Dynamic Mechanical Relaxation Research Articles

Biomass- and Carbon Dioxide-Derived Polyurethane Networks for Thermal Interface Material Applications.

Recent environmental concerns have increased demand for renewable polymers and sustainable green resource usage, such as biomass-derived components and carbon dioxide (CO2). Herein, we present crosslinked polyurethanes (CPUs) fabricated from CO2- and biomass-derived monomers via a facile solvent-free ball milling process. Furan-containing bis(cyclic carbonate)s were synthesized through CO2 fixation and further transformed to tetraols, denoted FCTs, by aminolysis and utilized in CPU synthesis. Highly dispersed polyurethane-based hybrid composites (CPU-Ag) were also manufactured using a similar ball milling process. Due to the malleability of the CPU matrix, enabled by transcarbamoylation (dynamic covalent chemistry), CPU-based composites are expected to present very low interfacial thermal resistance between the heat sink and heat source. The characteristics of the dynamic covalent bond (i.e., urethane exchange reaction) were confirmed by the results of dynamic mechanical thermal analysis and stress relaxation analysis. Importantly, the high thermal conductivity of the CPU-based hybrid material was confirmed using laser flash analysis (up to 51.1 W/m·K). Our mechanochemical approach enables the facile preparation of sustainable polymers and hybrid composites for functional application.

Advancing viscoelastic material modeling : Tackling time-dependent behavior with fractional calculus

A combination of inherent unique stress-strain response features, Viscoelastic Materials (VM) provide an integral part in many different fields of engineering. Although practical, these materials’ highly complex time-dependent methods according to non-linear loading scenarios are impossible to model using conventional viscoelastic models such as Maxwell and Kelvin-Voigt. The present study introduces a framework that pushes over traditional Maxwell and Kelvin-Voigt approaches by employing fractional calculus in order to enhance the prediction of VM performance. The mathematical representation makes use of the Caputo fractional derivative for expressing an artificial viscoelastic polymer’s non-linear and time-dependent responses. Dynamic Mechanical Analysis (DMA) and Stress Relaxation Tests (SRT) proved the polymer possessed 1500 MPa fractional modulus and 0.65 fractional order, respectively. The resulting model involved more significant computing resources, but contrasting testing indicated that it accurately depicted stress relaxation and dynamic responses. As mentioned, the technique integrates mathematical and actual viscoelasticity for industrial uses while offering a precise basis for advanced material analysis.

Dynamic heterogeneity and mechanical relaxation of Ti20Zr20Cu20Ni20Be20 high-entropy metallic glass probed by quasi-elastic neutron scattering and stress relaxation

Heterogeneity is an intrinsic property of metallic glasses (MGs) and is closely related to their physical properties. Relaxation dynamics provides a powerful approach to understanding the heterogeneity of MGs. State-of-the-art quasi-elastic neutron scattering (QENS) and stress relaxation are used to study the atomic relaxation dynamics, dynamic heterogeneity, diffusion coefficients and mechanical relaxation of Ti20Zr20Cu20Ni20Be20 high-entropy metallic glass (HE-MG). The dynamic susceptibility derived from the QENS self-intermediate scattering function was used to evaluate the presence of dynamic heterogeneity, which increases gradually with decreasing temperature. The average self-diffusion coefficient of Ni and Ti atoms in the Ti20Zr20Cu20Ni20Be20 HE-MG melt is much higher than that in the Ti13.8Zr41.2Cu12.5Ni10Be22.5 Vit1 MG melt. The Ti20Zr20Cu20Ni20Be20 HE-MG melt has higher atomic mobility with respect to the Vit1 MG melt. In addition, the activation energy for crystallization of the Ti20Zr20Cu20Ni20Be20 HE-MG is considerably larger than that of the Vit1 MG. In the stress relaxation test, both Ti20Zr20Cu20Ni20Be20 HE-MG and Vit1 MG exhibit a relaxation decoupling phenomenon, indicating a two-step relaxation process. The slow and fast relaxation processes of the two alloy ribbons have a stretched exponential decay. At the measured temperatures, the relaxation times for both of the relaxation processes for the Ti20Zr20Cu20Ni20Be20 HE-MG are larger than those for the Vit1 MG, meaning that the Ti20Zr20Cu20Ni20Be20 ribbon has a slower stress decay. This in-depth investigation will improve our understanding of the intrinsic heterogeneity and mechanical relaxation mechanism in HE-MG.

