Facile Fabrication of Anisotropic Colloidal Particles with Controlled Shapes and Shape Dependence of Their Elastic Properties

A facile technique with only one step for fabrication of anisotropic colloidal particles at the air/water interface is demonstrated. Anisotropic colloidal particles with controlled shapes can be easily obtained by tuning the incubation time in solvent vapor at room temperature. The formation of separate anisotropic particles is attributed to the lateral stretch on the particles by the interfacial forces and repulsion among the neighboring particles by the generation of the polymer solution flows. To further explain the proposed formation mechanism of the colloidal particles with shape anisotropy, an in situ experiment is designed for direct observation of the arrangement change of the colloidal particles. This fabrication technique is general and applicable to polymer colloidal particles with various initial sizes ranging from microscale to nanoscale. Moreover, the elastic properties of the anisotropic colloidal particles are measured which exhibit a prominent change with different shapes and the change trend of the elastic moduli is similar for particles with different original sizes. This work provides a versatile approach for fabrication of anisotropic colloidal particles with tunable shapes and sizes and establishes the interplay between particle shape and elastic property, which is much valuable for further research on the effect of particle parameters on drug delivery process.     

Polymer depletion effects near mesoscopic particles

The behavior of mesoscopic particles dissolved in a dilute solution of long, flexible, and nonadsorbing polymer chains is studied by field-theoretic methods. For spherical and cylindrical particles the solvation free energy for immersing a single particle in the solution is calculated explicitly. Important features are qualitatively different for self-avoiding polymer chains as compared with ideal chains. The results corroborate the validity of the Helfrich-type curvature expansion for general particle shapes and allow for quantitative experimental tests. For the effective interactions between a small sphere and a wall, between a thin rod and a wall, and between two small spheres, quantitative results are presented. A systematic approach for studying effective many-body interactions is provided. The common Asakura-Oosawa approximation modeling the polymer coils as hard spheres turns out to fail completely for small particles and still fails by about 10% for large particles.     

Entropic wetting and many-body induced layering in a model colloid-polymer mixture

We develop an efficient simulation scheme to study a model suspension of equally sized colloidal hard spheres and nonadsorbing ideal polymer coils, both in bulk and adsorbed against a planar hard wall. The many-body character of the polymer-mediated effective interactions between the colloids yields a bulk phase diagram and adsorption phenomena that differ substantially from those found for pairwise simple fluids; e.g., we find an anomalously large bulk liquid regime and, far from the bulk triple point, three layering transitions in the partial wetting regime prior to a transition to complete wetting by colloidal liquid.     

Effect of the stabilizing coating and the presence of free polymer on the rate of crystallization of colloidal systems

Visual observations and preliminary light scattering experiments on the crystallization process in various colloidal systems are reported. In the first place we studied the influence of the stabilizing coating of the colloidal particles on the rate of crystallization. The various coatings give rise to different ranges of the repulsive interaction. This is found to have a pronounced effect on the rate of crystallization. Furthermore we investigated the effect of free polymer added to the dispersion. It appears that addition of free polymer has a dramatic effect on the crystallization phenomena.     

Inter-lamellar interactions modulated by addition of guest components

We have investigated the effects of a guest component (polymer or spherical colloidal particle) confined between flexible lamellar slits on the inter-lamellar interaction by means of a small-angle X-ray scattering technique and a neutron spin echo technique. The dominant interaction between flexible lamellar membranes without guest components is the Helfrich mechanism. The addition of a neutral polymer into the lamellar phase induces an attractive inter-lamellar interaction and finally destabilizes the lamellar phase. On the other hand, spherical colloidal particles confined between flexible lamellar membranes reduce the undulational fluctuations of lamellae and bring a repulsive inter-lamellar interaction. The behavior of the layer compression modulus of the lamellar membrane containing colloidal particles is well described by the entropical repulsive inter-lamellar interaction driven by steric hindrance.Received: 26 March 2004, Published online: 4 May 2004PACS: 82.70.Uv Surfactants, micellar solutions, vesicles, lamellae, amphiphilic systems, (hydrophilic and hydrophobic interactions) - 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling - 83.80.Hj Suspensions, dispersions, pastes, slurries, colloids - 89.75.Fb Structures and organization in complex systems     

Quantum mechanics simulation and experimental study of adhesive interaction and aggregation of carbon-silicate nanoparticles—reinforcing fillers of polymer composites

