Experimental and numerical investigations of the anisotropic deformation behavior of low-density polymeric foams

Low-density porous materials and foams have been widely used for a variety of applications, such as light structural components, impact energy absorption, thermal insulation and sound absorption. The macroscopic deformation of such materials is strongly dependent on their inherent micro-cellular structure. This study investigated the compressive anisotropic deformation behavior of low-density polymeric foam by using X-ray computed tomography (CT) and the finite element method (FEM) in order to understand both the microscopic and macroscopic deformation behavior. The foams used in this study have a closed cell structure, with pores that are elliptical in shape. Three different types of expansion ratios were employed. The porosities of these materials were 93.5, 95, and 96%. From the observations using the X-ray CT method, the averaged pore heights were 1 mm and the aspect ratios were 2, 2.5, and 2.25, respectively. The foam demonstrated anisotropic deformation, dependent on the uni-axial compression direction. It was found that the deformation rigidity in the longitudinal direction was larger than that in the transverse direction. By using the X-ray CT method in situ, the microscopic deformation behavior when subjected to compressive loading was observed. Deformation and collapse of pores was observed for both directions during the loading. In conjunction with this, FEM computations were carried out to elucidate how such pore geometry undergoes elastoplastic deformation and leads to macroscopic deformation behavior. The FEM-created three-dimensional spatial structures were based on elongated rhombic dodecahedrons. It is revealed that the FEM computation shows relatively good agreement with the experimental results. Thus, our experimental and computational models may be useful for microstructural design using anisotropic cellular materials.     

Characterization of the compressive deformation behavior with strain rate effect of low-density polymeric foams

This study investigates the compressive deformation behavior of a low-density polymeric foam at different strain rates. The material tested has micron-sized pores with a closed cell structure. The porosity is about 94%. During a uni-axial compressive test, the macroscopic stress–strain curve indicates a plateau region during plastic deformation. Finite Element Method (FEM) simulation was carried out, in which the yield criterion considered both components of Mises stress and hydrostatic stress. By using the present FEM and experimental data, we established a computational model for the plastic deformation behavior of porous material. To verify our model, several indentation experiments with different indenters (spherical indentation and wedge indentation) were carried out to generate various tri-axial stress states. From the series of experiments and computations, we observed good agreement between the experimental data and that generated by the computational model. In addition, the strain rate effect is examined for a more reliable prediction of plastic deformation. Therefore, the present computational model can predict the plastic deformation behavior (including time-dependent properties) of porous material subjected to uni-axial compression and indentation loadings.     

Multiscale description of polymeric foam behavior: A new approach based on discrete element modeling

The mechanical behavior of polymeric foams depends on several parameters, such as temperature, material density, and strain rate. The studied foams are multiscale materials; agglomerated beads (bead scale is millimetric) are composed of microscopic closed cells (a few tens of microns). The response of the material to dynamic loading consists of three regions: an elastic phase, a plastic phase, and densification. The first part of this work has been the identification of the behavior of these multiscale foams in terms of density and strain rate. Some results are presented in this paper. From these first dynamic results, the second step has been the observation and the analysis of the physical phenomena initiated during the yield plateau. Buckling of the bead and cell wall and strong damage localization were studied with several devices and techniques such as high-speed camera, SEM, and microtomography. The final objective is the development of a model adapted to the multiscale structure of the foam. The first step of this numerical approach consists in the modeling of the microstructure. Due to the microscopic discrete aspect of the foam, a Discrete Element Model has been developed to study the relationship between microscopic properties and the macroscopic behavior of foam. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 6, pp. 1037–1050. This article was submitted by the authors in English.     

Characterization of irregular open-cell cellular structure with silicone pore filler

The paper presents the results of studying the influence of silicone polymer pore filler on the macroscopic quasi-static and dynamic compressive behaviour of aluminium foam with irregular open-cell structure. The study is based on a mechanical experimental testing programme, where the deformation mechanism and mechanical energy absorption capacity of aluminium foam with silicone pore filler have been observed for the first time. As plastic yielding is accompanied by significant heat energy dissipation, this study was additionally supported by thermal imaging, which enables visualization of plastification to better understand the deformation process of observed specimens. The influence of specimen size on the behaviour of aluminium foam specimens has also been investigated. The results show that introduction of silicone pore filler considerably increases the energy absorption capacity at almost unchanged densification strain under both quasi-static and dynamic loading conditions. The silicone pore filler also significantly influences the deformation behaviour of aluminium foam specimens, which is manifested in a different stress distribution and a significant transverse deformation with conical plastification front. However, only a minor difference in response of different size specimens has been observed.     

