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We explore the behavior of a supramolecular polymer subjected to repeated shear-startup runs. This interesting test was proposed 50 years ago as a measure of disentanglement kinetics in linear polymers. A key experimental finding was that the recovery time (originally attributed exclusively to the relaxation of partially disentangled state) decreased with increasing imposed shear rate. Recent modeling and simulations put the work in perspective by showing that the recovery time mainly reflects chain reorientation. However, for the present supramolecular polymer, the recovery time was found to be rate independent. This drastic difference is attributed to the different mechanism of stress relaxation, which involves a combination of bond breaking/recombination and reptation processes. Hence, the repeated shear-startup protocol may serve as a sensitive indicator of (supramolecular) polymer signatures and a tool to evaluate stress relaxation models.

We study the linear viscoelastic properties of polymeric networks formed by poly( n-butyl acrylate) telechelic stars end-capped with 2,2:6,2″-terpyridine (Star-PnBA-tpy4) and two types of metal-ligand cross-links with different lifetimes. The influence of interactions, mediated by temperature, nature of metal ions, and ion content, on the linear viscoelastic behavior of both single and double dynamics transient networks is systematically investigated by small amplitude oscillatory shear and creep rheometry. The experimental results reveal that the dynamics of networks with two different metal-ligand cross-links is much faster than expected, characterized by the average sticker lifetime rather than a discrete contribution of each metal-ligand complex. We model the dynamics with the help of our modified tube-based time marching algorithm by accounting for both association/dissociation dynamics of metal-ligand coordination and the entanglement dynamics. Two parameters are defined in the model, namely, the proportion of dangling ends and the average time during which a sticker is free. This allows us to quantify the transient dynamics of the network and, in particular, to determine how the sticker dynamics depend on temperature and ion content.

We investigate the linear viscoelastic properties of industrial pressure sensitive adhesives comprising double networks with an entangled acrylate-based polymer and two types of intermolecular associations (crosslinking), permanent (epoxide) and reversible (metal-chelate), having different compositions. A combination of shear rheometry and an appropriately modified version of the Time Marching Algorithm (TMA) allows to probe and analyze the behavior of the different double dynamic networks, in particular, the effects of the type and amount of crosslinks on their linear viscoelastic spectra. To this end, the dynamics of the double networks are compared with the respective individual responses of the polymeric component without crosslinks and the single networks (possessing only physical or only chemical crosslinks), in order to quantify their contributions to the relaxation mechanisms, particularly the interplay between disentanglement and bond association/dissociation processes. With the help of the TMA model, we also examine the respective roles of the lifetime of stickers, polydispersity, and molar mass. Triggered by the good comparison between predictions and experimental data, we propose a framework to tune material parameters in order to obtain a desired viscoelastic behavior.

The shape, breadth, and average molecular weight of the overall molecular weight distribution (MWD) largely define polymer properties. In conventional free-radical polymerization, control over this distribution is through the many competing kinetic pathways dominated by radical termination events. "Living"radical polymerization mechanistically minimizes these termination events, providing a facile route to a desired Gaussian distribution with the distribution breadth dependent upon the activity of the catalyst or modulating agent. However, producing unusually shaped distributions can only be achieved through modeling of the complex polymerization kinetics and invoking feeding and other methods. Here, we construct square, slanted, and chair-like MWDs by blending two to four polymers made using a low-reactive RAFT agent with dispersities close to 2. The synthesis of these polymers, unlike that of polymers made with high-reactive RAFT agents, is simple, scalable, and importantly reproducible as the MWD is independent of conversion, making this polymerization method virtually and kinetically model-free. The blending method described here overcomes many of the difficulties in producing unusually shaped MWDs and allows control over the shape and breadth of the MWD. The concept further provides a general synthetic strategy for studying important structure-property relationships of polymers with desired processing and performance characteristics. This is demonstrated by measurement and modeling analysis of the linear viscoelastic properties of selected samples, which provides a way to tailor the properties of polymers by controlling the form of their MWD via blending. Unlike conventional approaches analyzing the effects of the MWD, its actual shape is considered and its effect on the properties is addressed.

International audience; When a drop of fluid hits a small solid target of comparable size, it expands radially until reaching a maximum diameter and subsequently recedes. In this work, we show that the expansion process of liquid sheets is controlled by a combination of shear (on the target) and biaxial extensional (in the air) deformations. We propose an approach toward a rational description of the phenomenon for Newtonian and viscoelastic fluids by evaluating the viscous dissipation due to shear and extensional deformations, yielding a prediction of the maximum expansion factor of the sheet as a function of the relevant viscosity. For Newtonian systems, biaxial extensional and shear viscous dissipation are of the same order of magnitude. On the contrary, for thinning solutions of supramolecular polymers, shear dissipation is negligible compared to biaxial extensional dissipation and the biaxial thinning extensional viscosity is the appropriate quantity to describe the maximum expansion of the sheets. Moreover, we show that the rate-dependent biaxial extensional viscosities deduced from drop impact experiments are in good quantitative agreement with previous experimental data and theoretical predictions for various viscoelastic liquids.

