Blob Model Research Articles

Anomalous Small-Angle X-ray Scattering Study of Structure of Polymer Micelles Having Bromines in Hydrophobic Core

Anomalous small-angle X-ray scattering (ASAXS) near the absorption edge of bromine has been used to investigate the internal structure of polymer micelles comprising poly(4-bromostyrene)-block-poly(ethylene glycol)-block-poly(4-bromostyrene) (PBrS-b-PEG-b-PBrS) in aqueous solution. The ASAXS analysis revealed that the hydrophobic core composed of PBrS can be regarded as a homogeneous sphere with a radius of 10.2 nm and a decaying electron density at the surface. The aggregation number of micelle directly estimated on the basis of ASAXS analysis well agreed with that determined by field-flow fractionation coupled with multiangle light scattering, confirming the reliability of ASAXS analysis. The small-angle X-ray scattering (SAXS) profile from whole PBrS-b-PEG-b-PBrS micelle was further analyzed by using the result of ASAXS analysis. The micelles could be well described by a core–corona model in which the monomer concentration of PEG in the shell decreased as R–4.2/3, where R is the distance from the center of micelle. The index 4.2/3 is very close to that predicted by theory for star polymers (R–4/3), indicating that the PEG chains in the shell can be well described by the blob model in which chains are modeled as strings of blobs that extend radially. Thus, combination of ASAXS and conventional SAXS analyses permit to reveal precisely the internal structure and nature of the tethered chains of polymer micelles, and it could become a powerful tool for exploring internal structures of multiphase systems when they contain a probe atom for ASAXS.

Numerical Study of Linear and Circular Model DNA Chains Confined in a Slit: Metric and Topological Properties

Advanced Monte Carlo simulations are used to study the effect of nano-slit confinement on metric and topological properties of model DNA chains. We consider both linear and circularised chains with contour lengths in the 1.2--4.8 $\mu$m range and slits widths spanning continuously the 50--1250nm range. The metric scaling predicted by de Gennes' blob model is shown to hold for both linear and circularised DNA up to the strongest levels of confinement. More notably, the topological properties of the circularised DNA molecules have two major differences compared to three-dimensional confinement. First, the overall knotting probability is non-monotonic for increasing confinement and can be largely enhanced or suppressed compared to the bulk case by simply varying the slit width. Secondly, the knot population consists of knots that are far simpler than for three-dimensional confinement. The results suggest that nano-slits could be used in nano-fluidic setups to produce DNA rings having simple topologies (including the unknot) or to separate heterogeneous ensembles of DNA rings by knot type.

Length-dependent translocation of polymers through nanochannels

We consider the flow-driven translocation of single polymer chains through nanochannels. Using analytical calculations based on the de Gennes blob model and mesoscopic numerical simulations, we estimate the threshold flux for the translocation of chains of different number of monomers. The translocation of the chains is controlled by the competition between entropic and hydrodynamic effects, which set a critical penetration length for the chain before it can translocate through the channel. We demonstrate that the polymers show two different translocation regimes depending on how their length under confinement compares to the critical penetration length. For polymer chains longer than the threshold, the translocation process is insensitive to the number of monomers in the chain as predicted in Sakaue et al., Euro. Phys. Lett., 2005, 72, 83. However, for chains shorter than the critical length we show that the translocation process is strongly dependent on the length of the chain. We discuss the possible relevance of our results to biological transport.

Three-dimensional MHD simulation of two coronal mass ejections' propagation and interaction using a successive magnetized plasma blobs model

