Order-disorder transition in a quasi-two-dimensional colloidal system

A 2D colloidal system governed by repulsive dipolar forces tends to form a more ordered system when the interaction strength between the particles increases. Here we report an order-disorder transition of the colloidal system followed by chain formation upon increasing the dipolar interactions and show that the critical field scales with the density of colloids. Our system can do this by changing its dimensionality and therefore exhibits novel behavior that could help us understand colloidal ordering phenomena.     

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.     

A general theory of DNA-mediated and other valence-limited colloidal interactions

We present a general theory for predicting the interaction potentials between DNA-coated colloids, and more broadly, any particles that interact via valence-limited ligand-receptor binding. Our theory correctly incorporates the configurational and combinatorial entropic factors that play a key role in valence-limited interactions. By rigorously enforcing self-consistency, it achieves near-quantitative accuracy with respect to detailed Monte Carlo calculations. With suitable approximations and in particular geometries, our theory reduces to previous successful treatments, which are now united in a common and extensible framework. We expect our tools to be useful to other researchers investigating ligand-mediated interactions. A complete and well-documented Python implementation is freely available at http://github.com/patvarilly/DNACC.     

Thermosensitive, soft glassy and structural colored colloidal array in ionic liquid: colloidal glass to gel transition

A novel soft material comprising thermosensitive poly(benzyl methacrylate)-grafted silica nanoparticles (PBnMA-g-NPs) and the ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfonyl)amide ([C(2)mim][NTf(2)]), was fabricated. The thermosensitive properties were studied over a wide range of particle concentrations and temperatures. PBnMA-g-NPs in the IL underwent the lower critical solution temperature (LCST) phase transition at lower temperatures with a broader transition temperature range as compared to the free PBnMA solution. Highly concentrated suspensions formed soft glassy colloidal arrays (SGCAs) exhibiting a soft-solid behavior and angle-independent structural color. For the first time, we report a discrete change in the angle-independent structural color of SGCAs with temperature because of a temperature-induced colloidal glass-to-gel transition. The interparticle interaction changed from repulsive to attractive at the LCST temperature, and it was characterized by a V-shaped rheological response and a direct electron microscope observation of the colloidal suspension in the IL. With unique rheological and optical properties as well as properties derived from the IL itself, the thermosensitive SGCAs may be of interest as a new material for a wide range of applications such as electrochemical devices and color displays.     

NMR study of liquid to solid transition in a glass forming metallic system

Quadrupolar spin-lattice relaxation effect was used to study the temperature dependence of the correlation of electric field gradient (EFG) observed by (63)Cu and (65)Cu NMR in the liquid and supercooled liquid states of Pd(43)Cu(27)Ni(10)P(20) metallic glass forming system. The correlation time of EFG was shown to have a dramatic temperature dependence that cannot be accounted for by available theory. Analyzed in the context of mode coupling theory (MCT), it is shown that the correlation time of EFG follows the scaling equation of MCT and reveals a T(c), the critical temperature of MCT, at 700 K. Other NMR techniques such as chemical exchange line narrowing and stimulated echo pulse sequences were used to study motion of (31)P at lower temperatures. Combined together, these techniques cover the whole range of liquid to solid transition. By comparing the NMR results with data obtained by other techniques, a decoupling of motion for different types of atoms is revealed starting from T(c) and below. This essentially demonstrates a transition from liquidlike to solidlike motion at T(c) as predicted by MCT.     

Molecular simulation of the glass transition of polyphosphazenes

Molecular dynamics simulation (NPT ensemble) has been used to obtain specific volume as a function of temperature for four polyphosphazenes. From these results, the volumetric glass transition temperature has been determined as the temperature marking the discontinuity in slope of the V–T simulation data. The molecular mechanics force field used in this study was COMPASS (Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies) which has been extensively parameterized and validated for phosphazenes in a previous communication. The polyphosphazenes include poly[bis(2,2,2-trifluoroethoxy)phosphazene] (PTFEP) and three isomers of poly(dibutoxyphosphazene)—poly[bis(n-butoxy)phosphazene] (PnBuP), poly[bis(iso-butoxy)phosphazene)] (PiBuP), and poly[bis(sec-butoxy)phosphazene] (PsBuP). In all cases, there was reasonable agreement between experimental results and values of density and T_g obtained from the simulations. In the case of PnBuP, two different methods for amorphous cell building are compared to explore the relationship between the state of equilibration and the T_g obtained from simulation.     