This work addresses the shear response of entangled linear polymers by assessing the current state-of-the-art model and proposing alternative directions. In particular, we examine the performance of the Graham–Likhtman–McLeish–Milner (GLaMM) nonlinear tube model in fast shear flows. Predictions are compared against experimental data for well-characterized, monodisperse, entangled linear polystyrene chains. Unlike previous works using the GLaMM model, finite extensibility is accounted for. Comparison of model predictions and data not only reveals an overall reasonable performance but also highlights limitations. For example, the predictions significantly depend on whether the contribution of contour length fluctuations to the retraction rate is accounted for or not. This specific sensitivity of the model is enhanced as the Rouse–Weissenberg number increases. Possible improvement of the model by modification of the existing mechanisms/model assumptions and/or by incorporation of overlooked mechanisms such as chain tumbling and CCR-driven disentanglement (CCR-D) is discussed. As a possible way to overcome the existing limitations, we propose an alternative approach based on the time marching algorithm (TMA) and focus on the description of the steady-state regime. Compared to the GLaMM model, this approach allows us to keep track of the relaxation state of each entanglement segment in a simple way, while ensuring consistency between chain stretch and constraint release mechanisms. Moreover, it accounts for CCR-D in an indirect manner. A key ingredient is the use of the recently advanced concept of the shear blob and its dependence on the shear rate. Our simple modification of TMA reproduces the experimental steady-state viscosities accurately for all samples and for all shear rates examined.

Hydrogels that can respond to a number of external stimuli and at the same time show impressive rheological properties are promising materials for a wide range of bioapplications. Here, we present a series of well-defined linear amphiphilic pentablock hybrid polypeptides of the ABCBA type, where A is poly(l-lysine), B is poly(l-histidine)-co-poly(γ-benzyl-l-glutamate), and C is poly(ethylene oxide) (PEO). The polymers were synthesized by the sequential primary amine ring-opening polymerization of N-carboxy anhydrides using bis amine poly(ethylene oxide) (PEO) as a bifunctional macroinitiator, and the length of all of the blocks was varied. The resulting materials formed novel extrudable in situ forming quickly self-healing hydrogels, responsive to the alteration of pH and increase of temperature. The connection between the alteration of the secondary structure of the polypeptides with the viscoelastic behavior was revealed by means of rheology and circular dichroism. Small-angle neutron scattering and scanning electron microscopy were employed to shed light on the structure of the polymers and how it affects their rheological properties. The obtained polymers were subjected to enzymatic degradation tests with trypsin and leucine aminopeptidase. The results suggest that these biomaterials have the potential to be used in a number of bioapplications like drug delivery, 3D printing, and tissue engineering.

In some recent experiments on entangled polymers of stress growth in startup of fast shear flows an undershoot in the shear stress is observed following the overshoot, i.e., before approaching the steady state. Whereas tumbling of the entangled chain was proposed to be at its origin, here we investigate another possible cause for the stress undershoot, i.e., slippage at the interface between polymer and solid wall. To this end, we extend the primitive chain network model to include slip at the interface between entangled polymeric liquids and solid walls with grafted polymers. We determine the slip velocity at the wall, and the shear rate in the bulk, by imposing that the shear stress in the bulk polymers is equal to that resulting from the polymers grafted at the wall. After confirming that the predicted results for the steady state are reasonable, we examine the transient behavior. The simulations confirm that slippage weakens the magnitude of the stress overshoot, as reported earlier. The undershoot is also weakened, or even disappears, because of a reduced coherence in molecular tumbling. In other words, the disentanglement between grafted and bulk chains, occurring throughout the stress overshoot region, does not contribute to the stress undershoot., Comment: 38 pages and 9 figures

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 at fixed core radius (R(core)) and grafting density (or number of arms f), but varying arm degree of polymerization (N(arm)) show two distinctly different regimes of response. A colloidal Regime I at small N(arm) (large core volume fraction), with a predominant low-frequency solid-like colloidal plateau and ultraslow relaxation, and a polymeric Regime II at large N(arm) (small core volume fractions), with a response dominated by the star-like relaxation of partially interpenetrated arms. The transition between the two regimes is marked by a cross-over where both polymeric and colloidal modes are discerned albeit without a distinct colloidal plateau. Similarly, polybutadiene multiarm stars also exhibit the colloidal response of Regime I at very large f and small N(arm). The star arm relaxation model and a simple scaling model of nanoparticle escape from the cage of neighbors by overcoming a hopping potential barrier due to their elastic deformation, describe quantitatively the linear response of the polymeric and colloidal regimes, respectively, in all these cases. The dynamic behavior of hairy nanoparticles of different chemistry and molecular characteristics, investigated here and reported in the literature, can be mapped onto a universal dynamic diagram of [Formula: see text] as a function of [Formula: see text] , where [Formula: see text] is the monomeric volume. In this diagram, the two regimes are separated by a line where the hopping potential ΔU(hop) is equal to the thermal energy, k(B)T. ΔU(hop) can be expressed as a function of the overcrowding parameter x, i.e. the ratio of f to the maximum number of unperturbed chains with N(arm) that can fill the volume occupied by the polymeric corona, hence this crossing is shown to occur when x=1. For x>1 we have colloidal Regime I with overcrowded volume, stretched arms and ΔU(hop) > k(B)T, while polymeric Regime II is linked to x

We combine linear viscoelastic measurements and modelling in order to explore the dynamics of blends of the same-molecular-weight ring and linear polymers in the regime of the low volume fraction (0.3 or lower) of the ring component. The stress relaxation modulus is affected by the constraint release (CR) of both rings and linear components due to the motion of linear chains. We develop a CR-based model of ring-linear blends that predicts the stress relaxation function in the low fraction regime of ring component in excellent agreement with experiments. Rings trapped by their entanglements with linear chains can only relax by linear-chain-induced constraint release, resulting in much slower relaxation of rings than of linear chains. The relative viscosity [Formula: see text] of the blend with respect to the linear melt viscosity η(L) at ring overlap volume fraction [Formula: see text] increases proportionally to the square root of ring molecular weight [Formula: see text]. Our experimental results clearly demonstrate that it is possible to enhance the viscosity and simultaneously the structural relaxation time of linear polymer melts by adding a small fraction of ring polymers. These results not only provide fundamental insights into the physics of the CR process but also suggest ways to fine-tune the flow properties of linear polymers by means of adding rings.

