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

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

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

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

Non-isothermal model for the direct isotropic/smectic-A liquid crystalline transition

An extension to a high-order model for the direct isotropic/smectic-A liquid crystalline phase transition was derived to take into account thermal effects including anisotropic thermal diffusion and latent heat of phase-ordering. Multi-scale multi-transport simulations of the non-isothermal model were compared to isothermal simulation, showing that the presented model extension corrects the standard Landau-de Gennes prediction from constant growth to diffusion-limited growth, under shallow quench/undercooling conditions. Non-isothermal simulations, where meta-stable nematic pre-ordering precedes smectic-A growth, were also conducted and novel non-monotonic phase-transformation kinetics observed.Comment: First revision: 20 pages, 7 figure

Transverse momentum dependence of semi-inclusive pion production

Cross sections for semi-inclusive electroproduction of charged pions ($\pi^{\pm}$) from both proton and deuteron targets were measured for $0.2<x<0.5$, $2<Q^2<4$ GeV$^2$, $0.3<z<1$, and $P_t^2<0.2$ GeV$^2$. For $P_t<0.1$ GeV, we find the azimuthal dependence to be small, as expected theoretically. For both $\pi^+$ and $\pi^-$, the $P_t$ dependence from the deuteron is found to be slightly weaker than from the proton. In the context of a simple model, this implies that the initial transverse momenta width of $d$ quarks is larger than for $u$ quarks and, contrary to expectations, the transverse momentum width of the favored fragmentation function is larger than the unfavored one.Comment: 15 pages, 4 figures. Fit form changed to include Cahn effect Minor revisions. Added one new figur

The Onset of Quark-Hadron Duality in Pion Electroproduction

A large data set of charged-pion electroproduction from both hydrogen and deuterium targets has been obtained spanning the low-energy residual-mass region. These data conclusively show the onset of the quark-hadron duality phenomenon, as predicted for high-energy hadron electroproduction. We construct several ratios from these data to exhibit the relation of this phenomenon to the high-energy factorization ansatz of electron-quark scattering and subsequent quark-to- pion production mechanisms.Comment: 11 pages, 3 figures, accepted in Phys. Rev. Lett. Tables adde