CiteULike: dchen's polymer

In Situ Observation of Fringing-Field-Induced Phase Separation in a Liquid-Crystal--Monomer Mixture

Hongwen Ren — 2008-03-24T17:17:09-00:00

Physical Review Letters, Vol. 100, No. 11. (2008)

Fringing-field-induced phase separation dynamics in liquid-crystal–(LC-)monomer mixtures is investigated via a microscope. At a low LC concentration, the fringing field converts the randomly dispersed LC droplets to an ordered droplet array, while at a high LC concentration the fringing field converts the amorphous LC-monomer system to a composite film. Because the LC and monomer are immiscible, the converted morphologies are stable even after the voltage is removed. Using the fringing field-induced phase separation, it is possible to prepare different polymer-dispersed LC morphologies.

Directed Motion of Proteins along Tethered Polyelectrolytes

Katja Henzler — 2008-05-06T17:57:50-00:00

Physical Review Letters, Vol. 100, No. 15. (2008)

We present the first time-resolved investigation of motions of proteins in densely grafted layers of spherical polyelectrolyte brushes. Using small-angle x-ray scattering combined with rapid stopped-flow mixing, we followed the uptake of bovine serum albumin by poly(acrylic acid) layer with high spatial and temporal resolution. We find that the total amount of adsorbed protein scales with time as t1/4. This subdiffusive behavior is explained on the basis of directed motion of the protein along the polyelectrolyte chains.

Soft Nanopolyhedra as a Route to Multivalent Nanoparticles

Jiunn Roan — 2008-05-05T15:43:40-00:00

Physical Review Letters, Vol. 96, No. 24. (2006)

Computer simulations show that end-grafted immiscible homopolymers can confer multivalence to nanoparticles, resulting in soft nanopolyhedra with structures identical to those found in small clusters of colloidal microspheres. Unprecedented structure tunability is demonstrated by several structure transition sequences, including a reentrant transition, induced by varying composition, polymer lengths, or grafting patterns. These results suggest a new method for fabricating nanoparticles with precisely controlled numbers and locations of functional sites (i.e., multivalent nanoparticles).

Polymeric Quasicrystal: Mesoscopic Quasicrystalline Tiling in ABC Star Polymers

Kenichi Hayashida — 2008-05-04T16:23:48-00:00

Physical Review Letters, Vol. 98, No. 19. (2007)

A mesoscopic tiling pattern with 12-fold symmetry has been observed in a three-component polymer system composed of polyisoprene, polystyrene, and poly(2-vinylpyridine) which forms a star-shaped terpolymer, and a polystyrene homopolymer blend. Transmission electron microscopy images reveal a nonperiodic tiling pattern covered with equilateral triangles and squares, their triangle/square number ratio of 2.3 (4/), and a microbeam x-ray diffraction pattern shows dodecagonal symmetry. The same kind of quasicrystalline structures have been found for metal alloys (~0.5 nm), chalcogenides (~2 nm), and liquid crystals (~10 nm). The present result (~50 nm) confirms the universal nature of dodecagonal quasicrystals over several hierarchical length scales.

Spontaneous Formation of Complex Micelles from a Homogeneous Solution

Xuehao He — 2008-05-03T17:52:28-00:00

Physical Review Letters, Vol. 100, No. 13. (2008)

We present an extensive computer simulation study of structure formation in amphiphilic block copolymer solutions after a quench from a homogeneous state. By using a mesoscopic field-based simulation method, we are able to access time scales in the range of a second. A “phase diagram” of final structures is mapped out as a function of the concentration and solvent philicity of the copolymers. A rich spectrum of structures is observed, ranging from spherical and rodlike micelles and vesicles to toroidal and net-cage micelles. The dynamical pathways leading to these structures are analyzed in detail, and possible ways to control the structures are discussed briefly.

Simulation and theory of self-assembly and network formation in reversibly cross-linked equilibrium polymers

James Kindt — 2007-10-19T18:58:12-00:00

The Journal of Chemical Physics, Vol. 123, No. 14. (2005)

A simulation model of hard spheres capable of reversible assembly into chains, which then may reversibly cross-link into networks, has been studied through grand canonical Monte Carlo simulation. Effects of varying intra- and interchain bond strengths, chain flexibilities, and restrictions on cross-linking angle were investigated. Observations including chain-length distributions and phase separation could be captured in most cases using a simple model theory. The coupling of chain growth to cross-linking was shown to be highly sensitive to the treatment of cross-linking by chain ends. In some systems, ladderlike domains of several cross-links joining two chains were common, resulting from cooperativity in the cross-linking. Extended to account for this phenomenon, the model theory predicts that such cooperativity will suppress phase separation in weakly polymerizing chains and at high cross-link concentration. In the present model, cross-linking stabilizes the isotropic phase with respect to the nematic phase, causing a shift in the isotropic-nematic transition to higher monomer concentration than in simple equilibrium polymers. ©2005 American Institute of Physics