We present a systematic investigation of static and dynamic properties of block copolymer micelles with crosslinked cores, representing model polymer-grafted nanoparticles, over a wide concentration range from dilute regime to an arrested (crystalline) state, by means of light and neutron scattering, complemented by linear viscoelasticity. We have followed the evolution of their scattering intensity and diffusion dynamics throughout the non-ergodicity transition and the observed results have been contrasted against appropriately coarse-grained Langevin Dynamics simulations. These stable model soft particles of the core-shell type are situated between ultrasoft stars and hard spheres, and the well-known star pair interaction potential is not appropriate to describe them. Instead, we have found that an effective brush interaction potential provides very satisfactory agreement between experiments and simulations, offering insights into the interplay of softness and dynamics in spherical colloidal suspensions., Comment: This version corresponds only to the Supporting Information published in "Static and Dynamic Properties of Block-Copolymer Based Grafted Nanoparticles Across the Non-Ergodicity Transition". This information has been attached to the main manuscript, identified as arXiv:2011.06373

Polymer membranes are critical to many sustainability applications that require the size-based separation of gas mixtures. Despite their ubiquity, there is a continuing need to selectively affect the transport of different mixture components while enhancing mechanical strength and hindering aging. Polymer-grafted nanoparticles (GNPs) have recently been explored in the context of gas separations. Membranes made from pure GNPs have higher gas permeability and lower selectivity relative to the neat polymer because they have increased mean free volume. Going beyond this ability to manipulate the mean free volume by grafting chains to a nanoparticle, the conceptual advance of the present work is our finding that GNPs are spatially heterogeneous transport media, with this free volume distribution being easily manipulated by the addition of free polymer. In particular, adding a small amount of appropriately chosen free polymer can increase the membrane gas selectivity by up to two orders of magnitude while only moderately reducing small gas permeability. Added short free chains, which are homogeneously distributed in the polymer layer of the GNP, reduce the permeability of all gases but yield no dramatic increases in selectivity. In contrast, free chains with length comparable to the grafts, which populate the interstitial pockets between GNPs, preferentially hinder the transport of the larger gas and thus result in large selectivity increases. This work thus establishes that we can favorably manipulate the selective gas transport properties of GNP membranes through the entropic effects associated with the addition of free chains.

The thermodynamic and rheological properties of densely packed dendronized polymers (DPs) at water–air interfaces were studied here for first- and fourth-generation DPs (PG1, PG4) with both small (Pn ≈ 50) and large (Pn ≈ 500) backbone degrees of polymerization. The excellent control over the structural characteristics of these polymers enabled us to investigate how the interfacial properties change as we go from thin, flexible macromolecules toward thicker molecular objects that display colloidal features. The effects of the dendron generation, affecting the persistence length, as well as the degree of polymerization and surface pressure on the formation of DP layers at the water–air interface were studied. Surface pressure measurements and interfacial rheology suggest the existence of significant attractive interactions between the molecules of the higher generation DPs. While all DPs featured reproducible Π–A diagrams, successive compression–expansion cycles and surface pressure relaxation experiments ...

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. Those observations were made for physicochemically very different systems, having in common, however, the presence of self-associating units generating structures at large length scales. The nonmonotonic stress relaxation can be described phenomenologically by a generic model based on a redistribution of energy after the flow has stopped. When broken bonds are reestablished after flow cessation, the released energy is partly used to locally increase the elastic energy by the formation of deformed domains. If shear has induced order such that these elastic domains are partly aligned, the reestablishing of bonds gives rise to an increase of the overall stress. ispartof: Physical Review Letters vol:123 issue:21 ispartof: location:United States status: published

In the melt state at equilibrium, entangled nonconcatenated ring macromolecules adapt more compact conformations compared to their linear analogs and do not form an entanglement network. We show here that, when subjected to uniaxial stretching, they exhibit a unique response, which sets them apart from any other polymer. Remarkably, whereas both linear and ring polymers strain-harden, the viscosity of the rings increases dramatically (the melt thickens) at very low stretch rates due to the unraveling of their conformations along the stretching direction. At high rates, stretching leads to viscosity thinning similar to that of entangled linear polymers, albeit with subtle differences.

We investigate the influence of the environment on the relaxation dynamics of well-defined H-polymers diluted in a matrix of linear chains. The molar mass of the linear chain matrix is systematically varied and the relaxation dynamics of the H-polymer is probed by means of linear viscoelastic measurements, with the aim to understand its altered motion in different blends, compared to its pure melt state. Our results indicate that short unentangled linear chains accelerate the relaxation of both the branches and the backbone of the H-polymers by acting as an effective solvent. On the other hand, the relaxation of the H-polymer in an entangled matrix is slowed-down, with the degree of retardation depending on the entanglement number of the linear chains. We show that this retardation can be quantified by considering that the H-polymers are moving in a dilated tube at the rhythm of the motion of the linear matrix.

