Improved Dielectric Breakdown Strength of Covalently-Bonded Interface Polymer–Particle Nanocomposites

Interfacial covalent bonding is an effective approach to increase the electrical resistance of a polymer–particle composite to charge flow and dielectric breakdown. A bifunctional tether reagent bonded to an inorganic oxide particle surface assists with particle dispersion within a thermosetting epoxy polymer matrix but then also reacts covalently with the polymer matrix. Bonding the particle surface to the polymer matrix resulted in a composite that maintained the pure polymer glass transition temperature, compared to modified or unmodified particle dispersions that lacked covalent bonding to the polymer matrix, which depressed the polymer glass transition to lower temperatures. The added interfacial control, directly bonding the particle to the polymer matrix, appears to prevent conductive percolation across particle surfaces that results in a reduced Maxwell–Wagner relaxation of the polymer–particle composite and a reduced sensitivity to a dielectric breakdown event. The inclusion of 5 vol% particles of higher permittivity produces a composite of enhanced dielectric constant and, with surface modification to permit surface cross-linking into the polymer, a polymer–particle composite with a Weibull E ₀ dielectric breakdown strength of 25% greater than that of the pure polymer resulted. The estimated energy density for the cross-linked interface composite was improved 260% compared to the polymer alone, 560% better than a polymer–particle composite synthesized using bare particles, and 80% better than a polymer–particle composite utilizing bare particles with a dispersant.     

Phase transitions in binary mixtures of rodlike colloids and stiff polymers

A suspension of long rodlike colloids and long stiff polymers is modelled as a mixture of hard rods. The diameter of the colloid particle is finite, while the polymer is considered in the limit of zero diameter. Two types of first-order phase transition may occur in such mixtures: an isotropic-nematic phase transition if the density (or the pressure) is high enough, and a demixing transition involving two isotropic phases. The demixing transition has a critical point, and a triple point with one nematic and two isotropic phases may also exist. Phase diagrams are calculated. For the demixing isotropic-isotropic transition to be observed the ratio between the polymer length and the colloid length must exceed 0.36.     

Solvent‐Assisted Interfacial Tension Deformation of Spherical Particles for the Fabrication of Non‐Spherical Particle Arrays

A facile and efficient approach is developed for the fabrication of asymmetric non‐spherical polymer particle arrays. A specific amount of solvent is provided to interact with the spherical polymer particles to intensify the segmental mobility, thus suppressing the viscosity and the glass transition temperature of the polymer particles. The spherical polymer particles in the rubbery state are deformed into non‐spherical particle arrays at the gas/liquid interface. The upper parts of the polymer particles that protrude out of the liquid phase undergo deformation by interfacial tensions at the three‐phase contact line, allowing the formation of a ridge of polymer with a protrusion on the top surface. Simultaneously, the lower parts of the polymer particles submerged under the liquid phase are subjected to enormous surface tension at the contact points, leading to a non‐linear coalescence behavior of the neighboring polymer particles.     

Laser-Induced Phase Transition in a Monolayer of Polymer Particles Levitating in a Low-Pressure Gas-Discharge Plasma

We report on the results of analysis of the mean kinetic energy and the pair correlation function of polymer particles in a plasma–dust structure under the action of laser radiation. We have observed experimentally the crystal–liquid phase transition in the monolayer of particles levitating in the near-electrode layer of a capacitive high-frequency discharge. The coupling parameter of the dust system has been estimated. The results of analysis of the modification of the polymer dust particle surface after holding in the plasma are considered. We propose an explanation of the phase transition taking into account the role of the photophoretic force in the motion of macroparticles. The effect of the photophoretic force is associated with the modification of the dust particle surface in the plasma, as a result of which the particles can effectively absorb laser radiation.     

Effect of nanoparticle dispersion on glass transition in thin films of polymer nanocomposites

We present spectroscopic ellipsometry measurements on thin films of polymer nanocomposites consisting of gold nanoparticles embedded in poly(styrene). The temperature dependence of thickness variation is used to estimate the glass transition temperature, T _g . In these thin films we find a significant dependence of T _g on the nature of dispersion of the embedded nanoparticles. Our work thus highlights the crucial role played by the particle polymer interface morphology in determining the glass transition in particular and thermo-mechanical properties of such nanocomposite films.     

Mixtures of anisotropic and spherical colloids: Phase behavior, confinement, percolation phenomena and kinetics

Purely entropic systems such as suspensions of hard rods, platelets and spheres show rich phase behavior. Rods and platelets have successfully been used as models to predict the equilibrium properties of liquid crystals for several decades. Over the past years hard particle models have also been studied in the context of non-equilibrium statistical mechanics, in particular regarding the glass transition, jamming, sedimentation and crystallization. Recently suspensions of hard anisotropic particles also moved into the focus of materials scientists who work on conducting soft matter composites. An insulating polymer resin that is mixed with conductive filler particles becomes conductive when the filler percolates. In this context the mathematical topic of connectivity percolation finds an application in modern nano-technology. In this article, we briefly review recent work on the phase behavior, confinement effects, percolation transition and phase transition kinetics in hard particle models. In the first part, we discuss the effects that particle anisotropy and depletion have on the percolation transition. In the second part, we present results on the kinetics of the liquid-to-crystal transition in suspensions of spheres and of ellipsoids.     

