Thermal transport phenomena and limitations in heterogeneous polymer composites containing carbon nanotubes and inorganic nanoparticles

An Off-Lattice Monte Carlo model was developed to investigate effective thermal conductivities (K_eff) and thermal transport limitations of polymer composites containing carbon nanotubes (CNTs) and inorganic nanoparticles. The simulation results agree with experimental data for poly(ether ether ketone) (PEEK) with inclusions of CNTs and tungsten disulfide (WS₂) nanoparticles. The developed model can predict the thermal conductivities of multiphase composite systems more accurately than previous models by taking into account interfacial thermal resistance (R_bd) between the nanofillers and the polymer matrix, and the nanofiller orientation and morphology. The effects of (i) R_bd of CNT–PEEK and WS₂–PEEK (0.0232–115.8 × 10⁻⁸ m²K/W), (ii) CNT concentration (0.1–0.5 wt%), (iii) CNT morphology (aspect ratio of 50–450, and diameter of 2–8 nm), and (iv) CNT orientation (parallel, random and perpendicular to the heat flux) on K_eff of a multi-phase composite are quantified. The simulation results show that K_eff of multiphase composites increases when the CNT concentration increases, and when the R_bd of CNT–PEEK and WS₂–PEEK interfaces decrease. The thermal conductivity of composites with CNTs parallel to the heat flux can be enhanced ∼2.7 times relative to that of composites with randomly-dispersed CNTs. CNTs with larger aspect ratio and smaller diameter can significantly improve the thermal conductivity of a multiphase polymer composite.     

Light scattering by optically heterogeneous polymer systems: stress whitening in polypropylene materials

Summary A theoretical background is presented which allows the assessment of the physical nature of turbidity in heterogeneous polymeric materials. The theory (based on the diffusion approximation of the transfer theory and Kubelka-Munk theory) predicts a decreasing spectral dependence of turbidity with increasing wavelength for a matrix with embedded particles of slightly different refractive indices, but a flat dependence of this quantity for a matrix material with microvoids. It is demonstrated that the diffuse reflectance displays the same type of wavelength dependence for the thick layer approximation. Indeed, diffuse light reflectance experiments on bulk specimens using an integrating sphere accessory reveal the first type of behaviour for nondeformed neat and rubber-modified polypropylenes. On the other hand, the second type of behaviour was observed with stress-whitened neat and rubber-modified polypropylenes after solid-state drawing.     

General rate equations and their applications for cyclic reaction networks: multi-pathway systems

Many chemical reactions of industrial importance involve complex reaction pathways and networks; and the determinations of the reaction rate laws are very difficult. The accurate or proper rate expression is the most desired information in design phase however. In this study, the network reduction technique and the Bodenstein approximation of quasi-stationary behavior of reaction intermediates were systematically applied to derive general rate and instantaneous rate ratio equations for multi-pathway reaction networks in homogeneous catalysis or enzyme system. Multiple small reaction cycles in a main cycle, and multiple-pathways stemming from an intermediate and ending at different nodes in a cycle were considered. The general rate and rate ratio equations derived in this study are applicable for most homogeneous catalytic reactions and enzymatic reactions. Two examples of multi-pathway cyclic enzyme reaction were used to illustrate the applications of the general rate and rate ratio equations for network elucidation.     

General rate equations and their applications for cyclic reaction networks: multi-cycle systems

The network reduction technique and the Bodenstein approximation of quasi-stationary behavior of reaction intermediates were systematically applied to derive general yield ratio and rate equations for multi-cycle reaction networks in homogeneous catalysis. Dual-cycle reaction networks connected by a linear pathway, multi-cycle networks stemming from the same intermediate, and single-cycle with arbitrary number of pathways between two intermediates were considered. The general yield ratio and rate equations derived in this study are applicable for most enzymatic reactions and for homogeneous catalytic reactions. Examples of homogeneous catalysis were used to illustrate the application of the general yield ratio and rate equations for network elucidation.     

General rate equations and their applications for cyclic reaction networks: pyramidal systems

The King-Altman-Hill graphic method has been widely used to derive the rate laws of enzymatic reactions, but the compilation of all the possible pathways is very time-consuming and the reaction rates are not given explicitly. In this study, the network reduction and Y-to-delta transformation techniques were systematically used to derive the general rate equations for pyramidal reaction networks in homogeneous catalysis. The enzymatic reaction of 7,8-dihydrofolate and NADPH to form 5,6,6,8-tetrahydrofolate and NADP, catalyzed by dihydrofolate reductase was taken as an example to illustrate the application of the general reaction rate equations. The calculated overall reaction rate was compared with that obtained from the exact solution by matrix algebra and those obtained from the King-Altman-Hill graphic method.     