The state‐of‐the‐art understanding of entangled linear polymers is based on the concept of physical network formation from entanglements. The physical network is characterized by aplateau modulus in linear viscoelastic (LVE) measurements. However, linking the two free ends of a linear polymer, thereafter called a ring polymer, has dramatic consequences. For example, non-concatenated rings have much lower zero‐shear‐rate viscosity compared to their linear entangled counterparts. A plateau modulus is not observed in LVE measurements for ring polymers [1].Due to the difficulties in synthesis, which leads to very limited amount of samples, well‐defined ring polymers have never been studied in extensional flow. In this work, we present the first results of extensional rheology of a ring polystyrene (PS) melt with the molecular weight 185k(Ring‐185k). We show that the ring PS is surprisingly strain hardening in extensional flow, and reaches the same extensional steady state viscosity as its linear counterpart (Lin‐185k) when the stretch rate is fast enough. We further present the extensional rheology of blends made of Ring‐185k and Lin‐185k, with weight fraction of 5%, 20%, and 30% of Ring‐185k, respectively. We show that in the transient stress‐strain responses, stress overshoot is observed for the samples containing 20% and 30% Ring‐185k, while the stress overshoot is not observed for the pure Ring‐185k and Lin‐185k.The present results shed light into the fascinating flow properties of polymers without free ends, while they also advance the state‐of‐the‐art in polymer physics. At the same time, they open the route for understanding the response of folded proteins and chromosome territories under strong external fields.

The defense mechanism of hagfish against predators is based on its ability to form slime within a few milliseconds. Hagfish slime consists of two main components, namely mucin-like glycoproteins and long protein threads, which together entrap vast amounts of water and thus form a highly dilute hydrogel. Here, we investigate the mucin part of this hydrogel, in particular the role of the saline marine environment on the viscoelasticity and structure. By means of dynamic light scattering (DLS), shear and extensional rheology we probe the diffusion dynamics, the flow behavior, and the longest filament breaking time of hagfish mucin solutions. Using DLS we find a concentration-independent diffusion coefficient – characteristic for polyelectrolytes – up to the entanglement regime of 0.2 mg ml−1, which is about ten times higher than the natural concentration of hagfish mucin in hagfish slime. We also observe a slow relaxation process associated with clustering, probably due to electrostatic interactions. Shear rheology further revealed that hagfish mucin possesses pronounced viscoelastic properties at high concentrations (3 mg ml−1), showing that mucin alone achieves mechanical properties similar to those of natural hagfish slime (mucins and protein threads). The main effects of added seawater salts, and predominantly CaCl2 is to reduce the intensity of the slow relaxation process, which suggests that calcium ions lead to an ionotropic gelation of hagfish mucins., Soft Matter, 15 (42), ISSN:1744-683X, ISSN:1744-6848

The relaxation dynamics of molten monodisperse polymers are well described by tube-based theories in the linear viscoelastic regime. However, predicting their behavior under large shear rates is still a challenge today and many questions still need to be addressed. The objective of the present work is to investigate the role played by the molecular environment on the stretch and orientation state of linear polymer chains under strong shear flow. To this end, we diluted a long linear polystyrene (PS) of molar mass of 820 kg/mol into PS oligomers, at different concentrations varying from 5wt% to 50wt%. The transient shear behavior of these samples was then measured for a large range of shear rates (corresponding to Rouse-Weissenberg numbers from 0.09 to 90), using a strain controlled rotational rheometer equipped with a cone-partitioned plate geometry. From the comparison between the data and their analysis by means of the TMA tube-based model, we attempt at understanding and rationalizing the different scaling laws observed in the shear thinning regime, as well as the transient stress response and in particular the importance of the stress overshoot as function of the long chains concentration. We examine how shear flow affects the capability of the chains to create new entanglements and how this property depends on concentration. Our results provide a framework for developing a rigorous understanding the nonlinear shear rheology of entangled polymers.

We report on the viscoelastic properties of linear, monocyclic, and multicyclic polystyrenes with the same low molecular weight. All polymers investigated were found to exhibit unentangled dynamics. For monocyclic polymers without inner loops, a cyclic-Rouse model complemented by the contribution of unlinked chains (whose fraction was determined experimentally) captured the observed rheological response. On the other hand, multicyclic polymers with inner loops were shown to follow a hierarchical cyclic-Rouse relaxation with the outer loops relaxing first, followed by the inner loop relaxation. The influence of unlinked linear chains was less significant in multicyclic polymers with inner loops. The isofrictional zero-shear viscosity decreased with increasing number of constrained segments on the coupling sites, which was attributed to the decreasing loop size and the dilution effect due to the hierarchical relaxation.

We report on a novel synthetic route to synthesize relatively large quantities of polystyrene (PS) star polymers with targeted arm functionality and molar mass and their theological properties in the molten state. The synthetic route involves grafting styrene monomers onto a modified (aliphatic, alternating) polyketone backbone with a specific number of initiating grafting sites using controlled atom transfer radical polymerization (ATRP). Several polyketone precursors were used. This resulted in a large array of star polystyrenes with nonspherical cores and varying average arm length and number of arms. Their linear viscoelasticity was investigated and discussed in the context of the known response of anionically synthesized stars. Using a powerful characterization toolbox, including state-of-the-art interaction chromatography, rheometry, and tube modeling via the branch-on-branch (BoB) algorithm, we have assessed the viscoelasticity of these star polymers quantitatively. In particular, we have demonstrated a variability in molecular structure, which differs substantially from their anionically synthesized counterparts. Hence, whereas this new family of star polymers is not recommended for fundamental studies of polymer physics such as the molecular origin of relaxation mechanisms without prior extensive fractionation, they could be used in studies of mixtures as well as industrially relevant processing operations that require large amounts of polymeric stars.

