Dynamic Mechanical Analysis Tests Research Articles

Improving damping capabilities of composites structures by electroactive films containing piezoelectric and conductive fillers

In this paper, a passive vibration damping concept based on multifunctional materials was studied for thermoplastic composite structures. The synergy between piezoelectric and conductive particles brings a new contribution of energy dissipation based on the local transduction-dissipation phenomenon. While piezoelectric fillers ensure the conversion of mechanical energy into electrical energy (transduction), conductive particles locally dissipate the electric charges created avoiding saturation in the vicinity of piezoelectric particles. Here, the concept has been studied at material and structure scales for laboratory and preindustrial samples in order to bring solid proof of the damping concept. For this purpose, piezoelectric and electrically conductive particles were dispersed into engineering thermoplastics polyamide 12 and poly ether ketone ketone. Damping films were obtained by hot press and embedded in a composite sandwich beam and carbon fiber reinforced polymer (CFRP)-aluminum panels. Dynamic mechanical analysis and vibration tests were performed on bulk nanocomposite samples and in composite sandwich beams. The study of hysteresis loops and frequency response function showed strong nonlinear effects and vibration amplitude decrease up to 50%. Tests on CFRP-aluminum panels highlighted the structural damping increase demonstrating the potential capacity of this multifunctional material for energy dissipation in typical aerospace structures.

On the Mechanical Response of Silicon Dioxide Nanofiller Concentration on Fused Filament Fabrication 3D Printed Isotactic Polypropylene Nanocomposites.

Utilization of advanced engineering thermoplastic materials in fused filament fabrication (FFF) 3D printing process is critical in expanding additive manufacturing (AM) applications. Polypropylene (PP) is a widely used thermoplastic material, while silicon dioxide (SiO2) nanoparticles (NPs), which can be found in many living organisms, are commonly employed as fillers in polymers to improve their mechanical properties and processability. In this work, PP/SiO2 nanocomposite filaments at various concentrations were developed following a melt mixing extrusion process, and used for FFF 3D printing of specimens’ characterization according to international standards. Tensile, flexural, impact, microhardness, and dynamic mechanical analysis (DMA) tests were conducted to determine the effect of the nanofiller loading on the mechanical and viscoelastic properties of the polymer matrix. Scanning electron microscopy (SEM), Raman spectroscopy and atomic force microscopy (AFM) were performed for microstructural analysis, and finally melt flow index (MFI) tests were conducted to assess the melt rheological properties. An improvement in the mechanical performance was observed for silica loading up to 2.0 wt.%, while 4.0 wt.% was a potential threshold revealing processability challenges. Overall, PP/SiO2 nanocomposites could be ideal candidates for advanced 3D printing engineering applications towards structural plastic components with enhanced mechanical performance.

Multi–scale enhancement mechanisms of graphene oxide on styrene–butadiene–styrene modified asphalt: An exploration from molecular dynamics simulations

The objective of this research is to clarify the mechanism of interaction between graphene oxide (GO) and styrene–butadiene–styrene (SBS) modified asphalt. In this paper, the GO/SBS–modified asphalt was prepared using GO with contents of 0.1 wt%, 0.3 wt%, and 0.6 wt%. The dynamic mechanical analysis (DMA) test was applied to characterize the viscoelastic properties of the asphalt. Fluorescence microscopy and atomic force microscopy (AFM) were employed to observe the internal structure and interface enhancement. Fourier transform infrared spectroscopy (FTIR) was used to determine whether some compounds were removed from the base asphalt based on the variations of the functional group. Furthermore, molecular dynamic simulations were performed to understand the subject and conduct the performance evaluation of the experiments. The results explain the interaction mechanism between GO and SBS–modified asphalt and provide multi–scale insights into the performance evaluation for the design of modified asphalt polymer materials.

