Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y19: Focus Session: Polymer Colloids: Dynamics |
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Sponsoring Units: DPOLY DCMP Chair: Ken Schweizer, University of Illinois, Urbana-Champaign Room: B118-B119 |
Friday, March 19, 2010 8:00AM - 8:36AM |
Y19.00001: Multiarm Star Polymers as Model Soft Colloids Invited Speaker: Over the last decade, star polymers emerged as a useful model colloids that interpolate between polymers and hard sphere colloids. Together with microgels, they represent two benchmark soft colloidal systems, their internal structure being the key difference. Indeed, in the case of stars with open structure, the arms can interpenetrate in dense suspensions. The latter feature, that can be probed experimentally, is responsible for a number of interesting structural and dynamic properties of star polymers that set them apart from microgels. In this talk we present the basic properties of star polymers and focus on their extraordinary behavior in the highly concentrated regime, which is typically glassy. Our rheological and scattering experiments demonstrate unique features of the star glasses. Here we discuss two major ones: (i) Aging after pre-shear (the so-called rejuvenation) proceeds via a two-step process, associated with a fast arm engagement and a slow cooperative (cage) rearrangement. Remarkably, at extremely long times a steady state is observed and the terminal time in these systems can be experimentally accessible (and hence tailored at molecular level), as a consequence of the arms fluctuations. (ii) Multiple glassy states can be obtained when mixing stars with polymers or with other stars. Simultaneous theoretical and simulations work suggests that the softness is at the core of this unexpected behavior where depletion gives rise to glass melting and eventually re-entrant glasses are formed. Construction of a state diagram suggests kinetic pathways for tailoring the flow of soft colloids. These examples outline the importance of particle architecture on colloidal properties. Stars are a representative of a large class of hairy particles. The parallel important developments in mode coupling theory and its verses provide much needed predictive tools and rationalization for a number of phenomena such as those discussed here, as well as the complex rheological response. A wide range of applications in this exciting, fast growing field appear to emerge. Parts of this work reflect collaboration with M. Cloitre (ESPCI), B. Erwin (FORTH/ESPCI), C. N. Likos (Duesseldorf), G. Petekidis (FORTH), F. Sciortino (Rome), E. Stiakakis (FORTH), and E. Zaccarelli (Rome). Synthesis of particles by N. Hadjichristidis (Athens), M. Gauthier (Waterloo) and J. Roovers (NRC). [Preview Abstract] |
Friday, March 19, 2010 8:36AM - 8:48AM |
Y19.00002: Slow dynamics, ageing and crystallization of a multiarm star glass: New routes to equilibrium? Emmanuel Stiakakis, Agnieszka Wilk, Joachim Kohlbrecher, Dimitris Vlassopoulos, George Petekidis Multiarm star polymers are model systems with tunable intermediate colloid to polymer-like character, exhibiting rich phase behaviour, internal relaxations and flow properties [1]. 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 ageing, of the multiam star glass. This delayed glass to crystal transition establishes a novel pathway for star crystallization that might be generic in colloidal glasses [2]. \\[4pt] [1] Likos C.N., Phys. Rep., 348, 267, (2001). \\[0pt] [2] E. Zaccarelli et al., Phys Rev. Lett., 103, 135704, (2009). [Preview Abstract] |
Friday, March 19, 2010 8:48AM - 9:00AM |
Y19.00003: Activated transport, tunable fragility and thermal jamming in microgel suspensions Ken Schweizer, Jian Yang Microgels composed of crosslinked polymers are deformable objects of controllable softness that exhibit glassy dynamics that is a highly variable function of suspension volume fraction and single particle modulus. We have developed a microscopic theory for this class of polymer colloids that relates interparticle repulsive forces, structure, elasticity and relaxation. A kinetic arrest diagram is determined in the reduced temperature versus volume fraction plane; no activated dynamics regime is predicted below a critical microgel stiffness or above a critical temperature. As the microgel becomes softer the dynamical slowing down with growing volume fraction increasingly resembles a strong glass former. At fixed concentration, an increase of particle stiffness results in a broad range of dynamic fragilities which partition into two qualitatively different categories. A soft jamming crossover is predicted beyond which local packing order is reduced and the elastic modulus follows a gas-like concentration dependence. Comparison of the theoretical results with recent mechanical and relaxation experiments has been performed. [Preview Abstract] |
Friday, March 19, 2010 9:00AM - 9:12AM |
