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We present small-angle X-ray scattering measurements in model star homopolymer melts with high functionality, revealing liquid-like ordering. Monte Carlo simulations attribute this structure to the nonuniform intramolecular segmental mass distribution, which results in core-shell topology. This ordering is also reflected in their linear viscoelastic response as a new structural mode at low frequencies, due to cooperative rearrangements of the stars.

Photon correlation spectroscopy in the polarized (VV) and depolarized (VH) geometries has been utilized in order to investigate the aggregation dynamics in toluene solutions of a hairy-rod polymer, poly(p-phenylene) with flexible dodecyl side chains [Ph-Ph(C 12 H 25 ) 2 -Ph] n . Static and dynamic results from the dilute regime suggest that polymer molecules form small aggregates, typically trimers, even at very low concentrations. In the semidilute regime, results at different scattering wavevectors (q) in both the VV and VH geometry reveal two new relaxation processes (slow and ultra slow), in addition to the faster cooperative diffusion and reorientation of the trimers, respectively. They are attributed to the formation of large anisotropic clusters (of typical size 570 nm), with inherent crystallization, as revealed by X-ray scattering. After about 2 weeks the clusters settle under the action of gravity and form an opaque sediment. Assessment of the aggregation modes has been carried out by sediment redispersion. The first stages of this process, which exhibit multiple scattering, reveal a steeper than exponential relaxation, associated with a sedimentation-induced velocity gradient in a transient pseudonetwork formed by the dispersed clusters. In the single scattering limit, reached at later stages, the two relaxation processes are related to reorientation (slow) and number density fluctuation (ultraslow) of the anisotropic clusters. On the basis of simple thermodynamic arguments, we argue that rodlike macromolecules exhibit a larger activation energy barrier compared to their flexible counterparts, in agreement with the experimental evidence.

Linear viscoelastic, shear, and extensional rheological characterization of linear and branched Poly(Ethylene Terephthalate) resins (PET) was carried out by means of both a parallel-plate and capillary rheometers. Before loading into the rheometers, the polymer pellets were thoroughly dried at well-characterized conditions long enough to obtain consistent and reproducible results. Continuing polymer degradation and poly-condensation reactions in the relatively open environment of the parallel-plate rheometer were accounted for by correcting the data using material-time super-position. The rheological data obtained were used to fit by nonlinear optimization, the linear relaxation spectrum and nonlinear parameters of a multi-mode Phan-Thien and Tanner (PTT) constitutive relation. It was found that this model can represent rheological data for PET resins very well and as a result may be used in relevant processing flow simulations, i.e. film casting.

We report on a combination of experiments to determine the elastic moduli of a soft poly (dimethylsiloxane) rubber that was utilized in a smart experiment on resonant phononic modes [Liu et al., Science 289, 1734 (2000)] and whose reported moduli became widely used as a model system in theoretical calculations of phononic materials. We found that the most peculiar hallmark of these values, an extremely low longitudinal sound velocity, is not supported by our experiments. Anyhow, performing theoretical band structure calculations, we can reproduce the surprising experimental findings of Liu et al. even utilizing the correct mechanical parameters. Thus, the physical conclusions derived in the theoretical works do not require the use of an extremely low longitudinal velocity, but can be reproduced assuming only a low value of the shear modulus, in agreement with our experiments.

International audience; Under certain conditions, the addition of small non-adsorbing polymers to a suspension of large hard spheres yields a depletion gel. It is expected that, provided that solvent quality issues do not lead to phase separation, thermodynamics imposes an invariance of the hard-sphere depletion gel upon heating. We show here that this is not the case for depletion gels of soft hairy nanoparticles. Using multiarm star polymers as a model that encompasses the key features of a huge range of grafted colloids, we show that their microstructural details make the difference. In particular, their hybrid nature, consisting of a carbosilane core and a polymeric shell, gives rise to a mismatch in thermal expansion upon heating, which affects the nanoparticle's conformation in a way analogous to a dry-to-wet brush transition, and hence leads to thermal melting of the gel. These results open the route for the molecular design of new materials, providing new opportunities to colloid chemistry and processing.

The ability to purify cyclic polymers with liquid chromatography at the critical condition and the recent advances in theoretical modeling and advanced simulations have opened the route for determining and understanding their rheological properties, and hence resolving a nearly 30-years old controversy. In the unentangled regime, the zero-shear viscosity of cyclic polymers is half that of their linear counterparts and their rheology conforms to the Rouse model. The behavior of entangled rings conforms to the lattice animal configuration and is characterized by a self-similar stress relaxation with a power law exponent of about 0.4 extending over 3 decades in time. No entanglement plateau is observed. Their viscosity remains much smaller in comparison to linear polymers. Tiny amounts of linear chain contaminants (of the order of 0.1%, below their overlap concentration) dramatically alter the rheology of rings as they entropically penetrate them, forming a transient network of linear chains bridged by rings. Despite the significant progress, many outstanding challenges lie ahead and make this topic fascinating.

