Arrested swelling of highly entangled polymer globules

Upon aging, a collapsed long chain evolves from a crumpled state to a self-entangled globule which can be thought of as a large knot. Swelling of an equilibrium globule in good solvent is a two-step process: (i) fast swelling into an arrested stretched structure with conserved entanglement topology followed by (ii) slow disentanglement. Using computer simulation, we found both mass-mass (m-m) and entanglement-entanglement (e-e) power law correlations inside the swollen globule. The m-m correlations are characterized by a set of two exponents in agreement with a Flory-type argument. The e-e correlations are also characterized by two exponents, both of them larger (by approximately 0.3) than the related m-m exponents. We interpret this difference as evidence of distance-dependent repulsion E=-0.3ln((rho)k(B)T between entanglements sliding along the polymer chain.     

Dynamics of mass transfer caused by the photoinduced spatially inhomogeneous modulation of mobility in a multicomponent medium

The reasons behind the three-dimensional mass transfer of the inert components of a mixture in which photoinduced spatially inhomogeneous polymerization takes place are discussed. Such a mass transfer is responsible for the laser-induced formation of controllable diffraction gratings in polymer composites containing liquid crystals [R. L. Sutherland, V. P. Tondiglia, L. V. Natarajan, and T. J. Bunning, Chem. Mater. 5, 1533 (1993); R. L. Sutherland, V. P. Tondiglia, and L. V. Natarajan, Appl. Phys. Lett. 64, 1074 (1994); R. L. Sutherland, V. P. Tondiglia, L. V. Natarajan, et al., J. Nonlinear Opt. Phys. Mater. 5, 89 (1996); R. Caputo, A. V. Sukhov, and C. Umeton, Mol. Mater. 12, 192 (1999)]. The semiphenomenological model proposed in this work is based on the assumption that mass transfer results from conventional Fickian diffusion which, however, does not occur over the entire volume of the sample, but only in regions free from clusters of long polymer chains. It is shown that such a “restriction of the active volume” is responsible for the conventional diffusion of inert components of liquid crystals as well as reagents and low-molecular products (short chains), in spite of the initial spatial homogeneity of concentrations in the sample. The qualitative predictions of the model coincide with the experimental results [R. Caputo, A. V. Sukhov, and C. Umeton, Mol. Mater. 12, 192 (1999)].     

Temperature-responsive polymers in mixed solvents: competitive hydrogen bonds cause cononsolvency

If two good solvents become poor for a polymer when mixed, the solvent pair is called a cononsolvent pair. The sharp reentrant coil-to-globule-to-coil transition of a poly(N-isopropylacrylamide) chain observed in the mixed solvent of water and methanol is shown to be caused by the competitive hydrogen bonding by water and methanol molecules onto the polymer chain. On the basis of a new statistical-mechanical model for competitive hydrogen bonds, the mean square end-to-end distance is theoretically calculated and compared with experiment. The chain sharply collapses at the molar fraction xm approximately 0.2 of methanol, stays collapsed up to xm approximately 0.4, and finally recovers the swollen state at xm approximately 0.6. Such a reentrant coil-globule transition takes place because the total number of hydrogen bonds along the chain exhibits a similar square-well-type depression as a result of the competition.     

Stretching necklaces

Polyelectrolytes in poor solvents show a necklace structure where collapsed polymer pearls are linked to stretched strings. In the present paper the elasticity of such chains is studied in detail. Different deformation regimes are addressed. The first is the continuous regime, where many pearls are present. A continuous force extension relationship is calculated. The main contribution comes from the tension balance and the electrostatic repulsion of consecutive pearls. The main correction term stems from the finite size of the pearls, which monitors their surface energy. For a finite amount of pearls discontinuous stretching is predicted. Finally counterion effects are discussed qualitatively. Received 6 December 1999     

Shear-flow-induced unfolding of polymeric globules

The behavior of a single collapsed polymer under shear flow is examined using hydrodynamic simulations and scaling arguments. Below a threshold shear rate gamma[.]{*}, the chain remains collapsed and only deforms slightly, while above gamma[.]{*} the globule exhibits unfolding/refolding cycles. Hydrodynamics are crucial: In the free draining case, gamma[.]{*} scales with the globule radius R as gamma[.]{*} approximately R{-1}, while in the presence of hydrodynamic interactions gamma[.]{*} approximately R. Experiments on the globular von Willebrand protein confirm the presence of an unfolding transition at a well-defined critical shear rate.     

