Phonons in suspensions of hard sphere colloids: volume fraction dependence

The propagation of sound waves in suspensions of hard sphere colloids is studied as a function of their volume fraction up to random close packing using Brillouin light scattering. The rich experimental phonon spectra of up to five phonon modes are successfully described by theoretical calculations based on the multiple scattering method. Two main types of phonon modes are revealed: Type A modes are acoustic excitations which set up deformations in both the solid (particles) and the liquid (solvent) phases; for type B modes the stress and strain are predominantly localized near the interface between the solid particles and the surrounding liquid (interface waves). While the former become harder (increase their effective sound velocity) as the particle volume fraction increases the latter become softer (the corresponding sound velocity decreases).     

Fading memory of deformation history in carbon black-filled thermoplastic elastomers

Observations are reported on a carbon black–reinforced thermoplastic elastomer in multistep uniaxial tensile cyclic tests with a mixed deformation program (oscillations between maximum elongation ratios k_max and various minimum stresses σ_min with k_max monotonically increasing with number of cycles n). Fading memory of deformation history is demonstrated: when specimens are subjected to two loading programs that differ along the first n −1 cycles of deformation and coincide afterwards, their stress–strain diagrams become identical starting from the nth cycle. A constitutive model is developed in cyclic viscoplasticity with finite deformations, and its adjustable parameters are found by fitting the observations. Ability of the stress–strain relations to describe the fading memory phenomenon and to predict the mechanical response of polymer composites in multi-step cyclic tests with large strains is confirmed by numerical simulation.     

An investigation of the yield and postyield behavior and corresponding structure of poly(methyl methacrylate)

The mechanical behavior of glassy polymers is time and temperature dependent as evidenced by their viscoelastic and viscoplastic response to loading. The behavior is also known to depend strongly on the prior history of the material, changing with time and temperature without chemical intervention. In this investigation, we examine the effects of this process of physical aging on the yield and postyield behavior and corresponding evolution in the structural state of glassy polymers. This has been achieved through a systematic program of uniaxial, isothermal, constant strain–rate tests on poly(methyl methacrylate) (PMMA) specimens of different thermal histories and by performing positron annihilation lifetime spectroscopy (PALS) measurements prior to and after mechanical deformation. PALS is an indicator of the free volume content, probing size and density of free volume sites and can be considered to be a measurement of structural state. The results of the mechanical tests show that aging acts to increase both the initial yield stress and the amount of strain softening which occurs subsequent to yield. Moreover, the amount of strain softening was found to be independent of strain rate indicating that softening is related to an evolution in structure as opposed to deformation kinetics. Furthermore, after sufficient inelastic straining, the initial thermal history is completely erased as evidenced by identical values of flow stress following strain softening, for both annealed and quenched polymer. Strong confirmation of the structural state or free volume related nature of the strain softening process is obtained by our companion PALS measurements. PALS detects an increase in the size of free volume sites following inelastic deformation and finds the initially annealed and quenched specimens to posses the same post-deformation distribution. The size of sites is found to evolve steadily with inelastic strain until it attains a steady-state value. This evolution of free volume with strain follows the observed softening of the flow stress to a steady-state value. These results provide experimental evidence that an increase in free volume with inelastic straining accompanies the strain softening phenomenon in glassy polymers and that strain softening is indeed a de-aging process. Based on our experimental results a mechanistically based constitutive model has been formulated to describe the effects of thermal history on the yield and postyield deformation behavior of glassy polymers up to moderate strains. The model is found to successfully capture the effects of physical aging, strain softening, strain rate, and temperature on the inelastic behavior of glassy polymers when compared with experimental results. © 1993 John Wiley & Sons, Inc.     

