Phase-diagram for Irregular and Non-Symmetric Cross-linked Polymer Blends

We consider here a blend made of two types of polymers, $A$ and $B$, of different chemical nature. At high temperature the homogeneous mixture is cross-linked. As the temperature is lowered, the two species try to segregate but are kept together by the cross-links. We show that for inhomogeneous, non-regular and non-permanent cross-links, there is a complete segregation at low temperatures if the system is just weakly cross-linked and partial segregation, otherwise. We also demonstrate that there is no phase transition between the homogeneous phase and the microphase for non-symmetric systems. Our analysis is checked with the experiment.Comment: 10 pages, Revte

CLAS+FROST: new generation of photoproduction experiments at Jefferson Lab

A large part of the experimental program in Hall B of the Jefferson Lab is dedicated to baryon spectroscopy. Photoproduction experiments are essential part of this program. CEBAF Large Acceptance Spectrometer (CLAS) and availability of circularly and linearly polarized tagged photon beams provide unique conditions for this type of experiments. Recent addition of the Frozen Spin Target (FROST) gives a remarkable opportunity to measure double and triple polarization observables for different pseudo-scalar meson photoproduction processes. For the first time, a complete or nearly complete experiment becomes possible and will allow model independent extraction of the reaction amplitude. An overview of the experiment and its current status is presented.Comment: 6 pages, 7 figures. Invited paper NSTAR 2009 conferenc

On the Linear Viscoelastic Behavior of Polyolefins in the Molten State: Influence of Long Chain Branching

This contribution presents a survey on the influence of long chain branching on the linear viscoelastic properties. The aim of this article is to provide evidence that rheological testing is a potent tool for characterising polymers in the melts. The materials chosen are linear and slightly long- chain branched metallocene- catalysed polyethylene of narrow molecular mass distribution, as well as linear and highly long- chain branched polyethylene of broad molecular mass distribution, and other sample of the big family of polyolefin’s. The linear viscoelastic flow properties are determined in shear creep and recovery experiments by means of a magnetic bearing torsional creep apparatus. The interest of this study consists in reviewing various experimental, theoretical and simulation results of the some viscoelastic properties. The samples with a slight degree of long- chain branching exhibit a surprisingly high zero shear-rate viscosity in comparison to linear one, whereas the highly branched polymers have a much lower viscosity compared to linear samples. Slightly branched polyethylene have got a higher steady- state compliance in comparison to linear products of similar polydispersity, whereas the highly branched polyethylene of broad molecular mass distribution exhibit a surprisingly low elasticity in comparison to linear broad molecular one. The same remarks are obtained for the other samples. The actual framework leads us to say that the number of branches is less important than the topology itself. Therefore, the position and architecture of the branches along the main polymer chain are the principal factors that control the material rheology.This contribution presents a survey on the influence of long chain branching on the linear viscoelastic properties. The aim of this article is to provide evidence that rheological testing is a potent tool for characterising polymers in the melts. The materials chosen are linear and slightly long- chain branched metallocene- catalysed polyethylene of narrow molecular mass distribution, as well as linear and highly long- chain branched polyethylene of broad molecular mass distribution, and other sample of the big family of polyolefin’s. The linear viscoelastic flow properties are determined in shear creep and recovery experiments by means of a magnetic bearing torsional creep apparatus. The interest of this study consists in reviewing various experimental, theoretical and simulation results of the some viscoelastic properties. The samples with a slight degree of long- chain branching exhibit a surprisingly high zero shear-rate viscosity in comparison to linear one, whereas the highly branched polymers have a much lower viscosity compared to linear samples. Slightly branched polyethylene have got a higher steady- state compliance in comparison to linear products of similar polydispersity, whereas the highly branched polyethylene of broad molecular mass distribution exhibit a surprisingly low elasticity in comparison to linear broad molecular one. The same remarks are obtained for the other samples. The actual framework leads us to say that the number of branches is less important than the topology itself. Therefore, the position and architecture of the branches along the main polymer chain are the principal factors that control the material rheology

Dynamics of Diblock Copolymers in Dilute Solutions

We consider the dynamics of freely translating and rotating diblock (A-B), Gaussian copolymers, in dilute solutions. Using the multiple scattering technique, we have computed the diffusion and the friction coefficients D_AB and Zeta_AB, and the change Eta_AB in the viscosity of the solution as functions of x = N_A/N and t = l_B/l_A, where N_A, N are the number of segments of the A block and of the whole copolymer, respectively, and l_A, l_B are the Kuhn lengths of the A and B blocks. Specific regimes that maximize the efficiency of separation of copolymers with distinct "t" values, have been identified.Comment: 20 pages Revtex, 7 eps figures, needs epsf.tex and amssymb.sty, submitted to Macromolecule

