Temperature-dependent Storage Modulus Research Articles

Temperature-dependent storage modulus of polymer nanocomposites, blends and blend-based nanocomposites based on percolation and De Gennes’s self-similar carpet theories

Temperature-dependent storage modulus of polymer nanocomposites, blends and blend-based nanocomposites based on percolation and De Gennes’s self-similar carpet theories

Thermomechanical Characterization and Modeling of NiTi Shape Memory Alloy Coil Spring.

Today, shape memory alloys (SMAs) have important applications in several fields of science and engineering. This work reports the thermomechanical behavior of NiTi SMA coil springs. The thermomechanical characterization is approached starting from mechanical loading-unloading tests under different electric current intensities, from 0 to 2.5 A. In addition, the material is studied using dynamic mechanical analysis (DMA), which is used to evaluate the complex elastic modulus E* = E' - iE″, obtaining a viscoelastic response under isochronal conditions. This work further evaluates the damping capacity of NiTi SMA using tan δ, showing a maximum around 70 °C. These results are interpreted under the framework of fractional calculus, using the Fractional Zener Model (FZM). The fractional orders, between 0 and 1, reflect the atomic mobility of the NiTi SMA in the martensite (low-temperature) and austenite (high-temperature) phases. The present work compares the results obtained from using the FZM with a proposed phenomenological model, which requires few parameters for the description of the temperature-dependent storage modulus E'.

Compatibilized bio-based polybutylene-succinate blended composites filled with surface modified Si3N4 for improved rigidity and thermal performance

In this study, polybutylene succinate was melt blended with polypropylene (PP) to fabricate partially bio-based and miscible composites filled with various loading silicon nitride (Si3N4) particles. Oleic acid and PP-grafted maleic anhydride were used as a functionalizing agent for Si3N4 and as a compatibilizer for polymer blends, respectively. The surface functionalization of the Si3N4 particles enhanced the interfacial adhesion between the filler and the blended polymers, affecting the thermal and mechanical properties of the composites. The thermal conductivity of the composite filled with only 18.6 vol% surface-treated Si3N4 increased by 342% compared to that of the blended polymer. In addition, the surface treatment of Si3N4 significantly improved the Young’s modulus, tensile strength, and thermal stability of the corresponding composites. However, the temperature-dependent storage modulus was unaffected by the surface treatment of Si3N4 and increased as the loading content of the Si3N4 particles increased.

Hydrogel Formation by Glutamic-acid-based Organogelator Using Surfactant-mediated Gelation.

Hydrogels formed by low-molecular-weight gelators have reversible sol-gel transition and responsiveness to various stimuli, and are used in cosmetics and drug applications. It is challenging to obtain hydrogels using novel gelators because subtle differences in their molecular architecture affect gelation. Organogelators (which form organogels) are insoluble in water, and their use as hydrogelators has not previously been considered. However, a surfactant-mediated gelation method was reported in which organogelators were solubilized in water by surfactants to form hydrogels using 12-hydroxyoctadecanoic acid. To investigate whether this method can be applied with other organogelators, the formation of hydrogel using a glutamic-acid-based organogelator was studied here. Hydrogels were formed by solubilizing 1:1 mixtures of glutamate-based organogelators, N-lauroyl-L-glutamic acid dibuthylamide, and N-2-ethylhexanoyl-L-glutamic acid dibutylamide in aqueous micellar solutions of anionic surfactant (sodium lauroyl glutamate) and cationic surfactant (cetyltrimethylammonium chloride). The minimum gelation concentration of the hydrogel was ~0.2-0.6 wt%. By changing the molar fraction of cetyltrimethylammonium chloride in the mixed surfactant, either spherical or wormlike micelles were formed. The hydrogel with wormlike micelles had a higher sol-gel transition temperature than that with spherical micelles and formed fine self-assembled fibrillar networks. Additionally, the hydrogel with the spherical micelles was elastic, whereas that with wormlike micelles was viscoelastic, suggesting that networks of the organogelators and wormlike micelles coexisted in the hydrogel from the wormlike micellar solution. Moreover, the hydrogel suppressed the reduction in the storage modulus at higher temperatures compared with the micellar aqueous solution, indicating that the elastic properties of the organogelator networks were maintained at high temperatures. The gel fibers of the hydrogel partially formed a loosely aggregated structure as the temperature increased, the fibers bundled via hydrophobic interactions, and new cross-linking points formed spontaneously. This phenomenon corresponded with an inflection point in the temperature-dependent storage modulus of the hydrogel.