Deeper insights into the thermal properties of epoxy thermoset resins using torsion measurements

AbstractThe glass transition temperature (Tg) of epoxy thermosets is a critical material property that depends on the component chemistry, the final cross‐link density, and processing conditions. This study incorporates dynamic mechanical analysis (DMA) testing with a torsion clamp geometry on a TA Instruments DHR‐2 and differential scanning calorimetry (DSC) to characterize five different two‐component epoxy‐amine systems. Investigation of the Tg dependence on DMA frequency and heating shows that lowering the frequency from 1 to 0.01 Hz results in a Tg very similar to that measured using DSC, while a heating rate of 0.3°C/min using DMA gives a Tg comparable to the DSC measured value at 30°C/min. The DMA technique reveals secondary relaxation transitions and peak broadening in the tan(δ) plots of poorly mixed epoxy blends, quantified using full width at half maximum (FWHM) of tan(δ) peaks, and are indicative of a non‐homogeneous cross‐linked network and off‐ratio blending, respectively. The increase in the FWHM due to poor mixing ranges from 8% to 96%. These parameters are easily measurable and quantifiable in DMA, but are not observed in DSC. The additional DMA insights are valuable for process development and failure analysis, and can improve the understanding of epoxies.

Rubbery-Modified CFRPs with Improved Mode I Fracture Toughness: Effect of Nanofibrous Mat Grammage and Positioning on Tanδ Behaviour.

Carbon Fiber Reinforced Polymers (CFRPs) are widely used where high mechanical performance and lightweight are required. However, they suffer from delamination and low damping, severely affecting laminate reliability during the service life of components. CFRP laminates modified by rubbery nanofibers interleaving is a recently introduced way to increase material damping and to improve delamination resistance. In this work, nitrile butadiene rubber/poly(ε-caprolactone) (NBR/PCL) blend rubbery nanofibrous mats with 60 wt% NBR were produced in three different mat grammages (5, 10 and 20 g/m2) via single-needle electrospinning and integrated into epoxy CFRP laminates. The investigation demonstrated that both mat grammage and positioning affect CFRP tanδ behaviour, evaluated by dynamic mechanical analysis (DMA) tests, as well as the number of nano-modified interleaves. Double cantilever beam (DCB) tests were carried out to assess the mat grammage effect on the interlaminar fracture toughness. Results show an outstanding improvement of GI,R for all the tested reinforced laminates regardless of the mat grammage (from +140% to +238%), while the effect on GI,C is more dependent on it (up to +140%). The obtained results disclose the great capability of NBR/PCL rubbery nanofibrous mats at improving CFRP damping and interlaminar fracture toughness. Moreover, CFRP damping can be tailored by choosing the number and positioning of the nano-modified interleaves, besides choosing the mat grammage.

Guided wave monitoring of Nano-Fe3O4 reinforced thermoplastic adhesive in manufacturing of reversible composite lap-joints using targeted electromagnetic heating

Targeted heating of nano-Fe3O4 reinforced thermoplastics using electromagnetic (EM) radiations allows for rapid dis-assembly and re-assembly of bonded structural joints. Alternate EM field causes local heating of the dispersed ferromagnetic nanoparticles (FMNP), thereby melting the surrounding thermoplastic. However, it is essential to accurately measure the temperature of the adhesive since overheating may cause degradation and underheating may introduce inefficient bonding. This paper presents an ultrasonic guided wave (GW) technique for monitoring the adhesive state and provides feedback to control the electromagnetic process. Experiments were performed on single lap-shear joints FMNP reinforced thermoplastics and non-conductive glass fiber reinforced polymer (GFRP) adherends. GW were made to propagate across the bond-line of the joint by actuating and sensing them using miniature piezoelectric wafers. Dispersion relations and dynamic wave propagation were obtained using finite element (FE) modeling. Fundamental longitudinal mode L0 at 35 kHz was found optimal for bond process monitoring. Mapping between the FE-simulated transmission coefficient of L0 and actual temperature of the thermoplastic adhesive was established using the dynamic mechanical analysis (DMA) test data. Real-time GW measurements were used as a feedback in the discrete control of the induction heater to provide optimal bonding. The developed ultrasonic technique was successfully validated by a high resolution, optical frequency domain reflectometer (OFDR) based fiber-optic temperature sensor which was embedded inside the adhesive bond-line. Experiments demonstrated good agreement between GW measurements and OFDR system. Overall, the results indicate the potential of GW technique for in-situ monitoring and controlled bonding of reversible lap-joints using electromagnetic heating.