Two-dimensional dynamics of metal nanoparticles on the surface of thin polymer films studied with coherent X rays

X-ray photon-correlation spectroscopy is used to measure the dynamic structure factor f(q,tau) of gold particles moving on the surface of thin polymer films. Above the glass transition of the polymer the peculiar form f(q,tau) approximately exp[-(Gamma tau)(alpha)] is found with 0.7 < alpha < 1.5, depending on sample age and temperature. The relaxation rates Gamma scale linearly with q, excluding a simple Brownian diffusive motion. This type of behavior, already observed in aging bulk soft matter systems, is explained by a power law distribution of particle velocities due to ballistic motion.     

Microrheology of the liquid-solid transition during gelation

The viscoelastic properties of physical and chemical polymer gels are characterized through the liquid-solid transition using particle tracking microrheology. Measurements of the probe particle mean-squared displacement are shifted as the extent of gelation increases to generate master curves. From the shift factors, we determine the gel point and critical scaling exponents. Both systems exhibit a critical relaxation exponent n approximately 0.6, where G' approximately G' approximately omega n for the incipient gel, consistent with the Rouse model of dynamic scaling in the percolation universality class.     

Molecular dynamics simulation studies of the miscibility and thermal properties of PMMA/PS polymer blend

This investigation is an attempt to improve our understanding of the thermal properties of PMMA (Polymethyl methacrylate) by using PS(Polystyrene); the miscibility of PMMA/PS polymer blend is studied. Our work aims to study the impact of the percentage of PMMA/PS polymer blend on the simulated values of the glass transition temperature (Tg) using the dilatometric method. Compass was chosen as the force field (second category force field). The results reveal a single value of the glass transition temperature Tg that is found for all the curves of the PMMA/PS blend system (molar ratio: (50:50, 60:40, 54:46 and 80:20)); this could be a good criterion for predicting the miscibility. Additionally, the solubility parameters of PMMA and PS are calculated and used to obtain the Flory–Huggins parameter, and the morphology of our polymer blend is simulated using the dissipative particle dynamics method (DPD). Our results exhibit an increase in the Tg of PMMA whenever PS is added; hence, we can confirm the miscibility of the PMMA/PS polymer system.     

Free energy evaluation in field-theoretic polymer simulations

We present a thermodynamic integration method for free energy evaluation in field-theoretic simulations of classical fluids and polymers. The approach employs an Einstein crystal reference state, analogous to a method developed for particle simulations of crystals by Frenkel and Ladd, but applies equally well in the present context to ordered and disordered phases. Thermodynamic averages are computed using complex Langevin sampling, which is effective against the sign problem inherent to polymer field theories. Our method is illustrated in the context of a diblock copolymer melt, where we provide a demonstration of the experimentally observed transition between the cubic gyroid and disordered phases.     

Nanoparticle Distribution in Three‐Layer Polymer–Nanoparticle Composite Films: Comparison of Experiment and Theory

The distribution of hydrophobic nanoparticles deposited on a hydrophilic polymer film is investigated by scanning electron microscopy, transmission electron microscopy, and atomic force microscopy before and after spin‐coating a polymer solution on the particle film and drying it at room temperature. Various polymers and solvents are used. To reach equilibrium, all investigated systems are annealed additionally above the glass transition temperature (T_g) of the polymers. The compatibility of the interacting components is estimated by calculating their surface energy, solubility, and mutual interaction parameters. The experimental results show a redistribution of the particles on both interfaces of the polymer film. This corresponds to the calculated immiscibility of particles and polymers. The distribution of the nanoparticles at the interfaces is related mainly to the vapor pressure of the solvent, that is, kinetic effects during spin‐coating. Only minor contributions result from surface energy, solubility, and interaction parameters.     

Effect of shear on the symmetric diblock copolymer/nanorod mixture: A dissipative particle dynamics study

The phase behaviours of a lamellar diblock copolymer/nanorod composite under steady shear are investigated using dissipative particle dynamics.We consider a wide range of nanorod concentrations,where the nanorods each have a preferential affinity to one of the blocks.Our results suggest that shear not only aligns the orientations of the diblock copolymer templates and nanorods towards flow direction,but also regulates the distribution of the nanorods within the polymer matrix.Meanwhile,the shear-induced reorientation and morphology transitions of the systems also significantly depend on the nanorod concentration.At certain nanorod concentrations,the competitions between shearinduced polymer thinning and nanorods dispersion behaviours determine the phase behaviours of the composites.For high nanorod concentrations,no morphology transition is observed,but reorientation is present,in which the sheared nanorods are arranged into hexagonal packing arrays.Additionally,the orientation behaviour of nanorods is determined directly by the applied shear,also interfered with by the shear-stretched copolymer molecules.     