Nanoparticles are widely used as polymer composite-reinforcing additives—fillers. Understanding the interaction mechanisms and regularities responsible for nanoparticle aggregation is of great significance for elucidating the nature of reinforcing of polymer composites. The paper reports on quantum mechanics calculations and full-scale experimental study of adhesive interaction of carbon and silicate adsorption complexes (nanomodels of active filler particles of polymer composites). The quantum mechanics approach allowed describing the adhesive properties of particle aggregates reasoning from nanoscopic structure of their surface. The quantum mechanics data were checked for adequacy on schungite—a natural mineral containing carbon and silicate. Schungite microparticles were milled to nanosizes by colloidal grinding in various disperse liquid media (alcohol, acetone, water) and the structure and properties of aggregated schungite micro- and nanoparticles were studied; fractal analysis of their surface was performed. It is found that smaller aggregates of silicate and carbon particles with higher surface fractal dimension are formed in colloidal grinding with small molecular sizes of disperse media (in our case, ethanol or methanol) and this agrees with the data predicted by quantum mechanics calculations.     

Density functional study of fluorocarbon emulsions with adsorbed polymer surfactant

Applying density functional treatment based on wavelets, we have investigated the stability of polymer–colloidal solutions in which polymer surfactants can form globules or adsorb at colloidal particles. The results have indicated that the solution becomes unstable due to the aggregation and formation of dimers and trimers when colloidal concentration exceeds the critical one.     

紫外光引发还原制备金溶胶及其SERS活性的研究

文章报道了一种用紫外光引发还原制备金溶胶的新方法,其还原过程经历了自由基机理。用紫外可见光谱观察了不同反应时间溶胶状态的变化。结果表明,光照反应2h时明显出现金溶胶粒子,7h后氯金酸基本转化完毕。同时,研究了稳定剂聚乙烯吡咯烷酮(PVP)的加入对还原过程的影响,结果表明,PVP的加入不仅稳定了溶胶,而且降低了反应速率。用SEM观察了溶胶粒子聚集体的形貌。最后以1,4-bis(4-vinylpyridyl)phenylene为探针分子研究了这种溶胶的SERS活性。     

A Novel Ultraviolet Irradiation Technique for Fabrication of Polyacrylamide–metal (M = Au,Pd) Nanocomposites at Room Temperature

Polyacrylamide (PAM)–metal (M = gold, palladium) nanocomposites with metal nanoparticles homogeneously dispersed in the polymer matrix have been prepared via a novel ultraviolet irradiation technique at room temperature, which is based on the simultaneous occurrence of photo-reduction formation of the colloidal metal particles and photo-polymerization of the acrylamide (AM) monomer. The UV–vis absorption spectra and TEM were employed to characterize the M-PAM nanocomposites by different irradiation times. The average sizes of the colloidal gold and palladium particles dispersed in the nanocomposites were calculated by XRD patterns and TEM images. The present method may be extended to prepare other metal–polymer hybrid nanocomposite materials.     

The Effect of an Absorbed Polyelectrolyte Layer on the Dynamic Mobility of a Colloidal Particle

Electroacoustic measurements can be used for determining the characteristics of polymer layers on colloidal particles. In this paper a theoretical formula is derived for calculating the effect of a polyelectrolyte layer on the colloid electroacoustic signal. Representative calculations are presented, and the application of these results for polyelectrolyte characterisation are discussed.     

Structure transitions of polymer-mediated colloidal crystals

The self-assembled colloidal crystals with structure transitions were realized experimentally on substrates with polymer brushes. Such polymer-mediated colloidal crystals were found to experience a series of in-layer structure transitions including stable columnar, polycrystalline, mixed square-hexagonal, square, hexagonal structures when the mass ratio of colloidal particles to free polymers was adjusted from 0.125 to 10. Among these structures, a large area interfacial square structure was proved to be a body-centered tetragonal (BCT) structure in three dimensions in the polymer-mediated colloidal crystals.     