Effect of solvent annealing on the tensile deformation mechanism of a colloidal crystalline polymeric latex film

The influence of solvent annealing on microscopic deformational behavior of a styrene/n-butyl acrylate copolymer latex film subjected to uniaxial tensile deformation was studied by small-angle X-ray scattering. It was demonstrated that the microscopic deformation mechanism of the latex films transformed from a nonaffine deformation behavior to an affine deformation behavior after solvent annealing. This was attributed to the interdiffusion of polymeric chains between adjacent swollen latex particles in the film. It turns out that solvent annealing is much more efficient than thermal annealing due to a much slow evaporation process after solvent annealing.     

Flow of non‐newtonian fluids in porous media

The study of flow of non‐Newtonian fluids in porous media is very important and serves a wide variety of practical applications in processes such as enhanced oil recovery from underground reservoirs, filtration of polymer solutions and soil remediation through the removal of liquid pollutants. These fluids occur in diverse natural and synthetic forms and can be regarded as the rule rather than the exception. They show very complex strain and time dependent behavior and may have initial yield‐stress. Their common feature is that they do not obey the simple Newtonian relation of proportionality between stress and rate of deformation. Non‐Newtonian fluids are generally classified into three main categories: time‐independent whose strain rate solely depends on the instantaneous stress, time‐dependent whose strain rate is a function of both magnitude and duration of the applied stress and viscoelastic which shows partial elastic recovery on removal of the deforming stress and usually demonstrates both time and strain dependency. In this article, the key aspects of these fluids are reviewed with particular emphasis on single‐phase flow through porous media. The four main approaches for describing the flow in porous media are examined and assessed. These are: continuum models, bundle of tubes models, numerical methods and pore‐scale network modeling. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

The deformation and failure response of closed-cell PMDI foams subjected to dynamic impact loading

The present work aims to investigate the bulk deformation and failure response of closed-cell Polymeric Methylene Diphenyl Diisocyanate (PMDI) foams subjected to dynamic impact loading. First, foam specimens of different initial densities are examined and characterized in quasi-static loading conditions, where the deformation behavior of the samples is quantified in terms of the compressive elastic modulus and effective plastic Poisson's ratio. Then, the deformation response of the foam specimens subjected to direct impact loading is examined by taking into account the effects of material compressibility and inertia stresses developed during deformation, using high speed imaging in conjunction with 3D digital image correlation. The stress-strain response and the energy absorption as a function of strain rate and initial density are presented and the bulk failure mechanisms are discussed. It is observed that the initial density of the foam and the applied strain rates have a substantial influence on the strength, bulk failure mechanism and the energy dissipation characteristics of the foam specimens.     

Anisotropic compressive behavior of rigid PVC foam at strain rates up to 200 s−1

The elevated strain rate compressive response of closed-cell polyvinyl chloride (PVC) foam at various densities is investigated. Two loading directions, (i.e., parallel and perpendicular to foam rise direction) were considered to investigate structural anisotropy. The elevated strain rates tests (up to 200 s⁻¹) were performed using a customized drop tower device. Engineering stress/strain behavior, energy dissipation, and maximum stress capacity were obtained for each density and compared against each other. Except for the lowest density of 45 kg/m³, strain rate effects were clearly observed through increased compressive strength and plateau stress when loading in the foam rise direction. The strain rate effect is more evident at higher densities. However, no significant strain rate effect was observed when loading perpendicular to the foam rise direction. Scanning electron microscopy (SEM) analysis showed that plastic hinges are the primary deformation mechanism for PVC foam cells. An analytical model has been calibrated using the experimental results and successfully predicted the mechanical response of the foam. Shape anisotropy has been measured employing the SEM images. The analytical approach was also able to predict the foam's anisotropic mechanical response.     