A three-dimensional (3-D), time-dependent, numerical magnetohydrodynamic (MHD) model is used to investigate the evolution and interaction of two coronal mass ejections (CMEs) in the nonhomogeneous ambient solar wind. The background solar wind is constructed on the basis of the self-consistent source surface with observed line of sight of magnetic field and density from the source surface of 2.5 R(s) to Earth's orbit (215 R(s)) and beyond. The two successive CMEs occurring on 28 March 2001 and forming a multiple magnetic cloud in interplanetary space are chosen as a test case, in which they are simulated by means of a two high-density, high-velocity, and high-temperature magnetized plasma blobs model, and are successively ejected into the nonhomogeneous background solar wind medium along different initial launch directions. The dynamical propagation and interaction of the two CMEs between 2.5 and 220 R(s) are investigated. Our simulation results show that, although the two CMEs are separated by 10 h, the second CME is able to overtake the first one and cause compound interactions and an obvious acceleration of the shock. At the L1 point near Earth the two resultant magnetic clouds in our simulation are consistent with the observations by ACE. In this validation study we find that this 3-D MHD model, with the self-consistent source surface as the initial boundary condition and the magnetized plasma blob as the CME model, is able to reproduce and explain some of the general characters of the multiple magnetic clouds observed by satellite.

Long-Range Polymer Chain Dynamics of Pyrene-Labeled Poly(N-isopropylacrylamide)s Studied by Fluorescence

The long-range polymer chain dynamics (LRPCD) of poly(N-isopropylacrylamide) (PNIPAM) were studied by steady-state and time-resolved fluorescence. Two types of fluorescently labeled PNIPAM constructs were considered. One polymer set, named Py2–PNIPAM–Y, where 5.6 kDa < Y < 44 kDa is the molecular weight of the polymer, consisted of five monodisperse PNIPAMs that carried 1-pyrenyl-butyl groups at each chain end (functionalization level >75%). The second polymer set consisted of five polydisperse PNIPAMs 75 kDa < Mn < 104 kDa randomly labeled with pyrene (0.1 mol % < pyrene content < 6.0 mol %). Pyrene was selected as its ability to form an excimer from the encounter of two pyrenyl pendants covalently attached onto the polymer yields quantitative information about the LRPCD of a given backbone. The fluorescence data were analyzed according to the Birks scheme for the pyrene end-labeled PNIPAMs and the fluorescence blob model (FBM) for the randomly labeled PNIPAMs. The parameters describing the process of pyrene excimer formation were found to yield equivalent trends regardless of the PNIPAM construct as would be expected since both PNIPAM constructs are essentially pyrene-labeled PNIPAM in nature. Comparison of the parameters describing the LRPCD of PNIPAM and polystyrene obtained with either end- or randomly labeled polymers led to the conclusion that PNIPAM and polystyrene have similar LRPCD in solution as expected from the similarity in bulkiness of their side-chain. This study confirms the claim made first with polystyrene that randomly labeled polymers can be used to obtain quantitative information on the LRPCD of a given polymeric backbone and that they constitute an appealing alternative to end-labeled polymers.

Three-dimensional MHD simulation of the evolution of the April 2000 CME event and its induced shocks using a magnetized plasma blob model

A three-dimensional (3-D) time-dependent, numerical magnetohydrodynamic (MHD) model with asynchronous and parallel time-marching method is used to investigate the propagation of coronal mass ejections (CMEs) in the nonhomogenous background solar wind flow. The background solar wind is constructed based on the self-consistent source surface with observed line-of-sight of magnetic field and density from the source surface of 2.5 R-s to the Earth's orbit (215 Rs) and beyond. The CMEs are simulated by means of a very simple flux rope model: a high-density, high-velocity, and high-temperature magnetized plasma blob is superimposed on a steady state background solar wind with an initial launch direction. The dynamical interaction of a CME with the background solar wind flow between 2.5 and 220 Rs is investigated. The evolution of the physical parameters at the cobpoint, which is located at the shock front region magnetically connected to ACE spacecraft, is also investigated. We have chosen the well-defined halo-CME event of 4-6 April 2000 as a test case. In this validation study we find that this 3-D MHD model, with the asynchronous and parallel time-marching method, the self-consistent source surface as initial boundary conditions, and the simple flux rope as CME model, provide a relatively satisfactory comparison with the ACE spacecraft observations at the L1 point.