Swelling-collapse transition of self-attracting walks

We study the structural properties of self-attracting walks in d dimensions using scaling arguments and Monte Carlo simulations. We find evidence of a transition analogous to the Theta transition of polymers. Above a critical attractive interaction u(c), the walk collapses and the exponents nu and k, characterizing the scaling with time t of the mean square end-to-end distance approximately t(2nu) and the average number of visited sites approximately t(k), are universal and given by nu=1/(d+1) and k=d/(d+1). Below u(c), the walk swells and the exponents are as with no interaction, i.e., nu=1/2 for all d, k=1/2 for d=1 and k=1 for d>/=2. At u(c), the exponents are found to be in a different universality class.     

Pressure induced phase transformations in alkali metal hydrides

Abstract

A three body potential approach(Singh and Singh, Phys. Rev. B39,761(1989)) has been described and applied to investigate the pressure induced phase transformation from B1 (NaCl-Structure) to B2 (CsCl-Structure) phases for the partially ionic alkali-metal hydrides (LiH, NaH and KH). Our calculated values of the phase transition pressures and the associated volume collapses for NaH and KH are in reasonably good agreement with their experimental data. The variations of the relative volumes (V(P)/V(O)) with pressure for both B1 and B2 phases, have shown no phase transition upto 140 GPa in LiH. This feature is in agreement with the experimental observations.     

No confinement without Coulomb confinement

We compare the physical potential V(D)(R) of an external quark-antiquark pair in the representation D of SU(N), to the color-Coulomb potential V(coul)(R) which is the instantaneous part of the 44-component: of the gluon propagator in Coulomb-gauge D44(x-->,t)=V(coul)(|x-->|)delta(t)+(noninstantaneous). We show that if V(D)(R) is confining, lim(V(D)(R)=+ infinity, as is believed to hold in the absence of dynamical quarks, then the inequality V(D)(R)0 is the Casimir in the representation D. This implies that -V(coul)(R) is also confining.     

Finite Size Corrections to the Parisi Overlap Function in the GREM

We investigate the effects of finite size corrections on the overlap probabilities in the Generalized Random Energy Model in two situations where replica symmetry is broken in the thermodynamic limit. Our calculations do not use replicas, but shed some light on what the replica method should give for finite size corrections. In the gradual freezing situation, which is known to exhibit full replica symmetry breaking, we show that the finite size corrections lead to a modification of the simple relations between the sample averages of the overlaps \( Y_k \) between k configurations predicted by replica theory. This can be interpreted as fluctuations in the replica block size with a negative variance. The mechanism is similar to the one we found recently in the random energy model in Derrida and Mottishaw (J Stat Mech 2015(1): P01021, 2015). We also consider a simultaneous freezing situation, which is known to exhibit one step replica symmetry breaking. We show that finite size corrections lead to full replica symmetry breaking and give a more complete derivation of the results presented in Derrida and Mottishaw (Europhys Lett 115(4): 40005, 2016) for the directed polymer on a tree.     

Quantum superconductor-metal transition in a proximity array

A theory of the zero-temperature superconductor-metal transition is developed for an array of superconductive islands (of size d) coupled via a disordered two-dimensional conductor with the dimensionless conductance g = Planck's over 2 pi/e(2)R(square)>1. At T = 0 the macroscopically superconductive state of the array with lattice spacing b>d is destroyed at g相似文献   