A facile crosslinking strategy endows the traditional additive flame retardant with enormous flame retardancy improvement

With great chemical and physical properties, epoxy resins (EPs) are used almost everywhere in life and engineering, but poor flame-retardant performance hinders them for a wider use. Traditional flame retardants (the most commonly used and researched) usually have dispersion and migration problems (additive ones) or are difficult to synthesize (reactive ones). Herein, a facile crosslinking strategy is used to make traditional additive flame retardants exhibit much better properties. Firstly, a nitrogen/phosphorus containing additive flame retardant with nitrile groups is designed, successfully synthesized, and directly mixed with EP matrix, the secondary crosslinking reaction of nitrile groups inner the flame retardants will occur in cured EP thermosets which was driven by a facile thermal treatment procedure. Then the properties of the flame retardant mixed and crosslinked EP composites are measured by limiting oxygen index (LOI) test, thermogravimetric analyses (TG), UL-94 vertical burning tests, dynamic mechanical analysis and cone calorimeter test, the results show that the crosslink of the additive flame retardant greatly improved the flame-retardant properties, and with 20 wt% loading content of the crosslinked flame retardant, the LOI value reach 46.2% and the peak heat release rate decreases for 77.4% compared with neat EP, furthermore the heat resistance and dynamic mechanical property of the crosslinked samples also get promoted compared with the physical mixed ones.

Synthesis and Characterization of Block Copolymers of Poly(silylene diethynylbenzen) and Poly(silylene dipropargyl aryl ether).

Poly(silylene diethynylbenzene)–b–poly(silylene dipropargyloxy diphenyl propane) copolymer (ABA-A), poly(silylene diethynylbenzene)–b–poly(silylene dipropargyloxy diphenyl ether) copolymer (ABA-O), and a contrast poly(silylene diethynylbenzene) with equivalent polymerization degree were synthesized through Grignard reactions. The structures and properties of the copolymers were investigated via hydrogen nuclear magnetic resonance, Fourier transform infrared spectroscopy, Haake torque rheometer, differential scanning calorimetry, dynamic mechanical analysis, thermogravimetric analysis and mechanical tests. The results show that the block copolymers possess comprehensive properties, especially good processability and good mechanical properties. The processing windows of these copolymers are wider than 58 °C. The flexural strength of the cured ABA-A copolymer reaches as high as 40.2 MPa. The degradation temperatures at 5% weight loss (Td5) of the cured copolymers in nitrogen are all above 560 °C.

Structural silicone sealants after exposure to laboratory test for durability assessment

AbstractDuring the service life of structural sealant glazing (SSG) facades, the load‐bearing capacity of the silicone bonds needs to be guaranteed. Laboratory tests can assess the durability of SSG‐systems based on mechanical characteristics of the bond after simultaneous exposure to both climatic and mechanical loads. This article studies how the material characteristics of two common structural sealants are affected by laboratory and field exposure. Dynamic mechanical analysis (DMA) confirms a reduction in the dynamic modulus of exposed silicone samples. Results from thermogravimetric analysis, Fourier‐transform infrared spectroscopy, differential scanning calorimetry, and small‐angle X‐ray scattering/wide‐angle X‐ray scattering show differences between the two sealants and indicate no/minor changes in the composition and morphology of the laboratory and field exposed sealants. Mechanical characterization methods, such as DMA, and tensile and shear testing of the structural bond, are shown to be sensitive toward the combined climatic and mechanical loadings, and are hence suitable for studying degradation mechanisms of structural sealants.