Influence of oscillation strain on the dynamic mechanical relaxation of a La-based metallic glass

Influence of oscillation strain on the dynamic mechanical relaxation of a La-based metallic glass

Dynamic mechanical relaxation behavior of TiZrHfCu-Ni/Be/NiBe high-entropy metallic glasses

Dynamic mechanical relaxation behavior of TiZrHfCu-Ni/Be/NiBe high-entropy metallic glasses

Prominent β-relaxation and stress relaxation decoupling phenomenon in Gd-Ni-Al metallic glasses

GdxNi20Al80−x (x = 50, 60, 65, 70 at%) metallic glasses (MGs) with prominent β-relaxation were developed, which provide a new model system for studying some controversial issues concerning β-relaxation. The relaxation behavior and thermal stability of GdxNi20Al80−x MGs were probed by dynamic mechanical relaxation, static stress relaxation, and differential scanning calorimetry. In view of the temperature and frequency dependence of the dynamic mechanical relaxation spectrum, we investigated the β-relaxation and α-relaxation separation phenomena of Gd50Ni20Al30 and Gd60Ni20Al20 MGs. With increasing Gd/Al ratio, the activation energy of the β-relaxation decreased, which was conducive to the flow units and prominent β-relaxation. The Gd70Ni20Al10 alloy had the most significant β-relaxation phenomenon, although some nanocrystalline Gd3Ni phase was distributed in the amorphous matrix. The Gd50Ni20Al30 glass had a crystallization activation energy of 476 kJ/mol, corresponding to the highest thermal stability. At a constant strain, the static stress relaxation process of the Gd70Ni20Al10 alloy was analyzed over a wide time window and temperature range. A decoupling phenomenon of the relaxation into a two-step process occurred in the Gd70Ni20Al10 alloy, namely, a fast relaxation process and a slow relaxation process, at different time scales. Analysis of the relaxation dynamics and time scales of the relaxation confirmed that the fast relaxation process differed from the β-relaxation process. This study elucidates the β-relaxation behavior and stress relaxation decoupling phenomenon in Gd-Ni-Al MGs and should, therefore, help clarify the mechanical relaxation behavior and intrinsic characteristics of MGs in future work.

Physics-motivated fractional viscoelasticity model for dynamic relaxation in amorphous solids

Dynamic mechanical relaxation is an important metric to understand the mechanical/physical properties of amorphous solids which are of viscoelastic nature. Due to the heterogenous microstructure, the relaxation behavior of amorphous solids usually shows strong deviation from the Debye relaxation. The distribution of relaxation time derived from either the stretched exponential function (KWW function) or the power law form is probably the most adopted paradigm to describe the non-Debye relaxation. They are essentially the continuous spectrums given in analytical forms. However, whether a real amorphous material conforms to such distribution law remains to be discussed. Here we test the assumption in typical metallic glasses (MGs) as representatives of the general amorphous solids. The mechanical spectrum of a Cu46Zr47Al7 MG in wide frequency domain is probed by the dynamic mechanical analysis technique. It is found that both the KWW function and the modified fractional (MF) model based on power law can well describe the experimental data. As a step forward, we combine the quasi-point defect theory with the MF model to theoretically reveal the feature of temperature-dependent structural evolution in MG. Finally, the distribution of relaxation time corresponding to the experimental data is discretized to argue the theoretically predicted microstructural heterogeneity in the MGs.

Dynamical relaxation and stress relaxation of Zr-based metallic glass

The relationship among dynamic relaxation, deformation and microstructural heterogeneity of amorphous alloy is one of the important research contents in the field of amorphous physics. In this paper, we utilize Zr<sub>48</sub>(Cu<sub>5/6</sub>Ag<sub>1/6</sub>)<sub>44</sub>Al<sub>8</sub> bulk amorphous alloy with excellent glass forming capability and good thermal stability as a research carrier, and investigate the effects of temperature and structural relaxation on dynamic relaxation and deformation behavior through dynamic mechanical analysis and stress relaxation experiments. The results show that the dynamic relaxation spectrum of the model alloy is sensitive to temperature, showing four stages as the temperature increases, namely, iso configuration, aging, glass transition and crystallization. Based on the isothermal measurement of dynamic mechanical parameters under the glass transition temperature, the structural relaxation causes the amorphous alloy to migrate from the non-equilibrium high energy state to the low energy state, and the evolution of internal friction with aging time can be described by Kohlrausch-Williams-Watts (KWW) stretched exponential function. In addition, based on the KWW equation and activation energy spectrum, the activation of heterogeneous structure in the stress relaxation process of model alloy is analyzed, which involves the transformation from elastic deformation to nonelastic deformation. Owing to the microstructural heterogeneity and energy fluctuations in amorphous alloys, the activation of deformation units is not a single characteristic time, but follows a certain distribution. By considering that the characteristic time distribution of activation of deformation units is symmetric Gauss distribution or asymmetric Gumbel distribution on a logarithmic time scale, the heterogeneous activation process in stress relaxation response can be reconstructed.