Polymer localization in random potential

This work re-examines the classical problem of polymer collapse in a random system, using a Gaussian variational formalism to treat the “poor solvent” case. In particular we seek to clarify some of the disputed questions related to symmetry breaking between the replicas used to analyze such quenched systems, and the scaling of the globular collapse with the disorder strength. We map the random system to a chain with attractive interactions in the standard way, and conduct a variational analysis along with a detailed examination of the theory's stability to replica symmetry breaking. The results suggest that replica symmetry is in fact not broken by the collapse of the chain in a random-disordered system, and that inclusion of a positive third, or higher, virial coefficient is crucial to stabilize the theory. For three dimensions, we find the globule square radius falling as (α-2ω)², where α and ω are proportional to the second and third virial coefficients, respectively. This is in keeping with the earliest results of the replica-symmetric argument advanced by Edwards and Muthukumar [J. Chem. Phys. 89 (1988) 2435], but we anticipate a different scaling with dimensionality in other cases, with the globule square radius scaling as l(α,ω)^2/(d-2) for d<4, where l is some linear function. This result agrees a scaling argument regarding the chain in a random potential like that put forth by Cates and Ball [J. Phys. (France) 49 (1988) 2009], but with a repulsive third virial coefficient present.     

Finding the impedance of the organ of Corti

Measurements of the nonlinear response of the basilar membrane to a pure tone are shown to have a simple form for moderate membrane velocities: V(x,f;Vu)/Vu approximately [V(x,f)/Vu]v(x,f), f less than or equal to fc(x), where the response V is the velocity of the membrane at measurement position x, Vu is the umbo velocity, f is the frequency of the stimulus, and fc(x) is the local characteristic frequency. The frequency dependence of the functions v(x,f) and V(x,f) is determined from the data, and v(x,f) and ln V(x,f) are shown to be analytic functions in the lower half of the complex frequency plane, with Re [v(x,f)] a monotonically increasing function of f at fixed x. The linear limit of basilar membrane motion is characterized by a transfer function T(x,f) = (V/V1)v/(1-v), estimated by extrapolating V(x,f;Vu)/Vu to a small membrane velocity V1.T(x,f) and ln T(x,f) are shown to be analytic functions in the lower half of the complex frequency plane. The inverse of the amplitude of the transfer function, which has both a deep dip at f approximately fc(x) and a broad shoulder at lower frequencies, bears a striking resemblance to the neural threshold tuning curve. The functional form of T(x,f) is used to deduce the equation governing the motion of a section of the organ of Corti. Each section acts like a negatively damped harmonic oscillator stabilized at time t by a feedback force proportional to the velocity at the previous time t-tau. The time delay tau is proportional to the oscillator period [tau approximately 1.75/fc(x)]. Like a laser, the organ of Corti pumps energy into harmonic traveling waves. Unlike the laser, the direction of energy flow abruptly reverses as the traveling wave approaches the point of maximum membrane velocity [fc(x) approximately f]. All accumulated wave energy is then pumped back into a small section of the organ of Corti where transduction presumably occurs. Outer hair cells are conjectured to be active elements contributing to the negative damping and feedback of the cochlear amplifier.     

Casimir Interaction between a plate and a cylinder

We find the exact Casimir force between a plate and a cylinder, a geometry intermediate between parallel plates, where the force is known exactly, and the plate sphere, where it is known at large separations. The force has an unexpectedly weak decay approximately L/[H3 ln(H/R)] at large plate-cylinder separations H (L and R are the cylinder length and radius), due to transverse magnetic modes. Path integral quantization with a partial wave expansion additionally gives a qualitative difference for the density of states of electric and magnetic modes, and corrections at finite temperatures.     

On the mean-field spin glass transition

In this paper we analyze two main prototypes of disordered mean-field systems, namely the Sherrington-Kirkpatrick (SK) and the Viana-Bray (VB) models, to show that, in the framework of the cavity method, the transition from the annealed regime to a broken replica symmetry phase can be thought of as the failure of the saturability property (detailed explained along the paper) of the overlap fluctuations which act as the order parameters of the theory. We show furthermore how this coincides with the lacking of the commutativity of the infinite volume limit with respect to a, suitably chosen, vanishing perturbing field inducing the transition as prescribed by standard statistical mechanics. This is another step towards a complete theory of disordered systems. As a well known consequence it turns out that the annealed and the replica symmetric regions must coincide, implying that the averaged overlap is zero in this phase. Within our framework the finding of the values of the critical point for the SK and line for the VB becomes available straightforwardly and the method is of a large generality and applicable to several other mean field models     

