Changes In Dynamic Mechanical Properties Research Articles

Effect of Cross-Linking Density on Non-Linear Viscoelasticity of Vulcanized SBR: A MD Simulation and Experimental Study.

In recent years, there has been a growing interest in changes in dynamic mechanical properties of mixed rubber during dynamic shear, yet the influence of vulcanized characteristics on the dynamic shear behavior of vulcanized rubber, particularly the effect of cross-linking density, has received little attention. This study focuses on styrene-butadiene rubber (SBR) and aims to investigate the impact of different cross-linking densities (Dc) on dynamic shear behavior using molecular dynamics (MD) simulations. The results reveal a remarkable Payne effect, where the storage modulus experiences a significant drop when the strain amplitude (γ0) exceeds 0.1, which can be attributed to the fracture of the polymer bond and the decrease in the molecular chain's flexibility. The influence of various Dc values mainly resides at the level of molecular aggregation in the system, where higher Dc values impede molecular chain motion and lead to an increase in the storage modulus of SBR. The MD simulation results are verified through comparisons with existing literature.

Characterization on fatigue behavior and damage mechanism of C/BMI composites experienced thermal cycling

The multi-directional laminate CCF800H/AC631 bismaleimide composite material was exposed for a long time under the thermal-cycling environment (−60°C ∼+180°C), and the mass loss rate, FTIR spectra, DMA, tensile strength were tested. The fatigue stress level was determined according to the tensile strength and the fatigue performance of the before and after the thermal-cycling environment was tested. Macroscopic visual inspection and ultrasonic C-scan were used to characterize and analyze the fatigue damage of composite materials. The results show that with the increase in the number of thermal cycles, the mass loss of the composite material started with increased rapidly and then basically flat. The C/BMI composites underwent obvious thermal oxygen aging. After thermal-cycling, it would lead to changes in dynamic mechanical properties by a certain degree of post-curing, physical aging, and local interface debonding in composite materials. With the thermal cycles increased the composite material tensile strength first increased slightly and then decreased rapidly. After 300 thermal cycles, the composite materials occurred slightly damaged, and the fatigue life was apparently reduced compared with the original state. The fatigue failure modes of composite materials are mainly fiber fracture and multi-directional laminate delamination. At high stress levels, the stiffness of the specimen after thermal-cycling are lower decrease compared with original specimens, more stress levels would lead to more II stage rate of stiffness decline, and stiffness degradation curve and hysteretic energy recovery curve had enough effect to characterize damage effect of material environment induced by thermal-cycling environment factors.

Microfluidic tapered aspirators for mechanical characterization of microgel beads.

In this study, we report a microfluidic approach for the measurement of mechanical properties of spherical microgel beads. This technique is analogous to tapered micropipette aspiration, while harnessing the benefits of microfluidics. We fabricate alginate-based microbeads and determine their mechanical properties using the microfluidic tapered aspirators. Individual microgel beads are aspirated and trapped in tapered channels, the deformed equilibrium shape is measured, and a stress balance is used to determine the Young's modulus. We investigate the effect of surface coating, taper angle, and bead diameter and find that the measured modulus is largely insensitive to these parameters. We show that the bead modulus increases with alginate concentration and follows a trend similar to that of the modulus measured using standard uniaxial compression. The critical pressure to squeeze out the beads from the tapered aspirators was found to depend on both the modulus and the bead diameter. Finally, we demonstrate how temporal changes in bead moduli due to enzymatic degradation of the hydrogel could be quantitatively determined. The results from this study highlight that the microfluidic tapered aspirators are a useful tool to measure hydrogel bead mechanics and have the potential to characterize dynamic changes in mechanical properties.

Dynamic changes in mechanical properties of the adult rat spinal cord after injury.