Polymer chains attached by one end to an impenetrable surface at high coverage exemplify a tethered layer of mesoscopic dimensions. At equilibrium, thermal fluctuations of the segment density profile of the brushlike layer reflect the tethered chain dynamics; the probing of these fluctuations by evanescent-wave dynamic light scattering is reported. By utilizing a set of terminally attached layers with thicknesses (L 0 ) from 45 to 130 nanometers, it was found that there is a preferred wavelength of order L 0 of these fluctuations with a concurrent slowing down of their thermal decay rate. This technique could open the route for the investigation of the largely unexplored area of polymer surface dynamics.

Photon correlation spectroscopy in the polarized geometry has been used to systematically investigate the complex dynamics of a highly concentrated entangled polymer solution in a nominally good solvent, poly(butylacrylate) in dioxane. In addition to the well known fast cooperative diffusion process, a slow virtuallyq-independent mode is detected, whereq is the scattering wavevector, in agreement with previous experimental works on semidilute solutions. This mode is attributed to the viscoelastic nature of the transient physical network, formed by the entanglements, which relaxes its elastic stress induced by the concentration fluctuations, as confirmed by small amplitude oscillatory shear measurements; the latter reveal a terminal relaxation time comparable to the characteristic time of the slow relaxation process. Results, especially in terms of concentration and temperature dependence, are evaluated and discussed in view of the existing theoretical treatments in the field, predicting the existence of the slow viscoelastic relaxation. The relationship between dynamic light scattering and mechanical spectroscopy is established.

The thermorheological complexity at low frequencies in polymer blends near macrophase separation is quantitatively accounted for. The behaviour of both storage, G', and loss, G'', moduli at low frequencies as the temperature, T, approaches phase separation is due to the critical concentration fluctuations in the one-phase region. The temperature at maximum slope of the G' vs. T dependence at low frequencies is identified with the cloud point curve, whereas a plot of [(G'')2/(G'T)]2/3 vs. 1/T leads to the identification of the spinodal temperature.

The chain conformation of a series of polydisperse polyesters and poly(p-phenylenes) has been investigated by means of depolarized Rayleigh scattering in solution. By considering polydisperse worm-...

Polarized laser Raman and FTIR spectroscopies, as well as wide angle X-ray diffraction (WAXD), have been employed in order to investigate the distribution of molecular orientation in uniaxially drawn solution-cast films of well-characterized polyesters, bearing hexyl side chains, at different draw ratios. Both the second (〈P2(cos θ)〉) and fourth (〈P4(cos θ)〉, only with Raman) moments of the segment orientation distribution function have been determined. Results reveal physically meaningful trends of both P2 and P4 with draw ratio. A critical comparison among the three techniques confirms the sensitivity of Raman and FTIR to order at molecular level, when detecting the orientation of a particular segment, compared to similar WAXD results that provide information on the larger scale liquid crystalline domain orientation only and thus correspond to higher values of P2. Therefore, the corresponding dependencies of P2 on draw ratio are also different. A simplified approach for the analysis of the Raman spectra...

The analysis of the conformation of a stiff-chain polyester by magnetic birefringence is presented. The specific Cotton−Mouton constant, C2b, has been measured for five different molecular weights of the polyester. A general analysis of the data is presented, including the effect of polydispersity. A weak dependence of C2b on molecular weight is found, which shows that the persistence length of the polyester is approximately 11 nm. Estimates of the specific Cotton−Mouton constant per unit length by using group increments compare favorably with the measured value.

The shear rheology of a binary polymer blend exhibiting a lower critical solution temperature (LCST) phase diagram and a small dynamic asymmetry (difference of glass transition temperatures between its constituents) has been investigated in the vicinity of phase separation; it is a mixture of a random copolymer of styrene and maleic anhydrite and poly(methyl methacrylate). In the linear viscoelastic regime, the material functions are sensitive to phase separation, and the effects of critical concentration fluctuations, which dominate the mechanical response, are quantified, yielding both the binodal and spinodal curves. The weak dynamic asymmetry is apparently responsible for the reduced magnitude of the observed effects, compared to blends exhibiting much larger contrast in glass transition; therefore, this property affects to some degree the accuracy of the rheologically determined phase diagram. The steady shear properties are weakly sensitive to phase separation, and suggest that shear-induced demixing may be possible. They also indicate the importance of the amount of strain energy introduced to the blend in controlling the effects of flow on phase behavior.