Self‐recovery and fatigue of double‐network gels with permanent and reversible bonds

Double‐network (DN) gels subjected to cyclic deformation (stretching up to a fixed strain followed by retraction down to the zero stress) demonstrate a monotonic decrease in strain with time (self‐recovery). Observations show that the duration of total recovery varies in a wide interval (from a few minutes to several days depending on composition of the gel), and this time is strongly affected by deformation history. A model is developed for the kinetics of self‐recovery. Its ability to describe stress–strain diagrams in cyclic tests with various periods of recovery is confirmed by comparison with observations on several DN gels. Numerical simulation reveals pronounced enhancement of fatigue resistance in multi‐cycle tests with stress‐ and strain‐controlled programs when subsequent cycles of deformation are interrupted by intervals of recovery. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 438–453     

Toward a Cohesive Theory of Polymerization Volume Change, 1

Summary: Rational design of polymer‐based composites must include an understanding of how and why polymerization volume change occurs. Computational chemistry methods offer significant leverage in such processes. An obstacle to their use has been the meager amount of systematic volume change data collected under the same conditions and using the same methods. This work provides volume change data for eight oxiranes using the mercury dilatometry method. Densities of pure monomers are often unknown for newly synthesized compounds, but are required for the correction of the composite to monomer volume change. The densities have been estimated here by the application of a newly‐developed quantum mechanically‐based quantitative structure property relationship (QMQSPR). This computational chemistry model can be used to estimate densities of a large array of organic compounds with sufficient accuracy for most routine purposes. These results are presented herein.

Correspondence between experimental and QMQSPR calculated results for densities.     

A novel dynamic constitutive micromechanical model to predict the strain rate dependent mechanical behavior of glass/epoxy laminated composites

In the present research, a novel dynamic constitutive micromechanical (DCM) model was developed to predict the strain rate dependent mechanical behavior of laminated glass/epoxy composites. The present model is an integration of the generalized strain rate dependent constitutive model as a constitutive model for the neat polymer, the plasticity model of Huang as a micromechanical model, and dynamic progressive failure criteria. This model is able to predict the longitudinal and transverse tensile and in-plane shear behaviors of unidirectional glass/epoxy composites with arbitrary fiber volume fractions at arbitrary strain rates. The present model can also predict the stress-strain behavior of laminated composites with different layups and fiber volume fractions at arbitrary strain rates. A comparison between the results predicted by the present model and the available experimental data showed that the model predicts the strain rate dependent mechanical behavior of glass/epoxy composites with very good accuracy.     

Effect of tensile load on the actuation performance of pH‐sensitive hydrogels

pH‐responsive hydrogels are capable of converting chemical energy to mechanical work. To optimize their use as actuators, their response when operating against an external load must be fully characterized. Here, the actuation strain of a model pH‐sensitive hydrogel as a function of different constant loads is studied. The experimental actuation strain, produced by switching the pH from 2 to 12, decreases significantly and monotonically with increasing initial tensile load. Two models are developed to predict the actuation strain as a function of applied stress. Simple mechanical models based on the change in hydrogel modulus and cross sectional area due to the change in pH are unsatisfactory as they predict only a small change in actuation strain with increasing external stress. However, the model based on the elastic and mixing free energy functions derived from the Flory–Huggins theory is found to accurately account for the actuation strain as a function of stress. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 218–225     

Temperature and variable strain rate sensitivity in recycled HDPE

The recycling of post-consumer plastics and their utilization as raw materials to develop value-added products has become an important goal worldwide. The present work is concerned with the thermo-mechanical analysis of recycled high-density polyethylene (HDPE) under uniaxial tensile loading. The main focus is to propose a one-dimensional phenomenological model able to describe the influence of temperature and strain rate on the mechanical behavior. Tensile tests were performed over a wide range of temperatures (from 25°C to 100°C). Each experiment was performed under controlled strain rate varying from 7.25 × 10⁻⁵ s⁻¹ to 7.25 × 10⁻³ s⁻¹ in steps. It is shown that only one tensile test performed at three different temperatures is necessary to fully identify experimentally all material parameters that arise in the theory. Thus, with this experimental procedure, the number of tests used to evaluate the mechanical properties of recycled HDPE is significantly reduced. The experiments are compared with the model predictions and show good agreement.     

