The effects of temperatures and volumetric expansion on the diffusion of fluids through solid polymers

This study examines moisture sorption behaviors of two glassy polymers, epoxy and vinylester, immersed in different fluids at two temperatures below the glass transition temperatures of the polymers. The main purpose of this study is to understand the effect of volume‐dependent temperatures and deformations on the diffusion process of solid polymers. Diffusivity coefficients are first determined by assuming the diffusion to follow the classical Fickian diffusion. In some cases, moisture sorption led to quite significant changes of volume, and the diffusion process cannot be well described by the Fickian diffusion. In such situation, the coupled deformation–diffusion model for linear elastic isotropic materials presented by Gurtin 1 is adopted, as a first approximation. This coupled deformation‐diffusion model reduces to a Fickian diffusion model when the coupling parameters are absent and the volume changes in the solid polymers during diffusion are negligible. A finite difference method is used in order to solve for the coupled deformation‐diffusion model. The model is used to predict the one‐dimensional moisture diffusion in thin plates and the multiaxial three‐dimensional moisture diffusion in dogbone specimens. The multiaxial diffusion in the dogbone specimens is used to validate the calibrated material parameters from the standard thin plate diffusion characterization. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45151.     

A phenomenological constitutive model for foams under large deformations

A nonlinear phenomenological constitutive model applicable to structural foams subjected to large deformations is proposed. The five‐parameter model can fully capture the three fundamental features of stress‐strain response, i.e., linearity, plasticity‐like stress plateau, and densification phases, when subjected to compressive loads. Moreover, the parameters of the model can be systematically varied to capture the influence of initial density of foams that may be responsible for changes in yield stress and hardening‐like or softening‐like behavior under various confinement conditions. The model was successfully applied to capture the stress‐strain response of two structural foams of different initial densities when subjected to uniaxial compression without lateral confinement and unixial compression under rigid confinement. Polym. Eng. Sci. 44:463–473, 2004. © 2004 Society of Plastics Engineers.     

Degradation and recovery of adhesion properties of deformed metal–polymer interfaces studied by laser induced delamination

Adhesion properties of polymer coatings on metals are of great interest in various industrial applications, including packaging of food and drinks. Particular interest is focused on polymer–metal interfaces that are subjected to significant deformations during manufacturing process. In this work steel samples laminated with polyethylene terephthalate (PET) were subjected to uniaxial tensile deformations followed by annealing treatments. The measurements have demonstrated degradation of adhesion of the metal–polymer interface as the strain introduced by the deformation increased. Moreover, it was observed that within the geometry used in the experiments tensile deformations of the metal substrate introduced in-plane compressive stresses in the bulk of the coating. After applying a thermal treatment restoration of the adhesion has been achieved.Laser induced delamination technique was used to monitor the adhesion properties. In this technique a coating is subjected to a series of infrared laser pulses with a stepwise increase of intensity. Upon increasing the laser pulse intensity, the pressure which is formed inside the blisters reaches a critical value, resulting in further delamination of the coating. To process the experimental data an elastic model was developed. From the analysis of the experimental data the critical stresses required for the delamination and the practical work of adhesion are derived. The model accounts for the compressive in-plane stress present in the coating of the deformed samples.     

Influence of molecular weight on strain‐gradient yielding in polystyrene

Experimental observations have indicated that the presence of strain gradients has an influence on the inelastic behavior of polymers as well as in other materials such as ceramics and metals. The present study has experimentally quantified length‐scale effects in inelastic deformations of the polymer material polystyrene (PS) with respect to the molecular length. The experimental technique that has been used is nano‐indentation to various depths with a Berkovich indenter. The hardness has been calculated with the method by Oliver and Pharr, and also by direct measurements of the area from atomic force microscopy. The experiments showed that the length‐scale effects in inelastic deformations exist in polystyrene at ambient conditions. The direct method gave a smaller hardness than the Oliver‐Pharr method. It was also shown that the length‐scale parameter according to Nix and Gao increases with increasing molecular weight. For high molecular weights above a critical value of entanglement, there was no pertinent increase in the length‐scale parameter. The length‐scale parameter for strain‐gradient plasticity has a size of around 0.1 μm for polystyrene. Polym. Eng. Sci. 44:1987–1997, 2004. © 2004 Society of Plastics Engineers.     

