Transition from confined to bulk dynamics in symmetric star-linear polymer mixtures

We report on the linear viscoelastic properties of mixtures comprising multiarm star (as model soft colloids) and long linear chain homopolymers in a good solvent. In contrast to earlier works, we investigated symmetric mixtures (with a size ratio of 1) and showed that the polymeric and colloidal responses can be decoupled. The adopted experimental protocol involved probing the linear chain dynamics in different star environments. To this end, we studied mixtures with different star mass fraction, which was kept constant while linear chains were added and their entanglement plateau modulus ($G_p$) and terminal relaxation time ($\tau_d$) were measured as functions of their concentration. Two distinct scaling regimes were observed for both $G_p$ and $\tau_d$: at low linear polymer concentrations, a weak concentration dependence was observed, that became even weaker as the fraction of stars in the mixtures increased into the star glassy regime. On the other hand, at higher linear polymer concentrations, the classical entangled polymer scaling was recovered. Simple scaling arguments show that the threshold crossover concentration between the two regimes corresponds to the maximum osmotic star compression and signals the transition from confined to bulk dynamics. These results provide the needed ingredients to complete the state diagram of soft colloid-polymer mixtures and investigate their dynamics at large polymer-colloid size ratios. They also offer an alternative way to explore aspects of the colloidal glass transition and the polymer dynamics in confinement. Finally, they provide a new avenue to tailor the rheology of soft composites.Comment: 9 Figure

Depletion gels from dense soft colloids: Rheology and thermoreversible melting

Truzzolillo, D., Vlassopoulos, D., Munam, A., & Gauthier, M. (2014). Depletion gels from dense soft colloids: Rheology and thermoreversible melting. Journal of Rheology, 58(5), 1441–1462. https://doi.org/10.1122/1.4866592Upon addition of small nonadsorbing linear polymers, colloidal glasses composed of large hard spheres melt and eventually revitrify into the so-called attractive glass regime. We show that, when replacing the hard spheres by star polymers representing model soft particles, a reentrant gel is formed. This is the result of compression and depletion of the stars due to the action of the osmotic pressure from the linear homopolymers. The viscoelastic properties of the soft dense gel were studied with emphasis on the shear-induced yielding process, which involved localized breaking of elements with a size of the order of the correlation length. Based on these results, a phenomenological attempt was made at describing the universal rheological features of colloid/nonadsorbing polymer mixtures of varying softness. The star gel was found to undergo thermoreversible melting, despite the fact that conventional hard-sphere depletion gels are invariant to heating. This phenomenon is attributed to the hybrid internal microstructure of the stars, akin to a dry-to-wet brush transition, and is characterized by slow kinetics, on the time scale of the osmotic gel formation process. These results may be useful in finding generic features in colloidal gelation, as well as in the molecular design of new soft composite materials.Financial support from the EU (ITN-COMPLOIDS FP7-234810, FP7 Infrastructure ESMI, GA 262348 and FP7-SMALL-Nanodirect CP-FP-213948) and the Natural Science and Engineering Research Council of Canada (NSERC) is gratefully acknowledged

Wall slip in primitive chain network simulations of shear startup of entangled polymers and its effect on the shear stress undershoot

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 figure

Glassy States in Asymmetric Mixtures of Soft and Hard Colloids

© 2013 American Physical Society, available at: Truzzolillo, D., Marzi, D., Marakis, J., Capone, B., Camargo, M., Munam, A., … Vlassopoulos, D. (2013). Glassy States in Asymmetric Mixtures of Soft and Hard Colloids. Physical Review Letters, 111(20). https://doi.org/10.1103/PhysRevLett.111.208301By 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.This work has been supported by the EU (ITN-COMPLOIDS Grant No. 234810) and by the J. S. Latsis Foundation (Grant No. 0839-2012)

Asymmetric soft-hard colloidal mixtures: osmotic effects, glassy states and rheology

The following article has been accepted by Journal of Rheology. After it is published, it will be found at http://sor.scitation.org/journal/jorWhereas mixtures of colloids and non-adsorbing polymers have been studied in great detail in the last two decades, binary colloidal mixtures have not received much attention. Yet, fragmental evidence from asymmetric mixtures of hard spheres indicates a wide-ranging, complex behavior from liquid to crystal to single glass and to double glass, and respective rich rheology. Recently, we addressed the question of softness by investigating a mixture of soft and virtually hard colloidal spheres. We found an unprecedented wealth of states including repulsive single glass (RG), liquid, arrested phase separation (APS) and double glass (DG). This is a consequence of the coupling of softness and osmotic forces due to the hard component. We now report on the rheology of the different states with emphasis on the nonlinear response during start-up of stress at constant rate, its relaxation upon flow cessation, and large amplitude oscillatory shearing. Distinct features are identified, whereas comparison with single-colloid (soft or hard) glasses reveals some phenomenological universalities in yielding, residual stresses and periodic intra-cycle stress response. In brief, the DG exhibits much larger yield and residual stresses as compared to the RG and APS, whereas the yield strain is the same for all states. Two-step yielding is unambiguously evidenced for the APS whereas both yield stress and strain exhibit a weak dependence on Péclet number. Large amplitude oscillatory tests reveal large value of the intrinsic nonlinear parameters, reflecting the role of colloidal interactions. Moreover, RG exhibits intra-cycle stress overshoots, a feature that characterizes most of the soft glassy materials formed by interpenetrable particles and that vanishes as hard (nearly impenetrable) colloids are added in the mixtures. These results demonstrate the sensitivity of linear and nonlinear rheology to colloidal state transitions and, more importantly, the power of entropic mixing as a means to tailor the flow properties, hence performance and handling of soft composites.EU FP7 Infrastructure ESMI || GA262348 ITN SOMATAI || GA316866 Horizon 2020 COLLDENSE || GA642774 Natural Science and Engineering Research Council of Canada (NSERC