Uncovering the glass-transition temperature and temperature-dependent storage modulus of graphene-polymer nanocomposites through irreversible thermodynamic processes

Several recent experiments have shown that the glass-transition temperature and temperature-dependent storage modulus of graphene-polymer nanocomposites are dependent on the graphene loading, but at present no theory exists to explain these observations. In this paper, we take the view that both issues are closely tied to the principle of irreversible thermodynamics, and that, by considering the phase transition from the glassy to the rubbery state in the polymer, and the temperature-affected degradation of the interphase, two independent state variables can be chosen and implemented into a two-scale homogenization scheme. In this approach, we also adopt the temperature-dependent complex modulus in the reduced frequency scale as the homogenization parameter. The developed theory is highlighted with a direct comparison with experiments. It is demonstrated that, with the addition of graphene fillers, both theory and experiments show an increase of glass-transition temperature and effective storage and loss moduli, but that, within the glass-transition range, the storage and loss moduli decrease drastically. The present research could provide the directions to tune the glass-transition temperature and storage modulus of graphene-polymer nanocomposite through graphene loading and temperature.

Thermo-mechanical debonding map for repairing composite–metal adhesive joints

Adhesive bonding is one of the dominant joining technologies in Design for Assembly to obtain innovative engineering products with complex geometries. These permanently bonded joints could be disassembled for repair purposes using the thermo-mechanical debonding technique to salvage the high-cost composite components without any damage. In this work, the temperature dependent storage modulus, loss modulus and tensile properties of the bulk modified epoxy film adhesive were measured through instrumented tests. Glass fibre reinforced composite and Ti–6Al–4V alloy sheets were bonded with the modified epoxy film adhesive and the floating roller peel performance was studied as a function of temperature to understand the load-displacement and failure mechanisms for generating a debonding map. The debonding region was identified in the temperature range between 1.17 T g and 1.33 T g. The knock-down in storage modulus and initial peel load exhibited a similar trend with respect to the temperature.

ICE CRYSTAL DYNAMICS IN THE NANOCOMPOSITE NETWORKS

Water is abundant in every day's life and critically useful in many biological systems and in water-based mechanical devices. Freeze-thaw process is one of the inevitable dynamics especially for the materials working at sub-zero conditions where ice crystal changes the physical property of the whole crystal-embedded composite systems. However, still many phenomena have not been explained in terms of crystal control methodology in conjunction with mechanical properties. In this study, ice crystal dynamics occurring in network systems has been discussed. Small size of network structure contributes to crystal growth inhibition especially time-dependent recrystallization. This could be explained by nano-scale confinement effect at the initial nucleation/growth stage, controlling size and shape of ice crystallites. The physical property of crystal embedded-nanocomposite is dominated by ice crystal behaviors over the network. This includes freezing point depression, frequency-dependent and temperature-dependent storage modulus changes and cooling rate- dependent dynamics. This study sheds light on ice crystal control methodology which would be useful in various materials and machines working under freeze-thaw dynamics.

A temperature-dependent storage modulus model for filler-dispersed PEG/silica composites

In this study, a temperature-dependent storage modulus model is described and then implemented in polyethylene glycol (PEG)/silica composite systems. The study began by investigating the performance of four available models in predicting the storage modulus as a function of the temperature. Then, the best performing model, based on the simulations and the evaluations of the models, was modified by adding a new parameter C to accommodate the filler content variation. Parameter C is filler-dependent with the largest effect is on the filler composition but may also be associated with its type, size, morphology, and distribution. The best performing model was obtained by the fitting performance which was determined by calculating the minimum standard error (S). The dynamic mechanical analysis (DMA) experimental data of the PEG/silica composites were collected in the shear mode for two types of silica, i.e., quartz and cristobalite, and for three filler contents, i.e., 0, 20 and 40 wt%. The new model was implemented on all DMA data. The modeling of the storage moduli of the composites is in excellent agreement with the new approach.

A multi-branch thermoviscoelastic model based on fractional derivatives for free recovery behaviors of shape memory polymers

A multi-branch thermoviscoelastic model based on fractional derivatives is proposed for the prediction of temperature-dependent free recovery behaviors of amorphous shape memory polymers (SMPs). The model is similar to the generalized fractional Maxwell model, which gives an approximation of the temperature-dependent storage modulus, loss modulus and loss factor by introducing the shift factor, usually derived from time-temperature superposition of isothermal data obtained at different temperatures. The simulation results show a satisfactory agreement with the experimental free recovery curves from shape memory tests. Moreover, this method is also proved valid for the triple-shape memory effect and multi-shape memory effect of shape memory polymers. In addition, it is an effective approach for the prediction of free recovery behaviors of the SMP-based syntactic foam consisting of hollow glass microspheres dispersed in a shape memory polymer matrix. The application of the fractional derivatives to the thermoviscoelastic constitutive model can effectively reduce the number of the required parameters while give an accurate prediction of viscoelastic behaviors and free recovery behaviors of SMPs.

A 1D thermomechanical network transition constitutive model coupled with multiple structural relaxation for shape memory polymers

A new thermomechanical network transition constitutive model is proposed in the study to describe the viscoelastic behavior of shape memory polymers (SMPs). Based on the microstructure of semi-crystalline SMPs, a new simplified transformation equation is proposed to describe the transform of transient networks. And the generalized fractional Maxwell model is introduced in the paper to estimate the temperature-dependent storage modulus. In addition, a neo-KAHR theory with multiple discrete relaxation processes is put forward to study the structural relaxation of the nonlinear thermal strain in cooling/heating processes. The evolution equations of the time- and temperature-dependent stress and strain response are developed. In the model, the thermodynamical and mechanical characteristics of SMPs in the typical thermomechanical cycle are described clearly and the irreversible deformation is studied in detail. Finally, the typical thermomechanical cycles are simulated using the present constitutive model, and the simulation results agree well with the experimental results.