Small amplitude oscillatory shear rheology is employed in order to investigate the linear viscoelastic behavior of the lower critical solution temperature blend polystyrene/poly(vinyl methyl ether), PS/PVME, as a function of temperature and composition. At low temperatures, where the mixture is homogeneous, the dependence of the zero shear viscosity (η0) on concentration is measured and is well-described by means of a new mixing rule, based on surface fractions instead of volume fractions. Shift factors from time-temperature superposition (TTS) exhibit a Williams−Landel−Ferry (WLF) behavior. As the macrophase separation temperature is approached (the phase diagram being established by turbidity measurements), the blend exhibits a thermorheologically complex behavior. A failure of TTS is observed at low frequencies, both in the homogeneous pretransitional and in the two-phase regimes. Its origin is attributed to the enhanced concentration fluctuations, which exhibit a critical slowing down near the phase b...

By employing rheological experiments, mode coupling theory, and computer simulations based on realistic coarse-grained models, we investigate the effects of small, hard colloids on the glassy states formed by large, soft colloids. Multiarm star polymers mimic hard and soft colloids by appropriately varying the number and size of their arms. The addition of hard colloids leads, depending on their concentration, to either melting of the soft glass or the emergence of two distinct glassy states. We explain our findings by depletion of the colloids adjacent to the stars, which leads to an arrested phase separation when the repulsive glass line meets the demixing binodal. The parameter-free agreement between experiment, theory, and simulations suggests the generic nature of our results and opens the route for designing soft-hard colloidal composites with tunable rheology. © 2013 American Physical Society.

LDA experiments have been conducted in a two-dimensional steady streaming flow field in order to determine the secondary velocity profiles. We describe here an LDA system developed to resolve small secondary-flow velocities and to detect flow reversals due to viscoelasticity. Results compare well with theoretical predictions. A detailed analysis of the errors and uncertainties involved in the measurements confirms the reliability and reproducibility of the measurements. Although the method has been applied here to a specific flow field, the technique should be applicable to a number of secondary flows, such as those which can occur in curved pipes and in oscillating pipe flows.

We report on a dynamic-light-scattering study of the order parameter fluctuations in a binary mixture of poly(methylphenylsiloxane) and poly(styrene) in the glass transition region. We find that the interdiffusion freezes and the growth of composition fluctuations ceases when the time scales of the segmental motion in the poly(styrene)-rich environment approach the duration of the experiment. In contrast to the single relaxation process found in polymer blends well above Tg, the polymer mixture studied here exhibits an additional slower relaxation mode.

This paper presents a simple generalization of the single-mode Giesekus constitutive model with retardation time. It is based on the utilization of the Gordon-Schowalter derivatives for the stress and rate-of-deformation tensors, which account for the non-affine motion of polymer strands, and make the model more versatile in terms of its ability to predict the complex flow behaviour. The model is consistent with molecular kinetic theory, and has been tested in several well-characterized shear flows, whereas its ability to describe shear-free flows has been proved as well. Finally, a comparison against steady shear data of nearly monodisperse polybutadienes demonstrates its superiority with respect to other continuum or molecular models exhibiting non-monotonic constitutive behaviour.

PURPOSE The effect of ultraviolet (UV)-riboflavin cross-linking (CXL) has been measured primarily using the strip extensometry technique. We propose a simple and reliable methodology for the assessment of CXL treatment by using an established rheologic protocol based on small amplitude oscillatory shear (SAOS) measurements. It provides information on the average cross-link density and the elastic modulus of treated cornea samples. METHODS Three fresh postmortem porcine corneas were used to study the feasibility of the technique, one serving as control and two receiving corneal collagen cross-linking treatment. Subsequently, five pairs of fresh postmortem porcine corneas received corneal collagen cross-linking treatment with riboflavin and UVA-irradiation (370 nm; irradiance of 3 mW/cm2) for 30 minutes (Dresden protocol); the contralateral porcine corneas were used as control samples. After the treatment, the linear viscoelastic moduli of the corneal samples were measured using SAOS measurements and the average cross-linking densities extracted. RESULTS For all cases investigated, the dynamic moduli of the cross-linked corneas were higher compared to those of the corresponding control samples. The increase of the elastic modulus of the treated samples was between 122% and 1750%. The difference was statistically significant for all tested samples (P = 0.018, 2-tailed t-test). CONCLUSIONS We report a simple and accurate methodology for quantifying the effects of cross-linking on porcine corneas treated with the Dresden protocol by means of SAOS measurements in the linear regime. The measured dynamic moduli, elastic and viscous modulus, represent the energy storage and energy dissipation, respectively. Hence, they provide a means to assess the changing physical properties of the cross-linked collagen networks after CXL treatment.