Dynamics of diffusion with reversible binding in microscopically heterogeneous membranes: General theory and applications to dermal penetration

Essentially all biological membranes and tissues exhibit microscopic heterogeneity in the form of cellular, lamellar or other organization, and molecular diffusion in these materials is frequently slowed by binding to elements of the microstructure (“trapping”). This paper addresses situations where binding is describable as a linear reversible process at the microscale, with forward (“on”) and reverse (“off”) rate constants k_f(x) and k_r(x) that vary with position. Very commonly it is tacitly assumed that the macroscopically observable binding behavior should follow the same rate law with the substitution of appropriate effective (tissue-average) rate constants and . This assumption is probed theoretically for spatially periodic microstructures using a judicious application of numerical calculations and asymptotic analysis to prototypical one-dimensional transport problems. We find that smooth microscopic variations produce an anomalous macroscopic exchange between free and bound solute populations that is not well described by a single pair of forward and reverse rate constants, i.e., violates the usual paradigm. In contrast, discontinuous variations (as in two-phase composite media) are evidently well described by the usual paradigm. For the latter case we derive simple and general algebraic equations giving and , and generalize them to any three-dimensional unit cell representing the tissue microstructure. Validity of the formulas is demonstrated with reference to a concrete example describing molecular diffusion through the stratum corneum (barrier) layer of skin, comprising lipid (intercellular) and corneocyte (cellular) phases. Our analysis extends coarse-graining (homogenization, effective transport) theory for irreversible trapping systems to the reversible case.     

Preparation of gradient-index (GRIN) polymer fibers for imaging applications

A closed extrusion process combined with a core-shell separation die design is developed in this study for preparing gradient index (GRIN) polymer fibers with a quadratic distribution of the refractive index. The material system used in this investigation is methyl methacrylate (MMA, n = 1.49) and benzyl methacrylate (BzMA, n = 1.568). The refractive index differences between the center and the periphery (Δn) of the prepared polymer fibers increase from 0.0115 to 0.020 when BzMA in the reactant mixtures increases from 17 to 28%. This finding would suggest that increasing the high refractive index monomer in the reactant mixture increased Δn. The Δn values decrease from 0.018 to 0.0135 when the diffusion zone temperature increases from 70 to 90°C. The diffusion rates of the monomers increase with the temperature, thereby causing BzMA and MMA to distribute more uniformly at a higher temperature than at a lower one. This uniform distribution leads to the decrease of Δn with an increasing temperature. The prepared GRIN polymer fibers have potential applications as imaging lenses for scanners, fax machines, and copiers. © 1996 John Wiley & Sons, Inc.     

Nanotechnology: applications and potentials for heterogeneous catalysis

Nanotechnology offers the potential to design, synthesize, and control at nanometer length scale. While the catalysis community has many techniques at their disposal to synthesize catalytic materials at such a scale, the ability to fully design and control is still lacking. Examples are presented to illustrate what can nanotechnology do for heterogeneous catalysis to help achieve the goal of designing catalysts for perfect selectivity in a chemical reaction. Some current state-of-the-art approaches and potential limitations are discussed. Some examples of what can catalysis do for nanotechnology are also presented. However, this aspect is much less studied, although it offers rich opportunities for the catalysis community.     

Micro-Raman investigation of vanadium-oxide coated tubular carbon nanofibers for gas-sensing applications

Commercial tubular carbon nanofibers were uniformly coated with a 5 nm thick vanadium oxide layer via a modified approach to atomic layer deposition. The composition and microstructure of the resulting hybrid material was analyzed by micro-Raman spectroscopy. The effect of the post-synthesis thermal treatment in air at temperatures in the range of 25–375 °C was investigated in order to more deeply understand the behavior of the hybrid material in gas sensing devices. The obtained results demonstrate that the thermal treatment primarily affects the oxide coating-layer that is responsible for the sensing properties. The best sensor performance was obtained at the temperature at which the oxide layered-structure exhibits the highest structural order.     

Structure and mechanical properties of heterogeneous polymer blends

Material response under uniaxial stress has been reported for heterogeneous systems: low density poly(ethylene)–poly(styrene). The ystems differed in a component ratio and in a content of the macromolecular modifier type compatibilizer. Tensile properties have been determined for the systems in a molten state (473 K) and below T_g as a function of elongation rate. The modifier which diminishes the interfacial tension is advantageous for mechanical properties of the system—both in the melt and the solid state. The differences in mechanical characteristics have been related to a specific phase structure of polymer systems. © 1995 John Wiley & Sons, Inc.     

聚合物微乳液及其应用

综述了聚合物微乳液的制备以及聚合物微乳液的结构与性能特征，详细分析了聚合物微乳液的应用。     

聚合物微凝胶的合成及其应用

介绍了新型纳米材料聚合物微凝胶的合成方法，并对该类微粒子表面和内部结构的表征以及应用前景进行了讨论。     

Predicting elastic moduli of heterogeneous polymer compositions

A new method for predicting elastic moduli M of heterogeneous polymer compositions is proposed. It is based on a phenomenological adjustment between parallel and series models for upper and lower bound moduli M_U and M_L. Thus, where ?_H is the volume fraction of hard phase, ?_S is the volume fraction of soft phase, and n is the only adjustable parameter since the upper and lower bound moduli are given by and where M_H and M_S are the moduli of the pure hard and soft phases, respectively. Predicted values of M are in agreement with measured values in a number of systems which include polyblends and composite materials of fixed morphology. The significance of n is discussed relative to concentrations in the area of a phase transition for the polyblends or relative to phase morphology in the case of fixed morphology compositions. Interestingly, the relationship, by analogy, is in agreement with measured values of polyblend melt viscosities.     