Finite strain 3D thermoviscoelastic constitutive model for shape memory polymers

A 3D thermoviscoelastic model is proposed to represent the thermomechanical behavior of shape memory polymers. The model is based on a physical understanding of the material behavior and a mechanical interpretation of the stress–strain–temperature changes observed during thermomechanical loading. The model is thermodynamically motivated and is formulated in a finite strain framework in order to account for large strain deformations. Model predictions capture critical features of shape memory polymer deformation and, in some cases, provide very favorable comparisons with experimental results. POLYM. ENG. SCI. 46:486–492, 2006. © 2006 Society of Plastics Engineers.     

Failure of elastic‐plastic core–shell microcapsules under compression

Characterization of the failure behavior of microcapsules is extremely important to control the release of their core actives by mechanical forces. The strain and stress of elastic‐plastic uninflated core–shell microcapsules at failure (rupture or bursting) has been determined using finite element modeling (FEM) and micromanipulation compression experiments. The ductile failure of polymeric microcapsules at high deformations is considered to occur when the maximum strain in the shell exceeds a critical strain, resulting in their rupture. FEM has been used to determine the maximum strains present in the capsule wall at different deformations for three types of shell material: elastic, elastic—perfectly plastic and elastic—perfectly plastic with strain hardening at large strains. The results obtained were used to determine the failure strain and stress of melamine‐formaldehyde microcapsules, with average population values of ～0.48 and ～350 MPa, respectively. Thus, the elastic‐plastic stress–strain relationship has been determined for the core–shell microcapsules tested. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Structure development in oriented polyethylene films and microporous membranes as monitored by sound propagation

Microporous membranes of high‐density polyethylene were prepared by melt‐extrusion followed by annealing and uniaxial extension. Crystallization at a high melt flow rate and subsequent annealing of the spun films with fixed ends led to the formation of oriented hard‐elastic materials with a high modulus of elasticity and a considerable work recovery. Uniaxial stretching of such systems along the orientation direction induced the formation of microscopic pores due to the specific structure of the hard‐elastic material. At some critical values of the processing parameters, through‐flow channels were formed, converting the film into a microporous membrane permeable to liquids and vapors. Sound propagation, tensile measurements, and X‐ray diffraction techniques were used to characterize the structure and properties of the samples at individual stages of the process as a function of the processing parameters. In particular, it was shown that polar diagrams of the sound propagation velocity reflect sensitively the structural changes in the process of porous structure formation. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 214–222, 2001     

Thermomechanical Modeling of Laser‐Induced Structural Relaxation and Deformation of Glass: Volume Changes in Fused Silica at High Temperatures

A fully coupled thermomechanical model of the nanoscale deformation in amorphous SiO₂ due to laser heating is presented. Direct measurement of the transient, nonuniform temperature profiles was used to first validate a nonlinear thermal transport model. Densification due to structural relaxation above the glass transition point was modeled using the Tool‐Narayanaswamy (TN) formulation for the evolution of structural relaxation times and fictive temperature. TN relaxation parameters were derived from spatially resolved confocal Raman scattering measurements of Si–O–Si stretching mode frequencies. Together, these thermal and microstructural data were used to simulate fictive temperatures which are shown to scale nearly linearly with density, consistent with previous measurements from Shelby et al. Volumetric relaxation coupled with thermal expansion occurring in the liquid‐like and solid‐like glassy states lead to residual stresses and permanent deformation which could be quantified. However, experimental surface deformation profiles between 1700 and 2000 K could only be reconciled with our simulation by assuming a roughly 2 × larger liquid thermal expansion for a‐SiO₂ with a temperature of maximum density ~150 K higher than previously estimated by Bruckner et al. Calculated stress fields agreed well with recent laser‐induced critical fracture measurements, demonstrating accurate material response prediction under processing conditions of practical interest.     