International audience

Polymer colloids are important model systems for investigating fundamental aspects of colloid science. Traditionally rigid particles that have rather ‘simple’, for example, hard sphere-like or Yukawa, interaction potentials have been employed. Such systems have been reviewed frequently in the past. In this chapter we focus on recent developments: depletion interaction and soft spheres. Concerning the latter topic, two different types of materials are discussed: star polymers and responsive microgels.

Carefully prepared aqueous solutions of drag‐reducing polymers have been characterized by means of steady viscometric and molecular weight measurements. Solutions of anionic polyacrylamide (Cyanatrol‐750), 0.5–50 ppm, and nonionic poly (ethylene oxide) (Polyox WSR‐303), 100–1000 ppm, were investigated. Steady shear viscosity measurements, with a Rheometrics Stress Rheometer and a Contraves LS30 rheometer, reveal substantial shear thinning for Cyanatrol solutions above 2 ppm and Polyox solutions above 500 ppm, for the shear‐rate range investigated. Zero‐shear viscosity was accurately determined for all solutions. Intrinsic viscosity,hydrodynamic radius of gyration, and overlap concentration were also estimated. The Carreau and Carreau–Yasuda models successfully fit all shear viscosity data, and allow estimates of elasticity. Reduced variables were used for generalizing the shear viscosity data. Aging tests show that both polymers are stable. The molecular weight and polydispersity of Cyanatrol were measured using the technique of band sedimentation/low angle laser light scattering. The drag‐reducing capability of these polymers has been confirmed by pipe flow experiments.

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. The fact that the system fully relaxes and flows at long times has important consequences. The yield stress only exists in the limited range of frequencies or shear rates where solid-like behavior is observed. Aging is controlled by the total time elapsed after flow cessation and not by the time elapsed from flow cessation to the beginning of the measurement as in other glassy materials. Our results, which demonstrate the importance of particle architecture with respect to glassy dynamics, should be generic for long hairy particles.

We present experimental evidence confirming the recently established rich dynamic state diagram of asymmetric binary mixtures of soft colloidal spheres. These mixtures consist of glassy suspensions of large star polymers to which different small stars are added at varying concentrations. Using rheology and dynamic light scattering measurements along with a simple phenomenological analysis, we show the existence of re-entrance and multiple glassy states, which exhibit distinct features. Cooperative diffusion, as a probe for star arm interpenetration, is proven to be sensitive to the formation of the liquid pockets which signal the melting of the large-star-glass upon addition of small stars. These results provide ample opportunities for tailoring the properties of soft colloidal glasses.

A semimicroscopic derivation is presented of equations of motion for the density and the flow velocity of concentrated systems of entangled polymers. The essential ingredient is the transient force that results from perturbations of overlapping polymers due to flow. A Smoluchowski equation is derived that includes these transient forces. From this, an equation of motion for the polymer number density is obtained, in which body forces couple the evolution of the polymer density to the local velocity field. Using a semimicroscopic Ansatz for the dynamics of the number of entanglements between overlapping polymers, and for the perturbations of the pair-correlation function due to flow, body forces are calculated for nonuniform systems where the density as well as the shear rate varies with position. Explicit expressions are derived for the shear viscosity and normal forces, as well as for nonlocal contributions to the body force, such as the shear-curvature viscosity. A contribution to the equation of motion for the density is found that describes mass transport due to spatial variation of the shear rate. The two coupled equations of motion for the density and flow velocity predict flow instabilities that will be discussed in more detail in a forthcoming publication.

We present a static and quasi-elastic neutron scattering study on both the structure and dynamics of a ring polymer in a ring and linear polymer melt, respectively. In the first case, the ring structure proved to be significantly more compact compared to the linear chain with the same molecular weight. In the mixture, the ring molecules swell as was confirmed by small angle neutron scattering ( SANS) in accordance with both theory and simulation work. The dynamical behavior of both systems, which for the first time has been explored by neutron spin echo spectroscopy (NSE), shows a surprisingly fast center of mass diffusion as compared to the linear polymer. These results agree qualitatively with the presented atomistic MD simulations. The fast diffusion turned out to be an explicit violation of the Rouse model.

