Modeling of transport of small molecules in polymer blends: Application of effective medium theory

This article concerns itself with the prediction of effective diffusion coefficients of small permeants in binary polymer blends of varying degrees of miscibility and microstructural order. Several models have been critically evaluated with the help of previously published experimental data and in terms of consistency of morphological information provided by small permeants serving as morphological probes. Completely random, two-phase media have been modeled in terms of Effective Medium Theory with the coordination number (z) describing the average morphology. A comprehensive analysis of experimental data has shown a correlation between z and various physical situations. Near the percolation threshold, z attains a maximum value while, above it, z tends to decrease with increasing content of the conductive component. Accurate predictions of effective diffusivity can be made for volume fractions between 0.3 and 0.8 by letting z = 6. Evidence for phase inversion was studied in terms of models with ordered microstructure and transport data.     

Modified bead-spring theory of polymer solutions. IV. Extension to polymer melts

A constitutive equation which has proven quite successful in describing the nonlinear viscoelastic behavior of dilute polymer solutions is extended to the case of molten polymers. The techniques utilized and similar to those discussed by Ferry in a similar adaptation of the Rouse–Zimm Theory. The resulting model is found to quantitatively portray the shear rate dependence of the non-Newtonian viscosity and primary normal stress functions and the frequency dependence of the storage and loss moduli. Extensional flow data reported by Spearot and Metzner for two polyethylenes are well described, using parameters calculated from steady shearing measurements. Of major significance is the ability of the model to account for influences of molecular weight, molecular weight distribution, and temperature.     

Electrical properties of conductive adhesives as affected by particle compositions,particle shapes,and oxidizing temperatures of copper powders in a polymer matrix

The effects of particle compositions, particle shapes, and oxidation temperatures on the electrical properties of conductive adhesives have been investigated. Silver‐coated copper powders and uncoated copper powders with spherical and flake‐shaped particles are oxidized at temperatures such as 30, 175, 240, 300°C and 350°C for 2 h and dispersed in an epoxy matrix. The results of this study indicate that the electrical properties of the conductive adhesives are strongly affected by the particle compositions and oxidation temperatures and only slightly affected by the particle shapes. Silver‐coated copper powders show significantly greater oxidation resistance than uncoated copper powders. To understand how silver‐coated copper powders show such oxidation resistance, they are analyzed by the techniques of thermogravimetric analysis (TGA), X‐ray diffraction (XRD), and Auger spectroscopy to observe how metal oxides such as AgO, Cu₂O, and CuO affect the electrical properties of the conductive adhesives. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2045–2053, 2004     

Resolution of structure characteristics of AE signals in multiphase flow system—From data to information

This investigation was performed to study the underlying structure characteristics of acoustic emission (AE) signals, which could be helpful not only to understand a relatively complete picture of hydrodynamics in multiphase flow systems, but also to extract the most useful information from the original signals with respect to a particular measurement requirement. However, due to AE signals are made up of emission from many acoustic sources at different scales, the resolution of AE signals is often very complicated and appears to be relatively poorly researched. In this study, the structure characteristics of AE signals measured both in gas–solid fluidized bed and liquid–solid stirred tank were researched in detail by resorting to wavelet transform and rescaled range analysis. A general criterion was proposed to resolve AE signals into three physical‐related characteristic scales, i.e., microscale, mesoscale, and macroscale. Multiscale resolution of AE signals implied that AE signals in microscale represented totally the dynamics of solid phase and could be applied to measure particle‐related properties. Furthermore, based on the structure characteristics of AE signals, useful features related to particles motion were extracted to establish two new prediction models, one for on‐line measurements of particle size distribution (PSD) and average particle size in gas–solid fluidized bed and the other for on‐line measurement of the suspension height in liquid–solid stirred tank. The prediction results indicated that (1) measurements of PSD and average particle size using AE method showed a fairly good agreement with that using sieve method both for laboratory scale and plant scale fluidized beds, and (2) measurements of the suspension height using AE method showed a fairly good agreement with that using visual method. The results thus validated that the extracted features based on analyses of structure characteristics of AE signals were very useful for establishing effective on‐line measurement models with respect to some particular applications. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Multiscale micromechanical modeling of polymer/clay nanocomposites and the effective clay particle