Loading of polymer nanocarriers: Factors,mechanisms and applications

The progress in synthetic polymer chemistry has allowed the precise design of hybrid and multifunctional colloidal particles, which differ in type, size and shape, thus enhancing their possible applications as target-oriented carriers of low and high molar mass active species. This survey discusses the basic principles and factors, associated with the process of loading of polymeric nanoparticles. For the purpose of this review, the polymeric nano-carriers are divided into five most studied types: micelles, nanogels, capsules (incl. vesicles), dendrimers, and hybrid nanoparticles with porous cores. Factors influencing the loading are described and their importance discussed. An important trend is the synthesis of multicompartment carriers for the encapsulation of different types of therapeutics. Special attention is focused on the loading of biomacromolecules.     

Extinction of dispersed heterogeneous systems

We consider extinction of various dispersed systems. Isolated boron particles and boron particles in gases are studied. Stability analysis of steady-state thermal regimes of reacting heterogeneous systems for the case of two parallel reactions on the reaction surface using the Frank-Kamenetskii method gives extinction conditions in oxygen-containing media. Curves of the extinction particle size versus the ambient temperature, oxidizer concentration, and, for particles in gases, also versus the oxidizer-to-fuel ratio are plotted. Approximate analytical calculations showed that the extinction process can be most actively controlled by varying the combustion temperature: a decrease in the latter increases the extinction particle size and decreases the completeness of fuel combustion. It is shown that at low ambient temperatures the extinction particle size for suspensions is larger than that for isolated particles. This effect is caused by a decrease in the oxidizer concentration during combustion of suspensions. At high temperatures, the role of this factor weakens.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 6, pp. 12–19, November–December, 1996.     

Practical applications of polymer solutions

To chemists and engineers working in industry, the most important applications of polymer solutions are the experiments leading to the measurement of molecular weight, molecular-weight distribution, and other characteristic parameters of random-coil polymers. This article extends earlier reviews by describing such practical aspects of these measurements as preferred techniques;; commercially-available equipment; analysis cost, sample size, and time; limitations of the methods; and pitfalls in the interpretation of the data. For molecular-weight measurement, commonly-used techniques include end-group analysis, cryoscropy, ebulliometry, osmometry, “vapor-pressure osmometry”, light scattering, and ultracentrifugation. Measurement of certain transport properties correlating emprirically with molecular weight, such as the intrinsic viscosity, is useful if the limitations of the techniques are recognized. Currently, the more promising and powerful methods polymer fractopmatopm are cpmsodered to be column elution and gel permeation chromatography.     

工程塑料改性及成型加工技术特集之六液晶聚合物改性及应用

就利用液晶聚合物特性的新材料开发，对高循环成型材料和流动性和机械物性同时改性的填料应用、耐热新规格聚合物的骨架、精密成型材料及其回收再生适应性作了说明。     

One-dimensional conducting polymer nanocomposites: Synthesis, properties and applications

Intrinsically conducting polymers have been studied extensively due to their intriguing electronic and redox properties and numerous potential applications in many fields since their discovery in 1970s. To improve and extend their functions, the fabrication of multi-functionalized conducting polymer nanocomposites has attracted a great deal of attention because of the emergence of nanotechnology. This article presents an overview of the synthesis of one-dimensional (1D) conducting polymer nanocomposites and their properties and applications. Nanocomposites consist of conducting polymers and one or more components, which can be carbon nanotubes, metals, oxide nanomaterials, chalcogenides, insulating or conducting polymers, biological materials, metal phthalocyanines and porphyrins, etc. The properties of 1D conducting polymer nanocomposites will be widely discussed. Special attention is paid to the difference in the properties between 1D conducting polymer nanocomposites and bulk conducting polymers. Applications of 1D conducting polymer nanocomposites described include electronic nanodevices, chemical and biological sensors, catalysis and electrocatalysis, energy, microwave absorption and electromagnetic interference (EMI) shielding, electrorheological (ER) fluids, and biomedicine. The advantages of 1D conducting polymer nanocomposites over the parent conducting polymers are highlighted. Combined with the intrinsic properties and synergistic effect of each component, it is anticipated that 1D conducting polymer nanocomposites will play an important role in various fields of nanotechnology.     

Electric force microscopy of dielectric heterogeneous polymer blends

We report the applicability of Electric Force Microscopy (EFM) for the analysis of thin films of dielectric heterogeneous polymer blends constituted of polymers with both close and significantly different dielectric constants and offer a simple model that enables quantitative analysis of EFM images of such blends.