The steady streaming flow induced by a cylinder oscillating in a fluid otherwise at rest has been employed to characterize the rheological properties of drag-reducing fluids. Flow reversal and several features common with turbulence make this flow on alternative to conventional viscometric measurements, which do not provide adequate information at very low polymer concentrations. Relevant fluid relaxation times are determined by measuring the steady secondary velocity profiles and then fitting theoretical predictions to the data. Laser Doppler velocimetry has been used to measure steady streaming velocities of well-characterized solutions of polyacrylamide, polyethylene oxide, and a surfactant mixture. Results show that the effective fluid elasticity, or relaxation time, increases with concentration or with frequency of oscillation of the primary flow. Furthermore, the relaxation time appropriate for characterizing steady streaming has been found to be far smaller than the longest relaxation time obtained from viscometric flows. The sensitivity to frequency is important, especially when associating a relaxation time with drag-reducing capability. A comparison with drag-reduction experiments suggests that ranking additives according to elasticity provides information about drag-reducing effectiveness. Steady streaming is proposed as a useful tool for characterizing the response of polymers and surfactants in a variety of unsteady nonviscometric flows.

We attempt to elucidate the dynamics of multiarm star polymer solutions, which are representative of a large class of soft hairy colloids, over a wide range of concentrations. In addition to the usual β-relaxation (in-cage rattling) and α-relaxation (terminal cage escape), the relaxation of stars in the glassy state involves a mode associated with arm interpenetration. From linear and nonlinear rheological measurements, we establish a set of criteria for identifying the colloidal glass transition, including aging, yielding and elastic properties. Linear viscoelasticity and steady-shear measurements merge at low frequencies, indicating that terminal behavior is accessible, albeit at very long times. The transition from linear to nonlinear response is controlled by star elasticity and a balance between Brownian diffusion and flow advection. In the nonlinear regime, the flow curves collapse on a universal flow curve using a scaling that expresses a competition between solvent-mediated interactions and elastic forces. While applied to stars, our framework appears to be a generic tool for fingerprinting liquid-solid transitions in colloidal suspensions.

We use particle-based computer simulations to study the rheology of suspensions of high-functionality star polymers with long entangled arms. Such particles have properties which are intermediate between those of soft colloidal particles and entangled polymer chains. In the simulations, each star polymer is coarse-grained to a single particle. In order to faithfully reproduce dynamical properties, it is very important to not only include time-averaged interactions (potentials of mean force) but to also account for transient interactions induced by entanglements between the arms of different star polymers. Using a model which has all these features, it is found that, for sufficiently high shear rates, the start-up shear stress displays an overshoot. With increasing concentration, the core interactions increasingly dominate the initial stress response, leading to a maximum in the stress overshoot at relatively low strain values (0. 1 to 0. 5). Transient forces start to dominate after this initial stage. In a simulated experiment in which the shear rate is suddenly stepped-down from a high to a lower value, the stress shows a clear undershoot, with the minimum stress again at a relatively low strain value (based on the new shear rate). Finally, it is shown that a stress plateau develops in the flow curve. This plateau is absent when the transient forces between the polymer stars are not taken into account. © 2009 The Author(s).

Multiarm star polymers are model systems with tunable intermediate colloid to polymerlike character, exhibiting rich phase behavior, internal relaxations, and flow properties. An important puzzle for several years has been the lack of clear experimental proof of crystalline states despite strong theoretical predictions. We present unambiguous evidence via multispeckle dynamic light scattering (MSDLS) and small-angle neutron scattering (SANS) for such crystallization in a solvent of intermediate quality. An unexpected speed up of the short-time star diffusion observed in MSDLS was attributed by SANS to crystallization, via aging, of the multiam star glass. This delayed glass to crystal transition establishes a pathway for star crystallization that might be generic in colloidal glasses.

Multiarm star polymers were used as model grafted colloidal particles with long hairs, to study their size variation due to osmotic forces arising from added linear homopolymers of smaller size. This is the origin of the depletion phenomenon that has been exploited in the past as a means to melt soft colloidal glasses by adding linear chains and analyzed using dynamic light scattering experiments and an effective interactions analysis yielding the depletion potential. Shrinkage is a generic phenomenon for hairy particles, which affects macroscopic properties and state transitions at high concentrations. In this work we present a small-angle neutron scattering study of star/linear polymer mixtures with different size ratios (varying the linear polymer molar mass) and confirm the depletion picture, i.e., osmotic star shrinkage. Moreover, we find that as the linear/star polymer size ratio increases for the same effective linear volume fraction (c/c ∗ with c ∗ the overlapping concentration), the star shrinkage is reduced whereas the onset of shrinkage appears to take place at higher linear polymer volume fractions. A theoretical description of the force balance on a star polymer in solution, accounting for the classic Flory contributions, i.e. elastic and excluded volume, as well as the osmotic force due to the linear chains, accurately predicts the experimental findings of reduced star size as a function of linear polymer concentration. This is done in a parameter-free fashion, in which the size of the cavity created by the star, and from which the chains are excluded, is related to the radius of the former from first principles.