Polymer/clay nanocomposites have been observed to exhibit enhanced mechanical properties at low weight fractions (W_c) of clay. Continuum-based composite modeling reveals that the enhanced properties are strongly dependent on particular features of the second-phase ‘particles’; in particular, the particle volume fraction (f_p), the particle aspect ratio (L/t), and the ratio of particle mechanical properties to those of the matrix. These important aspects of as-processed nanoclay composites require consistent and accurate definition. A multiscale modeling strategy is employed to account for the hierarchical morphology of the nanocomposite: at a lengthscale of thousands of microns, the structure is one of high aspect ratio particles within a matrix; at the lengthscale of microns, the clay particle structure is either (a) exfoliated clay sheets of nanometer level thickness or (b) stacks of parallel clay sheets separated from one another by interlayer galleries of nanometer level height, and the matrix, if semi-crystalline, consists of fine lamella, oriented with respect to the polymer/nanoclay interfaces. Here, quantitative structural parameters extracted from XRD patterns and TEM micrographs (the number of silicate sheets in a clay stack, N, and the silicate sheet layer spacing, d₍₀₀₁₎) are used to determine geometric features of the as-processed clay ‘particles’, including L/t and the ratio of f_p to W_c. These geometric features, together with estimates of silica lamina stiffness obtained from molecular dynamics simulations, provide a basis for modeling effective mechanical properties of the clay particle. In the case of the semi-crystalline matrices (e.g. nylon 6), the transcrystallization behavior induced by the nanoclay is taken into account by modeling a layer of matrix surrounding the particle to be highly textured and therefore mechanically anisotropic. Micromechanical models (numerical as well as analytical) based on the ‘effective clay particle’ were employed to calculate the overall elastic modulus of the amorphous and semi-crystalline polymer-clay nanocomposites and to compute their dependence on the matrix and clay properties as well as internal clay structural parameters. The proposed modeling technique captures the strong modulus enhancements observed in elastomer/clay nanocomposites as compared with the moderate enhancements observed in glassy and semi-crystalline polymer/clay nanocomposites. For the case where the matrix is semi-crystalline, the proposed approach captures the effect of transcrystallized matrix layers in terms of composite modulus enhancement, however, this effect is found to be surprisingly minor in comparison with the ‘composite’-level effects of stiff particles in a matrix. The elastic moduli for MXD6-clay and nylon 6-clay nanocomposites predicted by the micromechanical models are in excellent agreement with experimental data. When the nanocomposite experiences a morphological transition from intercalated to completely exfoliated, only a moderate increase in the overall composite modulus, as opposed to the expected abrupt jump, was predicted.     

Characteristics of medical polymer based on an epoxy resin system—Curing reaction characteristics of biphenol epoxy monomer with phenolic functional hardeners

The reaction kinetics of a biphenyl epoxy/polyol system was characterized by an isoconversional mode under dynamic conditions using differential scanning calorimetry (DSC) measurements. The results showed that the curing of epoxy resins involves different reaction stages and the values of activation energy is dependent on the degree of conversion. At the initial stage of cure, the phenol hydroxyl‐epoxy reaction is predominant, then the hydroxyl group generated during curing can catalyze the reaction. The results were supported by the isothermal experiments. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1495–1503, 2001     

Electrical properties of composites as affected by the degree of mixedness of the conductive filler in the polymer matrix

The development of the electrical properties of composites as a function of the degree of mixedness of a conductive filler distributed into an insulating polymer is investigated. A wide‐angle X‐ray diffraction (WAXD)‐based quantitative phase analysis method was used to characterize the variations of the concentrations of the insulating binder and the conductive particles around their mean values as a function of mixing time in an intensive batch mixer. Increasing the time and hence, the specific energy input, during the mixing process results in a more homogeneous spatial distribution of the conductive filler in the polymeric matrix, which in turn results in a decrease of the volume conductivity of the composite. The decreasing conductivity of the composite is attributed to the better coating and hence the isolation of the conductive particles from each other, thus hindering the formation of a conductive network “percolation”. Overall, these results suggest that the control of the electrical properties of conductive composites could benefit from a good understanding and adequate control of the dynamics of the mixing process and the resulting degree of mixedness of the conductive particles in the polymer matrix.     

Effect of melting and crystallization on the conductive network in conductive polymer composites

Investigation on the effect of melting and crystallization of polypropylene (PP) on the conductive network of multi-wall carbon nanotubes (MWNTs) and carbon black (CB) in MWNT/PP and CB/PP composites is performed. The conductive networks formed by fillers with different aspect ratios (MWNTs and CB) are compared during melting and cooling experiments. The network is found to be deformed during melting and re-constructed again due to the re-agglomeration of fillers during isothermal annealing of the melt. Both deformation and re-construction of the network result in a substantial increase/decrease of the thermal resistivity of MWNT/PP and CB/PP composites. For the modelling of the dynamic network reformation three different approaches are tested: classic percolation theory, general effective medium theory (GEM) and Fournier equation.     