Form-stable electrospun nanofibrous mats as a potential phase change material

A series of Poly vinyl butyral–Poly (acrylic acid) (PVB-PAA) based form-stable phase change materials (PCMs) have been prepared for the use of thermal energy storage applications. Six types of formulations containing five different fatty alcohols were prepared by adding PVB to PAA. Using electrospinning to fabricate nanofibrous mats, our aim was to investigate their properties as form-stable PCMs. Fatty alcohols, 1-Tetradecanol, 1-Hexadecanol, 1-Octadecanol, 1-Eicosanol and 1-Docosanol, were added separately to base formulation. The structural characterization tests were performed by ATR-FTIR spectroscopy. Morphological tests were conducted using Scanning Electron Microscope (SEM). Thermal performances and phase change behaviors were tested by thermogravimetric analysis system (TGA) and differential scanning calorimetry (DSC). The heating cycle phase change enthalpy is measured between 223 and 241?J/g, and the freezing cycle phase change enthalpy is found between 215 and 239?J/g. The main decomposition PVB-PAA based PCMs started at 220?°C. This study suggested that PVB-PAA based PCMs possess well phase change properties and they were found to have an applicable temperature range. With the presented results these materials promise a great potential in thermal energy storage applications.     

Cavitation free energy for organic molecules having various sizes and shapes

Cavitation free energy DeltaG(cav), corresponding to the formation of an excluded volume cavity in water, is calculated for a large set of organic molecules employing the thermodynamic integration procedure, which is realized as the original two-step algorithm for growing the interaction potential between the hard cavity wall and the water molecules. A large variety of solute systems is considered. Their characteristic radii change in the range 3-7 A; spherical cavities with radii 3-6 A are also studied. The interaction between water molecules is described by the four-site nonpolarizable TIP4P model. The diversity of the trial molecular set is provided by using a specially formulated nonspherical criterion classifying the cavity shapes according to their deviation from a sphere. Molecular objects were partly taken from the data base NCI Diversity with the aid of this criterion. The so-computed free energies are approximated by the linear volume dependence DeltaG(cav)V = XiV, where V is the cavity volume. This relation works fairly well until the cavity size becomes very large (the effective radius larger than 7 A). The volume dependence valid for solutes of arbitrary shapes can be included in a calculation of the nonpolar free energy component as required in the implicit water model.     

Pressure Effects in Adsorption Systems

A study of the occurrence of large pressure and flow transients when a strongly adsorbed gas is fed to a column which is initially loaded with a lightly adsorbed gas is presented here. Under certain conditions, these transients can cause premature breakthrough and change the shape of the breakthrough curve. This will result in improper estimation of adsorption parameters by the dynamic column loading method and lower apparent adsorption capacity in a full scale unit. A data acquisition system was used to record the pressure and flow transients. An isothermal PDE model developed to study these transients agreed reasonably well with the nonisothermal experimental results. The PDE model predicts that pressure and flow transients will occur during step and pulse~tests conducted to obtain adsorption and mass transfer parameters by the chromatographic method. For instance, lower adsorption capacity will be realized during step tests due to lowering in column pressure. Oscillations were observed when columns are connected in series. The PDE model also predicts these oscillations. Simulations indicate that the extent of oscillations is dependent on the dead volume between columns.     

Investigation for impact behavior of acrylonitrile-butadiene-styrene amorphous thermoplastic

Acrylonitrile-Butadiene-Styrene (ABS) is an important terpolymer that find applications in numerous engineering fields due to its high impact resistance. Thereby, the experimental characterzation and numerical validation of its impact behavior is the main focus of this investigation. Impact tests were carried out using hemi-spherical impactor at three velocities of 4.43 m/s, 5.775 m/s and 6.264 m/s, respectively. The localized material change caused by microvoids was noticed near the impact zone on non-impacted surfaces for all impact velocities. The damage morphologies on the non-impacted surface for 4.43 m/s includes plastic deformation and crazes without any microvoids, whereas a combination of crazes and microvoids were discovered for other two velocities. Tensile tests at various strain rates, compression and shear tests were performed on ABS material at quasi-static conditions to utilize as an input to the SAMP-1 material model in impact simulations. The predicted impact histories and damage morphologies were compared with the experimental results.     