The β relaxation process of La-based amorphous alloy: Effect of annealing and strain amplitude

The dynamic relaxation process of amorphous alloys is an important issue to understand the diffusion behavior, plastic deformation as well as glass transition phenomenon. In the current research, (La<sub>0.6</sub>Ce<sub>0.4</sub>)<sub>65</sub>Al<sub>10</sub>Co<sub>25</sub> amorphous alloy with a pronounced β relaxation process was selected as a model system to study the dynamic mechanical relaxation processes. Influence of driving frequency, physical aging and applied strain amplitude on the β relaxation of the La-based metallic glass was probed process using dynamic mechanical analysis. The experimental results demonstrated that the peak of the β relaxation process shifts to high temperature by increasing the driving frequency. Physical aging below the glass transition temperature induces a decrease of the intensity of the β relaxation process. The “defects” of amorphous alloy decreases during the physical aging process, which is ascribed to the glassy system shifts to more stable state induced by physical aging treatment. In parallel, the intensity of the β relaxation process of the amorphous alloy increases by increasing strain amplitude. The research sheds new light on further understanding the physical origin of β relaxation process of the amorphous alloy.

Effect of the physical aging on the secondary β relaxation process in a La-based metallic glass

Dynamic mechanical relaxation processes, i.e., main (α) relaxation and secondary (β) relaxation, are important issues to understand mechanical deformation, atomic diffusion as well as glass transition phenomenon of metallic glasses. In current work, La68Ni15Al15Cu2 metallic glass was selected as a protocol glass system. Mechanical relaxation processes were probed by dynamic mechanical analysis. The effects of annealing at different temperatures were analyzed by Kohlrausch–Williams–Watts (KWW)-type equation. The Kohlrausch exponent βKWW reflects the deviation from a single Debye relaxation, indicating the fact that dynamics in metallic glass are actually heterogeneous originating from the structural heterogeneity. The effects of thermal treatments were also discussed, which provides a potential solution to tune the relaxation behaviors in metallic glasses.

Poly(lactic acid)/Poly(vinyl alcohol) Biodegradable Blends Using Monobutyl Maleate as a Plasticizer and Compatibilizer

The Covid-19 pandemic situation has contributed to sparking discussions about an increase in packaging combined with the nonrecommendation of reuse. For this reason, many countries have encouraged the use of biodegradable polymers. In this study, blends of poly(lactic acid) (PLA) and poly(vinyl alcohol) (PVAL) were prepared at 80/20 (w/w) in the presence of specific amounts of monobutyl maleate (MBM) as a compatibilizer. All of these components are proven biodegradable. PLA/PVAL/MBM blend films were obtained by thermopressing, and the thermal, mechanical, and morphological properties were evaluated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), tensile tests, stress relaxation, and scanning electron microscopy (SEM). DSC results suggest that MBM can act as a plasticizer, at least for the PLA matrix, reducing the Tg from 60.2 °C (without MBM) to 23.5 °C in the case of the highest quantity of plasticizer (20%). Due to increase in macromolecular mobility, MBM also affects PLA crystallization. As a consequence of brittleness of the other samples, only those containing 15 and 20% of MBM (in PLA basis) did not fail the tensile and relaxation tests, showing more than 25% of elongation at break. Both the elastic and viscous parameters and the equilibrium modulus (Eeq) of the Maxwell–Wiechert mechanical system show lower values for the sample with higher MBM content. The SEM images show that the presence of the compatibilizer improves the adhesion between the PLA-rich phase and PVAL-rich phase.

Influence of Microalloying Process on Dynamic Mechanical Relaxation of ZrCo-Based Amorphous Alloy

In this study, the dynamic mechanical spectroscopy was used to characterize the effects of minor Ni addition on the relaxation behavior of ZrCoAl bulk metallic glass (BMG). For this purpose, the Kohlrausch–Williams–Watts (KWW) function and quasi-point defect (QPD) model were used for evaluation of relaxation under the different aging temperatures. The results indicated that the Ni addition shifted the relaxation process to the higher temperatures. Moreover, the estimations showed that the activation energy of relaxation was 5.951 eV and 6.205 eV for ZrCoAl and ZrCoAlNi, respectively. It was also revealed that the microalloying process enhanced the structural defects in the system and led to the improvement of dynamic heterogeneity in the BMG. Comparing the physical models, it is suggested that a small change in the structural defects intensifies the dynamic heterogeneity in the material.