Shear stresses of colloidal dispersions at the glass transition in equilibrium and in flow

We consider a model dense colloidal dispersion at the glass transition, and investigate the connection between equilibrium stress fluctuations, seen in linear shear moduli, and the shear stresses under strong flow conditions far from equilibrium, viz., flow curves for finite shear rates. To this purpose, thermosensitive core-shell particles consisting of a polystyrene core and a cross-linked poly(N-isopropylacrylamide) shell were synthesized. Data over an extended range in shear rates and frequencies are compared to theoretical results from integrations through transients and mode coupling approaches. The connection between nonlinear rheology and glass transition is clarified. While the theoretical models semiquantitatively fit the data taken in fluid states and the predominant elastic response of glass, a yet unaccounted dissipative mechanism is identified in glassy states.     

Molecular dynamics simulation study of glass transition in hydrated Nafion

The thermal transition of Nafion is studied using a molecular dynamics simulation through a chemically realistic model. Static and dynamic properties of polymer melts with different water contents are investigated over a wide range of temperatures to obtain viscometric and calorimetric glass transition temperatures. The effect of cooling rate of the simulation on the glass transition of the hydrated polymer is also examined within the well‐known Williams–Landel–Ferry (WLF) equation. Variation of relaxation times versus temperature shows a fragile‐to‐strong transition. The hydration level has a significant impact on the static and dynamic properties of the polymer chains and water molecules confined in nanometric spaces between polymer chains. The results of this study are useful to predict the behavior of Nafion for various applications including fuel cells, sensors, actuators, and shape memory devices at different temperatures. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 907–915     

Atomistic simulation of the glass transition of di-substituted polysilanes

Molecular dynamics has been used to determine the glass transition temperature of the amorphous phase of five di-substituted polysilanes from plots of specific volume versus temperature. In each case, good agreement was obtained between the simulation values and the reported DSC results. The effect of amorphous cell dimensions and equilibration time on T_g has been investigated. The use of larger cells provides better agreement with experimental T_g and probably more accurate densities as suggested by earlier studies. The effect of pressure on the T_g of two different polysilanes was also investigated. Although experimental data for comparison is unavailable, values obtained for dT_g/dp are consistent with those reported for other polymers. Vectorial autocorrelation analysis was used to explore the mobility of the polysilane main chains and side groups relative to polyalkanes, polyphosphazenes, and polysiloxanes.     

Molecular dynamics investigation of a density-driven glass transition in a liquid crystal system

Molecular dynamics simulations are carried out to address the density-driven glass transition in a system of rodlike particles that interact with the Gay-Berne potential. Since crystallization occurs in this system on the time scale of the simulations, direct simulation of the glass transition is not possible. Instead, glasses with isotropic orientational order are heated to a temperature T, and the relaxation times by which nematic orientational order develops are determined. These relaxation times appear to diverge at a critical density rho(c); i.e., the system can equilibrate at rhorho(c) (at the temperature T). The relaxation times follow a power-law scaling as the critical density is approached, suggesting that this density-driven glass transition concurs with mode coupling theory.     

Flow curves of dense colloidal dispersions: schematic model analysis of the shear-dependent viscosity near the colloidal glass transition

A recently proposed schematic model for the nonlinear rheology of dense colloidal dispersions is compared to flow curves measured in suspensions that consist of thermosensitive particles. The volume fraction of this purely repulsive model system can be adjusted by changing temperature. Hence, high volume fractions (phi相似文献   

Single particle jumps in a binary Lennard-Jones system below the glass transition

