Role of local structure on motions on the potential energy landscape for a model supercooled polymer

We have conducted detailed Monte Carlo and molecular dynamics simulations of a model glass forming polymeric system near its apparent glass transition temperature. We have characterized the local structure of the glass using a Voronoi-Delaunay analysis of local particle arrangements. After a perturbative face elimination, we find that a significant fraction of Voronoi polyhedra consist of 12 pentagonal faces, a sign of icosahedral ordering. Further, we have identified metabasins of particle vibrations on the potential energy landscape on the basis of persistence of particle positions and neighbors over a simulated trajectory. We find that the residence times for vibrations are correlated with a particular Voronoi volume and number of neighbors of a particle; the largest metabasins correspond to particles whose average Voronoi volume is close to the value expected on the basis of the density, and whose approximate number of neighbors is close to 12. The local distortion around a particle, measured in terms of the tetrahedricity of the Delaunay simplices, reveals that the particles with a higher degree of local distortion are likely to transition faster to a neighboring metabasin. In addition to the transition between metabasins, we have also examined the influence of vibrations at inherent structures (IS) on the local structure, and find that the the low frequency modes at the IS exhibit the greatest curvature with respect to the local structure. We believe that these results establish an important connection between the local structure of glass formers and the activated dynamics, thereby providing insights into the origins of dynamic heterogeneities.     

Effect of polydispersity on the crystallization kinetics of suspensions of colloidal hard spheres when approaching the glass transition

We present a comprehensive study of the solidification scenario in suspensions of colloidal hard spheres for three polydispersities between 4.8% and 5.8%, over a range of volume fractions from near freezing to near the glass transition. From these results, we identify four stages in the crystallization process: (i) an induction stage where large numbers of precursor structures are observed, (ii) a conversion stage as precursors are converted to close packed structures, (iii) a nucleation stage, and (iv) a ripening stage. It is found that the behavior is qualitatively different for volume fractions below or above the melting volume fraction. The main effect of increasing polydispersity is to increase the duration of the induction stage, due to the requirement for local fractionation of particles of larger or smaller than average size. Near the glass transition, the nucleation process is entirely frustrated, and the sample is locked into a compressed crystal precursor structure. Interestingly, neither polydispersity nor volume fraction significantly influences the precursor stage, suggesting that the crystal precursors are present in all solidifying samples. We speculate that these precursors are related to the dynamical heterogeneities observed in a number of dynamical studies.     

Rheological aspects of colloidal gels in thermoresponsive microgel suspensions: formation,structure, and linear and nonlinear viscoelasticity

This review focuses on the rheological aspects of colloidal gels that are a three-dimensional sparse network made of aggregated attractive particles formed in the aqueous suspensions of microgels composed of thermoresponsive polymers. Heating changes the dominant interparticle interactions from repulsive to attractive because of the hydrophilic-to-hydrophobic transition. Under appropriate conditions, the hydrophobic microgel suspensions form colloidal gels behave as a yield fluid. The elastic and yielding features of the colloidal gels are considerably different from those of the repulsive glass which is formed by the dense packing of the hydrophilic microgels at low temperatures. The thermoresponsive microgel suspensions undergoing colloidal gelation have attracted much attention from not only the academic interests but also the potentials as a functional suspension because they show interesting and marked changes in viscoelasticity when subjected to temperature variation. We discuss the criteria and dynamics of colloidal gelation, the structure, and linear and nonlinear viscoelasticity of the colloid gels with an emphasis on the results of the experimental studies.     

Colloidal glasses

Colloidal suspensions are important model systems for the study of phase transitions. The glass transition, especially, can be followed more directly with colloidal systems and compared to theoretical predictions. At high volume fractions of the colloidal particles the density fluctuations are partially frozen in. This can be identified by the typical plateau values in the time correlation function. If one compares the experimental results with the mode coupling theory, a very good agreement can be obtained. Currently, some new experimental results concerning the dynamical heterogeneity in colloidal systems are under discussion and will certainly initiate new theoretical developments.     