Evolution of the distribution of flow units of a metallic glass under cyclic loading

Dynamic mechanical relaxation processes (i.e., β relaxation and α relaxation) are closely connected with the mechanical and physical properties of metallic glasses. In the current work, the β relaxation of La60Ni15Al25 metallic glass was studied in sequential heating and cooling experiments by dynamic mechanical analysis. The values of the loss modulus peak were found to reach a relatively stable state during the progress of the heating-cooling cycles. The evolution of the flow unit distribution during thermal cycling was revealed by stress relaxation tests. The experimental results of stress relaxation were well described by a generalized Maxwell model, allowing the analysis of the underlying relaxation time and activation energy distributions. We also show here a suppressed relaxation behavior observed under repeated loading cycles, which is attributed to the annihilation of flow units and the modification of the relaxation time spectrum obtained in the framework of the generalized Maxwell model.

New insight into dynamic mechanical relaxation in N-butyl-N-(2-nitroxy-ethyl) nitramine plasticized nitrocellulose through molecular dynamic simulations

We performed dynamic mechanical analysis (DMA) on nitrocellulose (NC) plasticized by an insensitive plasticizer N-butyl-N-(2-nitroxy-ethyl)nitramine (Bu-NENA). NC/Bu-NENA blend shows two mechanical relaxation processes in the temperature ranges of − 50 to − 40 °C and 30 ~ 40 °C, and their variations with deformation frequencies were studied. To explore further the effect of temperature on relaxation, the binary mixture model of NC/Bu-NENA was constructed, and molecular dynamic simulations were conducted. The simulated mean square displacements (MSD) show abrupt increase in the temperature range of − 50 to − 40 °C and 30 ~ 40 °C, which are consistent with those of the two relaxation processes observed in the DMA curves. Moreover, the free volume (Vfree) and torsion energy obtained from molecular dynamic simulations exhibit distinct increase at the temperature above 30 °C and − 50 °C respectively, reflecting the sudden enhancements on the mobility of polymer chain elements and the rotation of molecular bonds. Furthermore, the radial distribution function (RDF) associated with the intermolecular interactions reveals that the intensities of both hydrogen bond and van der Waals forces decrease with the increase of temperature, which is responsible for the decrease of storage modulus at high temperature. These computational and experimental studies reveal guidance to strengthening the NC base propellants in broad temperature range.

Dynamic relaxation characteristics and stress relaxation behavior of Pd-based<sub> </sub>metallic glass

As a potential functional and structural material, high-entropy metallic glasses have aroused tremendous research interest in condense matter physics and mechanics. The correlation between macroscopic mechanic properties and microstructure heterogeneity of high-entropy metallic glasses is one of the most important scientific issues in glassy solids. In the present research, Pd<sub>42.5</sub>Cu<sub>30</sub>Ni<sub>7.5</sub>P<sub>20</sub> metallic glass and Pd<sub>20</sub>Pt<sub>20</sub>Cu<sub>20</sub>Ni<sub>20</sub>P<sub>20</sub> high-entropy metallic glass are selected as the model alloys. Dynamic mechanical analysis (DMA) and stress relaxation are used to investigate the influences of temperature and physical aging on dynamic mechanical relaxation process and microstructure heterogeneity of the model alloys. The dynamic mechanical analysis results demonstrate that the Pd<sub>42.5</sub>Cu<sub>30</sub>Ni<sub>7.5</sub>P<sub>20</sub> metallic glass and Pd<sub>20</sub>Pt<sub>20</sub>Cu<sub>20</sub>Ni<sub>20</sub>P<sub>20</sub> high-entropy metallic glass both exhibit evident <i>β</i> relaxation process. In addition, the atomic mobility of model alloys is reduced in these processes, and the <i>β</i> relaxation shifts toward higher temperatures. In the stress relaxation process, the Gibbs free energy is reduced due to the high configurational entropy. This is the potential reason that high-entropy metallic glass possesses higher activation energy. In parallel, high-entropy metallic glass is more difficult to activate and needs to break through a higher energy barrier. With the increase of physical aging time, the flow unit in high-entropy metallic glass becomes smaller. This also benefits from the high-entropy effects that bring sluggish diffusion into high-entropy metallic glass. The change of activation volume under physical aging of high-entropy metallic glass is less sensitive to stress relaxation than that of metallic glass.

