Universal Polymeric-to-Colloidal Transition in Melts of Hairy Nanoparticles.

Two different classes of hairy self-suspended nanoparticles in the melt state, polymer-grafted nanoparticles (GNPs) and star polymers, are shown to display universal dynamic behavior across a broad range of parameter space. Linear viscoelastic measurements on well-characterized silica-poly(methyl acrylate) GNPs with a fixed core radius () and grafting density (or number of arms ) but varying arm degree of polymerization () show two distinctly different regimes of response. The colloidal Regime I with a small (large core volume fraction) is characterized by predominant low-frequency solidlike colloidal plateau and ultraslow relaxation, while the polymeric Regime II with a large (small core volume fractions) has a response dominated by the starlike relaxation of partially interpenetrated arms.

Glassy states in adsorbing surfactant-microgel soft nanocomposites.

Mixtures of polymer-colloid hybrids such as star polymers and microgels with non-adsorbing polymeric additives have received a lot of attention. In these materials, the interplay between entropic forces and softness is responsible for a wealth of phenomena. By contrast, binary mixtures where one component can adsorb onto the other one have been far less studied.

Nonmonotonic Stress Relaxation after Cessation of Steady Shear Flow in Supramolecular Assemblies.

Stress relaxation upon cessation of shear flow is known to be described by single-mode or multimode monotonic exponential decays. This is considered to be ubiquitous in nature. However, we found that, in some cases, the relaxation becomes anomalous in that an increase in the relaxing stress is observed.

Rheological Behavior and in Situ Confocal Imaging of Bijels Made by Mixing.

Bijels (bicontinuous interfacially jammed emulsion gels) have the potential to be useful in many different applications due to their internal connectivity and the possibility of efficient mass transport through the channels. Recently, new methods of making the bijel have been proposed, which simplify the fabrication process, making commercial application more realistic. Here, we study the flow properties of bijels prepared by mixing alone using oscillatory rheology combined with confocal microscopy and also squeezing flow experiments.

On the universality of the flow properties of soft-particle glasses.

We identify the minimal interparticle interactions necessary for a particle dynamics simulation to predict the structure and flow behaviour of soft particle glasses (SPGs). Generally, two kinds of forces between the particles must be accounted for in simulations of SPGs: viscous or frictional drag forces and elastic contact forces. Far field drag forces are required to dissipate energy in the simulations and capture the effect of the rheology of the suspending fluid.

Versatile Encapsulation Technology Based on Tailored pH-Responsive Amphiphilic Polymers: Emulsion Gels and Capsules.

We present a multipurpose technology to encapsulate hydrophobic substances in micron-sized emulsion droplets and capsules. The encapsulating agent is a comblike stimuli-responsive copolymer comprising side-chain surfactants attached to a methacrylic acid/ethyl acrylate polyelectrolyte backbone. The composition and structure of the hydrophobic moieties of the side chains are customized to tune the particle morphology and the processing conditions.

Particle-wall tribology of slippery hydrogel particle suspensions.

Slip is an important phenomenon that occurs during the flow of yield stress fluids like soft materials and pastes. Densely packed suspensions of hydrogel microparticles are used to show that slip is governed by the tribological interactions occurring between the samples and shearing surfaces. Both attractive/repulsive interactions between the dispersed particles and surface, as well as the viscoelasticity of the suspension, are found to play key roles in slip occurring within rheometric flows.

The glass and jamming transitions of soft polyelectrolyte microgel suspensions.

We explore the influence of particle softness on the state diagram of well characterized polyelectrolyte microgel suspensions using dynamic light scattering and rheology. Upon increasing the polymer concentration, we cross successively the well defined glass and jamming transitions which delimit four different states: dilute colloidal suspension, entropic glass, jammed glass, and dense glass. Each state has a specific dynamical fingerprint dictated by two key ingredients related to particle softness: elastic contact interactions, and osmotic or steric deswelling.

Dynamical role of slip heterogeneities in confined flows.

We demonstrate that flows in confined systems are controlled by slip heterogeneities below a certain size. To show this we image the motion of soft glassy suspensions in microchannels whose inner walls impose different slip velocities. As the channel height decreases, the flow ceases to have the symmetric shape expected for yield-stress fluids.

The viscosity of short polyelectrolyte solutions.