Viscoelasticity measured by shear wave elastography in a rat model of nonalcoholic fatty liver disease: comparison with dynamic mechanical analysis

BackgroundNonalcoholic fatty liver disease (NAFLD) is rapidly becoming one of the most common liver diseases. Ultrasound elastography has been used for the diagnosis of NAFLD. However, clinical research on steatosis by elastography technology has mainly focused on steatosis with fibrosis or non-alcoholic steatohepatitis (NASH), while steatosis without fibrosis has been poorly studied. Moreover, the relationship between liver viscoelasticity and steatosis grade is not clear. In this study, we evaluated the degree of liver steatosis in a simple steatosis rat model using shear wave elastography (SWE).ResultsThe viscoelasticity values of 69 rats with hepatic steatosis were measured quantitatively by SWE in vivo and validated by a dynamic mechanical analysis (DMA) test. Pathological sections were used to determine the steatosis grade for each rat. The results showed that the elasticity values µ obtained by the two methods followed the same trend, and µ is significantly correlated with liver steatosis. The Pearson’s correlation coefficients indicate that mu obtained by SWE is positively linear correlated with DMA (r = 0.628, p = 7.85 × 10–9). However, the viscosity values eta obtained by SWE were relatively independent of those obtained by DMA with a correlation coefficient of − 0.01. The combined Voigt elasticity measurements have high validity in the prediction of steatosis (S0 vs. S1–S4), with an AUROC of 0.755 (95% CI 0.6175–0.8925, p < 0.01) and the optimal cutoff value was 2.08 kPa with a sensitivity of 78% and specificity of 63%.ConclusionSWE might have the feasibility to be introduced as an auxiliary technique for NAFLD patients in clinical settings. However, the viscosity results measured by SWE and DMA are significantly different, because the two methods work in different frequency bands.

Multiscale characterization of ovariectomized rat femur

Estrogen deficiency activates bone resorbing cells (osteoclasts) and to a lesser extent bone forming cells (osteoblasts), resulting in a gap between resorption and formation that leads to a net loss of bone. These cell activities alter bone architecture and tissue composition. Thus, the objective of this study is to examine whether multiscale (10-2 to 10-7 m) characterization can provide more integrated information to understand the effects of estrogen deficiency on the fracture risk of bone. This is the first study to examine the effects of estrogen deficiency on multiscale characteristics of the same bone specimen. Sprague-Dawley female rats (6 months old) were obtained for a bilateral ovariectomy (OVX) or a sham operation (sham). Micro-computed tomography of rat femurs provided bone volumetric, mineral density, and morphological parameters. Dynamic mechanical analysis, static elastic and fracture mechanical testing, and nanoindentation were also performed using the same femur. As expected, the current findings indicate that OVX reduces bone quantity (mass and bone mineral density) and quality (morphology, and fracture displacement). Additionally, they demonstrated reductions in amount and heterogeneity of tissue mineral density (TMD) and viscoelastic properties. The current results validate that multiscale characterization for the same bone specimen can provide more comprehensive insights to understand how the bone components contributed to mechanical behavior at different scales.

Multiscale evaluation of the effect of recycled polymers on the long-term performance of bituminous materials

The use of recycled polymers as bitumen modifiers contributes to extending their application at a competitive cost being also a more sustainable solution. However, despite several studies proving their benefits in asphalt mixtures, their use is not yet widespread. Hence, this research explored the long-term performance of bituminous materials made with two of the most common recycled polymers (household polyethylene and a combination of polyethylene and crumb rubber). To achieve this, fatigue cracking, ageing and environmental effects (i.e. moisture and temperature susceptibility) were evaluated at the most significant scales (microstructure, bitumen, mortar and asphalt mixture) through: Atomic Force Microscopy (AFM), Multiple Stress Creep and Recovery (MSCR), Dynamic Mechanical Analysis (DMA) and UGR-FACT tests. Results proved that recycled polymers (especially for LDPE and Crumb Rubber combination) represent an improvement in durability at the binder level that thrives at all the scale levels tested thus providing greater confidence to extend its use.