Y19.00004: Slow dynamics, elasticity and soft jamming of dense suspensions of many arm stars and microgels Jian Yang, Kenneth Schweizer The naive mode coupling theory and nonlinear Langevin equation theory of activated dynamics is applied to suspensions of tunable soft repulsive colloids composed of many arm star-polymers and crosslinked microgels. For stars, the volume fraction for the onset of activated dynamics is a non-monotonic function of the number of polymer arms. The barrier for thermally activated hopping grows roughly linearly with arm number, the dynamic fragility decreases as particles become softer, and the shear modulus follows a power law dependence on volume fraction with an apparent exponent that grows with arm number. At very high volume fractions where the stars interpenetrate, the value of the pair correlation function near contact goes through a maximum, the activation barrier saturates, and the modulus becomes a linear function of concentration. This dramatic ``soft jamming'' crossover bears some similarity to jamming phenomena in fluids of crosslinked microgels but with distinctive differences due to the formation of overlapping clusters in star suspensions. [Preview Abstract] |
Friday, March 19, 2010 9:12AM - 9:24AM |
Y19.00005: The Rheology of Concentrated Cross-Linker-Free Polymeric Microgels: Particulate Dispersions or Polymer Solutions? Zhiyong Meng, Chinedum Osuji Microgel particles based on N-isopropylacrylamide and acrylic acid have been synthesized using emulsifier-free radical precipitation polymerization without a cross-linking agent. The resulting particles display very low modulus with pH-, ionic strength-, and thermo-sensitivity. The absence of a cross-linking agent results in dramatic volume \newline shrinkage and expansion in response to the aforementioned stimuli. We study the concentration and temperature dependent rheology of concentrated dispersions of these particles. Several features emerge, including a marked dependence of the yield strain on temperature as well as aging of the complex viscosity over time. We discuss these results in the context of the rheology of stiff particles and that of branched polymer solutions. [Preview Abstract] |
Friday, March 19, 2010 9:24AM - 9:36AM |
Y19.00006: ABSTRACT HAS BEEN MOVED TO X19.00013 |
Friday, March 19, 2010 9:36AM - 9:48AM |
Y19.00007: Unusual microscopic dynamics in melts of star-like polymer grafted nanoparticles and their binary mixtures Jaydeep Basu, S. Srivastava, Sivasurender C, A. Kandar, Sarika C, S. Narayanan, A. Sandy Star polymers have attracted wide attention due to their fascinating structural, dynamical and rheological behavior including observation of multiple glassy states in concentrated solutions [1]. We have shown recently [2] that the microscopic dynamics in melts of a novel type of star-like polymers created by grafting of linear polymer chains on nanoparticle surfaces shows an unusual dynamical arrest in the case of low number,$ f,$ of grafted chains as opposed to that predicted and observed so far for both melts and solutions of star polymers. Here we extend our studies further to include similar star polymers with large range of $f$ and their binary mixtures. Remarkably we find that the structural relaxation times of the star polymers becomes smaller with increasing arm number upto a certain value above which the relaxation time increases with $f$. Further, in binary mixtures of star polymers of two different sizes the relaxation time decreases dramatically with very low added small star fraction but shows dynamical arrest at significantly higher fraction of smaller stars. Reference: 1. C. Mayer et al Nature Materials 7, 780 (2008); 2. A. K. Kandar et al, J Chem Phys 130, 121102 (2009) [Preview Abstract] |
Friday, March 19, 2010 9:48AM - 10:00AM |
Y19.00008: Connecting nanoscale motion and rheology of gel-forming colloidal suspensions Hongyu Guo, Subramanian Ramakrishnan, James Harden, Robert Leheny We report a combined x-ray photon correlation spectroscopy (XPCS) and rheometry study of the evolution of concentrated suspensions of nanometer-scale colloids undergoing gelation and aging. The suspensions are comprised of silica colloids, 45 nm in diameter, stabilized with octadecyl-hydrocarbon chains in decalin at colloidal volume fractions near 0.20. At low temperature, the solvent quality is poor for the octadecyl chains, leading to a weak, temperature-dependent, short-range attraction between the colloids that drives a reversible gel transition. Following a quench through this transition, the shear modulus grows rapidly as a function of time after an extended induction period. The intermediate scattering function measured with XPCS displays two features, a plateau value that provides information about constrained local dynamics in the gel and a terminal relaxation time that provides information about relaxation of residual stress. From the wave-vector dependence of the plateau value, a localization length can be extracted. Except at early times, the relationship between the localization length and the shear modulus agrees quantitatively with the prediction of Chen and Schweitzer based on a simplified mode coupling theory. [Preview Abstract] |