Vacuum relaxation and annealing-induced enhancement of mobility of regioregular poly（3-hexylthiophene） field-effect transistors

In order to enhance the performance of regioregular poly(3-hexylthiophene) (RR-P3HT) field-effect transistors (FETs), RR-P3HT FETs are prepared by the spin-coating method followed by vacuum placement and annealing. This paper reports that the crystal structure, the molecule interconnection, the surface morphology, and the charge carrier mobility of RR-P3HT films are affected by vacuum relaxation and annealing. The results reveal that the field-effect mobility of RR-P3HT FETs can reach 4.17×10^ - 2~m²/(V.s) by vacuum relaxation at room temperature due to an enhanced local self-organization. Furthermore, it reports that an appropriate annealing temperature can facilitate the crystal structure, the orientation and the interconnection of polymer molecules. These results show that the field-effect mobility of device annealed at 150~℃ for 10 minutes in vacuum at atmosphere and followed by placement for 20 hours in vacuum at room temperature is enhanced dramatically to 9.00×10^ - 2 ~cm²/(V.s).     

Molecular mobility in polymer thin films

Fluorescence recovery after photobleaching was used to measure in-plane dye-probe diffusion coefficients, D, in thin films of monodisperse polystyrene supported on fused quartz substrates. The substrates were prepared with a high density of surface hydroxyl groups which interact favorably with repeat units of the polymer. The effects of temperature and film thickness were investigated, at temperatures above the bulk glass transition of the polymer, T(g), and in the range of film thicknesses from 1-10(2) times the radius of gyration (R(g)) of individual polymer molecules. As the film thickness decreases towards R(g) the value of D increases above the bulk values, with significant effects first appearing in films approximately 20R(g). In the thinnest films studied, about 4R(g), the values of D lie as much as two orders over bulk values. At the same time, the temperature dependence of D becomes much weaker than in bulk. Analysis by free volume theory indicates that apparent values of both T(g) and the thermal expansion coefficient for liquid state, alpha(L), decrease as the film thickness decreases. The possible effects of surface segregation of the dye probe are discussed.     

Density-fluctuation-induced swelling of polymer thin films in carbon dioxide

We report an anomalous swelling of polymer thin films in carbon dioxide (CO(2)) which is associated (in both locus and form) with the density fluctuation ridge that forms along the extension of the coexistence curve of gas and liquid in the P-T phase diagram. Neutron reflectivity results showed that CO(2) could be sorbed to a large extent ( approximately 60%) in thin polymer films even when the bulk miscibility of the polymer with CO(2) is very poor. The anomalous swelling is found to scale with the polymer radius of gyration (R(g)) and extends to a distance approximately 10 R(g).     

Acoustic instabilities in thin polymer films

The spatial confinement of a fluctuation spectrum leads to forces at the confining boundaries. While electromagnetic (EM) fluctuations lead to the well-known dispersion forces, the acoustic analogy has widely been neglected. We show that the strength of the forces resulting from confined acoustic modes may be of the same order of magnitude as van der Waals forces. Additionally, the predicted scaling behavior is identical to the non-retarded case of the EM fluctuations. Our results suggest that dewetting experiments using polymer films are strongly influenced by the acoustic dispersion forces. Received 5 March 2002 and Received in final form 21 May 2002     

Kondo effect in quantum dots at high voltage: universality and scaling

We examine the properties of a dc-biased quantum dot in the Coulomb blockade regime. For voltages V that are large compared to the Kondo temperature T(K), the physics is governed by the scales V and gamma, where gamma approximately V/ln(2)(V/T(K)) is the nonequilibrium decoherence rate induced by the voltage-driven current. Based on scaling arguments, self-consistent perturbation theory, and perturbative renormalization group, we argue that due to the large gamma the system can be described by renormalized perturbation theory in 1/ln(V/T(K))<1. However, in certain variants of the Kondo problem, two-channel Kondo physics is induced by a large voltage V.