Measurement of flexural modulus of polymeric foam with improved accuracy using moiré method

The flexural modulus of polymeric foams determined from three-point bending tests is usually inaccurate due to the local deformation undergone by the material during testing. The machine used in the test gives deflection values larger than the actual deflection of the foam specimen due to the deformation of the material at the loading point. This leads to errors in the computation of the modulus value. In this work, the deflection values of a beam made of polymeric foam in a three-point bending test were determined using the moiré method. The change in the moiré pattern at the neutral axis of the foam during loading was recorded and converted into deflection values. The deflection data were used to generate the stress–strain curve from which the flexural modulus of the foam material was determined. The proposed method was verified using aluminum beams, where a high correlation between the deflection data from the machine readings and the moiré method was obtained. The flexural modulus of the foam determined using the moiré method was found to be within 3% of the value published in the material data sheet.     

Low density polyethylene,expanded polystyrene and expanded polypropylene: Strain rate and size effects on mechanical properties

Polymeric foam materials may be used as energy absorbing materials for protection in impact scenarios, and design with these materials requires the mechanical properties of foams across a range of deformation rates, where high deformation rate testing often requires small samples for testing. Owing to their cellular macrostructure, and the large deformations that occur during loading of foams, the measured stress-strain response of a foam material may be influenced by the sample size. In this study, the mechanical properties of three closed-cell polymeric foams (Low Density Polyethylene, Expanded Polystyrene and Expanded Polypropylene) at two different densities were investigated over a range of deformation rates from 0.01 s⁻¹ to 100 s⁻¹. For each foam material, three different nominal sample sizes (10 mm, 17 mm and 35 mm) were tested. On average, the polymeric foam materials exhibited increasing stress with increasing deformation rate, for a given amount of strain.Density variation was identified at the sample level, with smaller samples often exhibiting lower density. Expanded Polystyrene demonstrated the highest variability in sample density and corresponding variability in mechanical response, qualitatively supported by observed variations in the macrostructure of the foam. Expanded Polypropylene exhibited variability in density with sample size, and observable variability in the material macrostructure; however, the dependence of the measured mechanical properties on sample size was modest. Low Density Polyethylene was found to have a relatively consistent cell size at the macrostructure level, and the material density did not vary significantly with sample size. In a similar manner, the dependence of measured mechanical properties on sample size was modest. The effect of sample size was identified to be material specific, and it is recommended that this be assessed using sample-specific density measurements and considering different sized samples when testing foam materials.     

Numerical Simulation on the Dissociation,Formation, and Recovery of Gas Hydrates on Microscale Approach

Investigations into the structures of gas hydrates, the mechanisms of formation, and dissociation with modern instruments on the experimental aspects, including Raman, X-ray, XRD, X-CT, MRI, and pore networks, and numerical analyses, including CFD, LBM, and MD, were carried out. The gas hydrate characteristics for dissociation and formation are multi-phase and multi-component complexes. Therefore, it was important to carry out a comprehensive investigation to improve the concept of mechanisms involved in microscale porous media, emphasizing micro-modeling experiments, 3D imaging, and pore network modeling. This article reviewed the studies, carried out to date, regarding conditions surrounding hydrate dissociation, hydrate formation, and hydrate recovery, especially at the pore-scale phase in numerical simulations. The purpose of visualizing pores in microscale sediments is to obtain a robust analysis to apply the gas hydrate exploitation technique. The observed parameters, including temperature, pressure, concentration, porosity, saturation rate, and permeability, etc., present an interrelationship, to achieve an accurate production process method and recovery of gas hydrates.     