Colloidal Core–Satellite Supraparticles via Preprogramed Burst of Nanostructured Micro‐Raspberry Particles

Colloidal molecules, or more general supraparticles, i.e., particles which are themselves assembled of smaller nanoparticles in a defined way, are known to be synthesizable via bottom‐up assembly techniques in colloidal dispersion. The amount of synthesizable particles is mostly limited to milligrams. Herein, a bottom‐up‐programed, triggerable top‐down process is reported to obtain core–satellite supraparticles, i.e., particles composed of a larger core particle surrounded by smaller satellite particles. The key is to prepare a nanostructured, microparticulate powder into which defined burst behavior is preprogramed. Once the system is mechanically triggered, it bursts into well‐defined nanosized core–satellite supraparticles. Scale‐up is easily feasible and several hundred grams per batch can be demonstrated. The product is a ready‐to‐use and flexibly processible powder. Upon simple mixing with a polymer, it disintegrates into the preprogramed core–satellite supraparticles, thus forming a highly sophisticated nanocomposite with the polymer matrix. A pure silica nanoparticle system and a silica–iron oxide nanoparticle hybrid system are presented to demonstrate the versatility of the approach. Enhanced mechanical and unexpected magneto‐optical properties with the particle system are found. The disintegration of the microparticles into individual core–satellite colloidal supraparticles is confirmed via in situ liquid cell transmission electron microscopy.     

Tuning crack pattern by phase separation in the drying of binary colloid–polymer suspension

Drying the colloidal suspension leads to the versatile crack patterns, where the microstructure formed during the drying process determines crack patterns. Here, the polymer is introduced into a silica suspension to tune the crack arrangement in the drying deposit. Five dominated types of crack patterns are defined by varying the ratio of nanoparticles to polymer. Two phase separation processes are proposed to explain the spatial characteristics of the crack patterns: the depletion-induced phase separation of particles and polymer in their mixture; and the separation of water and particle (or polymer) clusters by the water drainage.     

Surface rheology of two-dimensional percolating networks: Langmuir films of polymer pancakes

We report surface rheological measurements on Langmuir films of submicron-sized polymer coils (pancakes). The dynamics of the sol phase is found to be governed by free volume as in the simplest percolation problem. The observed rheology of the percolated phase (gel-like) is compatible with predictions for compressed arrangements of highly deformed soft particles interacting through their contact interfaces.     

Ultrasonic properties of nanoparticles-liquid suspensions

A polymer colloidal solution having dispersed nanoparticles of Cu and Au metals have been developed using a novel chemical method. Average size of the nanoparticles could be varied in the 4-10 nm range by conducting the reaction at an elevated temperature of 50-70 degrees C. Colloidal solutions of representative concentrations of 0.1-2.0 wt% Cu/Au contents in the primary solutions are used to study the modified ultrasonic attenuation and ultrasonic velocity in PVA polymer molecules on incorporating the Cu/Au particles. A characteristic behaviour of the ultrasonic velocity and the attenuation are observed at the particular temperature/particle concentration. The results demonstrate that the primary reaction during the nanoparticles-PVA colloidal formation occurs in divided groups in small micelles. The results are analyzed predicting the enhanced thermal conductivity of the samples.     

Physical origin of the expansion of polymer coils in a binary solvent in the vicinity of its demixing critical point

Critical fluctuations are known to induce a collapse of polymer chains in a mixed solvent upon approaching its liquid–liquid critical point, as originally predicted by Brochard and de Gennes. Recently, we have found that closer to the critical point this collapse is followed by a reswelling of the polymer coils well beyond the original dimensions, a phenomenon not predicted by the theory of Brochard and de Gennes. We submit that upon approaching the critical temperature more closely, the correlation length of the critical fluctuations inside the polymer coils can no longer further increase due to the finite size of the coils, resulting in the appearance of large critical Casimir forces that cause a significant expansion of the polymer coils. Eventually, micro-phase separation inside the coils will appear and the coils will reshrink. This entire process takes place while the bulk solution is still in the one-phase region.     

Effect of defects of plasmon resonance colloidal crystals on their extinction spectra

The effect of the temperature of a silver sol on its extinction spectra during formation of aggregates of plasmon resonance nanoparticles with varying degree of order and the effect exerted on these spectra by changes in the structure of a colloidal crystal during its heating, which lowers elastic properties of a polymer adsorption layer of particles are studied.     

Fluids of clusters in attractive colloids

We show that colloidal particles with attractive interactions induced by a nonadsorbing polymer exhibit a stable phase consisting of a fluid of clusters of particles. This phase persists even in the absence of any long-range repulsion due to charge, contrary to expectations based on simulation and theory. Cluster morphology depends strongly on the range of the interparticle attraction: With a shorter range, clusters are tenuous and branched; with a longer range, they are more compact.