Probing End-to-End Cyclization beyond Willemski and Fixman

A series of poly(ethylene oxide)s labeled at both ends with pyrene, (PEO(X)-Py(2), where X represents the number average molecular weight (M(n)) of the PEO chains and equals 2, 5, 10, and 16.5 K) was prepared together with one-pyrene-monolabeled PEO (PEO(2K)-Py). The process of end-to-end cyclization (EEC) was investigated by monitoring intramolecular excimer formation in seven organic solvents with viscosities (η) ranging from 0.32 to 1.92 mPa·s. The steady-state fluorescence spectra showed that excimer formation of PEO(X)-Py(2) decreased strongly with increasing η and M(n). The monomer and excimer time-resolved fluorescence decays were analyzed according to the traditional Birks' scheme. Birks' scheme analysis indicated that the decrease in excimer formation with increasing M(n) and η was due partly to a decrease in the rate constant of EEC, but most importantly, to a large increase in the fraction of pyrenes that did not form excimer (f(Mfree)). This result is in itself incompatible with Birks' scheme analysis which requires that f(Mfree) be the molar fraction of chains bearing a single pyrene at one chain end; in short, f(Mfree) does not depend on M(n) and η within the framework of Birks' scheme analysis. In turn, this unexpected result agrees with the framework of the fluorescence blob model (FBM) which predicts that quenching takes place inside a blob, which is the finite volume probed by an excited chromophore during its lifetime. Increasing M(n) and η results in a larger fraction of chains having a conformation where the quencher is located outside the blob resulting in an increase in f(Mfree). Equations were derived to apply the FBM analysis, originally designed to study randomly labeled polymers, to the end-labeled PEO(X)-Py(2) series. FBM analysis was found to describe satisfyingly the data obtained with the longer PEO(X)-Py(2) samples.

Extra formation of positronium in amorphous polymers at low temperature

AbstractPositron annihilation lifetime spectroscopy measurements of polystyrene and poly(methyl methacrylate) have been performed at 20 K during ∼400 h. An increase of the probability of positronium (Ps) formation with irradiation time is observed. This effect is due to the accumulation of shallow‐trapped electrons which are stable at low temperatures. Detailed analyses of the lifetime spectra within the blob model framework show that these electrons induced by the previous positron tracks act as extra blob electrons: they either recombine with a cation or form Ps. This kinetic analysis introduced by the blob model indicates that no formation of Ps occurs after the blob recombination in these amorphous polymers. An increase of the free positron lifetime with the measuring time is also observed. This effect can not be explained in terms of artifact of discrete analysis caused by the existence of an ortho‐positronium lifetime distribution.

Incorporation of the Magnetic Quenching Effect into the Blob Model of Ps Formation. Finite Sized Ps in a Potential Well

A self consistent interpretation of the positron lifetime experiments in water at different temperatures (2-93 0C) and magnetic fields (H ≤ 2 T) is given. By using the blob model of Ps for-mation we have obtained the contact density in the positronium atom in water, which is in agree-ment with the previous measurements.

Electron Transfer between Physically Bound Electron Donors and Acceptors: A Fluorescence Blob Model Approach

The present study reports on the applicability of the fluorescence blob model (FBM) to analyze the complex fluorescence decays obtained with DNA-intercalated ethidium bromide (EB) as it transfers an electron to copper cations bound to the DNA helix. Traditionally, the information retrieved about the electron transfer process taking place between an electron donor intercalated in DNA and an electron acceptor physically and randomly bound to DNA has been limited due to the distribution of distances that quenching can occur over, which leads to a distribution of rate constants resulting in complex fluorescence decays. These complications can be overcome by analyzing the fluorescence data with a fluorescence blob model (FBM) that allows for the study of fluorescence quenching between fluorophores and quenchers randomly spaced along a polymeric backbone. The fluorescence decays obtained for EB intercalated between two DNA base pairs (bp) as it transfers an electron to copper randomly bound to the DNA were well fit with the FBM. In the FBM analysis, electron transfer is characterized by the size of a blob in term of base pairs, N(blob), over which electron transfer occurs, as well as the rate constant of electron transfer inside a blob, k(blob). The present work demonstrates that electron transfer between intercalated EB and randomly bound copper occurs over an average distance that increases with increasing duplex length up to a duplex length of 12 bp, beyond which the distance over which electron transfer occurs remains constant with duplex length and equals 10.8 ± 0.4 bp.