On the dynamics of polyelectrolyte solutions

A general formalism to study the dynamics of polyelectrolyte solutions is presented. We show in particular that the Berne–Pecora equations for charged pointlike particles are obtained by neglecting the memory function and using the Debye–Huckel potential with the linear approximation exp(−U/kBT)≂1−U/kBT. We generalize Berne–Pecora results by introducing the effect of hydrodynamic interaction. Our calculations show a plasmon mode which corresponds to a nonzero frequency at zero scattering angle.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70342/2/JCPSA6-80-6-2762-1.pd

A parallel implementation on a multi-core architecture of a dynamic programming algorithm applied in cognitive radio ad hoc networks

Spectral resources allocation is a major problem in cognitive radio ad hoc networks and currently most of the research papers use meta-heuristics to solve it. On the other side, the term parallelism refers to techniques to make programs faster by performing several computations in parallel. Parallelism would be very interesting to increase the performance of real-time systems, especially for the cognitive radio ad hoc networks that interest us in this work. In this paper, we present a parallel implementation on a multi-core architecture of dynamic programming algorithm applied in cognitive radio ad hoc networks. Our simulations approve the desired results, showing significant gain in terms of execution time. The main objective is to allow a cognitive engine to use an exact method and to have better results compared to the use of meta-heuristics

Dissipation Capacity of Steel Building with Fiction Pendulum Base-Isolation System

Use of base isolators in the seismic design of structures has attracted considerable attention in recent years. The major concern in the design of these structures is to have enough lateral stability to resist wind and seismic forces. There are different systems providing such isolation, among them there are friction- pendulum base isolation systems (FPS) which are rather widely applied nowadays involving to both affordable cost and high fundamental periods. These devices are characterised by a stiff resistance against wind loads and to be flexible to the seismic tremors, which make them suitable for different situations. In this paper, a 3D numerical investigation is done considering the seismic response of a twelve-storey steel building retrofitted with a FPS. Fast nonlinear time history analysis (FNA) of Boumerdes earthquake (Algeria, May 2003) is considered for analysis and carried out using SAP2000 software. Comparisons between fixed base, bearing base isolated and braced structures are shown in a tabulated and graphical format. The results of the various alternatives studies to compare the structural response without and with this device of dissipation energy thus obtained were discussed and the conclusions showed the interesting potential of the FPS isolator. This system may to improve the dissipative capacities of the structure without increasing its rigidity in a significant way which contributes to optimize the quantity of steel necessary for its general stability

Interpretation of dynamic scattering from polymer solutions

The theoretical results available for the interpretation of the dynamic scattering from polymer solutions have been re-examined. The scattering law S(q, t) is formulated using the eigenfunction expansion method and the linear response theory. All previously known exact expressions of S(q, t) for a single unperturbed Gaussian chain have been re-derived using the first method to demonstrate the interrelationships among the various approaches to calculation of S(q, t). The results are cast into new forms which, in many cases, are more convenient for both numerical and analytical discussions. The infinite chain results are obtained from the exact closed expression of S(q, t) for ring polymers as a special case as N --> [infinity]. Questions like the effect of the draining parameter on the shape of S(q, t), the positive definiteness of the diffusion tensor, and the possibility of measuring the eigenvalue of the first internal mode through light scattering, have been included in the discussions.A new method has been proposed for the interpretation of the dynamic scattering experiments in terms of the initial slope, [Omega], of In S(q, t). The quantity [Omega] can also be identified as the first cumulant of S(q, t). The advantage of this method is that [Omega](q) can be calculated for all q values as a function of temperature and concentration by combining the linear response theory and the blob model of chain statistics. Consequently, one is not restricted to the asymptotic small- and intermediate-q regions in order to interpret the scattering experiments. The analytical and numerical results giving [Omega](q) under various conditions have been presented. Using infinite chain results it is shown that [Omega] acts as a characteristic frequency in the sense that in both the small- and intermediate-q regions, In S(q, t) can be scaled to a q-independent shape function when time is expressed as [Omega]t. This property facilitates the measurement of [Omega] from S(q, t)-data using a known shape function. The feasibility of the method has been demonstrated using light scattering data on polystyrene in toluene in the transition region between small- and intermediate q-regions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23191/1/0000118.pd