For many years, colloidal hard spheres and polymeric coils served as model limiting cases of soft matter behavior. Softening the potential of interactions has been an obvious possibility for altering properties and has been explored in great detail with sterically interacting charged colloids. As the behavior of such (and technologically relevant) systems is very complex, it is desired to isolate the role of interactions and use well-characterized systems. Today, it is possible to achieve this goal by taking advantage of the capabilities of macromolecular and colloidal chemistry. We show how to use well-defined soft sphere systems interacting via excluded volume repulsions to generate a rich variety of phase states and materials properties. This approach provides opportunities for bridging the gap between polymers and colloids in terms of property variation, and thus designing soft systems with desired properties. At the same time, previously unexplored aspects of important open problems such as glass transition and effects of solvent on the dynamics of correlated systems are addressed. Appropriate blending of the two main classes of soft matter and further directing their assembly and dynamic response has now become a challenging new task.

The rheological behavior of aqueous micellar solutions of a triblock copolymer, bearing two small hydrophobic end-blocks of polybutadiene and a large hydrophilic middle block of poly(sodium methacrylate), was studied in the presence of a cationic (dodecyl trimethyl ammonium bromide) or an anionic (sodium dodecyl sulfate) surfactant. Depending on the concentration and the charges of the interacting (with the water soluble middle block) surfactants, the rheological behavior of the triblock copolymer micelles (which resemble compact spheres, based on scattering studies) can be altered dramatically. The surfactant additives can either solidify a liquid-like system (low triblock concentration, dominated by loops) or alternatively liquefy a gel (high triblock concentration, dominated by bridges). Apparently, the hydrophobic tails of the surfactants prefer to join the hydrophobic polybutadiene cores of the triblocks, whereas by increasing the surfactant concentration the core functionality can be changed. In addition interactions between the oppositely charged hydrophilic surfactant heads and middle blocks can yield complexes producing new hydrophobic domains. These findings suggest possibilities for controlling complex fluid rheology.

The long-standing observations that different amorphous materials exhibit a pronounced enhancement of viscosity and eventually vitrify on compression or cooling continue to fascinate and challenge scientists(1), on the ground of their physical origin and practical implications. Glass formation is a generic phenomenon, observed in physically quite distinct systems that encompass hard and soft particles. It is believed that a common underlying scenario(2,3), namely cage formation, drives dynamical arrest, especially at high concentrations. Here, we identify a novel, asymmetric glassy state in soft colloidal mixtures, which is characterized by strongly anisotropically distorted cages, bearing similarities to those of hard-sphere glasses under shear. The anisotropy is induced by the presence of soft additives. This phenomenon seems to be generic to soft colloids and its origins lie in the penetrability of the constituent particles. The resulting phase diagram for mixtures of soft particles is clearly distinct from that of hard-sphere mixtures and brings forward a rich variety of vitrified states that delineate an ergodic lake in the parameter space spanned by the size ratio between the two components and by the concentration of the additives. Thus, a new route opens for the rational design of soft particles with desired tunable rheological properties.

After many years of intense research, most aspects of the motion of entangled polymers have been understood. Long linear and branched polymers have a characteristic entanglement plateau and their stress relaxes by chain reptation or branch retraction, respectively. In both mechanisms, the presence of chain ends is essential. But how do entangled polymers without ends relax their stress? Using properly purified high-molar-mass ring polymers, we demonstrate that these materials exhibit self-similar dynamics, yielding a power-law stress relaxation. However, trace amounts of linear chains at a concentration almost two decades below their overlap cause an enhanced mechanical response. An entanglement plateau is recovered at higher concentrations of linear chains. These results constitute an important step towards solving an outstanding problem of polymer science and are useful for manipulating properties of materials ranging from DNA to polycarbonate. They also provide possible directions for tuning the rheology of entangled polymers.

Stress relaxation behavior after a large step strain is reported for a polyisoprene comb with lightly entangled arms. The linear viscoelastic behavior of the comb is fit with the theoretical model of Kapnistos et al.[1] using characteristic values for the given chemistry and p2 = 1/12. Using time‐temperature superposition, stress relaxation mastercurves are reported that span the complete relaxation behavior of the comb, from branch to long time backbone behavior. The backbone of the comb is well entangled after relaxation of the branches, but does not exhibit damping behavior consistent with an entangled linear chain showing a change due to the presence of the branch points.

