Mechanical Behavior Of Polymers Research Articles

Drivers of vascular growth and remodeling: A computational framework to promote benign adaptation in the Ross procedure

In the sixties, Dr Donald Ross designed a surgical solution for young patients with aortic valve disease by using the patients’ own pulmonary valve. The Ross procedure is the only aortic valve replacement technique that can restore long-term survival and preserve quality of life. The main failure mode of the Ross procedure is wall dilatation, potentially leading to valve regurgitation and leakage. Dilatation occurs due to the inability of the pulmonary autograft to adapt to the sudden increase in loading when exposing to aortic pressures. Previous experimental data has shown that a permanent external support wrapped around the artery can prevent the acute dilatation of the arterial wall. However, the textile support leads to stress-shielding phenomena due to the loss of mechanical wall compliance. We present a pragmatic and modular computational framework of arterial growth and remodeling predicting the long-term outcomes of cardiovascular tissue adaptation, with and without textile wrapping. The model integrates mean, systolic and diastolic pressures and assumes the resulting wall stresses to drive the biological remodeling rules. Rather than a single mean pressure or stress deviation from the homeostatic state, we demonstrate that only pulsatile stresses can predict available experimental results. Therefore, we suggest that a biodegradable external support could induce benign remodeling in the Ross procedure. Indeed, a biodegradable textile wrapped around the autograft fulfills the trade-off between prevention of acute dilatation on the one hand and recovery of arterial wall compliance on the other hand. After further validation, the computational framework can set the basis for the development of an actual biodegradable external support for the Ross procedure with optimized polymer mechanical properties and degradation behavior.

Effect of temperature and strain rate on the formation of shear bands in polymers under quasi-static and dynamic compressive loadings: Proposed constitutive model and numerical validation

The mechanical behaviour of polymers is strongly influenced by temperature and strain rate conditions. Their effects are clearly observed in the Young modulus and Yield strength assessment. In some cases, a change in material failure mode can be noted, which can be both ductile and brittle. For polymers exhibiting shear bands during compression loading, test conditions may enhance or diminish this effect. In this study, isotactic Polypropylene has been considered due to its strong sensitivity to test conditions; with some combinations of strain rate and temperature, the material is able to undergo large compression with nearly uniform strain distribution, whereas significant shear bands can be observed with other strain rate and temperature combinations. This paper proposes a constitutive law for the mechanical characterization of polymers that show shear banding in compression loading. The proposed law extends a visco-plastic model present in literature, allowing to predict the onset or not of micro-shear banding compression instability. The dependence of constitutive parameters with test conditions is determined by applying the Ree-Eyring equation. The model parameters have been calibrated analytically according to experimental data obtained from compression tests performed in a wide range of strain rates (10−3 to 103 1/s) and temperatures (253–313 K). In addition, the constitutive model has been implemented in a commercial FEM code by means of a user-defined subroutine. Results show a good correspondence between experimental observations and numerical calculations, regarding both stress-strain relationship and shear band formation.

Influence of soapstone waste on the mechanical and rheological properties of high‐density polyethylene

AbstractSoapstone is an abundant mineral in Ouro Preto ‐ Minas Gerais, Brazil and its main destination is in the production of craftsmanship. Rock recovery in those activities is low and the waste disposal is done with little control, which can be hazardous to the environment. This work proposes an alternative use of such potentially harmful waste as reinforcement in a novel polymer matrix composite, which can be particularly attractive to the automotive industry and of which very little information is available elsewhere in the literature. Firstly, the characterization of the waste was performed. Particle size and shape parameters were determined by automated image analysis and the mineralogical composition was determined by X‐ray diffraction, infrared, and Raman spectroscopy. High‐density polyethylene was used as matrix and the composites were made in three matrix/filler ratios: 90/10, 80/20, and 70/30 by weight. Tensile and rheological properties were measured in order to determine the influence of the particles on the polymer mechanical behavior and processing conditions. The materials showed a pseudoplastic behavior and the filler's influence was more pronounced in the 70/30 composites, which showed higher viscosities than the neat polymer. The addition of particles resulted in more brittle and rigid composites, with higher values of tensile strength.