Dynamic Mechanical Relaxation Behavior and Thermal Stability of Tizrhfcu(Ni/Be/Nibe) High-Entropy Metallic Glasses

Dynamic Mechanical Relaxation Behavior and Thermal Stability of Tizrhfcu(Ni/Be/Nibe) High-Entropy Metallic Glasses

Split dynamic mechanical relaxation of an inhomogeneous dielectric and the insights into its anomalous characteristics

The split relaxations of an inhomogeneous polymer were investigated in this paper, with special attention paid to the application of classic dynamic mechanical analysis (DMA) to inhomogeneous dielectrics. The single relaxation peak of the pristine epoxy resin split to two peaks after thermal ageing, concerning its transition toward an inhomogeneous skin-core structure. Moreover, the relaxation corresponding to the skin-layer, which only took up a small proportion in volume, anomalously dominated the entire relaxation response. The core-layer’s response was even unidentifiable when its proportion was below 65 vol%. This was found strongly correlated with the non-uniform distribution of applied stress during DMA test which kinetically affected the viscoelastic response received from the specimen. Since potential misjudgment could be induced in this case, it should be a critical issue deserving more attention.

Tunable thermo-sensitive electrical conductivity of melt-mixed PA- 12/PP-MWCNT nanocomposites

In this study, the correlation of morphology, temperature dependent electrical conductivity and solid-state relaxation dynamics of melt-mixed polyamide 12 (PA-12)/polypropylene-multi walled carbon nanotube (PP-MWCNT) based ternary nanocomposites has been systematically investigated. Selectively confined functionally conductive masterbatch dispersed in nylon matrix with structurally ordered co-continuous morphology enhanced the electrical conductivity due to double percolation phenomenon by virtue of their differences in melt viscosity, polarity, surface energy and similar processing window of PA-12 and PP-MWCNT. The stability of the conducting polymer nanocomposites and the correlation of temperature sensitive electrical behaviour with the endothermic transitions and dynamic-mechanical relaxation transitions in the temperature range of −20 °C–200 °C is also explored. Polymer nanocomposite based thermo-sensitive materials substantiated four distinct regimes highlighting the network build-up or restoration assisted conductivity. Our study demonstrates the tunability of temperature dependence of electrical conductivity by controlling conducting channels via manipulating the ratio of PA-12 to PP-MWCNT composite filler for temperature dependent current terminating electrical devices. The reproducibility of such temperature sensitivity has also been explored.

Dynamic mechanical response of ZrCu-based bulk metallic glasses

The thermal stability and the dynamic mechanical relaxation behavior of (Zr50Cu40Al10)100-xDyx (at.%) (x = 0 or 2) metallic glasses were investigated by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). In the case of the (Zr50Cu40Al10)100-xDyx (x = 0 or 2) metallic glasses, atomic mobility increases by introduction of the Dy element. The activation energy of the the main a relaxation process and fragility index were discussed according to the DMA measurements. With the help of the time-temperature superposition (TTS) principle, the master curves of the model alloys were established, the Kohlrausch-Williams-Watts (KWW) function and quasi-point defects (QPD) theory were used to descirbe the master curves of the metallic glasses. The characteristic parameters related to microstructural heterogeneity in the KWW equation and the QPD model, Kohlrausch exponent βKWW and the correlation factor χ were evaluated. In parallel, the elastic, viscoelastic and viscoplastic responses of the model alloys have been analyzed based on the DMA results. Our investigations demonstrated that introduction of the Dy increases the structure heterogeneity of the (Zr50Cu40Al10)100-xDyx (x = 0 or 2) glassy system, which plays an important role in tailoring the dynamic relaxation behavior and mechanical properties of the metallic glasses.

Dynamic mechanical relaxation and thermal creep of high-entropy La30Ce30Ni10Al20Co10 bulk metallic glass

Dynamic mechanical relaxation is a fundamental tool to understand the mechanical and physical properties of viscoelastic materials like glasses. Mechanical spectroscopy shows that the high-entropy bulk metallic glass (La30Ce30Ni10Al20Co10) exhibits a distinct β-relaxation feature. In the present research, dynamic mechanical analysis and thermal creep were performed using this bulk metallic glass material at a temperature domain around the β relaxation. The components of total strain, including ideal elastic strain, anelastic strain, and viscous-plastic strain, were analyzed based on the model of shear transformation zones (STZs). The stochastic activation of STZ contributes to the anelastic strain. When the temperature or external stress is high enough or the timescale is long enough, the interaction between STZs induces viscous-plastic strain. When all the spectrum of STZs is activated, the quasi-steady-state creep is achieved.