Charge and salt-driven reentrant order-disorder and gas-solid transitions in charged colloids

Monte Carlo simulations have been performed for aqueous charged colloidal suspensions as a function of effective charge density (sigma) on the particles and salt concentration C(s). We vary the effective charge density in our simulations over a range where a reentrant solid-liquid transition in suspensions of silica and polymer latex particles has been reported by Yamanaka et al. (Phys. Rev. Lett. 80 (1998) 5806). We show that at low ionic strengths a homogeneous liquid-like ordered suspension undergoes crystallization upon increasing sigma. Further increase in sigma resulted once again in a disordered state, which is in agreement with experimental observations. In addition to this reentrant order-disorder transition, we observe an inhomogeneous-to-homogeneous transition in our simulations when salt is added to the disordered inhomogeneous state. This inhomogeneous-to-homogeneous disordered transition is analogous to the solid-gas transition of atomic systems and has not yet been observed in charged colloids. The reported experimental observations on charged colloidal suspensions are discussed in the light of present simulation results.     

Colloidal rotation near the colloidal glass transition

We compare, using single-particle optical imaging, trajectories of rotation and translation for micron-sized spheres in index-matched colloidal suspensions near their glass transition. Rotational trajectories, while they show intermittent caged behavior associated with supercooled and glassy behavior, explore a sufficiently wider phase space such that in the averaged mean-square angular displacement there appears no plateau regime, but instead sub-Fickian angular diffusion that follows an apparent power law in time. We infer translation and rotation time constants, the former being the time to diffuse a particle diameter and the latter being the time to rotate a full revolution. Correlation between time constants increases with increasing volume fraction, but unlike the case for molecular glasses, the rotation time constant slows more weakly than the translation time.     

Particle sizing of colloidal suspensions by low-coherence fiber optic dynamic light scattering

A low-coherence fiber optic dynamic light scattering technique is used to measure the particle size distributions of colloidal suspensions with different volume fractions. We detect electric field autocorrelation function of the singly backscattered light from a sample and use the CONTIN algorithm to obtain the particle size distributions. As a result, in the range of volume fractions from 0.01 to 0.10 of monodispersive colloidal suspensions, the mean particle size with the deviation within 4% and the polydispersity approximate 5% can be determined for particles of different radii. The results demonstrate that the low-coherence fiber optic dynamic light scattering technique is effective in measuring particle size of colloidal suspensions.     

One-dimensional cluster growth and branching gels in colloidal systems with short-range depletion attraction and screened electrostatic repulsion

We report extensive numerical simulations of a simple model for charged colloidal particles in suspension with small nonadsorbing polymers. The chosen effective one-component interaction potential is composed of a short-range attractive part complemented by a Yukawa repulsive tail. We focus on the case where the screening length is comparable to the particle radius. Under these conditions, at low temperature, particles locally cluster into quasi one-dimensional aggregates which, via a branching mechanism, form a macroscopic percolating gel structure. We discuss gel formation and contrast it with the case of longer screening lengths, for which previous studies have shown that arrest is driven by the approach to a Yukawa glass of spherical clusters. We compare our results with recent experimental work on charged colloidal suspensions (Phys. Rev. Lett. 2005, 94, 208301).     

Thermosensitive core-shell particles as model systems for studying the flow behavior of concentrated colloidal dispersions

We report on a comprehensive investigation of the flow behavior of colloidal thermosensitive core-shell particles at high densities. The particles consist of a solid core of poly(styrene) onto which a network of cross-linked poly(N-isopropylacrylamide) is affixed. Immersed in water the shell of these particles will swell if the temperature is low. Raising the temperature above 32 degrees C leads to a volume transition within this shell which leads to a marked shrinking of the shell. The particles have well-defined core-shell structure and a narrow size distribution. The remaining electrostatic interactions due to a small number of charges affixed to the core particles can be screened by adding 0.05M KCl to the suspensions. Below the lower critical solution temperature at 32 degrees C the particles are purely repulsive. Above this transition, a thermoreversible coagulation takes place. Lowering the temperature again leads to full dissociation of the aggregates formed by this process. The particles crystallize for effective volume fractions between 0.48 and 0.55. The crystallites can be molten by shear in order to reach a fluid sample again. The reduced shear stress measured in this metastable disordered state was found to be a unique function of the shear rate and the effective volume fraction. These reduced flow curves thus obtained can be described quantitatively by the theory of Fuchs and Cates [Phys. Rev. Lett. 89, 248304 (2002)] which is based on the mode-coupling theory of the glass transition.     