Spinal cord injury (SCI), a debilitating medical condition that can cause irreversible loss of neurons and permanent paralysis, currently has no cure. However, regenerative medicine may offer a promising treatment. Given that numerous regenerative strategies aim to deliver cells and materials in the form of tissue-engineered therapies, understanding and characterising the mechanical properties of the spinal cord tissue is very important. In this study, we have systematically characterised the spatiotemporal changes in elastic stiffness (elastic modulus, Pa) and viscosity (drop in peak force, %) of injured rat thoracic spinal cord tissues at distinct time points after crush injury using the indentation technique. Our results demonstrate that in comparison with uninjured spinal cord tissue, the injured tissues exhibited lower stiffness (median 3281 Pa versus 9632 Pa; P<0.001) but demonstrated elevated viscosity (median 80% versus 57%; P<0.001) at 3 days postinjury. Between 4 and 6 weeks after SCI, the overall viscoelastic properties of injured tissues returned to baseline values. At 12 weeks after SCI, in comparison with uninjured tissue, the injured spinal cord tissues displayed a significant increase in both elasticity (median 13698 Pa versus 9920 Pa; P<0.001) and viscosity (median 64% versus 58%; P<0.001). This work constitutes the first quantitative mapping of spatiotemporal changes in spinal cord tissue elasticity and viscosity in injured rats, providing a mechanical basis of the tissue for future studies on the development of biomaterials for SCI repair. STATEMENT OF SIGNIFICANCE: Spinal cord injury (SCI) is a devastating disease often leading to permanent paralysis. While enormous progress in understanding the molecular pathomechanisms of SCI has been made, the mechanical properties of injured spinal cord tissue have received considerably less attention. This study provides systematic characterization of the biomechanical evolution of rat spinal cord tissue after SCI using a microindentation test method. We find spinal cord tissue behaves significantly softer but more viscous immediately postinjury. As time passes, the lesion site gradually returns to baseline values and then displays pronounced increased viscoelastic properties. As host tissue mechanical properties are a crucial consideration for any biomaterial implanted into central nervous system, our results may have important implications for further studies of SCI repair.

Analysis of subsurface damage inhibition in magnetization-enhanced force-rheological polishing

Subsurface damage (SSD) during polishing silicon carbide (SiC) is an unavoidable problem in mould production. However, studies on the law of SSD generation in the magnetisation-enhanced force-rheological polishing process are limited. An SSD depth prediction model is established considering factors such as strain rate effect, elastic-plastic deformation, and dynamic changes in mechanical properties. A detection technology for the SSD depth of aspheric mould was proposed, and the influence mechanism of orthogonal tests on the SSD state of SiC mould was studied. The results show that the maximum deviation between the calculated and experimental results is less than 7%. The SSD depth dropped to the minimum threshold (< 0.5 µm) under a specific combination of parameters.

Properties of silica-filled rubber compounds vs. epoxidized oil content and degree of epoxidation

Epoxidized vegetable oils are well-known as biobased, sustainable additives for polymers. However, relatively little work has been done to explore their effects on silica-filled rubber composites in depth. In order to address this knowledge gap, high purity epoxidized soybean (Glycine max (L.) Merr.) oils with varying levels of epoxidation were synthesized and compounded with a model silica-filled styrene-butadiene rubber formulation. The green properties, process characteristics, and cured properties (quasi-static and dynamic mechanical characteristics) of the resultant compounds were then assessed. A ∼7 % decrease in mixing energy was observed with the inclusion of ∼0.8 vol% of 100 % epoxidized soybean oil. Changes in cure kinetics and dynamic mechanical properties were observed as a function of epoxy group concentration. The changes in dynamic mechanical properties are explained in terms of changes in silica dispersion state as a consequence of epoxidized soybean oil addition.

A Method to Analyze Plantar Stiffness Variation in Diabetes Using Myotonometric Measurements

Abstract Diabetes mellitus is a group of metabolic disease, which has become globally prevalent, and affects a large population in socio-economically backward countries in Asian continent. Chronic diabetes can lead to ulceration in the plantar region and may result in amputation. Assessment of mechanical properties of plantar tissues can aid in early diagnosis of ulceration. Myotonometry, a technique to measure dynamic stiffness, is preferred due to its noninvasiveness, easy employability, and rapid investigation. In this study, an attempt has been made to analyze the changes in biomechanical properties of plantar soft tissue in diabetes. MyotonPro, a handheld device, is used for this purpose. 43 diabetic subjects with varied duration of diabetes are recruited. Site-specific mechanical properties of the plantar region for both the feet are acquired and statistical analysis is performed. Results show that the MyotonPro is able to differentiate the stages of diabetes. It is seen that there is a spatial variability in the mechanical properties of the plantar. Additionally, it is observed that there is a significant increment in the plantar stiffness value in the group with higher diabetic age (p &amp;lt; 0.05). Further, significant changes in dynamic mechanical properties are also observed in submetatarsal region. Additionally, a right–left asymmetry has been observed in frequency and stiffness values for later stages of diabetes. This study demonstrated the feasibility of MyotonPro in discriminating the stages of diabetic period. Thus, the proposed approach could be useful in early diagnosis of foot ulceration for various clinical conditions.