We examine the rheological transition from an arrested to a fluid state for different mixtures of star polymers with varying functionality and size ratios. As a general trend, we find that addition of smaller star polymers in an arrested, concentrated solution of larger ones brings about melting of the large star glass. At the same time, the dependence of the amount of additives needed to melt the glass has a nontrivial dependence on the size ratio. Theoretical analysis, based on effective interactions and Mode Coupling Theory, reproduces the experimental results and helps identify two distinct types of glasses in the composite system.

Using rheological techniques, we investigate the evolution of the microstructure evolution during the early stages of crystallization of poly(1-Butene). In performing the measurements, use is made of an innovative experimental protocol, called “inverse quenching”, which allows stopping the crystallization process and producing a stable biphasic (crystalline/amorphous) system. In this way, very low frequency measurements at fixed degrees of crystallization are made possible. We find that crystallization, evidenced as a Liquid-to-Solid Transition (LST) under isothermal conditions, with characteristics of critical gel behavior, takes place at surprisingly low degrees of crystallinity (below 1.5%). The critical gel properties, which are found to depend on both crystallization temperature and molecular weight, can be reduced to a single master curve when the gel strength is plotted as a function of the relaxation exponent.. More importantly, the LST it is preceded by the development of a long, but finite relaxation process. This latter process, although not fully understood, brings analogies to the slow dynamics observed in hybrid colloid-polymer systems (block copolymer micelles or multiarm star polymers) as well as the recently suggested presence of dormant nuclei. It is clear, however, that the connectivity among crystallites, apparently via the amorphous segments, plays a key role in this new process

We examine the vitrification and melting of asymmetric star polymers mixtures by combining rheological measurements with mode coupling theory. We identify two types of glassy states, a {\it single} glass, in which the small component is fluid in the glassy matrix of the big one and a {\it double} glass, in which both components are vitrified. Addition of small star polymers leads to melting of {\it both} glasses and the melting curve has a non-monotonic dependence on the star-star size ratio. The phenomenon opens new ways for externally steering the rheological behavior of soft matter systems., 4 pages, 4 figures, accepted in Phys. Rev. Lett

Dense suspensions of multiarm star polymers are known to develop liquidlike microstructure, which has been attributed to the similarities between high functionality stars and colloidal particles interacting via soft, long ranged potentials. Recent experimental studies reported a counterintuitive solidification of suspensions with f=128 , upon increase of the temperature in marginal solvents. We present our results from molecular dynamics simulations of dense suspensions of multiarm star polymers. Star polymers are modeled as "soft spheres" interacting via a theoretically developed potential of mean field. Our results show a transition towards a "glassy" state at a temperature very close to the one reported experimentally. The features of the transition are consistent with those of ideal glass transitions, as described by ideal mode coupling theory. Furthermore, our findings illustrate the road to vitrification for these soft-colloidal suspensions. Higher temperatures result in arm expansion that causes jamming and more than compensates for faster short time, temperature induced kinetics.

The structure and dynamics of polymer-grafted two-dimensional silicate layers in solution were investigated. The geometry of the individual silicate layers was examined by looking at both polarized and depolarized light scattering from dilute solutions, while higher-concentration systems were used to study the interaction and dynamics of polymer-grafted silicate layers in suspension. The form factor for an oblate ellipsoid was used to fit the polarized intensity profile, and values of a approximately 80 nm and b approximately 380 nm for the semi-axes were obtained. The 80 nm value compares reasonably with the dimensions of the polymer brushes grafted on the surface of the silicate layers. The modulus of the grafted silicate in solution, as determined by Brillouin scattering, is of the order of 10 GPa. The cooperative diffusion mechanism, typical of interacting polymer chains, is suppressed due to the high polymer osmotic pressure. The osmotic pressure is also responsible for the weak interpenetration of the densely grafted polymer chains on the surface of the silicate layers. The scattering data indicates that the polymer-grafted nanoparticles move via collective diffusion and experience significant decrease in mobility above their overlap concentration.

We investigate the yielding and flow of hard-sphere colloidal glasses by combining rheological measurements with the technique of light scattering echo. The poly-methylmethacrylate particles used are sufficiently polydisperse that crystallization is suppressed. Creep and recovery measurements show that the glasses can tolerate surprisingly large strains, up to at least 15%, before yielding irreversibly. We attribute this behaviour to ‘cage elasticity’, the ability of a particle and its cage of neighbours to retain their identity under quite large distortion. Results from light scattering echo, which measures the extent of irreversible particle rearrangement under oscillatory shear, support the notion of cage elasticity. In the lower concentration glasses we find that particle trajectories are partly reversible under strains which significantly exceed the yield strain.