X-ray CT microtomography and mechanical response of foamed polysiloxane elastomers

3D X-ray computer microtomography (CT) experiments have been performed to assess the microstructure of scaled cellular polysiloxane elastomers and to predict how key morphological features alter as a function of compressive loading. In the work reported here, full scale (nominally 600 μm pore size) and half scale (nominally 300 μm pore size) polydimethylsiloxane foams (M97) were prepared using extractable urea particles, and tested. CT test methodology was developed to image foam microstructure at different levels of compression. 1D magnetic resonance imaging (MRI) experiments have also been performed on full scale foams for baseline characterisation. Material porosity, bulk density and dynamic mechanical analysis (DMA) stress/strain responses as a function of compression were recorded. Our results show that undesirable engineering stress responses are evident when the material microstructure (cell size and shape) is non-uniform and complex. This is particularly evident when non-spherical urea particles are used, leading to undesirable scaled foam microstructures with mechanical responses that do not match that shown by ‘full scale’ versions. Through the use of X-ray CT and MRI, our studies have provided insights into the link between manufacturing, polymer architecture (cell size/shape) and mechanical response of scaled M97 cellular materials. The data collected will support materials FEA (finite element model) code development activities, as well as help identify how the material architecture can be modified to achieve more controlled and uniform mechanical responses.     

Rheological properties of surface-modified nanoparticles-stabilized CO2 foam

This study investigates the rheological properties of surface-modified nanoparticles-stabilized CO₂ foam in porous media for enhanced oil recovery (EOR) applications. Due to the foam pseudo-plastic behavior, the foam apparent viscosity was estimated based on the power law constitutive model. The results show that foam exhibit shear-thinning behavior. The presence of surface-modified silica nanoparticles enhanced the foam bulk apparent viscosity by 15%. Foam apparent viscosity in the capillary porous media was four times higher than that in capillary viscometer, and foam apparent viscosity increased as porous media permeability increases. The high apparent viscosity of the surface-modified nanoparticles-stabilized foam could result in effective fluid diversion and pore blocking processes and enhance their potential applications in heterogeneous reservoir.     

Polypropylene modified with elastomeric metallocene‐catalyzed polyolefin blends: Fracture behavior and development of damage mechanisms

The fracture behavior and deformation mechanisms of polypropylene modified by elastomeric metallocene‐catalyzed polyolefin blends were investigated under both static and dynamic loading conditions. The fracture toughness was evaluated with the J integral approach. The development of damage mechanisms was studied by the examination of fracture surfaces with scanning electron microscopy and by the examination of single‐edge, double‐notch, four‐point‐bending or low‐impact‐energy fractured samples with optical microscopy. In addition, tensile dilatometry measurements were carried out to determine the nature of the deformation micromechanisms. The fracture behavior and the size and shape of the damage zones were drastically influenced by the elastomeric particles and the imposed constraint. The role of the elastomeric particles was different, depending on the strain rate. Under impact loading, particle pullout and crazing were responsible for the increased fracture toughness of polypropylene. Under quasistatic loading, stable fracture growth was caused by particle cavitation, which promoted ductile tearing of polypropylene before failure continued in an unstable fashion via crazing. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1075–1089, 2004     

X-ray computed tomography-based modeling of polymeric foams: The effect of finite element model size on the large strain response

A hybrid numerical–experimental approach is used to characterize the macroscopic mechanical behavior of polymeric foams. The method is based on microstructural characterization of foams with X-ray computed tomography (CT) and conversion of the data to finite element (FE) models. The 2D models are created from a 3D close-celled foam and subjected to compression loads. Since the large strain regime is explored, contact between elements is incorporated. It is shown that, for calculating the effective Young's modulus, a model consisting of at least 11²–12² cells in the model should be used, whereas for the large strain regime 12²–14² cells in the model are needed. Discretization had a significant influence on the results, where relatively coarse elements caused loss of connectivity in the cell walls and thickening of the cell walls. It is shown that at least three to four elements should be taken over the thickness of the cell walls for these structures. Finally, a good qualitative agreement is observed between the deformations found with the FE models and in situ compression experiments of an open-celled foam during X-ray CT. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1473–1482, 2010     