Alternate derivation of the effective medium conductance above and below the critical fill in a metal-filled polymer

Herein is more detail to microscopic aspects of electron transport through a conductor-filled polymer than has yet been considered in the literature (irrespective of quantum mechanical tunneling and thermal expansion). Some of these details only clarify earlier theories, but others lead to new discoveries such as the cubic conduction path coordination number z = 18 (contrasted with the classical geometric coordination number of z = 6). New also are simplified derivations of the high and low fill asymptotes of the master equation for conductance as a function of fill fraction in the effective medium theory of conductor-filled polymers.     

Development of thermally conductive polymer matrix composites by foaming‐assisted networking of micron‐ and submicron‐scale hexagonal boron nitride

Thermally conductive polymer matrix composite (PMC) foams with effective thermal conductivities (k_eff) higher than their solid counterparts have been developed for the first time. Using a material system consists of low density polyethylene and micron‐scale or submicron‐scale hexagon boron nitride platelets as a case example, this article demonstrates that foaming‐assisted filler networking is a feasible processing strategy to enhance PMC's k_eff, especially at a low hBN loading. Parametric studies were conducted to identify the structure‐to‐property relationships between foam morphology (e.g., cell population density, cell size, and foam expansion) and the PMC foam's k_eff. In particular, there exists an optimal cell size to maximize the PMC foam's k_eff for foams with up to 50% volume expansion. However, an optimal cell size is absent for PMC foams with higher volume expansion. X‐ray diffraction (XRD) analyses reveal that both the presence of hBN platelets and foam expansion promoted the crystallization of LDPE phase. Moreover, the XRD spectra also provide evidence for the effect of foam expansion on the orientation of hBN platelets. Overall, the findings provide new directions to design and fabricate thermally conductive PMC foams with low filler contents for heat management applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42910.     

Improvement of conductive network quality in carbon black‐filled polymer blends

Heat treatment of polymer‐based composites is critical for the enhancement of both stability and long‐term service life, especially when the materials function under an inconstant temperature environment. The present article discusses the effect of heat‐treatment conditions on the electrically conductive properties of carbon black (CB)‐filled low‐density polyethylene (LDPE) and ethylene–vinyl acetate copolymer (EVA) composites, which are candidates for positive temperature coefficient (PTC) materials. It was found that the dispersion mode of CB particles changes as a function of the matrix morphology. When the composites are irradiated to form crosslinked networks in the matrix for the elimination of negative temperature coefficient (NTC) behavior, some of the produced free radicals are also entrapped for quite a long time after the irradiation treatment. These residual radicals further enhance the interaction between CB and the matrix and further induce the crosslinking of the matrix so that the composites' conductivity changes with time as a result of the continuous variation in the contacts between the conductive fillers. To improve the quality of the conduction paths in the composites, appropriate post‐heat treatment should be carried out, which speeds up the formation of the above‐mentioned two kinds of crosslinked structures within a limited time. Annealing at 75°C for more than 10 h is believed to be an effective way. After the treatment, a balanced performance characterized by reduced room‐temperature resistivity and improved PTC intensity was obtained. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2768–2775, 2002     

Oxidation reaction in graphite—role of particle characterstics

Diffusion controlled reactions involve square of the particle size as a significant controlling factor and great attention should be given to characterize the particles. Common observations are that imperfections present within pores, cracks, voids, etc., play a critical role in further reactions, leading to anisotropic reaction behaviour.

Two different graphites of different physico-chemical properties and having different particle sizes were studied and it was found that the particles, irrespective of their initial particle characteristics, exhibit the same time period for completion of reaction, and the rate of reaction towards completion is variable with particle characteristics and atmosphere employed.     

Sonochemical polymerization of acrylic acid and acrylamide in the presence of a new redox system— A comparative study

Acrylic acid and acrylamide were polymerized by a peroxydisulfate–suberic acid redox system under a nitrogen atmosphere both in the presence and in the absence of ultrasound (at constant frequency). The rate of polymerization was determined for different concentrations of monomer, initiator, and activator and for different percentages of ultrasonic intensity. The polymers were characterized by X‐ray diffraction and ¹H‐NMR spectroscopy. A probable mechanism is proposed to explain the experimental results obtained. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3685–3692, 2003     

Estimating the permeability of cement pastes and mortars using image analysis and effective medium theory

A method to estimate permeability of cement-based materials using pore areas and perimeters from SEM images is presented. The pore structure is idealised as a cubic lattice having pores of arbitrary size. The hydraulic conductance of each pore is calculated using the hydraulic radius approximation, and a stereological factor is applied to account for the random orientation of the image plane. A ‘constriction factor’ is applied to account for variations in pore radius along the pore axis. Kirkpatrick's effective medium equation is then used to obtain an effective pore conductance, from which the macroscopic permeability is derived. The method was tested on forty-six pastes and mortars with different w/c ratio, cement, age and sand content. The permeabilities ranged from 3 × 10^? 18 m² to 5.8 × 10^? 16 m². It was found that 76% of the permeabilities were predicted to within a factor of ± 2, and 98% within a factor of ± 5 from measured values.     