Effect of machine compliance on mold deflection during injection and packing of thermoplastic parts

Minimizing mold deflection is essential when manufacturing plastic parts to tight tolerances. Both the mold and the machine are compliant and deform upon loading, which can affect the part quality. Therefore, understanding mold deflection during injection molding is critical for determining the final geometry of the part. It is also critical for secondary processes such as the in‐mold coating process. This article presents work in quantifying both mold deflection during an injection‐molding cycle and the effect of machine compliance on mold behavior. The mold cavity pressure obtained using MoldFlow? was used as input for the subsequent finite element mold deflection analysis. Two different structural models were used: the first model included only the mold, the mold base units and the ejector platen; the second model included the effect of the injection‐molding machine compliance. To validate the model, strain gage rosettes were placed on the mold and the machine. Validating experiments were conducted using process parameters identical to those used in the simulations. A comparison of the experimental and simulation results for both models is presented. POLYM. ENG. SCI., 46:844–852, 2006. © 2006 Society of Plastics Engineers     

Mechanical characterization of agarose micro-particles with a narrow size distribution

A micromanipulation technique was used for the mechanical characterisation of two types of agarose microspheres with different material properties. Narrow-size distributions having a mean diameter in the range of 15–22 µm were prepared using (a) conventional emulsification followed by filtration and (b) membrane emulsification. Single microspheres were compressed to a range of deformations at different speeds up to a maximum of ~ 70 µm/s, and then held at constant deformation to permit relaxation to occur. It was found that the loading data could be satisfactorily described by the Hertz equation up to 30% deformation. The Young's moduli calculated on this basis were found to correlate with the gel strength of the agarose which was used to prepare the microspheres. However, the values of the moduli increased with the compression speed and significant stress relaxation occurred. Consequently, a modified Hertz analysis was employed that accounts for the viscoelastic behaviour. Two relaxation times were sufficient to describe the stress relaxation function. The Young's moduli from the Hertz analysis corresponded to the long-time values of the stress relaxation function, which is reasonable given the relatively slow compression speeds used. The predominant process occurring at short times was ascribed to water transport from the interior of the microspheres and the process occurring at longer times was interpreted as that arising from the residual viscoelasticity of the polymer network. As a result of the stress relaxation during loading, the Tatara model could not be used to describe loading data at large deformations.     

Prediction of creep of polymer concrete

Polymer concrete possesses viscoelastic properties conditioned by relaxation processes in the polymer binder. Their acceleration with an increase of temperature (principle of time–temperature equivalence) is used in predicting the long‐term creep of polymer concrete. Physical aging of the polymer binder influences the creep of polymer concrete. To predict the long‐term creep accounting for the aging process, an attempt to improve the time–temperature equivalence principle was undertaken. As a result of the experimental study of polyester resin‐based concrete and its structural components (a resin unfilled and filled with diabase flour), it has been established that the changes in the creep compliance of the material follow according to the principle of the time–aging time equivalence with the reduction function depending on aging temperature. To predict the long‐term creep of polymer concrete, a function of the time–temperature–aging time reduction was applied. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1949–1952, 1999     

Modeling of visco‐hyperelastic behavior of transversely isotropic functionally graded rubbers

