Microcellular Materials Research Articles

Synthesis of Porous Emulsion-Templated Monoliths Using a Low-Energy Emulsification Batch Mixer

Emulsion-templated porous monoliths based on castor oil-in-black liquor emulsions have been prepared using a low-energy emulsification technique. Lignins from black liquor polymerize in the continuous phase of the high internal phase emulsion to obtain highly microcellular materials with interconnected open porous structures. A two-step emulsification operating mode was developed in order to increase the maximum value of the castor-oil dispersed volume fraction obtained with the one-step emulsification mode (limited to 53 % of the total emulsion volume due to the very high viscosities of the fluids). A rheological study of the black liquor and of the prepared Medium Internal Phase Emulsions, have been conducted. Depending on the emulsification operating mode used, the morphology of the porous monoliths will differ. A rather low mean droplet size (about 6 μm) is obtained with the one-step mode, the two-step technique leads to a broader void size distribution and a further increase of added castor oil (up to 69 %) do not favor the homogeneity of the material.

Efficient Solution Techniques for Multiscale Structural Optimization in Materials Science

ABSTRACTWe consider the modeling and simulation of multiscale phenomena which arise in finding the optimal shape design of microcellular composite materials with heterogeneous microstructures. The paper focuses on the solution of the resulting partial differential equation (PDE) constrained structural optimization problem and development of efficient multiscale numerical algorithms which are challenging tools in reducing the computational complexity. The modeling strategy is applied in materials science for microstructural ceramic materials of multiple constituents. Our multiscale method is based on the efficient combination of both macroscopic and microscopic models. The homogenization technique based on the concept of strong separation of scales and the asymptotic expansion of the unknown displacements is applied to extract the macroscopic information from the microscale model.In the framework of all-at-once approach we find a proper combination of the iterative procedure for the nonlinear problem arising from the first order necessary optimality conditions, also known as Karush-Kuhn-Tucker (KKT) conditions, and efficient large-scale solvers for the stress-strain constitutive equation. We use the path-following predictor-corrector schemes by means of Newton's method and fast multigrid (MG) solution techniques. The performance of two preconditioners, incomplete Cholesky (IC) and algebraic multigrid (AMG), for the resulting homogenized state equation is studied. The comparative analysis for both preconditioners in terms of number of iterations and computing times is presented and discussed. Our interests focus on the parallel implementation of the preconditioning techniques and the use of BoomerAMG as a part of the free software library Hypre developed at the Center for Applied Scientific Computing (CASC), Lawrence Livermore National Laboratory (LLNL).

Controlling arrangement of multi‐layer microcellular materials by using supercritical fluids

A series of semicrystalline microcellular materials were prepared by the foaming technology using supercritical fluids. The distribution of multi‐size microcellulars and multi‐layer arrangement was observed by scanning electronic microscopy. By controlling the foaming condition accurately, the alternated arrangement of variable in cellular size may be tuned. One key point was discussed here to understand the phenomenon: the prior sequencing of the crystal nucleation and cellular nucleation. As the supporting experiments, the gravimetric desorption data of CO2 were kinetically and thermodynamically evaluated by three diffusion equations. The concepts of local crystal nucleation and local cellular nucleation were introduced first, and the mechanism of multi‐layer arrangement was proved. Copyright © 2012 John Wiley & Sons, Ltd.

Modeling of Phase Separation Mechanism in Polycaprolactone/Dioxane Binary Systems

Unidirectional freezing followed by freeze-drying is a way to produce microcellular material from a polymer solution for biomedical application. As a distinctive feature of this type of process, bundles of channels are observed with an average diameter of hundreds of microns. Variations in porous morphology, particularly in porosity, density, and degree of regularity of spatial organization of pores, have been observed when polymer concentrations and quenching temperature are changed. To examine these issues in more detail the thermally induced phase separation of a polycaprolactone/dioxane solution was studied as a function of polymer concentration and quenching temperature in connection with the ultimate morphology of the micro-cellular material. We prepared microcellular samples of polycaprolactone by freeze/freeze-drying technique. The microstructure of the material was observed by scanning electron microscopy. Moreover, a mathematical model for the prediction of the temperature profile and morphology was developed. A microstructural disorder region inside the samples was sometimes observed in connection with process parameters. The developed model is able to capture the formation of such a microstructural disorder region as a direct consequence of the slowing down of the solid-liquid interface. Predictions of the model as a function of freezing rate and concentration are in excellent agreement with experimental observation.