Dynamic Mechanical Analysis and Thermal Stability of Neem Wood Veneer Plastic Composites

The neem wood veneer is used as core materials in many of the wood structural applications. In this research work, neem wood polymer composite (NWPC) were prepared to understand the effect of weight fraction, stacking sequence, and interfacial bonding between neem wood veneer and epoxy material. NWPC were prepared conventional compression molding and Thermo-mechanical properties of wood epoxy composite samples were investigated by the dynamic mechanical analyzer over the temperature range from 30 to 180°C. Storage modulus (G’) and Loss modulus (G”) of NWPC samples exhibited significant effect with respect weight fraction of wood veneer. Similarly epoxy has good interfacial bonding agent to enhance the strength of multilayered wood polymer composites. However, the stacking sequence having major changes in dynamic mechanical properties, G’ & G” increases along the wood fiber direction with negligible load bearing capacity in transfers direction. By changing the orientation of the wood veneer properties improved in both the direction. Additionally, scanning electron microscopy (SEM) evidently shows the interfacial bonding efficiency

Evaluation of the dynamic mechanical property changes in porcine crystalline lenses during induced cataract formation

Evaluation of the dynamic mechanical property changes in porcine crystalline lenses during induced cataract formation

Selective decomposition of hexabromocyclododecane in polystyrene and recyclability improvement of its polymeric component

Selective flame retardant decomposition and polymer recycling in a polystyrene (PS) material containing hexabromocyclododecane (HBCD) were studied using a paint-type copper phthalocyanine modified TiO2 (CuPcTiO2)/polyethylene oxide (PEO) photocatalyst system with three kinds of hindered amine light stabilizer (HALS) loadings. They were performed using visible light irradiation and additional thermal hybrid treatments. Mobility of the HALS with lightest weight was highest and caused the bleeding from the matrix during the irradiation treatment, leading to a lower decomposition ratio and larger changes of dynamic mechanical properties of the recycled PS. On the other hand, the higher molecular weight HALS loadings provided the higher ratios and blocked the changes. In particular, the properties in the HALS loading with a dormant (TEMPO) bond were hardly changed by such hybrid treatment. The py-GC/MS spectra of the extraction residue showed that the HALS adequately worked depending on the treatment situation as radical scavenger and initiator. The tensile strength and strain at break values of the recycled PS kept 90% of the pristine one. In addition, there were no significant differences between the SEM images. The results showed that it had sufficient performance to be used as the post-consumer material.

Dynamic changes of mechanical properties induced by friction in the Archard wear model

Fretting is small-amplitude, oscillatory motion between two bodies leading to surface damage. During the fretting process, a tribologically transformed structure (TTS) which has different mechanical properties and microstructure than the starting material is formed on the surface. The commonly-used Archard wear equation relating wear volume to the hardness of the worn material does not account for changes in mechanical properties induced by friction in fretting. To investigate that effect, ball-on-plane fretting tests were conducted on three engineering materials (type 316 stainless steel, pure copper, and titanium alloy Ti-6Al-4V) against an alumina ball to generate TTS layers. The evolution of mechanical properties and microstructures of TTS layers were investigated using nanoindentation and focused ion beam-scanning electron microscope (FIB-SEM). Wear volumes after different fretting cycles were measured with a white light interference microscope. Results show that the mechanical properties of TTS layers evolve differently on different materials during the fretting process. Microstructures of TTS layers also vary from one material to the other. A modified wear model that accounts for friction-induced dynamic changes in mechanical properties is proposed. In these tests the modified model was able to predict the wear volume of 316 steel and pure copper more accurately than the classical Archard model, but it was less successful in predicting wear on Ti6Al4V where there is added complexity from changing microstructure, oxidation, porosity and cracking.