Star polymers with a high number of arms, f = 263, become kinetically trapped when dispersed in an athermal solvent at concentrations above the overlapping one, forming physical gels. We show that the addition of linear chains at different concentrations and molecular weights reduces the modulus of the gel, eventually melting it. We explain this linear polymer-induced gel-liquid transition in terms of effective interactions and star depletion. In the limit of very high linear-chain molecular weight a "reentrant gelation" is detected and attributed to bridging flocculation, analogous to that observed in colloidal dispersions.

Crowded solutions of multiarm star polymers, representing model colloidal spheres with ultrasoft repulsive interactions, undergo a reversible gelation transition upon heating in solvents of intermediate quality (between good and Theta). This unusual phenomenon is due to the kinetic arrest of the swollen interpenetrating spheres at high temperatures, forming clusters, in analogy to the colloidal glass transition. In this work we demonstrate that the choice of the solvent has a dramatic effect on the gelation transition, because of the different degree of star swelling (at the same temperature) associated with the solvent quality. We construct a generic kinetic phase diagram for the gelation of different stars in different solvents (gelation temperature against effective volume fraction, phi) and propose a critical "soft sphere close packing" volume fraction phi(c) distinguishing the temperature-induced (for phiphi(c)) glass-like gelation. We conclude that appropriate selection of the solvent allows for manipulation of the sol-gel transition in such ultrasoft colloids.

We present from simulations and experiments results on the linear and nonlinear rheology of a moderate functionality, low molecular weight unentangled polystyrene (PS) star melt. The PS samples were anionically synthesized and close to monodisperse while their moderate functionality ensures that they do not display a pronounced core effect. We employ a highly coarse-grained model known as Responsive Particle Dynamics where each star polymer is approximated as a point particle. The eliminated degrees of freedom are used in the definition of an appropriate free energy as well as describing the transient pair-wise potential between particles that accounts for the viscoelastic response. First we reproduce very satisfactorily the experimental moduli using simulation. We then consider the nonlinear response of the same polymer melts by implementing a start-up shear protocol for a wide range of shear rates. As in experiments, we observe the development of a stress overshoot with increasing shear rate followed by a steady-state shear stress. We also recover the shear-thinning nature of the melt, although we slightly overestimate the extent of shear-thinning with simulations. In addition, we study relaxations upon the removal of shear where we find encouraging agreement between experiments and simulations, a finding that corroborates our agreement for the linear rheology.

Oligomeric peptides can be designed which undergo one-dimensional self-assembly in solution to form beta-sheet tapes a single molecule in thickness and micrometers in length.(1) In this paper, we present the first systematic investigation of the size, shape, dynamics, and interactions of beta-sheet tapes formed by a self-assembling 24-residue peptide, K24, in 2-chloroethanol, over a wide range of peptide concentrations c (c: 10(-7)-1 mM), using photon correlation spectroscopy. The tapes behave like semiflexible chains with persistence lengths of several hundred nanometers and much longer contour lengths, even at c approximately 0.1 nM. The polarized q-dependent light-scattering intensity I fits a model of a prolate object with major and minor axes alpha approximately 630 nm and beta approximately 40 nm. This is an unexpected result in view of the previous theoretical predictions that tapelike polymers could form oblate coinlike structures in solution.(2) This experimentally observed behavior is attributed to the pronounced twist and bend of the beta-tapes, which do not allow them to form the coinlike structures, but instead they favor the formation of elongated polymers. At c approximately 10(-2) mM, the tapes are seen to start overlapping and forming networks with unusually large mesh sizes (e.g., ca. 400 nm at 15 mciroM), much larger than those of conventional polymers. With increasing peptide concentration the mesh size decreases and the network becomes a physical gel at c approximately 0.4 mM. These semidilute solutions are characterized by one main relaxation mode associated with the cooperative diffusion of the entangled tape network, and a weaker slower mode, associated with gel cluster formation. The concentration dependence of xi (xi(c) approximately c(-0.34)) is much weaker compared to the expected scaling for Gaussian or swollen chains (xi(c) approximately c(-1) for Gaussian chains, or xi(c) approximately c(-3/4) for swollen ones), but is not inconsistent with the expected scaling for rigid rods. On the basis of the concentration dependencies of the light-scattering intensity I and of the cooperative diffusion coefficient D, the cooperative friction coefficient f(c) is found to display a stronger concentration dependence (f(c) approximately c(1.34)) than in the case of semidilute flexible and semiflexible polymer solutions (f(c) approximately c(0.5)).(3) Thus, we may conclude that the network of entangled tapes approximates in its behavior that of semirigid polymers.