In this article, visco‐hyperelastic constitutive model is developed to describe the rate‐dependent behavior of transversely isotropic functionally graded rubber‐like materials at finite deformations. Zener model that consists of Maxwell element parallel to a hyperelastic equilibrium spring is used in this article. Steady state response is described by equilibrium hyperelastic spring and rate‐dependence behavior is modeled by Maxwell element that consists of a hyperelastic intermediate spring and a nonlinear viscous damper. Modified and reinforced neo‐Hookean strain energy function is proposed for the two hyperelastic springs. The mechanical properties and material constants of strain energy function are graded along the axial direction based on exponential function. A history‐integral method has been used to develop a constitutive equation for modeling the behavior of the model. The applied history integral method is based on the Kaye‐BKZ theory. The material constant parameters appeared in the formulation have been determined with the aid of available uniaxial tensile experimental tests for a specific material and the results are compared to experimental results. It is then concluded that, the proposed constitutive equation is quite proficient in forecasting the behavior of rubber‐like materials in different deformation and wide ranges of strain rate. POLYM. ENG. SCI., 56:342–347, 2016. © 2016 Society of Plastics Engineers     

Experimental Study and DEM Simulation of Micro–Macro Behavior of TATB Granules During Compaction Using X‐ray Tomography

In this study, a noninvasive experimental method and a discrete element method (DEM) model were used to investigate the micro–macro behavior of TATB granules including global deformations, contact forces, stress distributions, local coordination number, local pore ratio, particle kinematics, and granule displacement during compacting process. The TATB granule deformation and structural evolution during compaction process were studied by X‐ray tomography and the deformation characteristics of deformable material. The size and number of granules with different sizes were determined from the CT images. The digital microstructure can be directly used for the numerical simulation setup. The DEM method was used for the numerical simulation of TATB granules during compaction. The contact forces and displacement vectors in compaction process were obtained. The behaviors and stress distributions at different pressures were also derived from the simulation. This study evaluated the possibility to better describe the behaviors of TATB granules.     

Influence of pores arrangement on stability of photonic structures during sintering

Discrete Element Method (DEM) has been used for numerical investigation of sintering-induced structural deformations occurring in inverse opal photonic structures. The influence of the initial arrangement of template particles on the stability of highly porous inverse opal α-Al₂O₃ structures has been analyzed. The material transport, densification, as well as formation of defects and cracks have been compared for various case studies. Three different stages of defects formation have been distinguished starting with local defects ending with intrapore cracks. The results show that the packing of the template particles defined during the template self-assembly process play a crucial role in the later structural deformation upon thermal exposure. The simulation results are in very good agreement with experimental data obtained from SEM images and previous studies by ptychographic X-ray tomography.     

Prediction and experimental verification of bubble and processing characteristics in blown‐film extrusion

Blown‐film modeling is useful to the flexible packaging industry for predicting process and bubble characteristics, such as freeze line height (FLH), bubble diameter, and film thickness. The use of a suitable rheological equation to describe material properties is critical in simulating the blown‐film process. In this article, we present an improved rheological constitutive equation, which incorporates more realistic parameters of stress and deformation properties of the materials by combining the Hookean model with the Phan‐Thien Tanner (PTT) model. The proposed PTT–Hookean model is aimed at enhancing the viscoelastic behavior of the melt during biaxial stretching in the blown‐film extrusion. Predictions of the blown‐film bubble characteristics and FLH obtained with the PTT–Hookean model agreed well with the experimental data of this study and previous studies with different materials and different die geometries. The justification for combining the Hookean model with the PTT model in the blown‐film process is also reported here. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

The application of digital image techniques to determine the large stress–strain behaviors of soft materials

Understanding the mechanical properties of soft materials such as stress–strain behavior over a large deformation domain is essential for both mechanical and biological applications. Conventional measurement methods have limited access to these properties because of the difficulties in accurately measuring large deformations of soft materials. In this study, we optimized digital image correlation (DIC) method to measure the large‐strain deformations by considering referencing scheme and frame rate. The optimized DIC was utilized to estimate strain in characterizing the stress–strain behavior of a polydimethylsiloxane (PDMS) elastomer as a model soft material. A series of comparative experimental studies and finite element analysis were performed; they indicated the advantages of optimized DIC over conventional methods such as robustness to slip, insensitivity to boundary conditions, and the ability to yield consistent and reliable results. These advantages enabled the optimized DIC to perform an in‐depth analysis of the behavior of soft materials at large strain domain. An empirical constitutive equation to describe the large stress–strain behavior of PDMS was proposed and verified by finite element simulations that show excellent agreements with experimental results. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Simulation of shale gas transport and production with complex fractures using embedded discrete fracture model