PROPRIETES STRUCTURALES DE COPOLYMERES BISEQUENCE ET TRISEQUENCE LINEAIRES ET DE LEUR HOMOLOGUE CYCLIQUE EN SOLUTION DILUEE STRUCTURAL PROPERTIES OF LINEAR DIBLOCK AND TRIBLOCK COPOLYMERS AND THEIR CYCLIC HOMOLOGOUS IN DILUTE SOLUTION

SUMMARYThe structural properties of linear block copolymers in solution aroused a considerable interest and are the subject of manytheoretical and experimental investigations. However, their cyclic counterparts received only a limited attention fromresearchers mainly because of difficulties related to their synthesis with high-molecular weights and a low polydispersity. Thisstudy was justified by the relatively recent light scattering measurements carried out by Amis et al. on a cyclic diblockcopolymer and its linear triblock counterpart made up of polystyrene (PS) and poly(dimethylsiloxane) (PDMS). In this work,the adopted theoretical approach is based on the random phase approximation when the polymeric systems under study areplaced in similar conditions of temperature, concentration, composition and mass than those covered experimentally by Amisand others. The main objective of this study is intended to present a theoretical formalism allowing a systematic analysis of thescattering data, relating to block copolymers of arbitrary architectures, and to suggest some indications relating to theirtreatment. In terms of thermodynamic and structural properties, the results obtained reveal differences between the two typesof copolymers. In particular, contrary to what was brought back by Amis and his collaborators, the discrepancies, shownbetween the radii of gyration of linear block copolymers and their cyclic counterpart, being sufficiently large, it should bepossible to highlight them in experiments by light scattering.RESUMELes propriétés structurales de copolymères séquencés linéaires en solution ont suscité un intérêt considérable et fait l'objet denombreuses investigations théoriques et expérimentales. Cependant, leurs homologues cycliques n'ont retenu l'attention qued'un nombre limité de chercheurs principalement du fait de difficultés liées à leur synthèse avec des poids moléculaires élevéset une faible polydispersité. Cette étude a été motivée par des mesures de diffusion de lumière relativement récentes réaliséespar Amis et al. sur un copolymère biséquencé cyclique et son homologue triséquencé linéaire constitué de polystyrène (PS) etde poly(diméthylsiloxane) (PDMS). Dans ce travail, l'approche théorique adoptée est basée sur l'approximation de la phasealéatoire en plaçant les systèmes polymériques considérés dans des conditions de température, de concentration, decomposition et de masse similaires à celles couvertes expérimentalement par Amis et autres. L’objectif premier de la présenteétude consiste à présenter un formalisme théorique permettant une analyse systématique de données de diffusion, relatives àdes copolymères séquencés d'architectures arbitraires, et de suggérer quelques indications concernant le traitement de celles-ci.En termes de propriétés thermodynamiques et structurales, les résultats obtenus révèlent des écarts entre les deux types decopolymères. En particulier, contrairement à ce qui a été rapporté par Amis et ses collaborateurs, les différences, relevées entreles rayons de giration de copolymères séquencés linéaires et de leur homologue cyclique, étant suffisamment importantes, ildevrait être possible de les mettre en évidence expérimentalement par diffusion de lumière

Frequency Dispersion of Sound Propagation in Rouse Polymer Melts via Generalized Dynamic Random Phase Approximation

An extended generalization of the dynamic random phase approximation (DRPA) for L-component polymer systems is presented. Unlike the original version of the DRPA, which relates the (LxL) matrices of the collective density-density time correlation fumctions and the corresponding susceptibilities of polymer concentrated systems to those of the tracer macromolecules and so-called broken links system (BLS), our generalized DRPA solves this problem for (5xL)x(5xL) matrices of the coupled susceptibilities and time correlation functions of the component number, kinetic energy and flux densities. The presented technique is used to study propagation of sound and dynamic form-factor in disentangled (Rouse) monodisperse homopolymer melt. The calculated sound velocity and absorption coefficient reveal substantial frequency dispersion. The relaxation time is found to be N times less than the Rouse time (N is the degree of polymerization), which evidences strong dynamic screening because of interchain interaction. We discuss also some peculiarities of the Brillouin scattering in polymer melts. Besides, a new convenient expression for the dynamic structural function of the Rouse chain in (q,p)-representation is found.Comment: 37 pages, 2 appendices, 48 references, 1 figur