A coarse-grained explicit solvent simulation of rheology of colloidal suspensions

We use a simple extension of the dissipative particle dynamics (DPD) model to address the dynamical properties of macrosolutes immersed in complex fluid solvents. In this approach, the solvent particles are still represented as DPD particles, thereby retaining the time and length scale advantages offered by the DPD approach. In contrast, the solute particles are represented as hard particles of the appropriate size. We examine the applicability of this simulation approach to reproduce the correct hydrodynamical characteristics of the mixture. Our results focus on the equilibrium dynamics and the steady-state shear rheological behaviors for a range of volume fractions of the suspension, and demonstrate excellent agreement with many published experimental and theoretical results. Moreover, we are also able to track the glass transition of our suspension and the associated dynamical signatures in both the diffusivities and the rheological properties of our suspension. Our results suggest that the simulation approach can be used as a one-parameter model to examine quantitatively the rheological properties of colloidal suspensions in complex fluid solvents such as polymeric melts and solutions, as well as allied dynamical phenomena such as phase ordering in mixtures of block copolymers and particles.     

Investigation of particle interactions in concentrated colloidal suspensions using frequency domain photon migration: monodisperse systems

Frequency domain photon migration (FDPM) measurements were conducted to assess particle interactions of concentrated, monodisperse, polystyrene samples obtained directly from industry by using multiple scattering light. The angle-integrated static structure factor, S(q), and static structure factor at small wave vector q, S(0), were obtained from FDPM measurements at high volume fractions ranging from 0.05 to 0.3, and were compared with those obtained from the monodisperse hard sphere Percus-Yevick (HSPY) model. Effects of different colloid sizes on structure factor evaluated at two different wavelengths were also investigated. Results show that the monodisperse HSPY model is suitable for accounting for particle interactions and local microstructures in these colloidal suspensions. Upon using the HSPY model, particle sizes of polystyrene suspensions were recovered from FDPM measurements at high volume fractions (up to 0.3), which agree well with the DLS measurement of diluted sample ( approximately 0.001). The study of polydispersity effect shows that the FDPM method can be successfully used for recovering the mean particle size of polydisperse colloidal suspension with low polydispersity (<16%) under the assumption of monodisperse hard sphere systems.     

Drying dissipative structures of colloidal crystals of silica spheres coated with polymer brushes of poly(carboxymethyl betaine)

Drying patterns of colloidal crystals of colloidal silica spheres coated with the brushes of zwitterionic poly(carboxymethyl betaine) (SiP-PCMB) and their parent silica spheres (SiP) were studied on a cover glass, a watch glass, and a Petri glass dish. Crystal structures kept the whole process of dryness of the suspensions of SiP-PCMB and SiP. Crystal structures of the dried films of SiP-PCMB were kept stable even when the initial suspensions contained 5 mM of sodium chloride, which is the important role of the excluded volume effects of the shells of the polymer brushes. On the other hand, crystal structures of SiP spheres in the dried films were much unstable and melted in the presence of 5 mM sodium chloride. In the suspension state, colloidal crystallization of SiP-PCMB took place stably by the contribution of the excluded volume effects besides the extended electrical double layers compared with that of SiP spheres, where only the double layer effect contributes to the crystallization. The fractal patterns of the complexation of SiP-PCMB or SiP spheres with sodium chloride were observed microscopically in the dried films. Several kinds of dissipative crystallization such as array and/or accumulation of the crystallites were observed, and the importance of the convectional and sedimentation processes during the course of dryness was demonstrated.     

Electroviscous effect of moderately concentrated colloidal suspensions under overlapping conditions

In this work we present a theoretical model for the calculation of the electroviscous coefficient of a colloidal suspension. The treatment is not limited for dilute suspensions and includes the contribution of the overlapping between adjacent ionic layers. The development here used is based on a cell model, which is applicable to Newtonian suspensions under low shear conditions and without crystalline ordering. Also presented are a complete study of the new numerical results and comparisons with previous results. We find new behaviors for the case of moderate volume fractions that do not appear in the dilute limit.     

Re-entrant kinetic arrest and elasticity of concentrated suspensions of spherical and nonspherical repulsive and attractive colloids

We have designed and studied a new experimental colloidal system to probe how the weak shape anisotropy of uniaxial particles and variable repulsive (Coulombic) and attractive (van der Waals) forces influence slow dynamics, shear elasticity, and kinetic vitrification in dense suspensions. The introduction of shape anisotropy dramatically delays kinetic vitrification and reduces the shear elastic modulus of colloidal diatomics relative to their chemically identical spherical analogs. Tuning the interparticle interaction from repulsive, to nearly hard, to attractive by increasing suspension ionic strength reveals a nonmonotonic re-entrant dynamical phase behavior (glass-fluid-gel) and a rich variation of the shear modulus. The experimental results are quantitatively confronted with recent predictions of ideal mode coupling and activated barrier hopping theories of kinetic arrest and elasticity, and good agreement is generally found with a couple of exceptions. The systems created may have interesting materials science applications such as flowable ultrahigh volume fraction suspensions, or responsive fluids that can be reversibly switched between a flowing liquid and a solid nonequilibrium state based on in situ modification of suspension ionic strength.     