Detailing the visco-elastic origin of thermo-mechanical training of twisted and coiled polymer fiber artificial muscles

Artificial muscles made be twisting and coiling polymer fibers provide outstanding performance. However, these materials show inconsistency in their non-loaded length that depends on their thermo-mechanical history. Typically, this behavior has been treated by “training” the samples before any actuation testing. A change in sample length occurs during training but remains consistent during subsequent heat/cool cycles at the same applied load. In this study, the training effect is investigated for a twisted and coiled nylon yarn heated over two temperature ranges: 25–50°C and 50–75°C. The training effect was most obvious in the lower temperature range, but nearly absent in the higher temperature range. When loaded below the glass transition temperature (T_g ~ 40°C) the viscoelastic strain occurs slowly but is rapidly released when the sample is first heated above T_g. The net effect of the first heating through T_g after loading is a small length change because the contraction due to actuation is offset by the expansion due to the release of the viscoelastic strain. A simple spring and dashpot model was developed and by changing only two relaxation times it was possible to simulate the observed training phenomena.     

Characterizing capsule-substrate adhesion in presence of osmosis

The adhesion of a cellular entity (liquid-filled microcapsule) to a flat glass substrate in response to an osmosis change is studied. A sensitive microscope visualization instrument has been developed to measure the cell–substrate contact area and inflated capsule volume. A theoretical model is developed to quantitatively correlate the adhesion energy to the contact area and osmotic inflation of cell volume. The results show that the contact area increased with increasing adhesion energy, while it shrank in dimension as cell inflation was enlarged. This observed phenomenon is consistent with the theoretical prediction. This work demonstrates the possibility of obtaining quantitative interfacial adhesion energy by using the present technique and represents the first step in extending this approach to study more complicated system such as cell–substrate interaction.     

Pollutants leaching behaviour from solidified wastes: a selection of adapted various models

Leaching tests are essential in the environmental assessment of stabilized wastes. Research programmes were conducted on their interpretation in order to develop tools for the evaluation of long term release of pollutants contained in solidified wastes. Models for the leaching of porous materials are discussed in this paper according to the specificity of the chemical species (i.e. transport model with total dissolution of species-diffusional model; transport model with progressive dissolution of species due to limitation of solubility-shrinking core model; and the model coupling transport and chemical phenomena). The leaching behaviour of pollutants (i.e. lead) solidified in a cement matrix was studied under different chemical conditions. Results have shown that the release of species whose solubilities depend on the physico-chemical conditions, and especially the pH (e.g. amphoteric metals), is governed by the solubility of the species in the pore water at local conditions and by the pH evolution within the matrix. A coupled dissolution/diffusion model was developed to describe the release of chemically complex species contained in a porous medium in contact with water. Leaching tests of cement matrices and artificial porous matrices containing calcium hydroxide and pollutants were conducted in order to validate the coupled dissolution/diffusion model. A good assessment of the retention of some pollutants contained in cement matrices could then be obtained by the association of two tests: solubilization of the pollutants related to the chemical context (pH) under steady state conditions and monolithic long term dynamic leaching tests in order to characterize the evolution of the chemical context (pH) and consequently the release of pollutants. The objective is to integrate this approach in the standardization process (CEN TC 292- WG 6, in progress).     

Removal of 2,4-Dichloro phenoxy acetic acid pesticide by solvent sublation: Experimental and theoretical studies

The removal of the pesticide 2, 4 D from water using solvent sublation process was investigated in this paper. A lab scale unit was set up and various experimental runs were carried out to study the efficiency of the removal process. The experimental findings show that the method is very effective (>90% removal) in removing traces (ppb level) of the pesticide which is not easily removable by simple air stripping. In addition a mathematical model was developed to describe the experimental findings. Some parameters of the model were measured or calculated while others such as the aqueous mass transfer coefficient and the solute partition coefficient were adjusted to fit the experimental data. The calibration of the model was carried out using the experimental results of change in gas flow rate (the easiest parameter to vary). A numerical sensitivity analysis was carried out using the calibrated model to study the effect of various parameters such as the bubble radius, aqueous phase drag-up by air, column radius and ratio of organic to aqueous volume phases.     