Molecular motors in materials science

Abstract

Mechanical and Dynamic Mechanical Studies on Epoxy-Cobaltous Sulfate Polymer Hybrids

Cobaltous sulfate heptahydrate (CoSO4·7H2O) was incorporated as filler into diglycidyl ether of bisphenol A (DGEBA) based epoxy resin system, to prepare organic-inorganic polymer hybrid materials. Mechanical tensile studies and dynamic mechanical analysis (DMA) were carried out in order to study the static and dynamic mechanical properties of the prepared hybrid films. Mechanical tensile studies were carried out at room temperature, at a test speed of 30 mm/min. Highest tensile strength of 24.74±2.42 MPa was achieved for 4.44 wt% filler level (FL), along with an increase in the value of Young’s modulus. Storage modulus (E′), loss modulus (E″), damping factor (tan δ) were obtained by DMA studies. Glass transition temperature (Tg) was obtained for pure epoxy and filled epoxy, for various FLs varying from 0.28 wt% to 5.00 wt%. Pure epoxy showed highest Tg value compared to filled epoxy hybrids. Highest storage modulus of 9.5 GPa was obtained for 2.22 wt% FL, which also showed highest loss modulus peak. Parameters like effectiveness coefficient (C) and crosslink density were calculated from the storage modulus data. Loss modulus and tan δ curves were analyzed to study the energy dissipation properties of prepared hybrid films. Activation energy (Ea) value for glass transition was obtained from damping factor (tan δ), which showed highest Ea value of 630.5 kJmol-1, for 4.44 wt% FL. DMA studies for various FLs were carried out at different test frequencies in order to study the changes in dynamic mechanical properties of the prepared hybrid materials with respect to frequency

Effect of Free-Volume Holes on Dynamic Mechanical Properties of Epoxy Resins for Carbon-Fiber-Reinforced Polymers

Six types of matrices of carbon-fiber-reinforced polymers were prepared from different epoxies, amines, and thermoplastics at different curing conditions. Dynamic mechanical analysis measurements were performed to investigate the dynamic mechanical properties of storage modulus E′, loss modulus E″, damping factor tan δ, and complex viscosity |η*|. Positron annihilation lifetime (PAL) spectroscopy was carried out to evaluate each size and fraction of free-volume holes in the sample. The correlations between the dynamic mechanical properties and relative free-volume fraction were studied by using the Williams–Landel–Ferry equation. With increasing relative free-volume fraction, regular changes of dynamic mechanical properties are revealed: log[E′(T)] and log[|η*|(T)] decrease linearly in the temperature range of Tg(PAL) < T < Trub(E′) (Tg(PAL) is the glass transition temperature determined by PAL measurements; Trub(E′) is the lowest temperature where E′(T) curve coincides with its fitting line in the rubber...

Time-dependent changes in dynamic mechanical properties of irradiated bone.

The increased use of allograft tissue for musculoskeletal repair has brought more focus to the safety of allogenic tissue and the efficacy of various sterilization techniques. The currently available literature contains few examples of studies on long-lasting strains of bones but no example for irradiated bones. In this study the bovine femurs from a 2-year-old animal were machine cut and irradiated with the doses of 10, 15, 25, 35, 45 and 50kGy. The dynamic mechanical analysis was performed at 1Hz at the room temperature in a 3-point bending configuration for 2880minutes. The final values of E' and E″ were dose independent but they were reached at different periods. For this reason, so called "critical point" was introduced for the further analysis. All the examined sample groups were characterized by statistically significant lower values of the critical point in comparison with the control samples (p<0.05) but the biggest differences were observed between the control samples and the samples irradiated with the doses of 10, 15 and 25kGy. Current results and literature review suggest that the dose of 35kGy is the optimal dose for ionizing radiation sterilization.

Probing regional mechanical properties of embryonic tissue using microindentation and optical coherence tomography.

Physical forces regulate morphogenetic movements and the mechanical properties of embryonic tissues during development. Such quantities are closely interrelated, as increases in material stiffness can limit force-induced deformations and vice versa. Here we present a minimally invasive method to quantify spatiotemporal changes in mechanical properties during morphogenesis. Regional stiffness is measured using microindentation, while displacement and strain distributions near the indenter are computed from the motion of tissue labels tracked from 3-D optical coherence tomography (OCT) images. Applied forces, displacements, and strain distributions are then used in conjunction with finite-element models to estimate regional material properties. This method is applicable to a wide variety of experimental systems and can be used to better understand the dynamic interrelation between tissue deformations and material properties that occur during time-lapse studies of embryogenesis. Such information is important to improve our understanding of the etiology of congenital disease where dynamic changes in mechanical properties are likely involved, such as situs inversus in the heart, hydrocephalus in the brain, and microphthalmia in the eye.