Combining mechanical and thermal surface fourier transform analysis to follow the dynamic fatigue behavior of polymers

This work investigates the phenomena of self-heating, also called intrinsic heating, and thermoelastic coupling during non-linear dynamic mechanical fatigue testing via surface temperature measurement coupled with the mechanical behavior of polymers. Static tensile tests and dynamic strain controlled fatigue tests under tension/tension were performed at a frequency of ω1/2π = 5 Hz, as well as in the low cycle fatigue regime at ω1/2π = 0.2 Hz, on six polymers: high density polyethylene (HDPE), low density polyethylene (LDPE), ultra high molecular weight polyethylene (UHMWPE), polyamide 6 (PA6), and two grades of polypropylene (PP).In dynamic testing, the surface temperature rises to a plateau value (ΔT) when an equilibrium between the viscous/plastic dissipated energy and heat convection is reached. Power-law correlations were found between the strain amplitude (ε0) and ΔT, as well as between ε0 and the calculated dissipated energy density (Wdiss,p) obtained from the mechanical stress response, with similar exponents for both correlations. Thermoelastic coupling is firstly investigated in uniaxial tension, revealing a linear relation between the strain rate and the rate of temperature decrease, which is more distinct with decreasing polymer chain mobility. In dynamic fatigue testing, the surface temperature was found to oscillate with an amplitude T1, which was analyzed via Fourier transform. A direct relation between T1 and ε0 at small deformations was observed. At large strain amplitudes, T1 (ε0) follows a similar trend as the complex modulus E*(ε0). At low frequencies and large strain amplitudes, additional higher harmonics at two (T2) and three (T3) times the fundamental frequency were also detected as fingerprints of plastic deformation, resulting in additional heat dissipated during the loading half cycle. From the results obtained, the advantages of the calculated dissipated energy density over the surface temperature analysis was analyzed to predict the fatigue behavior. This analysis is believed to be valid for all materials due to the mathematical/physical principles involved. The results are thus expected to hold for other materials such as composites, rubbers, ceramics and metals.

An MD study of the polymer–polymer adhesion via connector chains: some aspects of the competition between bulk dissipation in the interphase and pull-out of interface-connecting molecules

Atomistic simulation approach was used to analyze the separation behavior of polymer–polymer interfaces. This was achieved using coarse-grained molecular dynamics (MD) simulations of some connector chains embedded in two adjacent polymer entangled melts. To separate the contribution of polymer molecules pull-out from the micro-bulk dissipation in the viscoplastic interphase, surrounding the surface of separation, two kinds of simulations were considered: (a), simulations of the pull-out mechanism, where the extraction of the connectors out of the melts takes place through a complex mechanism of forced reptation only; (b), simulations of the full debonding process where the micro-bulk dissipation contributes to the overall crack opening and can skew the description of the interface separation by pure connector molecules pull-out. From the MD simulation results, some relevant parameters (e.g., a cohesive strength or an adhesion energy) of the separation behavior of the interface being modelized were extracted. In addition, one was able to separate quantitatively connector pull-out from micro-bulk dissipation (in the surrounding bulk region of the interface). This means that we were able to emphasize: (i) how the straining divides into interface opening and bulk deformation and (ii), how the macroscopic opening rate diffuses throughout the melts and the interface opening rate, since the mechanical behavior of polymers is strain-rate dependent. A comparison between two kinds of MD simulations, where a system is stretched along one direction, is shortly discussed as a necessary prelude to future investigations.

Experimental and numerical investigation into mechanical degradation of polymers

Polymers have myriad uses in the modern world, but their advantageous properties can be compromised by degradation induced via moisture absorption. This work presents a novel reduced order numerical model for the mechanical behavior and degradation of polymers, along with experimental results, illustrating the influence of water absorption on 3D printed and injection molded nylon 6. The nylon 6 specimens are immersed in water at 21∘C and 70∘C, temperatures above and below the glass transition temperature. Both the water absorption behavior and the influence of the absorbed water on thermomechanical and tensile properties are investigated. The hygroscopic nylon 6 absorbs up to 10% water when immersed at 21∘C. The rate of absorption for the 3D printed specimens is higher than that of the injection molded specimens at both temperatures. This absorbed water enhances polymer chain mobility and results in significantly lower storage modulus and glass transition temperatures after only 1 day of immersion. Consequently, as much as 30% reduced modulus and strength are observed during this period. The constitutive model capably captures the degradation of polymer mechanical behavior as a function of water concentration for both materials and temperatures investigated.