陶瓷泡沫空气过滤器涂覆中孔TiO2薄膜用于室内空气净化中高效光催化降解NO

由于人们80%的时间呆在室内,室内空气的质量直接影响人类健康,因此近年来室内空气质量越来越受到人们的关注.室内污染物包括CO氮氧化物(NOx)和挥发性有机化合物(VOCs),它们给人体健康带来众多负面影响.更为重要的是,考虑到节能,现代建筑的空气密闭性大都较高,但这种减少吸入新鲜空气的设计直接导致室内各种污染物的累积.有些家用电器,如燃气灶和热水器,在使用的时候会涉及到煤、油和天然气的燃烧,特别是通风较差的情况下会成为室内主要的污染源.常规的治理技术,包括吸附和过滤,其成本相对较高,也不适用于低浓度污染物的治理.尤其是更换不及时的过滤器在排风系统中可能会成为VOCs的一个来源.因此,很有必要开发一种新型的技术以降低室内污染物的浓度和保持一个清洁的室内空气环境,从而保障人们的身体健康.光催化是去除室内空气污染物的有效方法.例如, TiO2、钛酸铋和钛酸锶等具有强氧化能力和稳定的光催化活性,因而是高效的光催化剂.一般而言,通常报道的TiO2光催化剂是高度分散的、或悬浮于液体介质中的细小颗粒或粉末.然而,粉末状的TiO2光催化剂不适宜于室内空气净化,因为它变得可吸入而对人体健康造成不利的影响.因此,人们尝试将TiO2颗粒作为薄膜固定在不同的刚性载体上,如玻璃、不锈钢和铝合金板.对基体进行涂覆可显著影响光催化时反应物的表面吸附行为.一般而言,光催化薄膜通常涂覆在平面上,如蜂窝空气过滤器.三维(3D)多孔的陶瓷泡沫对气体通过具有非常好的流体性质,因此本文以它作为涂覆的基体.这种陶瓷泡沫具有3D多孔结构,多种孔密度、比表面积和化学性质.3D多孔陶瓷泡沫空气过滤器的床层空隙率较高,因此使用时压降较低,且不像蜂窝空气过滤器,它具有复杂多变的孔结构,可增强流体的扰动和混合.另外,3D多孔陶瓷泡沫空气过滤器的开发多孔和网状的结构使得在催化体系具有非常好的气体动力学性质,催化剂表面和气体反应物有充分的接触.多孔材料在液相或气相催化反应中具有独特的优势,因此,陶瓷泡沫、多孔的氧化铝、多孔硅胶.分子筛和活性炭经常被用作催化剂载体.在固体基体上TiO2膜的形成可能使得TiO2光催化剂的有效比表面积降低,从而导致其光催化活性下降.然而,由于具有中孔结构的TiO2薄膜的比表面积大,其用于催化反应的活性位也更多,因此使用时仍然具有较高的活性.前期研究表明,涂覆在平面玻璃、不锈钢和氧化铝基体上的中孔TiO2薄膜用于环境净化时表现出增强的光催化效率.另外,室内环境中NO和NO2的浓度一般分别为几百个ppb之内和100 ppb以下.可见, NO是主要的室内空气污染物,对人体健康危害较大.基于此,本文首次采用反胶束法将中孔锐钛矿TiO2薄膜均匀一地涂覆在3D多孔高比表面积的泡沫过滤器上,采用X射线衍射、扫描电镜、X射线光电子能谱、N2吸附-脱附、紫外-可见光光谱和原子力显微镜对所制样品进行了表征,并将样品用于紫外光下催化降解NO,以揭示所制的中孔TiO2涂层具有高的比表面积和高的光催化活性,从而克服使用TiO2粉末所带来的不足.结果表明,由于中孔TiO2薄膜涂层具有较大的有效比表面积,其表面存在很多吸附活性位,用于吸附在反应过程中形成的水蒸汽、气相反应物和产物,因而具有更高的光催化活性,因此在陶瓷泡沫空气净化系统中可以高效地光催化NO降解：在所考察的不同孔密度的陶瓷泡沫过滤器涂覆的TiO2上400 ppb的NO单程转化率均在92.5%以上,高于涂覆在平面陶瓷砖上的TiO2.该陶瓷过滤器的3D多孔特性可增强流体的扰动和混合,使得气相反应物与光催化剂表面有着充分的接触;其大的孔密度也导致高的光催化速率.另外,本文所制样品在所有反应过程中均保持较高且稳定的NO降解速率,这表明其在NO降解反应中没有失活.     