We propose a simple, robust method to measure both the first and second normal stress differences of polymers, hence obtaining the full set of viscometric material functions in nonlinear shear flow. The method is based on the use of a modular cone-partitioned plate (CPP) setup with two different diameters of the inner plate, mounted on a rotational strain-controlled rheometer. The use of CPP allows extending the measured range of shear rates without edge fracture problems. The main advantage of such a protocol is that it overcomes limitations of previous approaches based on CPP (moderate temperatures not exceeding 120 °C, multiple measurements of samples with different volume) and yields data over a wide temperature range by performing a two-step measurement on two different samples with the same volume. The method was tested with two entangled polystyrene solutions at elevated temperatures, and the results were favorably compared with both the limited literature data on the second normal stress difference and the predictions obtained with a recent tube-based model of entangled polymers accounting for shear flow-induced molecular tumbling. Limitations and possible improvements of the proposed simple experimental protocol are also discussed.

Ultrasoft colloids typically do not spontaneously crystallize, but rather vitrify, at high concentrations. Combining in situ rheo--small-angle-neutron-scattering experiments and numerical simulations we show that shear facilitates crystallization of colloidal star polymers in the vicinity of their glass transition. With increasing shear rate well beyond rheological yielding, a transition is found from an initial bcc-dominated structure to an fcc-dominated one. This crystal-to-crystal transition is not accompanied by intermediate melting but occurs via a sudden reorganization of the crystal structure. Our results provide a new avenue to tailor colloidal crystallization and the crystal-to-crystal transition at the molecular level by coupling softness and shear.

International audience; The equilibrium mechanical properties of a cross-linked gel of telechelic star polymers are studied by rheology and Brownian dynamics simulations. The Brownian dynamics model consists of cores to which Rouse arms are attached. Forces between the cores are obtained from a potential of mean force model developed by Likos and co-workers. Both experimentally and in the simulations, networks were created by attaching sticker groups to the ends of the arms of the polymers, which were next allowed to form bonds among them in a one to one fashion. Simulations were sped up by solving the Rouse dynamics exactly. Moreover, the Rouse model was extended to allow for different frictions on different beads. In order to describe the rheology of the non-cross-linked polymers, it had to be assumed that bead frictions increase with increasing bead number along the arms. This friction model could be transferred to describe the rheology of the network without any adjustments other than an overall increase of the frictions due to the formation of bonds. The slowing down at intermediate times of the network rheology compared to that of the non-cross-linked polymers is well described by the model. The percentage of stickers involved in forming inter-star bonds in the system was determined to be 25%, both from simulations and from an application of the Green Tobolsky relation to the experimental plateau value of the shear relaxation modulus. Simulations with increasing cross-link percentages revealed that on approaching the gel transition the shear relaxation modulus develops an algebraic tail, which gets frozen at a percentage of maximum cross-linking of about 11%.

We investigate the linear and nonlinear viscoelastic response of a supramolecular gelator formed by association of a bisurea monomer (EHUT) via hydrogen bonds, in a nonpolar organic solvent (dodecane). Past experimental studies reported contradictory results concerning the concentration dependence and the magnitude of the terminal relaxation time. The discrepancies among data were attributed to the presence of water and, in particular, its chain stopper effects on the EHUT supramolecular assembly. Here, we present new measurements with this system under conditions of controlled humidity, achieved by means of a simple setup developed for this purpose. We resolve the discrepancy and demonstrate that humidity (both during sample storage and measurement) can substantially affect the linear viscoelastic response (frequency spectra). The magnitude of the terminal relaxation time, τt, is significantly reduced (by as much as 1 order of magnitude) and its concentration dependence follows τt ∼ c0.77 ± 0.06. This scaling differs from the theoretical prediction with nearly double exponent (1.25). We tentatively attribute this difference to the polydispersity in the supramolecular chain length, which results mainly from the EHUT self-assembly association mechanism, and propose that constraint release effects of smaller chains should be considered as they may weaken the concentration dependence of the terminal relaxation. On the other hand, the magnitude of the plateau modulus G0 is affected by humidity to a lesser degree (not exceeding 50%), whereas its concentration dependence is in accordance with predictions, G0 ∼ c2 ± 0.21. Finally, the nonlinear viscoelastic properties are also affected, again experiencing weakening in the presence of humidity.We investigate the linear and nonlinear viscoelastic response of a supramolecular gelator formed by association of a bisurea monomer (EHUT) via hydrogen bonds, in a nonpolar organic solvent (dodecane). Past experimental studies reported contradictory results concerning the concentration dependence and the magnitude of the terminal relaxation time. The discrepancies among data were attributed to the presence of water and, in particular, its chain stopper effects on the EHUT supramolecular assembly. Here, we present new measurements with this system under conditions of controlled humidity, achieved by means of a simple setup developed for this purpose. We resolve the discrepancy and demonstrate that humidity (both during sample storage and measurement) can substantially affect the linear viscoelastic response (frequency spectra). The magnitude of the terminal relaxation time, τt, is significantly reduced (by as much as 1 order of magnitude) and its concentration dependence follows τt ∼ c0.77 ± 0.06. This sc...