Effects of 1°, 2°, and 3° Thiols on Thiol–Ene Reactions: Polymerization Kinetics and Mechanical Behavior

The effect of thiol substitution in radical thiol–ene reactions has been studied by using model, monofunctional thiols as well as multifunctional thiol monomers along with the assessment of their subsequent polymerization reactions and polymer mechanical behavior. FT-IR was used to monitor the polymerization rate and quantify the overall conversion. While the total conversion was observed to range from 70% to 100%, the polymerization rate was found to decrease by as much as 10-fold as the thiol substitution was changed from primary to tertiary. Analogous multi-thiol monomers of similar structure but varying substitution were synthesized to observe the effect of substitution type on polymerization kinetics and polymer behavior. Methylation at the α-carbon was varied from primary to tertiary to observe these differences. Mechanical properties were assessed by using dynamic mechanical analysis and water sorption experiments, where the glass transition temperatures were found to be within 1–2 °C as thiol substitution varied. Furthermore, primary thiol films absorbed 1–3% more water than secondary thiol films. Resin shelf stability experiments were performed by using rheometry to measure storage time-dependent viscosity changes, and it was found that secondary thiol films remained relatively stable for up to 100 times longer than their primary counterparts. It was concluded that while there are differences under relatively slow initiation conditions, at typical initiation rates all three thiol substitutions may be made to react at similar rates for both monofunctional and polymeric systems.

Dynamics of Entangled Networks in Ultrafast Perforation of Polystyrene Nanomembranes

The mechanical behavior of polymers at ultrahigh strain rates can be very different from their typical behavior because of the inertia effect, wave propagation, adiabatic processes, and rate-dependent polymer chain dynamics. Polymer chain entanglements play a significant role in the mechanical properties of polymers. By employing microscopic ballistic testing in vacuum, the high-strain-rate intrinsic behavior of polystyrene (PS) is demonstrated depending on its molecular weight and microprojectile’s perforation speed. Based on the experimentally quantified correlation between the specific penetration energy and the entanglement density, we predict the upper limits of the specific penetration energy of PS.

“Green” environmental crazing of polymers in oil-in-water emulsions with high water content

This work addresses “green” environmental crazing of polymers in biphase oil-in-water (O/W) emulsions with high water content (above 95 vol %), which can serve as an ecologically safe alternative to pure organic solvents (hydrocarbons and aliphatic alcohols as oils) with respect to their effect on the mechanical behavior of polymers and stress-induced development of porosity via environmental crazing. This approach is shown to be universal for both modes of environmental crazing as intercrystallite crazing for semicrystalline (high-density polyethylene, polytetrafluoroethylene) and classical crazing for amorphous glassy polymers (PET). The mechanism behind the action of the O/W emulsions on polymers is discussed. The applied advantages of the use of the O/W emulsions with high water content for the preparation of mesoporous and nanocomposite polymer materials are highlighted.

Effect of the build orientation on the mechanical behaviour of polymers by stereolithography

The use of Rapid Prototyping techniques for the production of end-use parts is increasing in rapid manufacturing. The mechanical properties of stereolithography parts become the most important factor restricting its wide application. The mechanical properties of stereolithography parts can be not only influenced by the material characteristics, but also by the parameters of manufacturing. The purpose of this study is to investigate the influences of the build orientation on mechanical properties of stereolithography parts. Specimens were fabricated with different build orientations related to layout (flat or on an edge), axis and build angle. Mechanical testing including tensile, compressive and impact tests on stereolithography specimens were conducted. The results indicated that layout had a significant effect on tensile and impact properties, and the axis had no significant effect on tensile and impact properties. Also, build angles play an important factor in the compressive strength. From these results, it appears that related characteristics of layer interface have a significant influence on mechanical properties of stereolithography specimens.

Thermo-elastic-viscoplastic-damage model for self-heating and mechanical behavior of thermoplastic polymers

A thermo-elastic-viscoplastic-damage model based on thermodynamics is developed to describe the self-heating and stress-strain behavior of thermoplastic polymers under tensile loading. The constitutive model considers temperature-dependent elasticity, nonlinear viscoplastic flow and damage evolution. The model includes the important self-heating of a polymer caused by the viscoplasticity and the often neglected damage energy dissipation during deformation. The model parameters are calibrated from the monotonic and repeated loading tests for polyamide 6 under different temperatures and loading rates. In particular, the damage evolution is evaluated by the stiffness reduction in the repeated loading tests. The proposed model is implemented in a finite element package to predict the self-heating and mechanical behavior of the polymer at a high loading rate. The predicted temperature and load evolution curves agree with the experimental data.