Hemocompatibile Thin Films Assessed under Blood Flow Shear Forces

The aim of this study was to minimize the risk of life-threatening thromboembolism in the ventricle through the use of a new biomimetic heart valve based on metal–polymer composites. Finite volume element simulations of blood adhesion to the material were carried out, encompassing radial flow and the cone and plane test together with determination of the effect of boundary conditions. Both tilt-disc and bicuspid valves do not have optimized blood flow due to their design based on rigid valve materials (leaflet made of pyrolytic carbon). The main objective was the development of materials with specific properties dedicated to contact with blood. Materials were evaluated by dynamic tests using blood, concentrates, and whole human blood. Hemostability tests under hydrodynamic conditions were related to the mechanical properties of thin-film materials obtained from tribological tests. The quality of the coatings was high enough to avoid damage to the coating even as they were exposed up to maximum loading. Analysis towards blood concentrates of the hydrogenated carbon sample and the nitrogen-doped hydrogenated carbon sample revealed that the interaction of the coating with erythrocytes was the strongest. Hemocompatibility evaluation under hydrodynamic conditions confirmed very good properties of the developed coatings.     

The Nonlinear Time‐Dependent Response of Semicrystalline Polymers: Correlations Between the Configurational Entropy and the Rate of Rearrangement of Chains

Two series of tensile creep tests are performed on isotactic poly(propylene) in the sub‐yield region of deformations at room temperature. In the first series, injection‐molded specimens are used as produced, whereas in the other series the samples are preloaded (five loading–unloading cycles with the maximal strain 0.01). A constitutive model is derived for the viscoelastic and elastoplastic behavior of semicrystalline polymers. A polymer is treated as an equivalent transient network of chains bridged by junctions. Active chains separate from their junctions and dangling chains merge with the network at random times when they are thermally activated. The network is modelled as an ensemble of meso‐regions (MRs) with various activation energies for detachment of chains from temporary nodes (a distribution function for the activation energies entirely determines the configurational entropy of the ensemble). Rearrangement of chains in the network reflects the viscoelastic response. The elastoplastic behavior is attributed to sliding of junctions with respect to their reference positions and to changes in the distribution of activation energies (driven by fine and coarse slip of lamellar blocks). Stress–strain relations for a semicrystalline polymer are determined by four adjustable parameters that are found by fitting the experimental data. It is demonstrated that the attempt rate for detachment of active chains from their nodes is proportional to the configurational entropy of the ensemble of MRs.     

Critical strains in tensile deformed polyamide 6 and 6/66 copolymer

The recovery properties of dry and water saturated polyamide 6 (PA6) and its copolymer PA6/66 (ratio 4:1 by mol) were studied at elevated temperatures above the glass‐transition temperature in uniaxial tensile tests. The data yield critical points along the true stress–strain curves at which the differential compliance and the recovery property change. These critical points include the onset of the plastic deformation (point A), the yield point (B), and the point where the elasticity of the samples reaches a plateau value (C). The strains at points A and B remain constant, whereas the strain at point C varies with temperature. The invariance of the critical strains at points A and B is assumed to be the result of the homogeneous strain distribution in the system and the general activation of the intralamellar block slip mechanism at low deformations. The strain at point C, being related to the properties of the entangled network, varies because the effective entanglement density of the network changes due to the change in the hydrogen bond number with temperature. With the Gaussian model of Haward and Thackray, we calculated the network moduli. From these data, we derived that the network stress remains constant at point C. At point C, the deformation mechanism starts to change from the block slip mechanism to a stress‐induced melting–recrystallization process. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 87–96, 2005