The goal of this study is to develop a new model to simulate gas and water transport in shale nanopores and complex fractures. A new gas diffusivity equation was first derived to consider multiple important physical mechanisms such as gas desorption, gas slippage and diffusion, and non‐Darcy flow. For complex fractures, a state‐of‐the‐art embedded discrete fracture model (EDFM) was implemented. Numerical model is verified against a commercial reservoir simulator for shale gas simulation with multiple planar fractures. After that, a series of simulation studies was performed to investigate the impacts of complex gas transport mechanisms and various fracture geometries on well performance. The critical parameters controlling well performance are identified. The simulation results reveal that modeling of gas production from complex fractures as well as modeling important gas transport mechanisms in shale gas reservoirs is extremely significant. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2251–2264, 2018     

A Numerical Study of Structural Change and Anisotropic Permeability in Compressed Carbon Cloth Polymer Electrolyte Fuel Cell Gas Diffusion Layers

The effect of compression on the actual structure and transport properties of the carbon cloth gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC) are studied here. Structural features of GDL samples compressed in the 0.0–100.0 MPa range are encapsulated using polydimethylsiloxane (PDMS) and by employing X‐ray micro‐tomography to reconstruct direct digital 3D models. Pore size distribution (PSD) and porosity data are acquired directly from these models while permeability, degree of anisotropy and tortuosity are determined through lattice Boltzmann (LB) numerical modelling. The structural models reveal that structural change proceeds through a three‐step process, while PSD data suggests a characteristic peak in the pore diameter of 10–14 μm and a decrease in the mean pore diameter from 33 to 12 μm over the range of tested pressures. A mathematical relationship between compression pressure and permeability is determined based on the Kozeny–Carman equation, revealing a one order of magnitude reduction in through‐plane permeability for a two order of magnitude increase in pressure. The results also reveal that the degree of anisotropy peaks in the range of 0.3–10.0 MPa, suggesting that in‐plane permeability can be maximised relative to through‐plane permeability within a material‐specific range of compression pressures.     

Thermally treated low density polyethylene biodegradation by Penicillium pinophilum and Aspergillus niger

Differential scanning calorimetry (DSC), X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used to determine morphological, structural and surface changes (biodegradation) on thermo‐oxidized (80°C, 15 days) low‐density polyethylene (TO‐LDPE) incubated with Aspergillus niger and Penicillium pinophilum fungi, with and without ethanol as cosubstrate for 31 months. TO‐LDPE mineralization by fungi was also evaluated. Significantly morphological and structural final changes on biologically treated TO‐LDPE samples were observed. Decreases to three units on crystallinity and crystalline lamellar thickness (0.4–1.8 Å), and increases in small‐crystals content (up to 3.2%) and mean crystallite size (8.4–14 Å) were registered. An oxidation decrease (almost twice) on samples without ethanol with respect to the control was observed, while in those with ethanol it was increased (up to 2.5 times). Double bond index increased more than twice from 21 to 31 months. The higher TO‐LDPE changes and fungi‐LDPE interaction was observed in samples with ethanol, suggesting that ethanol favors the TO‐LDPE biodegradation, at least in case of P. pinophilum, probably by means of a cometabolic process. Mineralization of 0.50 % and 0.57 % for A. niger, and of 0.64 % and 0.37 % for P. pinophilum were obtained, for samples with and without ethanol, respectively. A model to explain morphological and structural changes on biologically treated TO‐LDPE is also proposed. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 305–314, 2002