Inorganic layered double hydroxides as a drug delivery system—intercalation and in vitro release of fenbufen

Layered double hydroxides (LDHs), or so-called anionic clays, consist of cationic brucite-like layers and exchangeable interlayer anions. Because of their biocompatibility, some LDHs, such as Mg/Al, Zn/Al, Fe/Al and Li/Al-LDH, can be used as host materials for drug-LDH host–guest supramolecular structures. The anti-inflammatory drug fenbufen has been intercalated into layered double hydroxides for the first time by co-precipitation under a nitrogen atmosphere. The product has been characterized by powder X-ray diffraction (XRD), FT-IR spectroscopy, elemental analysis and thermogravimetry (TG) and shows an expanded LDH structure, indicating that the drug has been successfully intercalated into LDH. In addition, the dependence of the nature of the fenbufen intercalation process on conditions such as pH value and chemical composition of the host has been systematically investigated. The interlayer distance in the intercalated materials increases with increasing pH value, resulting from a change in the arrangement of interlayer anions from monolayer to interdigitated bilayer. Drug release characteristics of the pillared LDH materials were investigated by a dissolution test in a simulated intestinal fluid (buffer at pH 7.8). The results show that the drug release of supramolecular LDH materials was a slow process, especially in the case of Mg/Al intercalated materials, suggesting that these drug-inorganic hybrid materials can be used as an effective drug delivery system.     

Oxidative polymerization of N‐vinylcarbazole in polymer matrix

A new class of soluble conductive poly(N‐vinylcarbazole) (PVCz) compounds has been developed by oxidative matrix polymerization of N‐vinylcarbazole (NVCz) by Ce(IV) in the presence of poly(ethylene glycol) (PEG). PEG was found to be a more suitable matrix with which to obtain a stable homogenous ternary complex solution when compared with poly(acrylic acid) (PAA) and poly(vinylpyrrolidone) (PVP). The role of PEG, NVCz and Ce(IV) concentration, order of component addition, the structure of the polymer matrix, molecular weight of polymer and the effect of solvent have been investigated. Obtaining soluble PEG–Ce(III)–PVCz ternary complexes was shown by cyclic voltammetric measurements, and the initial rate of formation NVCz cation radicals as calculated using UV–visible spectrophotometry. Advantageously with these soluble complexes, conductivities could be measured both in solution and in the solid state. © 2001 Society of Chemical Industry     

Modification of the ELPI to measure mean particle effective density in real-time

A new modification of electrical low pressure impactor (ELPI) for the particle effective density measurement is presented. The system is capable of real-time operation and it is based on the serial measurement of mobility and aerodynamic diameter. In the studied configuration, a zeroth order mobility analyser is installed inside of the ELPI-instrument. The system is feasible for single modal distributions. For several particle materials and varying size distributions, the measured average density values were within 15% of the values obtained with a reference method.     

Using polymer compounds in place of metal material to make cost‐effective parts

The use of plastic materials in place of metals is attractive because of their versatility and ease of batch fabrication, which reduces costs. This article investigates the possibility of using nylon compounds in place of metal materials to make cost‐effective parts through microinjection molding. Experimental results showed that the plastic parts were well formed with high accuracy and reproducibility through microinjection molding. The average tolerance in the dimensions of the plastic parts was less than 20 μm. The resulting composites with 15 wt % carbon particles exhibited the optimum improvement in accuracy, reproducibility, and wear resistance. In addition, the wear loss of the metal parts without lubrication was 4–10 times higher than that of the polymer compounds. The results revealed that the ball‐screw plates made with nylon compounds exhibited high accuracy, reproducibility, and wear resistance, could be produced at low cost, and they could successfully replace S316 metal parts in microinjection molding. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1645–1652, 2006     

Heat treatment‐induced multiple melting behavior of carbon black‐filled polymer blends in relation to the conductive performance stabilization

Polymer blend‐based electrical conductive composites are provided with more possibilities for tailoring the performance in comparison with single polymer systems. To find an optimum heat treatment temperature of the composites, which is critical to practical applications, detailed thermal analyses of the related materials were carried out as a function of different annealing conditions. Based on the discussion of the morphological variation during treatment in terms of multiple melting behavior, it was found that an annealing temperature of 75°C is able to stabilize the resistivity of the composites within a reasonable period of time, as only solid‐state crystallization of LDPE and uniformization of EVA crystalline size are involved. In contrast to treatment at a temperature higher than 75°C, the ultimate equilibrium resistivity resulting from the above annealing procedure approaches the resistivity of the composites as‐manufactured. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1267–1273, 2001