Friday, March 19, 2010 10:00AM - 10:12AM |
Y19.00009: Self-diffusion of nanoparticles in a crowded environment H.D. Ou-Yang, Joseph Junio, Yi Hu Diffusion is one of the most fundamental and important transport phenomena in a host of different chemical and biological processes. In crowded systems, neighboring particles can induce hydrodynamic, charge, and even entropic interactions that hinder free diffusion, complicating exact analysis. Using fluorescence correlation spectroscopy (FCS), we were able to investigate self-diffusion of fluorescently labeled tracer particles in a reservoir of non-fluorescent particles at varying mixing proportions. By using different sizes mixtures of colloids and polymers, we also explored regimes where suspensions were known to spontaneously phase separate due to entropic depletion, as diffusion has been found to both limit and induce such separation. [Preview Abstract] |
Friday, March 19, 2010 10:12AM - 10:24AM |
Y19.00010: Dynamics of nanospheres near the free surface of dibutyl phthalate C$_{16}$H$_{22}$O$_{4}$ Marcin Sikorski, Suresh Narayanan, Christian Gutt, Birgit Fischer, Ivan Kuzmenko, Alec Sandy We present results from the study of the influence of free surface on the properties of colloidal suspensions. We studied the suspension of silica spheres in organic glass former, dibutyl phthalate DBP. A comprehensive picture of processes governing the relaxation of the colloidal system in the vicinity of liquid/vapor interface has been obtained using various surface-sensitive synchrotron-based experimental techniques. Measurements were performed over a wide temperature range from 300 K down to few degrees above the glass transition temperature T$_{g}$ of DBP. The surface data are compared to the measurements in transmission geometry. GISAXS results indicate that the structure of the suspension is altered in the vicinity of the interface as compared to the bulk. This effect is more pronounced as the temperature of the sample is lowered. These findings agree well with the reflectivity data, showing dramatic increase of electron density in the direction normal to the surface upon approaching T$_{g}$ of the solvent. Lowering the temperature is accompanied by a transition of the particle motion from diffusive at higher temperatures to ballistic-like close to T$_{g}$. Observed phenomena are reversible and behave in a systematic manner upon subsequent heating and cooling. [Preview Abstract] |
Friday, March 19, 2010 10:24AM - 10:36AM |
Y19.00011: On the Dynamics of Spin-Coating of Rapidly Dried Colloidal Suspensions Maximiliano Giuliani, Wenceslao Gonz\'alez-Vi\~nas, Kristin Poduska, Anand Yethiraj The dynamics during the spin-coating of rapidly dried colloidal suspensions was studied. High-speed photography of the temporal evolution of long-range orientational order in both the fluid and dried phases shows three distinct symmetry transitions. Radial non-uniformity in the thickness of the spun suspension was measured from interference fringes, and from this the thinning rates as a function of radial position and time were calculated. A transition between two regimes is observed in the drying front speed. This transition is correlated with changes in the long-range orientational order (dried structure) as well as in the thickness profiles (in the fluid and dried structures). [Preview Abstract] |
Friday, March 19, 2010 10:36AM - 10:48AM |
Y19.00012: Polymeric stabilization of colloidal asphaltenes Sara Hashmi, Abbas Firoozabadi Asphaltenes, the heaviest component of crude oil, cause many problems in petroleum extraction and recovery. Operationally defined as insoluble in long chain alkanes but soluble in toluene, asphaltenes have been described by bulk thermodynamic models such as the Flory-Huggins theory. However, bulk models work well only for asphaltenes in good solvents. Characterization of asphaltenes in poor solvents remains elusive: molecular scale asphaltenes readily aggregate to the colloidal scale and become highly unstable in solution. We investigate the ability of polymers to stabilize colloidal asphaltene suspensions in heptane. In the absence of added polymer, sedimentation measurements reveal dynamics reminiscent of collapsing gels. Adding polymers to colloidal asphaltene suspensions can delay the characteristic sedimentation time by orders of magnitude. Light scattering results suggest that the mechanism of stabilization may be related to a decrease in both particle size and polydispersity as a function of added polymer. [Preview Abstract] |
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