Rapid tooling for injection molding inserts

Injection molding is a widespread manufacturing technology for mass production of polymeric parts. Conventionally, fused polymers are injected at high pressure in a metallic mold. This tool is typically characterized by high manufacturing costs and times, making the injection molding process not affordable for small batches or prototypal applications. Additive Manufacturing represents a practical solution to cut down tooling costs and times of molds and inserts. In this work, FDM (Fused Deposition Technology) has been considered as candidate technology to produce polymeric inserts for injection molding. Considering the commercially available filaments for FDM, a PEI (Polyetherimide) grade has been selected as tooling material for the injection of a part made of Polypropylene. The PEI grade represents a good compromise between manufacturing costs and thermo-mechanical properties required for the application. The PEI grade has been characterized with DSC (Differential Scanning Calorimetry), DMA (Dynamical Mechanical Analysis) and compression tests. The data gathered were used to set up 2D simplified thermo-mechanical finite element analyses, simulating the response of the PEI inserts subjected to repeated injection molding cycles. The simulations confirmed that the PEI grade is a good candidate tooling material but the progressive tool heating could lead to prolonged cooling time of the Polypropylene part. Finally, some PEI inserts were 3D printed with FDM and tested in a real injection molding machine injecting POM. In total, 20 POM parts have been injected correctly without relevant damaging of the PEI inserts.

Effect of zeolitic metal–organic framework on thermal, mechanical, and electrical properties of Ca/Zn‐stabilized polyvinyl chloride

AbstractIn order to enhance the heat stability of polyvinyl chloride (PVC), several heat stabilizing parameters such as metal content, antioxidant percentage, primary‐to‐secondary antioxidant ratios, stearic‐to‐acetylacetonate ratio, the amounts of pentaerythritol and β‐diketone were studied using oven and Congo red tests. Results indicated that the percentage contribution of the calcium‐to‐zinc molar ratio in the static heat stability was 60.78%. Due to the synergistic effects of the calcium/zinc stabilizer and the zeolitic metal–organic framework (MOF) as an effective co‐stabilizer, the heat stability was increased from 109 to 148 min. The EDX analysis was carried out to determine the mechanism of HCl gas absorption by the zeolitic imidazolate framework‐8 (ZIF‐8) existing in the PVC compound. Furthermore, the Ca/Zn thermal stabilizer showed poor electrical insulating properties, but the ZIF‐8 improved electrical resistivity of the PVC compounds. Volume resistivity of ZIF‐8 was significantly higher than that of the commonly used Ca/Zn stabilizers. Dynamic mechanical thermal analysis and tensile test of the PVC compound containing the ZIF‐8 showed increased Tg and enhanced toughness.

Influence of Thermal and Mechanical/Powder Processing on Microstructure and Dynamic Stiffness of Fe-Mn-Si-Cr-Ni Shape Memory Alloy

The influence of thermal processing and mechanical alloying of powder particles on the dynamic behavior and microstructural changes occurring in an Fel4Mn6Si9Cr5Ni (mass%) alloy is investigated. Five batches of 66Fe-14Mn-6Si-9Cr-5Ni (mass%) powder mixtures were produced via powder metallurgy. After powder mixing, different volume fractions (from 0 to 40 vol%) were mechanically alloyed, remixed with the rest of as-blended particles, compressed, sintered and subsequently hot rolled to a thickness of 1 mm. They were then subjected to a thermal treatment (annealing) at different temperatures and subsequently quenched into water. Thus, we had 25 different specimens for experimentation. The specimens were then machined and loaded in tension and subjected to dynamic mechanical analysis (DMA) by varying the temperature and strain amplitude. In the DMA tests, the samples were cycled between the ambient temperature and 673 K keeping the amplitude constant, while varying the temperature. These results were then correlated to the thermomagnetic properties that were measured in the same range of temperatures. Based on these data, the internal friction (tan δ) maxima, correlated with storage modulus (E′) discontinuities, were associated with the multistage reverse transformations of martensite (e, hcp) to austenite, superimposed over the antiferromagnetic–paramagnetic transitions and accompanied by magnetization peaks (Neel temperature). The temperature ranges pertaining to tan δ maxima, E′ discontinuities and magnetization peaks were determined and discussed in terms of thermal processing temperature and volume fraction of mechanically alloyed powder particles. The variation of storage modulus with strain for three different cycles (with strain amplitude varying from zero to maximum) was traced for three different temperatures: (i) T = RT; (ii) T Ttanδ (max). The role of magnitude of thermal treatment temperature and volume fraction of mechanically alloyed powder particles on the storage modulus plateaus were given prominence for these temperatures (RT, below tanδmax and above tanδmax) as they influence the evolution of the microstructure.