Microrheology of cross-linked polyacrylamide networks

Bivash Dasgupta — 2008-04-23T21:44:04-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 71, No. 2. (2005)

Experiments investigating the local viscoelastic properties of a chemically cross-linked polymer are performed on polyacrylamide solutions in the sol and the gel regimes using polystyrene beads of varying sizes and surface chemistry as probes. The thermal motions of the probes are measured to obtain the elastic and viscous moduli of the sample. Probe dynamics are measured using two different dynamic light scattering techniques, diffusing wave spectroscopy (DWS) and quasielastic light scattering (QELS) as well as video-based particle tracking. Diffusing wave spectroscopy probes the short-time dynamics of the scatterers while QELS measures the dynamics at larger times. Video-based particle tracking provides a way to investigate the local environment of the individual probe particles. A combination of all the techniques results in a larger range of frequencies that can be probed compared to conventional bulk measurements while providing local information at the level of individual probes. A modified algebraic form of the generalized Stokes-Einstein equation is used to calculate the frequency-dependent moduli. A comparison of microrheological measurements with bulk rheology exhibits striking similarity, confirming the applicability of microrheology for chemically cross-linked polymeric systems.

Microrheology Probes Length Scale Dependent Rheology

J Liu — 2008-01-20T23:36:22-00:00

Physical Review Letters, Vol. 96, No. 11. (2006)

We exploit the power of microrheology to measure the viscoelasticity of entangled F-actin solutions at different length scales from 1 to 100 µm over a wide frequency range. We compare the behavior of single probe-particle motion to that of the correlated motion of two particles. By varying the average length of the filaments, we identify fluctuations that dissipate diffusively over the filament length. These provide an important relaxation mechanism of the elasticity between 0.1 and 30 rad/sec.

Phase Behavior and Charge Regulation of Weak Polyelectrolyte Grafted Layers

Peng Gong — 2008-03-19T17:31:41-00:00

Physical Review Letters, Vol. 98, No. 1. (2007)

The stability of weak polyelectrolytes end grafted to a planar surface has been studied with a molecular theory. The effective quality of the solvent is found to depend on the interplay between polymer grafting density, acid-base equilibrium, and salt concentration. Our results reveal that increasing salt concentration results in a thermodynamically more stable layer. This reverse salt effect is due to the competition between the solvent quality and the dual role of the ionic strength in screening the electrostatic interactions (reducing stability with increasing salt concentration), and regulating the charge on the polymer (increasing charge with increasing salt concentration). Grafted weak polyelectrolyte layers are found to be thermodynamically unstable at intermediate surface coverages. Additionally, it is established that the increased solubility of the layer at low surface coverage is due to the relatively large charge of the grafted polymers. The range of stability of the film with regard to polymer surface coverage, temperature, bulk pH and salt concentration is demonstrated.

Two-Dimensional Dynamics of Metal Nanoparticles on the Surface of Thin Polymer Films Studied with Coherent X Rays

S Streit — 2008-03-19T17:03:19-00:00

Physical Review Letters, Vol. 98, No. 4. (2007)

X-ray photon-correlation spectroscopy is used to measure the dynamic structure factor f(q,) of gold particles moving on the surface of thin polymer films. Above the glass transition of the polymer the peculiar form f(q,)~exp[-()] is found with 0.7<<1.5, depending on sample age and temperature. The relaxation rates scale linearly with q, excluding a simple Brownian diffusive motion. This type of behavior, already observed in aging bulk soft matter systems, is explained by a power law distribution of particle velocities due to ballistic motion.

Molecular Theory of Physical Aging in Polymer Glasses

Kang Chen — 2008-03-18T23:12:48-00:00

Physical Review Letters, Vol. 98, No. 16. (2007)

A molecular level theory for the physical aging of polymer glasses is proposed. The nonequilibrium time evolution of the amplitude of long wavelength density fluctuations, and its influence on activated barrier hopping, plays an essential role. The theory predicts temperature-dependent apparent power-law aging of the segmental relaxation time and logarithmic aging of thermodynamiclike properties, in good accord with experiments. A physical origin for the quantitative nonuniversal aspects based on the amplitude of quenched density fluctuations is suggested.

Rheological Observation of Glassy Dynamics of Dilute Polymer Solutions near the Coil-Stretch Transition in Elongational Flows

T Sridhar — 2008-03-18T23:11:06-00:00

Physical Review Letters, Vol. 98, No. 16. (2007)

It has long been conjectured that the macroscopic dynamics of dilute polymer solutions may exhibit a glasslike slowdown caused by ergodicity breaking, in the vicinity of the coil-stretch transition in elongational flows. We report experimental observations using a filament stretching rheometer that confirm the existence of such glassy states. It is observed that different time-dependent elongational strain-rate profiles lead to a pronounced history dependence and aging effects within a narrow range of strain rates. The results have a direct bearing on the analysis and design of processes employing dilute polymer solutions, such as ink-jet printing, surface coating, and turbulent-drag reduction.

