudorotaxanes containing rigid ring-and-filament conjugates: synthesis and solution studies

By using the "gathering-and-threading" effect of copper(I) with rigid ring-and-string conjugates, daisy-chain-type [3]- and [4]pseudorotaxanes could be prepared in high yields. The organic fragment used consisted of a 2,9-diphenyl-1,10-phenanthroline (dpp)-containing ring attached to a coordinating filament capable of threading through the ring of another molecule by coordination to copper(I). The bidentate chelate introduced in the axis was also a 1,10-phenanthroline (phen) derivative with two methyl groups ortho to the nitrogen atoms of the phen unit. The organic component was prepared following a multistep strategy, one of the key steps being the attachment of the ring to the lateral axis. This connection was done by a condensation reaction between an ortho dione located at the back of a ring-incorporated phen and an aromatic aldehyde, which was the end-function of the thread. An oxazole nucleus was obtained after the condensation, which provided a rigid connection between the ring and the axis. In this way, the coordination axes of the ring-incorporated bidentate chelate and of the ligand belonging to the lateral filament were approximately orthogonal to one another. The design was such that the tetrameric complex, a [4]pseudorotaxane, seemed to be the most stable species, owing to the mutual geometrical arrangement of the filament and the ring. Various spectroscopic techniques, such as (1)H NMR spectroscopy, including DOSY, and electrospray ionization mass spectroscopy (ESI-MS), clearly demonstrated that a mixture of cyclic [3]- and [4]pseudorotaxane was obtained; the proportions of both components depended on concentration and temperature. Copper(I) was not the only metal center leading to the formation of cyclic pseudorotaxanes. A similar effect was observed with silver(I) as the templating metal: quantitative formation of threaded species was observed, with a higher proportion of trimer over tetramer than in the copper(I) case. Concentration and temperature effects were investigated for both series of Cu(I) - and Ag(I) -complexed threaded species showing that formation of the trimer was favored upon dilution or heating of the solution.     

Surface-dependent chemical equilibrium constants and capacitances for bare and 3-cyanopropyldimethylchlorosilane coated silica nanochannels

We present a combined theoretical and experimental analysis of the solid-liquid interface of fused-silica nanofabricated channels with and without a hydrophilic 3-cyanopropyldimethylchlorosilane (cyanosilane) coating. We develop a model that relaxes the assumption that the surface parameters C(1), C(2), and pK(+) are constant and independent of surface composition. Our theoretical model consists of three parts: (i) a chemical equilibrium model of the bare or coated wall, (ii) a chemical equilibrium model of the buffered bulk electrolyte, and (iii) a self-consistent Gouy-Chapman-Stern triple-layer model of the electrochemical double layer coupling these two equilibrium models. To validate our model, we used both pH-sensitive dye-based capillary filling experiments as well as electro-osmotic current-monitoring measurements. Using our model we predict the dependence of ζ potential, surface charge density, and capillary filling length ratio on ionic strength for different surface compositions, which can be difficult to achieve otherwise.