Disentangling the gamma-ray emission of NGC 1275 and that of the Perseus cluster

(Abridged). The gamma-ray emission from galaxy clusters hosting active galaxies is a complex combination of diffuse and point-like emission with different spectral and spatial properties. We discuss the case of the Perseus cluster containing the radio-galaxy NGC 1275 that has been detected as a bright gamma-ray source by the Fermi-LAT experiment. We provide a detailed study of the gamma-ray emission coming from the core of Perseus by modeling the central AGN emission with a multiple plasma blob model, and the emission from the cluster atmosphere with both a Warming Ray (WR) model and Dark Matter (DM) neutralino annihilation models. We set constraints on both the central galaxy and cluster SED models by using multi-frequency data including the observations obtained by Fermi and MAGIC. We find that: i) in all the viable models for the cluster gamma-ray emission, the emission detected by Fermi from the Perseus core is dominated by the active galaxy NGC 1275, that is found in a high-emission state; ii) the diffuse gamma-ray emission of the cluster, in the WR model and in the DM models with the highest allowed normalization, could be detected by Fermi if the central emission from NGC1275 is in a low-emission state; iii) Fermi can have the possibility to resolve and detect the diffuse gamma-ray flux coming from the outer corona of the Perseus atmosphere at r> 800 kpc. Our results show that a simultaneous study of the various emission mechanisms that produce diffuse gamma-rays from galaxy clusters and those producing gamma-rays from active galaxies residing in the cluster atmospheres is crucial first to disentangle the spectral and spatial characteristics of the gamma-ray emission and secondly to assess the optimal observational strategy in the attempt to reveal the still elusive diffuse gamma-ray emission widely predicted for the atmospheres of large-scale structures.

Estimate of convective radial transport due to SOL turbulence as measured by GPI in Alcator C-Mod

The convective radial transport effects of SOL turbulence have been estimated using recent turbulence data from the gas puff imaging (GPI) camera diagnostic on Alcator C-Mod. The average radial turbulence speed within the region 1–2cm outside the separatrix near the outer was calculated by a 2-D cross-correlation technique to be Vr∼0.2–0.3km/s. Assuming this to be the local convective plasma velocity, the density SOL width λn was evaluated using a simple convective model to be λn∼4–7cm, which is ∼2–3 times higher than that measured using a Langmuir probe. This convective velocity was also ∼2–3 times lower than the velocities estimated from analytic blob models, but showed a similar scaling with plasma current at constant q95. The measured blob speeds were lower than both the convective speeds and the analytic blob model speeds.

Theory of microphase separation in crosslinked polymer blends immersed in a $ \theta$ -solvent

The aim of this work is a theoretical study of the effects of the solvent quality on the microphase separation in crosslinked polymer blends, from a static and kinetics point of view. More precisely, we assume that the crosslinked mixture is trapped in a θ-solvent. The static microphase properties are studied through the static structure factor. The latter is computed using an extended blob model, where the crosslinked unlike chains can be viewed as sequences of blobs. We demonstrate that the presence of the θ-solvent simply leads to a multiplicative renormalization of these properties, and the renormalization factors are powers of the overall monomer volume fraction. Second, we investigate the early kinetics of the microphase separation, via the relaxation rate, τ(q), which is a function of the wave number q (at fixed temperature and monomer volume fraction). We first show that the kinetics is entirely controlled by local motions of Rouse type, since the slow motions are frozen out by the presence of crosslinks. Using the blob model, we find an explicit form for the growth rate Ω(q) = τ(q)⁻¹, which depends, in addition to the wave number q , on the overall monomer volume fraction, Φ. Also, we discuss the effect of initial entanglements that are trapped when the system is crosslinked. In fact, these play the role of true reticulation points, and then, they quantitatively contribute to the microseparation phenomenon. Finally, the results are compared to their homologous relatively to the molten state and to the good solvent case. The main conclusion is that the quality of the solvent induces drastic changes of the microphase properties.