An Adam‐Gibbs based model for the temperature behavior of polymers near glass transition

AbstractThe mechanical behavior of polymers and other glass forming materials is strongly time‐ and temperature‐dependent. Based on the rheological Poynting‐Thomson Model and the Adam‐Gibbs equation, a shift function is developed which relates the time constants of a material to temperature. The behavior of this function is compared to the established Williams‐Landel‐Ferry and Arrhenius equations in the glassy and rubbery temperature regimes.

Advanced Short‐Term Approaches to Estimate the Long‐Term Mechanical Behavior of Polymers at Static Loading Conditions

Conventional methods for testing the long‐term resistance against creep and slow crack growth (SCG) become more and more unusable for highly improved polymers due to the required long times of often more than 1 year. Therefore, there is a great need for developing new time‐saving methods for predicting the lifetime of such polymers. In this study, the authors describe one of these methods in more detail. The strain hardening method (SHM) is based on uniaxial tensile testing to estimate the time to failure. It has been shown that the strain hardening modulus can be easily determined using SHM with testing times of only a few hours. Due to the same molecular processes during SHM and conventional methods such as full notch creep test (FNCT) and Pennsylvania edge notch tensile (PENT) test, the strain hardening modulus covers the SCG resistance in pipe‐grade high‐density polyethylenes (PE‐HD) to a high degree.

Analyzing the Mechanical Behavior of Polymer and Composite Materials by Means of Unique Method of Deformation Calorimetry

Results are presented from long-term investigations of a wide range of polymer systems, varying from elastomers and thermoplastic elastomers to plastics and fibers. The thermophysical properties of both initial and modifying additive–containing polysiloxanes, block copolymers, and poleolefins that differ in chemical nature, structure, and composition are analyzed. It is shown that deformation calorimetry allows the simultaneous registration of mechanical (from 5 × 10−3 kg) and thermal effects (at a sensitivity of 2 × 10‒7 J/s), and the determination of changes in enthalpy, internal energy, and intra- and intermolecular contributions to the formation of the tensile stress response. In other words, it provides a unique opportunity to analyze the deformation mechanism of investigated systems and its dependence on the changing parameters.

Experimental investigation on the ballistic resistance of polymer-aluminum laminated plates

An experimental investigation on four kinds of polymer-aluminum laminated plates impacted by blunt- and ogival-nosed projectiles was carried out. The polycarbonate (PC) layer (2 mm) and the polymethyl methacrylate (PMMA) layer (2 mm) were used to make an array with the AA2024-T4 layer (2 mm), respectively, constituting these double layered polymer-aluminum laminated plates with no adherence. To better understand the strength and ductility difference among these materials, quasi-static tensile tests were conducted for them. The impact tests of pure AA2024-T4 layer (3 mm) which has close areal density with the bi-layered plates were compared in ballistic resistance. In each impact test, the projectile was driven by a gas-powered gun at impact velocities above and below the ballistic limit velocity of the target plates, mainly ranging from 60–150 m/s. And the initial and residual velocities of the projectiles impacting the targets were measured by a Photron FASTCAM SA5 high-speed camera. Based on these results, the ballistic limit velocities of seven kinds of structures (including three kinds of different thickness aluminum layer) impacted by blunt- and ogival-nosed projectiles were obtained. And after the impact tests, the plastic deformation of the aluminum alloy in each bi-layered structure was measured.It is shown that the polymer layer order, nose shape and polymer mechanical behavior affects the performance of the polymer-aluminum laminated plates. The PC is much better than the PMMA in the bi-layer plates in terms of improving the ballistic resistance of the aluminum. Furthermore, the structures with the polymer placed in the impact side performed better than that in the back side for blunt-nosed projectiles, which is opposite to that for ogival-nosed projectiles. And the polymer-aluminum laminated plates with the PC placed in the impact side for blunt-nosed projectiles have advantages comparing with pure aluminum alloy at same mass in this study.