The Mechanics of Change

Stem Cells As embryonic stem cells differentiate toward specific cell fates, shifts in the gene regulatory network accompany progression through the different states: from pluripotency, where cells are competent to differentiate into any somatic cell; to a transition state; and finally to a differentiated state where cells are irreversibly committed to a particular cell fate. Pagliara et al. used atomic force microscopy to compare the elastic stiffness of mouse cells in each of these different fates. When compressing the cytoplasm of cells in all of these states, a 5 to 10% increase in the cytoplasmic area was observed. However, only nuclei of cells in the transition state became smaller upon compression. When materials become denser with such mechanical stress, they are termed auxetic. Differences in chromatin condensation were observed by electron microscopy of the three cell states with considerably less-condensed nuclei during transition relative to the pluripotent or differentiated state. This work shows that the nucleus undergoes dynamic changes in mechanical properties and chromatin structure as cells transition toward a differentiated state. It is proposed that this auxeticity may be important for cell mechanotransduction. Nat. Mater. 10.1038/NMAT3943 (2014).

Changes in Large Pulmonary Arterial Viscoelasticity in Chronic Pulmonary Hypertension

Conduit pulmonary artery (PA) stiffening is characteristic of pulmonary arterial hypertension (PAH) and is an excellent predictor of mortality due to right ventricular (RV) overload. To better understand the impact of conduit PA stiffening on RV afterload, it is critical to examine the arterial viscoelastic properties, which require measurements of elasticity (energy storage behavior) and viscosity (energy dissipation behavior). Here we hypothesize that PAH leads to frequency-dependent changes in arterial stiffness (related to elasticity) and damping ratio (related to viscosity) in large PAs. To test our hypothesis, PAH was induced by the combination of chronic hypoxia and an antiangiogenic compound (SU5416) treatment in mice. Static and sinusoidal pressure-inflation tests were performed on isolated conduit PAs at various frequencies (0.01–20 Hz) to obtain the mechanical properties in the absence of smooth muscle contraction. Static mechanical tests showed significant stiffening of large PAs with PAH, as expected. In dynamic mechanical tests, structural stiffness (κ) increased and damping ratio (D) decreased at a physiologically relevant frequency (10 Hz) in hypertensive PAs. The dynamic elastic modulus (E), a material stiffness, did not increase significantly with PAH. All dynamic mechanical properties were strong functions of frequency. In particular, κ, E and D increased with increasing frequency in control PAs. While this behavior remained for D in hypertensive PAs, it reversed for κ and E. Since these novel dynamic mechanical property changes were found in the absence of changes in smooth muscle cell content or contraction, changes in collagen and proteoglycans and their interactions are likely critical to arterial viscoelasticity in a way that has not been previously described. The impact of these changes in PA viscoelasticity on RV afterload in PAH awaits further investigation.

Thermally Induced Changes in Dynamic Mechanical Properties of Native Silks

Dynamic mechanical thermal analysis (DMTA) on individual native silk fibers demonstrates changes in the dynamic mechanical properties of storage modulus and loss tangent as a function of temperature and temperature history ranging from -100 to 250 °C. These property changes are linked quantitatively to two main types of change in the silk structure. First, the evaporation of water with increasing temperature up to 100 °C increases the storage modulus and removes two characteristic loss tangent peaks at -60 and +60 °C. Second, various discrete loss tangent peaks in the range 150-220 °C are associated with specific disordered silk structures that are removed or converted to a limiting high-temperature relaxed structure by the combination of increasing temperature and static load in the DMTA tests. The results identify important origins of silk filament quality based on the analysis of measurements that can be traced back to differences in production and processing history.

Effects of 2,2,2-trifluoroethyl methacrylate on properties of autopolymerized hard direct denture reline resins

This study evaluated the effects of the fluorinated monomer of 2,2,2-trifluoroethyl methacrylate (TFEMA) on the properties of autopolymerized hard direct denture reline resins. Iso-butyl methacrylate (i-BMA) and 2-hydroxyethyl methacrylate (2-HEMA) containing 30% TFEMA by weight were used as monomers, while poly(ethyl methacrylate) was used as a powder. Setting characteristics, dynamic mechanical properties, and changes over time, as well as wettability were determined by use of an oscillating rheometer, dynamic viscoelastometer, and contact angle meter. Water absorption and solubility were also measured according to ISO specifications. The reline resin based on i-BMA had greater elasticity and stiffness properties, while that based on 2-HEMA showed fewer dynamic mechanical property changes over time with the addition of TFEMA. Furthermore, water absorption and solubility tended to be reduced and contact angle increased. The results of this study suggest that TFEMA improves mechanical properties and durability of reline resins over time.