Nanodomain Formation in Lipid Membranes Probed by Time-Resolved Fluorescence

Time-resolved fluorescence measurements on liposomes prepared with 1 mol % pyrene-labeled lipids (PLLs) with a headgroup bearing either an alcohol (PSOH) or an imido diacetic acid (PSIDA) and 99 mol % 1-palmitoyl-2-oleyl-3-sn-phosphatidylcholines (POPC) or 99 mol % distearylphosphatidylcholine (DSPC) were performed to investigate how lipids phase separate within the membrane bilayer. Global analysis of the fluorescence decays with the fluorescence blob model (FBM) led to the conclusion that the PLLs were homogeneously distributed on the surface of POPC vesicles while the PLLs phase-separated in the DSPC vesicles. The analysis yielded the fraction of aggregated pyrenes, f(agg). The large f(agg) values found for PSIDA suggest that the imido diacetic acid headgroup of PSIDA induces self-aggregation and phase separation in both membranes. The addition of external cations such as Cu(2+) and La(3+) was shown to hinder diffusional encounters between PSIDAs. The cations seem to target preferentially unassociated PSIDAs rather than aggregated PSIDA clusters. Accounting for the quenching of pyrene by Cu(2+) enables one to use PSIDA to probe the microviscosity of the lipid membrane. Using this effect, the environment of PSIDA in the DSPC membrane was found to be about 6 times more viscous than that in the POPC membrane. This difference is attributed to the difference in viscosity of the fluid POPC membrane and the gel-like DSPC membranes.

How switching the substituent of a pyrene derivative from a methyl to a butyl affects the fluorescence response of polystyrene randomly labeled with pyrene

A series of polystyrenes randomly labeled with 1-pyrenebutanol were prepared by copolymerizing styrene and 1-pyrenebutylacrylate yielding the CoBuE–PS series. Solutions of CoBuE–PS were prepared in nine organic solvents having viscosities ranging from 0.36 to 5.5 mPa·s and the fluorescence spectra and pyrene monomer and excimer fluorescence decays were acquired. Analysis of the fluorescence spectra yielded the IE/IM ratio, whereas analysis of the fluorescence decays with the fluorescence blob model (FBM) yielded the parameters N blobo , &lt;kblob × Nblob&gt; , and k blobo . These parameters were compared to those obtained with two other series of pyrene-labeled polystyrenes, which had been studied earlier, namely CoA–PS and CoE–PS where pyrene was attached to the polymer backbone via a methylamide and benzyl methylether linker, respectively. Although the parameters IE/IM, N blobo , &lt;kblob × Nblob&gt;, and k blobo took different values according to the specific nature of the linker connecting pyrene to the polystyrene backbone, they exhibited trends that were quite similar for all the pyrene-labeled polystyrene constructs. The excellent agreement between the parameters retrieved for the three different types of pyrene-labeled polystyrenes suggests that the FBM accounts satisfyingly for differences in the nature of the label used, while still retrieving information pertinent to the polymer of interest.

Theory of microphase separation in charged crosslinked polymer blends with arbitrary charge distribution

In this paper, we are interested in the phase behavior and scattering properties of charged crosslinked polymer blends in solution. The system undergoes a microphase separation, below some critical temperature. To study such a transition, use is made of the standard de Gennes theory based on an analogy with a dielectric medium. This analogy is extended to include the effects of the initial composition fluctuations in order to improve its agreement with experimental data in the small wave vector range. The excluded-volume interactions are explicitly introduced through the blob model. The charge effects on the phase behavior are examined, for any charge distribution of polyions and for any salt concentration. This completes a previous study which was concerned with the situation where only one species is charged. The early kinetics of microphase separation is discussed, and the charges contribution to the growth rate is also evaluated.

Comparison of the long range polymer chain dynamics of polystyrene and cis-polyisoprene using polymers randomly labeled with pyrene

The ability of the Fluorescence Blob Model (FBM) to probe the chain dynamics of a polymer backbone was verified for the first time by investigating whether it responds to known differences in backbone flexibility for flexible cis-polyisoprene (PIP) and more sterically hindered polystyrene (PS), both randomly labeled with pyrene. For comparable pyrene contents, the steady-state fluorescence spectra indicated that Py-PIP formed considerably more excimer than the Py-PS samples. Analysis of the fluorescence decays with the FBM provided the rate constant for intramolecular excimer formation. This rate constant was much larger for Py-PIP than for Py-PS, in agreement with the enhanced excimer formation observed for Py-PIP. The enhanced ability of Py-PIP to form excimer is attributed to the PIP backbone being less sterically hindered than the PS backbone. This study is the first example of a comparison of the long range polymer chain dynamics for two different polymeric backbones randomly labeled with pyrene.