Glasses formed from nano- and microparticles form a fascinating testing ground to explore and understand the origins of vitrification. For atomic and molecular glasses, a wide range of fragilities have been observed; in colloidal systems, these effects can be emulated by adjusting the particle softness. The colloidal glass transition can range from a superexponential, fragile increase in viscosity with increasing density for hard spheres to a strong, Arrhenius-like transition for compressible particles. However, the microscopic origin of fragility and strength remains elusive, both in the colloidal and in the atomic domains. Here, we propose a simple model that explains fragility changes in colloidal glasses by describing the volume regulation of compressible colloids in order to maintain osmotic equilibrium. Our simple model provides a microscopic explanation for fragility, and we show that it can describe experimental data for a variety of soft colloidal systems, ranging from microgels to star polymers and proteins. Our results highlight that the elastic energy per particle acts as an effective fragility order parameter, leading to a universal description of the colloidal glass transition.

We present unique nonlinear shear and extensional rheology data of unentan-gled amorphous polyester ionomers based on polyethers and sulphonated phthalates with sodium/ lithium counterions. Previous linear viscoelastic (LVE) measurements1 showed significant elasticity in these ionomers due to formation of strong ionic aggre-gates. These ionomer melts exhibit viscoelastic properties similar to well-entangled melts with an extended rubbery plateau. To evaluate the effects of nonlinear deforma-tion, the rheology of these ionomers was investigated using uniaxial extension and shear. The measurements were performed on an updated filament stretching rheome-ter and on a strain controlled shear rheometer equipped with a cone-partitioned-plate (CPP) setup. In extension, ionomer samples exhibited a decreasing strain hardening trend with increasing extension rates. The presence of high solvating poly (ethylene oxide) (PEO) along the backbone in the coionomer with poly ( tetramethylene glycol) PTMO, increases the maximum Hencky strain at fracture thus adding ductility to the brittle PTMO-Na ionomer. As a result, the coionomer deforms much more compared to PTMO-Na but both fracture eventually. On the other hand, whereas PTMO-Na can-not be sheared due to slip at the wall, the coionomer deforms in shear and eventually suffers from edge fracture instabilities. From the above, a picture emerges suggesting that PEO coionomers enhance ductility, make fracture smoother and offers a compro-mise of mechanical performance and ion conduction.

A library of poly(methyl methacrylate)-based dendronized polymers with generation numbers g = 1-3 was prepared, which contain different degrees of dendritic substitution (0-50%) with strongly hydrogen bonding 2-ureido-4[1H]-pyrimidinone (UPy) moieties at their respective g = 1 levels. Our rheological and thermal studies demonstrate that the strong intermolecular UPy interactions are suppressed for g = 2 and essentially eliminated for g = 3. Focusing on samples with short backbone degrees of polymerization (P-n approximate to 40), the linear viscoelastic response alters from liquid-like in the absence of UPy to gel-like with ever increasing moduli as the UPy content increases. Nonlinear rheological measurements indicate a transition from ductile to brittle behavior and, in parallel, a transition from shear strain thinning to shear strain hardening. This unique behavior makes UPy-DPs promising candidates for the design of new functional materials.

We form films of carboxylated polystyrene particles (C-PS) at the air-water interface and investigate the effect of subphase pH on their structure and rheology by using a suite of complementary experimental techniques. Our results suggest that electrostatic interactions drive the stability and the structural order of the films. In particular, we show that by increasing the pH of the subphase from 9 up to 13, the films exhibit a gradual transition from solid to liquidlike, which is accompanied by a loss of the long-range order (that characterizes them at lower values of pH). Direct optical visualization of the layers, scanning electron microscopy, and surface pressure isotherms indicate that the particles deposited at the interface form three-dimensional structures involving clusters, with the latter being suppressed and a quasi-2D particle configuration eventually reached at the highest pH values. Evidently, the properties of colloidal films can be tailored significantly by altering the pH of the subphase.

We present a systematic investigation of well-characterized, experimentally pure polystyrene (PS) rings with molar mass of 161 000 g/mol in dilute solutions. We measure the ring form factor at θ - and good-solvent conditions as well as in a polymeric solvent (linear PS of roughly comparable molar mass) by means of small-angle neutron scattering (SANS). Additional dynamic light scattering (DLS) measurements support the SANS data and help elucidate the role of solvent quality and solution preparation. The results indicate the increase of ring dimensions as the solvent quality improves. Furthermore, the experimental form factors in both θ -solvent and linear matrix behave as ideal rings and are fully superimposable. The nearly Gaussian conformations of rings in a melt of linear chains provide evidence of threading of linear chains through rings. The latter result has implications for the dynamics of ring-linear polymer mixtures.

Semifluorinated alkanes (C(n)F(2n+1)C(m)H(2m+1)), short FnHm display local phase separation of mutually incompatible hydrocarbon and fluorocarbon chain moieties, which has been utilized as a structure-forming motif in supramolecular architectures. The packing of semifluorinated alkanes, nominally based on dodecyl subunits, such as perfluoro(dodecyl)dodecane (F12H12) and perfluoro(dodecyl)eicosane (F12H20), as well as a core extended analogue, 1,4-dibromo-2-((perfluoroundecyl)methoxy)-5-(dodecyloxy)benzene) (F11H1-core-H12), was studied at the air/water interface. Langmuir monolayers were investigated by means of neutron reflectivity directly at the air/water interface and scanning force microscopy after transfer to silicon wafers. Narrowly disperse surface micelles formed in all three cases; however, they were found to bear different morphologies with respect to molecular orientation and assembly dimensionality, which gives rise to different hierarchical aggregate topologies. For F12H12, micelles of ca. 30 nm in diameter, composed of several circular or "spherical cap" substructures, were observed and a monolayer model with the fluorocarbon block oriented toward air is proposed. F12H20 molecules formed larger (ca. 50 nm diameter) hexagonally shaped surface micelles that were hexagonally, densely packed, besides more elongated but tightly interlocked wormlike structures. Conversely, F11H1-core-H12 films organized into linear rows of elongated surface micelles with comparable width, but an average length of ca. 400 nm, apparently formed by antiparallel molecular packing.