Effect of layering pattern and fiber hybridization on viscoelastic properties of PALF/COIR hybrid epoxy composites

To design and develop a hybrid biocomposite material for structural applications, it becomes necessary to determine the optimum fibers layering pattern. Therefore, in this research work, the different layered hybrid biocomposite boards i.e. bilayer pineapple/coir (P/C), trilayer (PCP, CPC), and intimately mixed (IM) were developed and characterized for viscoelastic properties. The composites were made by hand lay-up method, keeping the volume ratio of PALF and COIR 1:1 and the total fiber volume fraction is 0.40 volume of composite. Dynamic mechanical thermal analysis test was employed to characterize the viscoelastic behavior in terms of storage modulus, loss modulus, loss damping factor, and the glass transition temperature. Amongst all the different layered hybrid composites, the trilayer CPC has lowest value (0.635) of effectiveness coefficient with highest stiffness and activation energy (40.54 kJ/mole). It confirms the better fiber-matrix interaction at the interfacial region. The glass transition temperature of CF-EP and PF-EP was increased by 8.74% and 13.15% respectively by the synergistic hybridization of cellulosic fibers. The PCP layered composite possesses lowest value of phase transition energy (9.17 kJ/mole) and this was because of the poor fiber-matrix interfacial adhesion.

Synthesis and application of low-cost layered double hydroxides intercalated by gluconic acid anion for flame retardancy and tensile strength conservation of high filling epoxy resin

Epoxy resin (EP) is a polymer that is widely used in different aspects of life, but its flammability property limits its fields of applications. Most flame retardants at present cannot be applied practically in scale due to their toxicity, incompatibility in polymers, degraded mechanical property or high cost of raw materials and comprehensive preparation process. Layered double hydroxides intercalated by gluconic acid anion (GLDHs) may serve as a new approach. GLDHs with Mg/Al ratio of 3/1 and 2/1 were first coprecipitated with low-cost green reactants, MgCl2·6H2O, AlCl3·6H2O, NaOH and sodium gluconate. Their structures were confirmed by X-ray diffractions (XRD), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR) and elemental analysis; their thermal properties were analyzed by thermo gravimetric analysis (TGA). Composites containing 40 wt.% GLDHs were easily manufactured with normal magnetic stirring without any filler precipitation. The combustion properties of the composites filled with 40 wt.% GLDHs2 are as follows: the limit oxygen index (LOI) could rise to 29.8% from 25.2% of pristine EP; UL-94 can reach V-1 level with total burning time of only 12.1 s without dropping; compared to pristine EP, the heat release rate peak (PHRR) could drop to 30% with heavy decrease in the smoke production rate and CO production rate. Dynamic mechanical thermal analysis (DMA) tests showed that the addition of 40 wt.% GLDHs had little impact on the glass transition temperature of the composites and could slightly improve their rigidity and toughness. Tensile strength of the composites filled with 40 wt.% GLDHs2 was almost close to 88% of the tensile strength of pristine EP. Above all, GLDHs with good compatibility in polymers can serve as a promising environmental friendly and low-cost flame retardant for EP and other heterochain polymers.

Regulating the mechanical properties and microporous structures of hydroxypropyl methylcellulose based microporous photophobic films by adjusting the l-ethyl-3-methylimidazolium acetate content