Tracking multiple people under occlusion and across cameras using probabilistic models

Tracking multiple people under occlusion and across cameras is a challenging question for discussion. Furthermore, the cameras in this study are used to extend the field of view, which are distinguished from the same field of view. Such correspondence between multiple cameras is a burgeoning research subject in the area of computer vision. This paper effectively solves the problems of tracking multiple people who pass from one camera to another and segmenting people under occlusion using probabilistic models. The probabilistic models are composed of blob model, motion model and color model, which make the most of the space, motion and color information. First, we present a color model that uses maximum likelihood estimation based on non-parametric kernel density estimation. Second, we introduce a blob model based on mean shift, which segments the body into many regions according to the color of each person in order to spatially localize the color features corresponding to the way people are dressed. Clothes can be any mixture of colors. Third, we bring forward a motion model based on statistical probability which indicates the movement position of the same person between two successive frames in a single camera. Finally, we effectively unify the three models into a general probabilistic model and attain a maximization likelihood probability image, which is used to segment the foreground region under occlusion and to match people across multiple cameras.

One-Dimensional Dynamics and Transport of DNA Molecules in a Quasi-Two-Dimensional Nanoslit

We report an unusual phenomena of attraction between DNA and the sidewall of a two-dimensional (2d) glass nanoslit with height, h, less than the Kuhn length l K . The DNA molecules are stretched and diffuse along the wall. The scaling analysis reveals that the wall-bound DNA molecules exhibit one-dimensional (Id) diffusion. A modified blob model is able to predict the chain extension down to 30 nm, well into the subpersistence length regime where the chain deflection length should be considered. This discrepancy may be due to the partially bound confinement of chains near the sidewall. In contrast, the scaling analysis of DNA molecules far from the sidewall exhibit 2d dynamics that can be described by de Gennes and Odijk model for h > l K and h < l K , respectively. We further apply the unusual wall attraction to trap and stretch DNA molecules around posts in a nanoslit. We demonstrate that the DNA molecules exhibit trapping-escaping movement, and they can be transported from post to post with an electric field.

Effect of Time on the Rate of Long Range Polymer Segmental Intramolecular Encounters

The kinetics of encounters between the pyrene pendants randomly attached along a polystyrene chain (Py-PS) were monitored with a fluorescence blob model (FBM) as an external quencher was added to the solution to decrease the lifetime of the excited pyrene. The fluorescence decays acquired with the Py-PS samples yielded a measure of the volume Vblob probed by an excited pyrene during its lifetime in the form of N0blob, the number of monomers found within Vblob, and K0blob, which is inversely proportional to Vblob. Both N0blob and K0blob(-1) were found to increase with increasing probing time as the excited pyrene was allowed to probe a larger Vblob volume. The rate constant for pyrene-pyrene encounters was obtained from the product (KblobNblob). (KblobNblob) was found to decrease with increasing probing time, in agreement with scaling arguments suggesting that, as the probing time increases, the exited pyrene probes a larger Vblob where the local concentration of ground-state pyrenes in the polymer coil, [Py]loc, is smaller. K0blob, which is inversely proportional to Vblob, was found to scale as N0blob(alpha), where alpha equaled -1.5 and -1.2 in DMF and THF, respectively. The alpha exponents found for the Py-PS samples are in the same range as those found for other polymers exhibiting a random polymer coil conformation in solution and where much smaller than those obtained with more compact structured alpha-helical polypeptides randomly labeled with pyrene. Master curves were also constructed that describe how K0blob and the product (KblobNblob) scale as a function of solvent viscosity, probing time, and N0blob. These scaling laws illustrate the opposite effects that probing time and viscosity have on N0blob, V0blob, and the product (KblobNblob).