We study a binary Lennard-Jones system below the glass transition with molecular dynamics simulations. To investigate the dynamics we focus on events (jumps) where a particle escapes the cage formed by its neighbors. Using single particle trajectories we define a jump by comparing for each particle its fluctuations with its changes in average position. We find two kinds of jumps: "reversible jumps," where a particle jumps back and forth between two or more average positions, and "irreversible jumps," where a particle does not return to any of its former average positions, i.e., successfully escapes its cage. For all investigated temperatures both kinds of particles jump and both irreversible and reversible jumps occur. With increasing temperature, relaxation is enhanced by an increasing number of jumps and growing jump lengths in position and potential energy. However, the waiting time between two successive jumps is independent of temperature. This temperature independence might be due to aging, which is present in our system. We therefore also present a comparison of simulation data with three different histories. The ratio of irreversible to reversible jumps is also increasing with increasing temperature, which we interpret as a consequence of the increased likelihood of changes in the cages, i.e., a blocking of the "entrance" back into the previous cage. In accordance with this interpretation, the fluctuations both in position and energy are increasing with increasing temperature. A comparison of the fluctuations of jumping particles and nonjumping particles indicates that jumping particles are more mobile even when not jumping. The jumps in energy normalized by their fluctuations are decreasing with increasing temperature, which is consistent with relaxation being increasingly driven by thermal fluctuations. In accordance with subdiffusive behavior are the distributions of waiting times and jump lengths in position.     

Graphene arrested in laponite-water colloidal glass

Graphene production in water from graphite sources is an important technological route toward harvesting the unique properties of this material. Graphene forms thermodynamically unstable dispersions in water, limiting the use of this solvent due to aggregation. We show that graphene-water dispersions can be controlled kinetically to produce graphene by using laponite clay. Laponite exhibits rapid gelation kinetics when dispersed in water above its gelation concentration, allowing graphene aggregation to be halted after exfoliation in water at ambient conditions. The transparency of laponite colloidal glass and films is important in examining the extent of graphene exfoliation.     

不同构型聚丙烯的玻璃化转变温度的分子模拟

应用分子力学和分子动力学的方法对3种不同构型聚丙烯高分子的玻璃化转变温度进行了模拟.用NPT(等温等压)分子动力学模拟获得聚丙烯(PP)在不同温度下的特征体积,通过对模拟得到的V-T做图,求得玻璃化转变温度,其结果与实验值吻合较好.并分析了聚丙烯主链柔顺性和立构规整度对高分子玻璃化转变的影响.     

Computer simulation study of bulk atactic polystyrene in the vicinity of the glass transition

Molecular dynamics (MD) simulations of bulk atactic polystyrene have been performed in a temperature range from 100 K to 650 K at atmospheric pressure. Local translational mobility has been investigated by measuring the mean square translational displacements of monomers. The long-time asymptotic slope of these dependencies is 0.54 at T>T_g, showing Rouse behavior. Cross-over from motion in the cage to Rouse like dynamics has been studied at T>T_g with a characteristic crossover time follows a power law behavior as a function of T, as predicted by mode-coupling theory (MCT). Local orientational mobility has been studied via the orientational autocorrelation functions, ACFs, (Legendre polynomials of the first and second, order) of both the main-chain and side-group bonds. The relaxation times of the orientational α-relaxation follow the same power law (γ∼2.9) as the characteristic translational diffusion time. Below T>T_g both types of dynamics are described by the same activated law. The ACFs time-distribution functions reveal the existence of activated local rearrangements already above T>T_g.     

Local chain dynamics of a model polycarbonate near glass transition temperature: A molecular dynamics simulation

Constant pressure constant temperature molecular dynamics method is employed to investigate the atomistic scale dynamics of a model Bisphenol A polycarbonate in the vicinity of its glass transition temperature. First, the glass transition temperature and the thermal expansion coefficients of the polymer are predicted by performing simulations at different temperatures. To explore the significance of different modes of motion, various types of time correlation functions are utilized in analyzing the trajectories. In these nanosecond scale simulations, the motion of the chain segments is found to be highly localized with little reorientation of the vectors representing these segments. Detailed analysis of trajectories and the correlation functions of the backbone dihedrals and side methyl groups indicates that they exhibit numerous conformational transitions. The activation energies of the conformational transitions obtained from the simulation are generally larger than the potential barriers for the rotations of these dihedrals, however, both show the same trend. We also have estimated the phenylene ring flip activation energy as 12.6 kcal/mol and the flip frequency as 0.77 MHz at 300 K. These values fall either fall within the range determined by various NMR spectroscopy experiments or slightly out of the range. The study shows that the conformational transitions between the adjacent dihedrals are strongly correlated. Three basic cooperative modes are identified from the simulation. They are: a positive synchronous rotation of two phenylene rings, a negative synchronous rotation of two phenylene rings, and a carbonate group rotation. Above the glass transition temperature, the large scale cooperative motions become much more significant.     