X-ray diffraction, small angel X-ray scattering, scanning electron microscopy, moisture determination, dynamic mechanical analysis, whiteness assay and mechanical testing were used to study the effects of different doses of [EMIM]Ac on the microstructures and physical properties of HPMC/MSP film. By adjusting the 1-ethyl-3-methylimidazolium acetate ([EMIM]Ac) content (0 %–8 %), the mechanical properties and microstructures could be regulated. The elongation of the film increased from 5.91 % to 12.83 % with the increasing [EMIM]Ac content from 0% to 8% without affecting the tensile strength and modulus significantly. The average pore area decreased from 0.82 μm2 to 0.05 μm2 and the pore number increased with the increasing [EMIM]Ac content from 0% to 8%. Moreover, with the increasing [EMIM]Ac content, Tg decreased. 6% and 8% [EMIM]Ac decreased the compactness and smoothness of the fractal structure of the film greatly. [EMIM]Ac improved the ductility of HPMC/MSP film, which was mainly due to the decreased pore size and Tg. The improved photophobic films are both flexible and with easily changeable porous structure, which can be used as controllable packaging to carry and control-release different functional agents for different light sensitive foods.

Microstructure and dynamic mechanical properties epoxy/asphaltene composites

Asphaltenes tend to aggregate to nanoparticles or clusters in crude oil and solvents over a wide concentration and temperature range. In the present paper, asphaltenes extracted from the base asphalt was used as a filler to introduce into epoxy resin. The microstructure and evolution of asphaltenes aggregation in the epoxy resin were observed using laser scanning confocal microscopy. Furthermore, the effect of asphaltenes on the viscosity, dynamic mechanical behavior, thermostability, mechanical properties of epoxy resin was evaluated by Brookfield rotational viscometer, dynamic mechanical analysis, thermogravimetric analysis and universal testing machine. The presence of asphaltenes increased the viscosity of the neat epoxy during all stages of cure reaction. The viscosity of epoxy/asphaltenes composites increased with the filler concentration. Fractal asphaltenes aggregation formed in the composites with 1 mass% asphaltenes. Network microstructures of asphaltenes aggregation appeared in the epoxy phase with a further increase of asphaltenes content. Moreover, the increase of asphaltenes loading resulted in denser network microstructures in the epoxy matrix. Aggregation evolution revealed that asphaltenes particles redispersed evenly in the epoxy resin in the form of some aggregates at the beginning of curing. During the cure reaction of epoxy, asphaltenes aggregates started to agglomerate and grow to network microstructures. The presence of asphaltenes led to the enhancement of the storage modulus of the neat epoxy at the rubbery stage. The glass transition temperature (Tg) of the epoxy composites slightly increased with the increase of asphaltenes loading. The epoxy composite with 5 mass% asphaltenes had higher Tg than the neat epoxy. The inclusion of asphaltenes had a negligible effect on the damping properties and thermal stability of the neat epoxy. The aggregation and heterogeneous dispersion of asphaltenes resulted in the decrease of the tensile strength and elongation at break of the neat epoxy. However, the inclusion of asphaltenes significantly enhanced Young’s modulus of the neat epoxy. Young’s modulus of the neat epoxy was increased by more than fourfold with the addition of 5 mass% asphaltenes.

The mechanical characterization of blood vessels and their substitutes in the continuous quest for physiological-relevant performances. A critical review.

During the last 50 years, novel biomaterials and tissue engineering techniques have been investigated to produce alternative vascular substitutes that recapitulate the unique elastic mechanical features of blood vessels. A large variation in mechanical characterization, including the test type, protocol, and data analysis, is present in literature which complicates the comparison among studies and prevents the blooming and the advancement of this field. In addition, a limited mechanical assessment of the substitute for the intended application is often provided. In this light, this review presents the mechanical environment of blood vessels, discusses their mechanical behavior responsible for the suited blood flow into the body (non-linearity, anisotropy, hysteresis, and compliance), and compares the mechanical properties reported in literature (obtained with compression, tensile, stress-relaxation, creep, dynamic mechanical analysis, burst pressure, and dynamic compliance tests). This perspective highlights that the mechanical properties extracted through conventional tests are not always suitable indicators of the mechanical performance during the working life of a vascular substitute. The available tests can be then strategically used at different stages of the substitute development, prioritizing the simplicity of the method at early stages, and the physiological pertinence at later stages, following as much as possible ISO standards in the field. A consistent mechanical characterization focused on the behavior to which they will be subdued during real life is one key and missing element in the quest for physiological-like mechanical performance of vascular substitutes.