High strain rate behaviour of polypropylene microfoams

Microcellular materials such as polypropylene foams are often used in protective applications and passive safety for packaging (electronic components, aeronautical structures, food, etc.) or personal safety (helmets, knee-pads, etc.). In such applications the foams which are used are often designed to absorb the maximum energy and are generally subjected to severe loadings involving high strain rates. The manufacture process to obtain polymeric microcellular foams is based on the polymer saturation with a supercritical gas, at high temperature and pressure. This method presents several advantages over the conventional injection moulding techniques which make it industrially feasible. However, the effect of processing conditions such as blowing agent, concentration and microfoaming time and/or temperature on the microstructure of the resulting microcellular polymer (density, cell size and geometry) is not yet set up. The compressive mechanical behaviour of several microcellular polypropylene foams has been investigated over a wide range of strain rates (0.001 to 3000 s −1 )i n order to show the effects of the processing parameters and strain rate on the mechanical properties. High strain rate tests were performed using a Split Hopkinson Pressure Bar apparatus (SHPB). Polypropylene and polyethylene-ethylene block copolymer foams of various densities were considered.

Study on Foaming Behavior of Microcellular PP/Inorganic Powder Composites

The modified inorganic powder material (one-dimensional MgSO4 whisker, two-dimensional Mica, zero-dimensional SiO2 ) was added into polypropylene with 5% wt. The microcellular foam PP composite was prepared under the condition of twice-open mold. According to the principle of interfacial adhesion，Liquid-solid interface free energy of PP composite system was calculated by means of the contact angle. Based on heterogeneous nucleation theory, foaming behavior of microcellular foam material was characterized quantitatively by interfacial behavior. The effects of different inorganic powder material on foaming behavior were analyzed in microcellular foam polypropylene. The results indicated that the minimum nucleation energy barrier of PP/Mica composite system is 46.94N.mwith the nucleation rate in 5.53×1011. After foaming, the cell diameter of the composite is 22.1μm and the cell density is 6.92×108 months/cm3.

Metal foams – towards microcellular materials

AbstractVarious techniques to manufacture low-density metallic foams containing sub-millimetre or even micrometre-sized pores are discussed and first trial experiments presented. Three strategies are evaluated: use of an intrinsic blowing agent, foaming under high pressure and foam control by mechanical pressure manipulation. In all three cases, average pore diameters well below 1 mm could be achieved for some aluminium or zinc-based foams while keeping the relative density in a range between 20 % and 50 % of the full metal density.

MICRO-CONSTITUENT BASED VISCOELASTIC FINITE ELEMENT ANALYSIS OF BIOLOGICAL CELLS

A viscoelastic analysis of the biological cell considering the microcellular material properties is carried out in this work. Three separate regions of the cell: the actin cortex, cytoplasm and nucleus are considered. The outer cortex and cytoplasm are modeled using standard linear viscoelastic model (SLS) and standard neo-Hookean viscoelastic solid, and a linear elastic material model is considered for the nucleus. The effect of the material properties of cytoplasm and actin cortex on the derivable parameters from three major experimental studies of magnetic twisting cytometry (MTC) and atomic force microscopy (AFM) and micropipette aspiration (MPA) are analyzed using the finite element method. The bead center displacement for the MTC, reaction force for AFM, and aspiration length ratio for the MPA are the major quantities derived from the finite element analysis. A number of parametric studies are also conducted and it is observed that SLS and SnHS models predict nearly identical results for the material constants.

Foaming Behaviour and Compressive Properties of Microcellular Nanostructured Polystyrene

A batch foaming process has been employed to obtain microcellular materials from polystyrene plus a SBM copolymer (polystyrene-co-1,4-polybutadiene-co-poly(methyl methacrylate). In the first part of the process, raw materials were mixed and extruded in a proportion 90:10 to obtain the precursor materials, leading to a nanostructured assembly in which SBM self-organizes in the polystyrene matrix. In a second stage, foaming was carried out by means of supercritical CO2 in a single step-process. Foamed samples were produced using a technique based on the saturation of the polymer under scCO2, and final properties were controlled by varying the temperature. The swelling in scCO2 was performed at 300 bar during 16 h, and subsequently releasing the gas with a fixed depressurization rate of 60 bar/min. Temperature was varied from 30 °C to 80 °C, leading to densities from 1.0 g/cm3 to 0.5 g/cm3 and cell sizes from 2 micron to 100 micron. In this part of the work, a comparison between foaming behaviour of neat PS and a nanostructured PS+SBM blend is reported, investigating the role of the nanostructured phase as nucleating agents for microcellular foaming. Finally, low rate compression tests were carried out, analyzing the dependence of mechanical parameters such as elastic modulus, yield stress and densification strain with density.