We consider the viscosity of solutions of highly charged short polyelectrolytes. Our system is a poly(styrene-maleic acid) copolymer solution (SMA) with various added salt concentrations in dilute and semidilute regimes. The SMA solutions show some particular features: (i) variations of the specific viscosity measured for different values of concentration and ionic strength can be rescaled on two universal curves when plotted as a function of the effective volume fraction; (ii) the reduced viscosity is proportional to the Debye length.

Microscopic origin of internal stresses in jammed soft particle suspensions.

The long time persistence of mechanical stresses is a generic property of glassy materials. Here we identify the microscopic mechanisms that control internal stresses in highly concentrated suspensions of soft particles brought to rest from steady flow. The persistence of the asymmetric angular distortions which characterize the pair distribution function during flow is at the origin of the internal stresses.

Order-disorder transition in supramolecular polymers.

In supramolecular polymers, directional interactions control the constituting units connectivity, but dispersion forces may conspire to make complex organizations. Here we report on the long-range order and order-disorder transition (ODT) of main-chain supramolecular polymers based on poly(propylene oxide) (PPO) spacers functionalized on both ends with thymine. Below the ODT temperature (T(ODT)), these compounds are semicrystalline with a lamellar structure, showing nanophase separation between crystallized thymine planes and amorphous PPO layers.

A micromechanical model to predict the flow of soft particle glasses.

Soft particle glasses form a broad family of materials made of deformable particles, as diverse as microgels, emulsion droplets, star polymers, block copolymer micelles and proteins, which are jammed at volume fractions where they are in contact and interact via soft elastic repulsions. Despite a great variety of particle elasticity, soft glasses have many generic features in common. They behave like weak elastic solids at rest but flow very much like liquids above the yield stress.

Unique slow dynamics and aging phenomena in soft glassy suspensions of multiarm star polymers.

We use time-resolved rheology to elucidate the slow dynamics and aging in highly concentrated suspensions of multiarm star polymers. The linear and nonlinear rheological properties exhibit a terminal regime corresponding to a well-defined maximal relaxation time. Terminal relaxation is driven by arm relaxation which speeds up the escape of stars from their cages.

Doubly responsive polymer-microgel composites: rheology and structure.

Mixtures of alkali swellable microgels and linear PNIPAm chains exhibit doubly responsive properties both with pH and temperature. Below the lower critical solution temperature (LCST), the linear chains of PNIPAm are soluble and increase the osmotic pressure outside the microgels, which causes them to deswell. Above the LCST, the PNIPAm chains become insoluble and form spherical colloidal particles confined between the microgels that subsequently reswell.

Colloidal phase separation of concentrated PNIPAm solutions.

In the concentration range of 1-6 wt %, solutions of a thermosensitive polymer (poly-N-isopropylacrylamide (PNIPAm), Mw = 1.4 x 10(5) g.mol(-1)) are shown to phase separate in the form of dense stable colloids of nearly pure polymer.

Control of the chemical cross-linking of gelatin by a thermosensitive polymer: example of switchable reactivity.

Chemical cross-linking of gelatin is achieved using a thermosensitive reactive copolymer based on N-isopropylacrylamide (NIPAM). The copolymer bears 5 mol % acrylic acid units which form amide bonds with the amino groups of gelatin in the presence of a water-soluble carbodiimide. The cross-linking reaction occurs only below the LCST congruent with 34 degrees C (lower critical solution temperature), i.

Slip and flow in soft particle pastes.

Concentrated dispersions of soft particles are shown to exhibit a generic slip behavior near smooth surfaces. Slip results from a balance between osmotic forces and noncontact elastohydrodynamic interaction between the squeezed particles and the wall. A model is presented that predicts the slip properties and provides insight into the behavior of the bulk paste.

Glassy dynamics and flow properties of soft colloidal pastes.

The local dynamics and the nonlinear rheology of soft colloidal pastes are shown to exhibit a remarkable universal behavior in terms of a unique microscopic time scale. This variable is associated with structural relaxation under the combined action of local frictional forces and elastic driving forces. These results establish a link between the local dynamics of pastes and their nonlinear flow behavior and provide a unified description of paste rheology.

Static and dynamic properties of highly turbid media determined by spatially resolved diffusive-wave spectroscopy.

We show that spatially resolved backscattering can be used for simultaneous measurements of static and dynamic properties of highly turbid media. The spatial variation of the backscattered intensity gives access to the transport men free path. The decay of the temporal intensity-intensity correlation function depends on the point of observation.