We investigated the rheology of a series of anionically synthesized, model symmetric Cayley tree poly(methylmethacrylates) having from 1 to 4 generations of identical molar mass. The hierarchical relaxation of the different generations was assessed from the linear data by accounting for both the plateau modulus and the characteristic relaxation times. Using a tube-model time-marching analysis based on the concept of hierarchy of motion, we described quantitatively the frequency spectra without adjustable parameters. We also performed uniaxial extensional measurements using the Sentmanat extensional rheometer fixture. The samples tested exhibited significant strain hardening compared to their linear analogs at lower and intermediate Hencky strain rates. This hardening was more pronounced with increasing number of generations, hence branch points. The extracted effective steady extensional viscosity was found to scale with the extensional rate with a power exponent of about -0.5, in agreement with earlier findings with linear polystyrenes. We also extended the time-marching-algorithm to predict the extensional behavior of these polymers using the conceptual framework developed recently for pom-pom and Cayley-tree polymers. Comparison between theoretical and experimental results was satisfactory when the maximum stretch which can be supported by the molecule was estimated by accounting for the recently proposed inter-chain pressure effects. (C) 2010 The Society of Rheology. [DOI: 10.1122/1.3368724]

Stable block copolymer micelles with crosslinked rubbery cores serve as model soft colloids with tunable rheology. We demonstrate this by studying systematically two such micelles: one with a small polyisoprene core and long polystyrene hairs that dominate the response of the system, and another with a larger polybutadiene core and shorter polystyrene hairs, exhibiting colloid-like behavior. In addition to mass concentration, we use temperature as a means to vary the volume fraction and show that achieving the same volume fraction by these two different routes yields different macroscopic properties. The number density of the particles and osmotic pressure effects are the origin of the difference. The response of these micelles to varying solvent quality conditions has unexpected rheological consequences: remarkably, for concentrated colloid-like micelles, the viscosity exhibits a non-monotonic temperature dependence. This is attributed to particle swelling and changing particle interactions with temperature. It is thus evident that such systems provide ways for tailoring the flow of soft colloids.

We use multi-arm star polymers as model soft colloids with tuneable interactions and explore their behaviour in the glassy state. In particular, we perform a systematic rheological study with a well-defined protocol and address aspects of ageing and shear melting of star glasses. Ageing proceeds in two distinct steps: a fast step of O (10 3 s) and a slow step of O (10 4 s). We focus on creep and recovery tests, which reveal a rich, albeit complex response. Although the waiting time, the time between pre-shear (rejuvenation) of the glassy sample and measurement, affects the material’s response, it does not play the same role as in other soft glasses. For stresses below the yield value, the creep curve is divided into three regimes with increasing time: viscoplastic, intermediate steady flow (associated with the first ageing step) and long-time evolving elastic solid. This behaviour reflects the interplay between ageing and shear rejuvenation. The yield behaviour, as investigated with the stress-dependent recoverable strain, indicates a highly nonlinear elastic response intermediate between a low-stress Hookean solid and a high-stress viscoelastic liquid, and exemplifies the distinct characteristics of this class of hairy colloids. It appears that a phenomenological classification of different colloidal glasses based on yielding performance may be possible.

International audience; Thermoplastic elastomers (TPEs) based on segmented block-copolymers containing poly(tetrahydrofuran) (pTHF) and terephthalate-based diamide groups (T4T) were synthesized via poly-condensation. While pTHF is known to be flexible and amorphous at rest, the more rigid T4T crystallized in different ways depending on both chain composition and sample preparation conditions. Increasing the content of hard-segments (HS) from 5 to 20% in weight leads to a substantial increase of the melting point T-m by more than 60 degrees C. We have systematically investigated the multiscale (1 A - 50 nm) organization of the HSs (of fractions from 5% to 20%) by means of DSC, WAXS and (ultra) small angle X-ray scattering (U)SAXS. By increasing HS content, hence the rigidity of the chain, scattering experiments unambiguously show the formation of bigger and better defined ribbon-like crystallites, as well as the densification of the network they form. We propose a scenario for rationalizing the local T4T-HSs packing and the crystallites anisotropy at the mesoscale (1-10 nm) for HS fraction above 5%. Moreover, following in-situ the crystallization of TPEs with large HS fraction (20%), we highlight the presence of ``persistent aggregates'' present at T > T-m and study the ribbon-like crystallites growth mechanism during cooling from the melt state. (C) 2016 Elsevier Ltd. All rights reserved.

We present systematic, unique linear and nonlinear shear rheology data of an experimentally pure ring polystyrene and its linear precursor. This polymer was synthesized anionically and characterized by interaction chromatography and fractionation at the critical condition. Its weight-average molar mass is 84 kg/mol; i.e., it is marginally entangled (entanglement number Z ≈ 5). Its linear viscoelastic response appears to be better described by the Rouse model (accounting for ring closure) rather than the lattice-animal-based model, suggesting a transition from unentangled to entangled ring dynamics. The failure of both models in the terminal region may reflect the remaining unlinked linear contaminants and/or ring–ring interpenetration. The viscosity evolution at different shear rates was measured using a homemade cone-partitioned plate fixture in order to avoid edge fracture instabilities. Our findings suggest that rings are much less shear thinning compared to their linear counterparts, whereas both obey...