N2‐filled hollow glass beads as novel gas carriers for microcellular polyethylene

AbstractN2‐filled hollow glass beads (HGB) were first used as novel gas carriers to prepare microcellular polymers by compression molding. Dicumyl peroxide was acted as crosslink agent to control the produced microcellular structure of low density polyethylene (LDPE)/HGB. The effect of temperature, pressure and the content of gel on the embryo‐foaming, and final‐foaming structure are investigated. Scanning electronic microscopy shows that the average cell size of microcellular LDPE ranges from 0.1 to 10 μm, and the foam density is about 109–1011 cells/cm3. A clear correlation is established between preserving desirable micromorphologies of microcellular LDPE in different processing stage and tuning processing factors. The pertinent foaming mechanism of microcellular materials foamed with HGB is proposed. Because of the good mechanical strength, low density, weak water‐absorption, and excellent heat insulate ability, microcellular LDPE has great potential application in energy building materials. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Microcellular processing and relaxation of poly(ether ether ketone)

AbstractThe semicrystalline microcellular closed‐cell foams are prepared by a two‐stage batch foaming process from poly(ether ether ketone) and characterized by scanning electronic microscopy. It can be observed that there are two kinds of cells with obviously different cellular sizes in the same transect and the distribution of larger cells (about 7 μm) looks like sandwich. The effects of foaming temperatures and transfer times on the cellular sizes and cell densities of porous materials were discussed. Particular emphasis was given to the effects of crystalline on the microcellular morphology. The relaxation mechanism of microcellular materials was systemically investigated by dynamic mechanics analysis. A plain on the storage modulus curve before Tg was observed due to the densification of cells. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2890–2898, 2007

Elaboration of open‐cell microcellular nanocomposites

AbstractStable water‐in‐oil high internal phase emulsions, containing styrene and divinylbenzene monomers and exfoliated montmorillonite, were prepared and polymerized to obtain nanocomposite microcellular materials. The porous structure was investigated by scanning electron microscopy, mercury intrusion porosimetry, and nitrogen adsorption/desorption analyses. The exfoliation of clay was investigated by X‐ray diffraction and transmission electron microscopy analyses. The presence of inorganic filler did not modify the microcellular structure of the composite, while the use of modified clay significantly enhanced its mechanical properties. No influence on the thermal degradation was noted, except for materials with high clay content that tended to deteriorate at lower temperature than the other materials. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4193–4203, 2007

Microcellular polymers and composites. Part I. Types of foaming agents and technologies of microcellular processing

In the review the principles of technology of microcellular polymers preparations were presented. The process consists of three steps: the bubbles nucleation, growth and stabilization. The examples physical foaming agents most often used such as carbon dioxide or nitrogen were given. Attention is paid to the fact that choice of foaming agent influences the structures of the foams obtained. An important group of chemical foaming agents, being organic or inorganic solid substances decomposing during the foaming process with carbon dioxide or nitrogen release, was also described. Three basic technologies used for microcellular materials preparation, i.e. MuCell (Fig. 1 and 2), Optifoam (Fig. 3) and ErgoCell (Fig. 4) were discussed in detail. The examples of applications of the materials prepared by the technologies mentioned above were given (Fig. 5-9).