Liquidus and glass transition temperatures of the ammonium nitrate-dimethylsulfoxide-water system

The liquidus temperature was measured in the ammonium nitrate-dimethylsulfoxide-water system over in the concentration range 0–60 mole% ammonium nitrate. The probable formation of the NH₄NO₃·nDMSO solvate with n=1.3–1.5 and the mixed solvate NH₄NO₃·DMSO·H₂O at 30 mole% ammonium nitrate and a DMSO:H₂O ratio of 4∶1 are indicated. The glass transition temperatures T _g were measured over a salt concentration range of 0–50 mol% ammonium nitrate and at various compositions of the mixed solvent (y _DMSO =0.1–0.9 mole fraction). At a constant mixed solvent composition, the dependence of the glass transition temperature on the salt concentration can be approximated by a linear relationship, as can its dependence on the DMSO content in the solution at constant salt concentration. The glass-forming composition regions were found and the limits of this region are discussed.     

Computer simulation of colloidal dispersions

This paper discusses recent applications of statistical mechanics to dispersions with particular emphasis on the computer simulation of the dynamic properties.Fundamental to any computation on a colloidal dispersion is the knowledge of the potential of mean force for at least a pair of suspended particles. At low-to-moderate particle concentrations for stable dispersions, statistical mechanical calculations based on the normal DLVO pair potential produce reasonable agreement with experiment for a number of equilibrium properties of simple latex dispersions. This phenomenon indicates that under these conditions the DLVO pair potential is a reasonable effective pair potential. However, recent Monte Carlo simulations and experimental measurements with liquids of spherical molecules suggest that the force between a pair of dispersed particles at very small separation may differ significantly from that predicted by DLVO theory.The computation of dynamic properties of dispersions involves problems not encountered in the above equilibrium calculations. In particular, one must include the effects of indirect hydrodynamic as well as direct interactions among the particles. This computation may be easily accomplished at moderately low particle concentrations and the results of such calculations are able to give a very detailed analysis of the results of Photon Correlation Spectroscopy measurements on ion exchanged polystyrene latex suspensions at low concentration. These computations also, once again, emphasize the usefulness of DLVO pair potentials as effective pair potentials for systems of strongly interacting particles.     

Solubility and glass transition in the system α-D-trehalose/water

Precipitation of trehalose dihydrate in water is observed at room temperature for trehalose concentrations higher than 47.5%w/w. Direct observations of crystal melting in water and measures of the solution density determine the thermal variations of trehalose saturationS(T) (mM) in water: ln(S(T))=ln(0.1223)-(1330/T) withR ² =0.9982. The glass transition (T_g) curve measured by DSC is lower at low concentrations and higher at high concentrations than previously reported.T _g is also measured as a function of the cooling/warming rates. Analysis of specific heat changes atT _g and associated activation energy leads to identify a most stable glassy state around the second eutectic concentration.     

Progressive transition from resonant to diffuse reflection in anisotropic colloidal films

The synthesis and characterization of highly ordered three‐dimensional photonic crystals have been the subjects of intense study over the past two decades due to the unique ability of these structures to control light at the nanoscale. Building on that work in recent years, increasing interest is now focused on the unique optical properties of disordered and quasi‐ordered photonic structures. We present a study of the effects of shape anisotropy and disorder on the specular reflection properties of polymer‐based colloidal films comprised of rod‐shaped subunits of varying aspect ratio. We characterize the specular reflectance properties of these films as a function of their increasing levels of disorder, demonstrating progressive transition from resonant reflection to diffuse reflection. The onset of the diffuse reflection is governed by particle size. © 2014 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys. 2014 , 52, 611–617