Network dynamics in nanofilled polymers

It is well accepted that adding nanoparticles (NPs) to polymer melts can result in significant property improvements. Here we focus on the causes of mechanical reinforcement and present rheological measurements on favourably interacting mixtures of spherical silica NPs and poly(2-vinylpyridine), complemented by several dynamic and structural probes. While the system dynamics are polymer-like with increased friction for low silica loadings, they turn network-like when the mean face-to-face separation between NPs becomes smaller than the entanglement tube diameter. Gel-like dynamics with a Williams-Landel-Ferry temperature dependence then result. This dependence turns particle dominated, that is, Arrhenius-like, when the silica loading increases to similar to 31 vol%, namely, when the average nearest distance between NP faces becomes comparable to the polymer's Kuhn length. Our results demonstrate that the flow properties of nanocomposites are complex and can be tuned via changes in filler loading, that is, the character of polymer bridges which 'tie' NPs together into a network.We thank Leon Serc (ETH Zurich) for help with FTIR. Enlightening discussions with Ulrich Jonas are gratefully acknowledged. Partial support has been provided by the EU FP7 (ETN Supolen GA-607937, Infrastructure ESMI GA-262348) and the Greek General Secretariat for Research and Technology (Thalis-380238 COVISCO). M.R. acknowledges financial support from the National Science Foundation under grants DMR-1309892, DMR-1436201 and DMR-1121107, the National Institutes of Health under grants P01-HL108808 and 1UH2HL123645 and the Cystic Fibrosis Foundation. D.Z., S.G., R.H.C. and S.K.K. gratefully acknowledge the National Science Foundation grant DMR-1408323 for financial support

Rheological Link Between Polymer Melts with a High Molecular Weight Tail and Enhanced Formation of Shish-Kebabs

Presence of an ultra high molecular weight (UHMw) fraction in flowing polymer melts is known to facilitate formation of oriented crystalline structures significantly. The UHMw fraction manifests itself as a minor tail in the molar mass distribution and is hardly detectable in the canonical characterization methods. In this study, alternatively, we demonstrate how the nonlinear extensional rheology reveals to be a very sensitive characterization tool for investigating the effect of the UHMw-tail on the structural ordering mechanism. Samples containing a UHMw-tail relative to samples without, exhibit a clear increase in extensional stress that is directly correlated with the crystalline orientation of the quenched samples. Extensional rheology, particularly, in combination with linear creep measurements, thus, enables the conformational evolution of the UHMw-tail to be studied and linked to the enhanced formation of oriented structures

Short and soft: multi-domain organization, tunable dynamics and jamming in suspensions of grafted colloidal cylinders with small aspect ratio

The yet virtually unexplored class of soft colloidal rods with small aspect ratio is investigated and shown to exhibit a very rich phase and dynamic behavior, spanning from liquid to nearly melt state. Instead of nematic order, these short and soft nanocylinders alter their organization with increasing concentration from isotropic liquid with random orientation to one with preferred local orientation and eventually a multi-domain arrangement with local orientational order. The latter gives rise to a kinetically suppressed state akin to structural glass with detectable terminal relaxation, which, on increasing concentration reveals features of hexagonally packed order as in ordered block copolymers. The respective dynamic response comprises four regimes, all above the overlapping concentration of 0.02 g/ml: I) from 0.03 to 0.1 g/mol the system undergoes a liquid-to-solid like transition with a structural relaxation time that grows by four orders of magnitude. II) from 0.1 to 0.2 g/ml a dramatic slowing-down is observed and is accompanied by an evolution from isotropic to multi-domain structure. III) between 0.2 and 0.6 g/mol the suspensions exhibit signatures of shell interpenetration and jamming, with the colloidal plateau modulus depending linearly on concentration. IV) at 0.74 g/ml in the densely jammed state, the viscoelastic signature of hexagonally packed cylinders from microphase-separated block copolymers is detected. These properties set short and soft nanocylinders apart from long colloidal rods (with large aspect ratio) and provide insights for fundamentally understanding the physics in this intermediate soft colloidal regime, as well as and for tailoring the flow properties of non-spherical soft colloids