Surface Diffusion Dynamics of a Single Polymer Chain in Dilute Solution

Hu Qian — 2008-03-18T21:08:38-00:00

Physical Review Letters, Vol. 99, No. 6. (2007)

Comprehensive three-dimensional dissipative particle dynamics simulations are carried out to elucidate the diffusion mechanism of a strongly adsorbed polymer chain on a solid surface in dilute solutions. We find Rouse and reptation dynamics for polymer chain diffusing on smooth and rough surfaces (with obstacles or sticking points), respectively. Combining with scaling analysis, we find that the interactions between the surface and the fluid screen the hydrodynamic interaction. The different scaling as found for a polymer chain diffusing on a fluid membrane [Phys. Rev. Lett. 82, 1911 (1999)] and on a solid surface [Nature (London) 406, 146 (2000)] may be explained by the solid surface inhomogeneity that induces reptation.

Chirality and Equilibrium Biopolymer Bundles

Gregory Grason — 2007-08-29T11:52:49-00:00

Physical Review Letters, Vol. 99, No. 9. (2007)

We use continuum theory to show that chirality is a key thermodynamic control parameter for the aggregation of biopolymers: chirality produces a stable disperse phase of hexagonal bundles under moderately poor solvent conditions, as has been observed in in vitro studies of F actin [O. Pelletier et al., Phys. Rev. Lett. 91, 148102 (2003)]. The large characteristic radius of these chiral bundles is not determined by a mysterious long-range molecular interaction but by in-plane shear elastic stresses generated by the interplay between a chiral torque and an unusual, but universal, nonlinear gauge term in the strain tensor of ordered chains that is imposed by rotational invariance.

Slippery or Sticky Boundary Conditions: Control of Wrinkling in Metal-Capped Thin Polymer Films by Selective Adhesion to Substrates

Hugues Vandeparre — 2008-03-18T02:07:36-00:00

Physical Review Letters, Vol. 99, No. 18. (2007)

Wrinkling patterns at the metallized surface of thin polymer films are shown to be sensitive to the sticky or slippery character of the polymer-substrate interface. Existing theoretical models were expanded to specific boundary conditions (adhesive versus slippery) in order to rationalize these observations. Based on this concept, we were able to propose a new and simple method to orient the wrinkles by chemically patterning the substrate with regions of high and low adhesion.

Rouse Dynamics of Colloids Bound to Polymer Networks

Joris Sprakel — 2008-03-18T01:58:14-00:00

Physical Review Letters, Vol. 99, No. 20. (2007)

We present experimental evidence of a transition in the short-time Brownian motion of colloids from diffusive to subdiffusive, Rouse-like. This transition is seen for particles that are bound, through physical adsorption, to transient polymer networks. The characteristic Rouse scaling of the mean square particle displacement with , found in the experiments, is rationalized using an analytical bead-spring model of a large particle anchored to a set of polymer chains.

Scaling Equations for a Biopolymer in Salt Solution

Erik Geissler — 2008-03-18T01:37:41-00:00

Physical Review Letters, Vol. 99, No. 26. (2007)

The effect of the simultaneous presence of monovalent and divalent cations on the thermodynamics of polyelectrolyte solutions is an incompletely solved problem. In physiological conditions, combinations of these ions affect structure formation in biopolymer systems. Dynamic light scattering measurements of the collective diffusion coefficient D and the osmotic compressibility of semidilute hyaluronan solutions containing different ratios of sodium and calcium ions are compared with simple polyelectrolyte models. Scaling relationships are proposed in terms of polymer concentration and ionic strength J of the added salt. Differences in the effects of sodium and calcium ions are found to be expressed only through J.

Density-Functional Theory for Polymer Fluids with Molecular Weight Polydispersity

Clifford Woodward — 2008-03-17T22:37:46-00:00

Physical Review Letters, Vol. 100, No. 9. (2008)

We develop a density-functional theory for polydisperse polymer fluids satisfying the Schulz-Flory distribution. The resulting equations are remarkably simple and quickly solved, the computational effort scaling with the polydispersity index, rather than the average molecular weight. Equilibrium, or “living”, polymers enter naturally as very polydisperse samples. We illustrate the importance of polydispersity on colloid stability by investigating interactions between adsorbing and nonadsorbing surfaces. Significant free energy barriers are present in monodisperse samples, but these diminish as the degree of polydispersity increases.

Heterogeneous nature of the dynamics and glass transition in thin polymer films

Merabia — 2006-05-23T21:54:44-00:00

The European Physical Journal E - Soft Matter, Vol. 15, No. 2. (October 2004), pp. 189-210.

Does equilibrium polymerization describe the dynamic heterogeneity of glass-forming liquids?

Jack Douglas — 2008-03-01T17:39:54-00:00

The Journal of Chemical Physics, Vol. 125, No. 14. (2006)