Design of liquid-crystalline aqueous suspensions of rutile nanorods: evidence of anisotropic photocatalytic properties

TiO2 rutile nanorods of average length L = 160 +/- 40 nm and average diameter D = 15 +/- 5 nm have been synthesized through a seed-mediated growth process by TiCl4 hydrolysis in concentrated acidic solution. These nanorods were dispersed in water to yield stable (aggregation-free) colloidal aqueous suspensions. At volume fractions phi > 3%, the suspensions spontaneously display a phase separation into an isotropic liquid phase and a liquid-crystalline phase identified as nematic by X-ray scattering. At phi > 12%, the suspensions form a nematic single phase, with large order parameter, S = 0.75 +/- 0.05. Very well aligned rutile films on glass substrate were produced by spin-coating, and their photocatalytic properties were examined by monitoring the decomposition of methylene blue under UV light. We found that UV-light polarized along the quadratic axis of the rutile nanorods was most efficient for this photocatalytic reaction.     

Tracer diffusion in colloidal suspensions under dilute and crowded conditions with hydrodynamic interactions

We consider tracer diffusion in colloidal suspensions under solid loading conditions, where hydrodynamic interactions play an important role. To this end, we carry out computer simulations based on the hybrid stochastic rotation dynamics-molecular dynamics (SRD-MD) technique. Many details of the simulation method are discussed in detail. In particular, our choices for the SRD-MD parameters and for the different scales are adapted to simulating colloidal suspensions under realistic conditions. Our simulation data are compared with published theoretical, experimental and numerical results and compared to Brownian dynamics simulation data. We demonstrate that our SRD-MD simulations reproduce many features of the hydrodynamics in colloidal fluids under finite loading. In particular, finite-size effects and the diffusive behavior of colloids for a range of volume fractions of the suspension show that hydrodynamic interactions are correctly included within the SRD-MD technique.     

Microchemical inhomogeneity to characterize atomic configurations in the heating and quenching of a CuHf2 alloy

Based on the constructed Cu-Hf interatomic potential, Monte Carlo simulations were conducted to reveal the atomic configurations in heating and quenching of a CuHf(2) alloy through scrutinizing the evolution of microchemical inhomogeneity. Simulations show that the CuHf(2) crystalline structure becomes more homogeneous during heating but an obvious drop in microchemical inhomogeneity appears when reaching its melting point. During the quenching process, the CuHf(2) melt becomes increasingly inhomogeneous and shows a change in the slope in the microchemical inhomogeneity around glass transition temperature. Simulation results were evidenced by the atomic packing analysis through the Voronoi tessellation method. The implications of our study suggest that the glass transition could be visualized as a process involving increase of microchemical inhomogeneity from the liquid to glassy state.     

Thermal analysis of PMMA/gel silica glass composites

Thermal analysis of poly-methylmethacrylate (PMMA) impregnated porous gel silica glasses confirms that the PMMA chains form hydrogen bonds with the pore surface silanol groups. The adopted conditions for the insitu polymerisation result in about 4% of residual monomers trapped in the polymer, most of them in the amorphous structure. The polymer and monomer mixture takes up the whole of the free pore volume. Most of the residual monomer polymerises during the DSC scans above the glass transition temperature providing an excellent probe for the weak glass transition. Polymerisation in the gel silica glass medium affects the glass transition temperature, the length of polymer chains, and the degree of polymerisation.     

A Single Glass Fiber with Ultrathin Layer of Carbon Nanotube Networks Beneficial to In-Situ Monitoring of Polymer Properties in Composite Interphases

Electrophoretic deposition (EPD) is used to deposit multiwalled carbon nanotube networks (CNTs) onto electrically insulating glass fiber surfaces. We found that the thin networks on a single glass fiber surface exhibit semiconducting properties. This enables us to realize a single CNT-glass fiber as a probe with novel multifunctional capabilities for in-situ monitoring of various chemical/physical transitions, particularly in the interphase region between polymer and glass fiber. Because of the intimate interaction between CNTs and polymers in the vicinity of a glass fiber, our CNT probe can rapidly sense the local changes of fundamental polymer properties, such as glass transition, reaction activation energy, cross-linking reaction, and crystallization.