Ketoprofen Loaded in Natural Rubber Latex Transdermal Patch for Tendinitis Treatment

Ketoprofen is an analgesic with potent anti-inflammatory activity against acute inflammation, subacute inflammation, for the acute and long-term treatment of various inflammatory pathologies, as rheumatoid arthritis and colonic adenocarcinoma. In order to minimize the incidence of systemic events related to ketoprofen, the transdermal drug delivery system development has been most important. The advantages of using natural rubber latex membranes include not only the reduction of adverse systemic events, but also the suitability of the low cost of the material together with its physicochemical properties such as flexibility, mechanical stability, surface porosity and water vapor permeability, and besides being a biocompatible material also presents biological activity to stimulate the angiogenesis, being able to be used in tissue repair. This study demonstrated that ketoprofen was successfully incorporated into natural latex membranes for drug delivery. FTIR indicated that the drug did not interact chemically with the membrane. Moreover, the natural latex membranes released 60% of the ketoprofen incorporated in 50 h. SEM images indicated that a portion of the drug was present on the membrane surface, being this portion responsible for the burst release. The tensile tests showed that the addition of the drug into the natural latex membrane did not influence on the polymer mechanical behavior. In addition, drug-natural latex membranes presented no red blood cell damaging effects. Our data shows that the ketoprofen loaded natural latex membranes is a promising system for sustained drug delivery which can be used to minimize the adverse side effects of high dose systemic drug delivery.

Failure of SLS polyamide 12 notched samples at high loading rates

The failure load of polymeric notched samples is necessarily affected by the loading rate due to the extreme influence of strain rate on the mechanical behaviour of polymers, and consequently, the characteristic stress in the Theory of Critical Distances (TDC) should be influenced by the loading rates. The objective of this work is to study the effect of strain rate on the parameters determined by the TDC using notched samples in additive manufactured polyamide PA12. To this end, tensile specimens have been prepared by Selective Laser Sintering (SLS) with different notch radius. Experiments in a wide range of strain rate have been carried out, from those representatives to static conditions in electromechanical machines to Hopkinson bar tests at very high strain rates.

Injection molding simulation with solid semi-crystalline polymer mechanical behavior for ejection analysis

Injection molded products, produced from semi-crystalline polymers, may include undercut features which can introduce distortion to the shape of the product during ejection. A thermo-mechanical modeling approach for simulating these advanced ejection problems is developed. The approach is formed by combining a method for three-dimensional residual stress prediction and an advanced material model for modeling the solid visco-elasto-plastic mechanical behavior. The task of this work is to assess, by analyzing a plaque-like product, the performance of the approach in the absence of the distortive ejection effects. The numerically predicted product shrinkage and mass at different packing pressure settings are compared to experimental results. The effect of packing pressure on product shrinkage and mass was reproduced by the model and the final residual stress field was found to be in accordance with the expectations. This confirms that the methodology could be used to analyze advanced ejection problems.

Damage study of Acrylonitrile Butadiene Styrene test pieces subjected to uniaxial loading

In industrial sectors such as: aeronautics, automotive or general mechanical industries, etc. Mechanical components are subjected to several types of stresses: thermal, chemical, mechanical and cyclical. These constraints cause a progressive degradation of the characteristics of the material, leading to a sudden rupture. For this purpose we have studied the mechanical behavior of an amorphous polymer, Acrylonitrile Butadiene Styrene (ABS), thought uniaxial tensile testing on pierced test pieces and with different notch lengths ranging from 1 to 14 mm. The proposed approach consists in analyzing the evolution of the global geometry of the strain curves obtained by considering the zones and the characteristic points of these curves and also taking into account the effect of the damage on the mechanical behavior of the polymers namely the ABS

Study on Mechanical Characteristics of Nanoclay Reinforced Polymer Composites

The present work is focused on analysis and influence of nanoclay on the mechanical behavior of polymers viz., epoxy and polyester. Different types of nanocomposites are prepared using 5 wt% and 10 wt% nanoclay with 95 wt% and 90 wt% epoxy and polyester respectively. Nanoclay is mixed with the resin using a mechanical stirrer for 2 hours to ensure the proper distribution of the reinforcement. Casting technique is used to prepare the specimens according to ASTM standard. The specimens are tested for tensile strength and hardness. 5 wt% of nanoclay increased the tensile strength of resins, but 10 wt% decreased the tensile strength. Hardness value of epoxy resin increased by increasing the wt% of nanoclay, whereas there is no much change in the hardness value of the polyester resin.