A series of homologous dendronized polymers (DPs) with generations (g) 1–3 and backbone nominal degrees of polymerization (Pn) in the range 50–3000 have been synthesized and characterized in order to investigate the g and Pn dependent viscoelastic properties and packing of this class of densely grafted associating and effectively “thick” macromolecules in their molten state. Rheological measurements reveal an unusually long thermal equilibration time attributed to (i) the tendency of DPs to minimize local density gradients as realized via their mutual weak interpenetration and (ii) the intermolecular DP–DP correlations and inter and intramolecular hydrogen bonding and p–p stacking interactions. With the help of simulations and X ray scattering measurements a scenario emerges according to which DPs interact via local cooperative rearrangements of the dendrons akin to a Velcro fastening process. In this picture neighboring bonds accelerate the local interpenetration process. Results from X ray scattering show increased lateral backbone–backbone correlations with a columnar arrangement of backbones and a liquid crystalline underlying order. Linear viscoelasticity is characterized by plateau moduli which originate from intermolecular bonding and whose extent in frequency and absolute value depends on Pn and g and can be lower than or comparable to that of the backbone. Very long relaxation times can be probed (sometimes via creep measurements) and attributed to the lifetime of the bonds. The nonlinear shear rheology data suggest a resemblance in behavior to unentangled linear chains with finite extensibility and point to reduced deformability of the DPs in flow. These findings indicate that DPs constitute a promising class of functional macromolecules with tunable properties.

In the linear viscoelastic regime, multiarm star chains having more than 24 arms bound by a soft core exhibit fast relaxation due to motion of individual arms and slow relaxation related to motion of the soft cores (structural rearrangement in a liquidlike order). In this regard, the multiarm stars are qualitatively similar to randomly dispersed polystyrene−polyisoprene (PS−PI) and polybutadiene−polystyrene (PB−PS) copolymer micelles, the former having rigid (glassy) PS cores and the latter with soft (rubbery) PB cores. However, the multiarm star PB chains and the copolymer micelles were found to exhibit significant rheological differences in the nonlinear regime possibly because of the differences in their core−arm (core−corona) compatibility and core softness. In long time scales, the nonlinear damping of the relaxation modulus measured for the multiarm star PB was much weaker than that for the micelles and somewhat similar to the damping of a critical gel. In contrast, the damping in short time scales ...

The flow of aqueous cellulose ether solutions through a bead packing is investigated using magnetic resonance imaging and filtration measurements. A rather complex behavior dominated by jamming (clogging) and unjamming phenomena in time is observed. With the help of several characterization techniques (laser grain sizing, dynamic light scattering, optical microscopy, and rheometry), we find that the particular methyl(hydroxyethyl) cellulose prepared with a specific protocol, tends to form aggregates in water, even at the lowest achievable concentration. These aggregates are highly polydisperse, ranging from 100 nm to 100 μm in size, and are deformable. Their origin appears to be the hydrophobic links among molecules and the related local crystallization. It is suggested that these features play a key role in the observed jamming/unjamming during filtration tests. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3923–3935, 2015

We present extensive experimental and theoretical investigations on the structure, phase behavior, dynamics and rheology of model soft–hard colloidal mixtures realized with large, multiarm star polymers as the soft component and smaller, compact stars as the hard one. The number and length of the arms in star polymers control their softness, whereas the size ratio, the overall density and the composition are additional parameters varied for the mixtures. A coarse-grained theoretical strategy is employed to predict the structure of the systems as well as their ergodicity properties on the basis of mode coupling theory, for comparison with rheological measurements on the samples. We discovered that dynamically arrested star–polymer solutions recover their ergodicity upon addition of colloidal additives. At the same time the system displays demixing instability, and the binodal of the latter meets the glass line in a way that leads, upon addition of a sufficient amount of colloidal particles, to an arrested phase separation and reentrant solidification. We present evidence for a subsequent solid-to-solid transition well within the region of arrested phase separation, attributed to a hard-sphere-mixture type of glass, due to osmotic shrinkage of the stars at high colloidal particle concentrations. We systematically investigated the interplay of star functionality and size ratio with glass melting and demixing, and rationalized our findings by the depletion of the big stars due to the smaller colloids. This new depletion potential in which, contrary to the classic colloid–polymer case, the hard component depletes the soft one, has unique and novel characteristics and allows the calculation of phase diagrams for such mixtures. This work covers a broad range of soft–hard colloidal mixture compositions in which the soft component exceeds the hard one in size and provides general guidelines for controlling the properties of such complex mixtures.

In the last two decades, advances in synthetic, experimental and modeling/simulation methodologies have considerably enhanced our understanding of colloidal suspension rheology and put the field at the forefront of soft matter research. Recent accomplishments include the ability to tailor the flow of colloidal materials via controlled changes of particle microstructure and interactions. Whereas hard sphere suspensions have been the most widely studied colloidal system, there is no richer type of particles than soft colloids in this respect. Yet, despite the remarkable progress in the field, many outstanding challenges remain in our quest to link particle microstructure to macroscopic properties and eventually design appropriate soft composites. Addressing them will provide the route towards novel responsive systems with hierarchical structures and multiple functionalities. Here we discuss the key structural and rheological parameters which determine the tunable rheology of dense soft deformable colloids. We restrict our discussion to non-crystallizing suspensions of spherical particles without electrostatic or enthalpic interactions.

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.

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.

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