Microencapsulation of TiO2 Nanoparticles with Polymer by Rapid Expansion of Supercritical Solution

A novel preparation method is reported for the microencapsulation of TiO₂ nanoparticles by rapid expansion of supercritical solution with a nonsolvent. A suspension of TiO₂ nanoparticles in carbon dioxide containing a cosolvent and dissolved polymer is sprayed through a nozzle to atmospheric pressure. After rapid expansion, polymer microspheres were obtained. The microspheres do not tend to agglomerate, since the pure cosolvent is a nonsolvent for the polymer. The structure and morphology of microspheres were investigated by SEM, TEM and XRD. The obtained polymer microspheres are microcapsules of TiO₂ nanoparticles. The mean particle diameter and particle size distribution of microcapsules, could be controlled by changing the polymer concentration, pre-expansion pressure, temperature and injection distance. The polymer feed compositions are more effective than other factors on the control of particle size.     

Heat transport in a two-dimensional complex (dusty) plasma at melting conditions

The heat transport in a two-dimensional complex (dusty) plasma undergoing a phase transition was studied experimentally. A single layer of highly charged polymer microspheres was suspended in a plasma sheath. A part of this lattice was heated by two counterpropagating focused laser beams that moved rapidly around in the lattice and provided short intense random kicks to the particles. Above a threshold, the lattice locally melted. The spatial profiles of the particle kinetic temperature were analyzed to find a thermal conductivity, which did not depend on temperature.     

小角x射线散射结晶聚合物过渡层厚度的测定

由于结晶聚合物中的过渡层是弥散的，而修正的Porod定律是解决这种问题的有效方法之一. 把相关函数法和Porod定律法求结晶聚合物的过渡层厚度进行了比较，它们的计算结果是一致的. 关键词： 小角x射线散射 结晶聚合物 过渡层厚度 相关函数法 Porod定律     

Morphology and thermal properties of clay/PMMA nanocomposites obtained by miniemulsion polymerization

Miniemulsion polymerization was used as the synthetic method to produce clay/poly(methyl methacrylate) nanocomposites. Two kinds of interfacial interactions clay–polymer particle were observed by electron microscopy, one where the polymer particles are adhered on the surface of the larger fragments of clay, and another where nanometric fragments of clay are encapsulated by polymer particles. Variations in the glass transition temperature (T_g) and thermomechanical properties of the matrix, as function of clay content, were observed. In particular, at the highest clay loading (1.0 wt%) depression of T_g and thermomechanical properties were observed. The increased clay–polymer matrix interfacial area appears to be the conditioning factor that determines such behavior.     

Polymer depletion interaction between a particle and a wall

The attractive depletion interaction between a spherical particle and a planar wall in a dilute solution of long flexible nonadsorbing free polymer chains is found to depend crucially on the particle to polymer size ratio . While the polymer-induced force between particle and wall decreases monotonically with increasing distance for large , for small it has a maximum at a distance of the order of the polymer size. For ideal chains we study the crossover from large to small behavior in full quantitative detail. Besides the free energy of interaction and the force, we also discuss the spatial variations of the densities of chain-ends and chain-monomers near the wall and particle. Two independent procedures, (1) solving directly the diffusion equation for the density of ends in terms of planar and spherical waves and (2) minimizing the Ginzburg-Landau functional of the “magnetic analog” of the polymer problem, are used to obtain results numerically for a broad range of ratios of the three lengths particle size, polymer size and distance of particle from the wall. Besides previously known cases, we find two more interesting limiting regions of the length ratios for which analytical results can be obtained. [2mm] Received 11 December 1998     

Application of transition radiation in a small dihedral angle to detection of relativistic particles

The features of transition radiation excited by a relativistic particle in a dihedral angle with an opening comparable to the angular divergence of transition radiation are considered. It is shown that the radiation distribution in the dihedral angle is more sensitive to the direction of emitting particle motion and to the position of the surface intersection by the particle, than the radiation excited when a plane surface is intersected. It is indicated that the spectral radiation density in the small dihedral angle is higher than the density of radiation excited when a plane surface is intersected. These features offer additional opportunities to use transition radiation in systems for measuring particle parameters.     

Equivalence between polymer nanocomposites and thin polymer films: Effect of processing conditions and molecular origins of observed behavior

Recently we established a quantitative equivalence in thermomechanical properties between polystyrene-silica nanocomposites and planar freestanding polystyrene thin films. This equivalence was quantified by drawing a direct analogy between film thickness and an appropriate experimental particle spacing. Using these findings, here we unequivocally show that the glass transition process in confined geometries is controlled by the mean volume fraction of polymer that is affected by the presence of surfaces. Since separate signatures of the bulk and the surface layers are never found, we can clearly rule out any simple “two layer” model which postulates the existence of surfaces which are dynamically decoupled from the bulk. Rather, we argue that the modification of properties at the surfaces propagates into the bulk through a spatial gradient: macroscopic experimental techniques average over these gradients and yield a broadened signature relative to the bulk polymer. In a second aspect of this paper we focus on the role of processing conditions on the results obtained. We have developed a new method of processing the nanocomposites which results in a better dispersion of the nanoparticles in the matrix. However, these samples did not show the unique glass transition behavior seen in the first set of nanocomposites discussed above. This indicates that processing conditions can profoundly affect the nature of the particle-polymer interface which controls the macroscopic behavior of these important systems.