Microcellular Wood Fibre Reinforced PP Composites

Abstract Microcellular wood fibre reinforced polymers have practical significance given the possibility of reducing the density of automotive components due to their microcellular structure, as well as processing and performance advantages. A microcellular foaming process with a chemical foaming agent was applied at an experimental stage to injection moulding, extrusion and compression moulding of wood fibre reinforced polypropylene composites. The focus of the research was to investigate these processes using a chemical foaming agent and to perform comparative studies of the physico-mechanical properties of microcellular materials. The effects of the presence of the chemical foaming agent (exothermic) and variation of its content on density, microvoid content, mechanical properties (tensile and flexural), odour concentration and cell morphology of microcellular polypropylene-wood fibre composites were studied. The morphology, cell size, shape and distribution of the microcells were investigated using scanning electron micrographs. Injection moulding process produced finer microcellular structures in comparison with the other processes. As compared to the non foamed composites, the density reduced maximum 30% (0.741 g/cm3), 20% (0.837 g/cm3) and 22% (0.830 g/cm3) for the injection moulding, extrusion and compression moulding process respectively. The chemical foaming agent reduced the odour concentration in relation to the same non foamed composites. Injection moulding showed better performance in comparison with extrusion and compression moulding in terms of cell morphology, density reduction, odour concentration and mechanical properties.

An Overview of the Study on Morph-Genetic Materials in State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University

Morph-genetic synthesis, a preparation method using bio-structures as templates for fabricating micro-cellular materials, has attracted a considerable interest in recent years. In this report, we summarize our obtained results on synthesizing oxides and carbides based on wood templates (for TiC and SiC) and cotton templates (for Al2O3, SnO2 and SiC), respectively. The final products are observed to be faithfully retaining the micro-morphologies of their original natural counterparts, with the template bodies absolutely or partially replaced by the chemically synthesized compounds. These results suggest a new and handy way to fabricate materials with various microstructures, with deliberately choosing desired bio-structures from billions of different species as templates. Moreover, results on Al infiltrated wood templates, as a composite system, are also presented.

Preparation of ultra-low-density microcellular materials

Microcellular polymeric materials can be obtained by the polymerization of a high-internal-phase emulsion. These materials are good candidates as targets toward inertial confinement fusion. This application requires severe specifications, including a very low density and a small cell size. In this study, we examined the influence of parameters such as emulsification conditions, surfactant nature, and the presence of a porogen on the obtainment of those requirements. It was possible to obtain microcellular polymeric foams with apparent densities as low as 0.0126 g/cm3. However, it was difficult to obtain very low material density and still maintain a small average pore size. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2053–2063, 2005

Permeability of open-pore microcellular materials

A simple microstructure-based model is proposed for the permeability of open-pore microcellular materials. The permeability of aluminium open-pore foams produced using the replication process is measured using water or glycerine as the fluid, varying the average cell size (75 and 400 μm) and the relative density from 12% to 32%. Data show the expected dependence on the square of the pore size and agree with other experimental data in the literature for coarser aluminium foams produced by a casting process. The analysis agrees well with present and published experimental data over the entire density range. The model also agrees with the predictions of Du Plessis et al. for low-density foams.

Generation of microcellular biodegradable polycaprolactone foams in supercritical carbon dioxide

AbstractMicrocellular foaming of low‐Tg biodegradable and biocompatible polycaprolactone (PCL) in supercritical CO2 has been studied. The purpose is to apply microcellular materials to drug containers and medical materials for artificial skin or bone. Effects of a series of variable factors on the foam structures, such as saturation temperature, saturation pressure, saturation time, and depressurization time were studied. The experimental results indicate that, while keeping other variables unchanged, higher saturation temperature leads to reduced bulk densities and different saturation pressures result in different nucleation processes. In addition, saturation time has a profound effect on the structure of the product. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 593–597, 2004

Generation of Microcellular Materials via Self-Assembly in Carbon Dioxide

Fluoroalkyl urea compounds were designed to generate foams via dissolution in CO2, self-assembly in solution, and critical point drying via pressure reduction. The fluoroalkyl groups imparted CO2-philicity while hydrogen bonding between urea groups led to the formation of macromolecular structures. Monourea and bisurea compounds yielded collapsed foams of overlapping fibers because the association site density was not sufficient to form a rigid macrostructure. Triureas with a symmetric spacer in the hydrogen bonding core formed stable foams most readily. These foams were able to support their own weight and exhibited density values of 0.03−0.09 g/cm3 and a thermal conductivity of 0.04 W/m·K. Although a fluorinated polyacrylate urea copolymer was designed to exhibit the greatest density of urea sites for associations, the high proportion of these CO2-phobic groups diminished CO2 solubility. Further, strong intramolecular associations prevented the intermolecular self-assembly required for the generation of foams.

Toward Manufacturing of Micro-cellular Materials

Toward Manufacturing of Micro-cellular Materials