The rheology of pseudoplastic fluids in porous media using network modeling

This paper considers the rheology of pseudoplastic (shear thinning) fluids in porous media. The central problem studied is the relationship between the viscometric behavior of the polymer solution and its observed behavior in the porous matrix. In the past, a number of macroscopic approaches have been applied, usually based on capillary bundle models of the porous medium. These simplified models have been used along with constitutive equations describing the fluid behavior (usually of power law type) to establish semiempirical macroscopic equations describing the flow of non-Newtonian fluids in porous media. This procedure has been reasonably successful in correlating experimental results on the flow of polymer solutions through both consolidated and unconsolidated porous materials. However, it does not allow an interpretation of polymer flow in porous media in terms of the flows on a microscopic scale; nor does it allow us to predict changes in macroscopic behavior resulting from variations at a microscopic level in the characteristics of the porous medium such as pore size distribution. In this work, we use a network approach to the modeling of non-Newtonian rheology, in order to understand some of the more detailed features of polymjer flow in porous media. This approach provides a mathematical bridge between the behavior of the non-Newtonian fluid in a single capillary and the macroscopic behavior as deduced from the pressure drop-flow rate relation across the whole network model. It demonstrates the importance of flow redistribution within the elements of the capillary network as the overall pressure gradient varies. As an example of a pseudoplastic fluid in a porous medium, we consider the flow of xanthan biopolymer. This polymer is important as a displacing fluid viscosifier in enhanced oil recovery applications and, for that reason, a considerable amount of experimental data has been published on the flow of xanthan solutions in various porous media.     

Construction by molecular dynamics modeling and simulations of the porous structures formed by dextran polymer chains attached on the surface of the pores of a base matrix: characterization of porous structures

Significant increases in the separation of bioactive molecules by using ion-exchange chromatography are realized by utilizing porous adsorbent particles in which the affinity group/ligand is linked to the base matrix of the porous particle via a polymeric extender. To study and understand the behavior of such systems, the M3B model is modified and used in molecular dynamics (MD) simulation studies to construct porous dextran layers on the surface of a base matrix, where the dextran polymer chains and the surface are covered by water. Two different porous polymer layers having 25 and 40 monomers per main polymer chain of dextran, respectively, are constructed, and their three-dimensional (3D) porous structures are characterized with respect to porosity, pore size distribution, and number of conducting pathways along the direction of net transport. It is found that the more desirable practical implications with respect to structural properties exhibited by the porous polymer layer having 40 monomers per main polymer chain, are mainly due to the higher flexibility of the polymer chains of this system, especially in the upper region of the porous structure. The characterization and analysis of the porous structures have suggested a useful definition for the physical meaning and implications of the pore connectivity of a real porous medium that is significantly different than the artificial physical meaning associated with the pore connectivity parameter employed in pore network models and whose physical limitations are discussed; furthermore, the methodology developed for the characterization of the three-dimensional structures of real porous media could be used to analyze the experimental data obtained from high-resolution noninvasive three-dimensional methods like high-resolution optical microscopy. The MD modeling and simulations methodology presented here could be used, considering that the type and size of affinity group/ligand as well as the size of the biomolecule to be adsorbed onto the affinity group/ligand are known, to construct different porous dextran layers by varying the length of the polymeric chain of dextran, the number of attachment points to the base matrix, the degree of side branching, and the number of main polymeric chains immobilized per unit surface area of base matrix. After the characterization of the porous structures of the different porous dextran layers is performed, then only a few promising structures would be selected for studying the immobilization of adsorption sites on the pore surfaces and the subsequent adsorption of the bioactive molecules onto the immobilized affinity groups/ligands.     

Novel drug delivery system by chitin derivative

A porous chitin foam was regenerated from chitin dope in calcium chloride dihydrate saturated methanol. The porous chitin foam was shown to have cationic property, because chitin foam tended to adsorb anionic dyes through ionic binding and hydrophobic interaction. A pendant type of polymeric drug was prepared applying peptide spacer composed of phenylalanine at amino end and two step hydrolyses of polymeric drug were shown to release active drug at the final step using lysozyme and chymotrypsin in vitro.