Numerical evaluation of ABS parts fabricated by fused deposition modeling and vapor smoothing

Fused deposition modeling (FDM) is one of the popular 3D printing technologies due to an inexpensive extrusion machine and multi-material printing. However, FDM could only use thermoplastics such as ABS and PLA and has a problem related to the post-processing. In this study, themechanical properties of ABS parts fabricated by FDM were measured. The ABS parts were divided into one with vapor smoothing process and the other without the vapor smoothing process which is one of the the post-processing methods. Using dynamic mechanical analysis (DMA) anddilatometer, temperature-dependent storage modulus and CTE for ABS specimens were observed. Based on the measured thermo-mechanical properties of ABS parts, finite element analysis was performed for an automotive bumper made of ABS. Moreover, response surface methodologywas applied to study the relationship among design parameters of thickness of the bumper, ambient temperature, and application of the vapor smoothing process. In result, a design guideline for a ABS product could be provided without time-consuming experiments.

Temperature-dependent mechanical properties of ABS parts fabricated by fused deposition modeling and vapor smoothing

3D printing technologies have gotten an attention as a viable option for future manufacturing. Among them, FDM is the most popular one because it is inexpensive and can process with multiples materials. Layered surface, that has high roughness, is obtained with the technology due to layer-by-layer based process. This result highly decreases value of the final product. Various methods for postprocessing were proposed to achieve fine surface. Among them, vapor smoothing process is one of powerful methods because of its cost-effectiveness and usefulness. However, this process could affect mechanical property of the printed structure. In this study, we investigated the effect of the vapor smoothing technique with 3D printed structures in terms of thermal-dependent mechanical property. ABS structure was fabricated with FDM and applied into the post-processing. Then, temperature-dependent storage modulus and tan δ of the structure were measured with dynamic mechanical analysis (DMA) in the variation of amount of acetone. The results showed that the process highly affects to the thermal stability. Below 50°C, any differences were not observed. However, lower modulus and higher tan δ were shown in the higher temperature. This experiment provides very useful data for FEM simulation to predict mechanical property of a 3D printed structure.

Temperature-frequency-dependent mechanical properties model of epoxy resin and its composites

Temperature-frequency-dependent dynamic mechanical properties of epoxy resin and glass/epoxy composites were studied at different loading modes by dynamic mechanical analysis. An improved temperature-dependent storage modulus model was developed to describe the storage modulus of the epoxy resin and glass/epoxy composites. A new and simple loss modulus model including two specific physical parameters was also developed. In addition, a model that can describe the temperature-frequency-dependent mechanical properties was established by combining the storage modulus model or loss modulus model with the Arrhenius equation. All of the model predictions showed very good agreements with the experimental results.

Long-Term Viscoelastic Response of E-glass/Bismaleimide Composite in Seawater Environment

The effect of seawater absorption on the long-term viscoelastic response of E-glass/BMI composite is presented in this paper. The diffusion of seawater into the composite shows a two-stage behavior, dominated by Fickian diffusion initially and followed by polymeric relaxation. The Glass transition temperature (Tg) of the composite with seawater absorption is considerably lowered due to the plasticization effect. However the effect of water absorption at 50 °C is found to be reversible after drying process. The time-temperature superposition (TTS) was performed based on the results of Dynamic Mechanical Analysis to construct the master curve of storage modulus. The shift factors exhibit Arrhenius behavior when temperature is well below Tg and Vogel-Fulcher-Tammann (VFT) like behavior when temperature gets close to glass transition region. As a result, a semi-empirical formulation is proposed to account for the seawater absorption effect in predicting long-term viscoelastic response of BMI composites based on temperature dependent storage modulus and TTS. The predicted master curves show that the degradation of storage modulus accelerates with both seawater exposure and increasing temperature. The proposed formulation can be applied to predict the long-term durability of any thermorheologically simple composite materials in seawater environment.

Electromechanical properties of multi-walled carbon nanotube/gelatin hydrogel composites: effects of aspect ratios, electric field, and temperature.

The effects of multi-walled carbon nanotube (MWNT) aspect ratio, electric field strength and temperature on the electromechanical properties of MWNT/gelatin hydrogel composites were investigated. The highest aspect ratio of MWNT provides the composites with the highest dynamic moduli under electric field. The MWNT/gelatin hydrogel composites of 0.01, 0.1, 0.5, and 1 vol.% and the pure gelatin hydrogel possess the storage modulus sensitivity values of 0.69, 1.23, 0.94, 0.81 and 0.47, respectively, at 800 V/mm. The results can be interpreted in terms of the enhanced polarizability between the carboxyl groups of gelatin under the presence of MWNT. The effect of temperature on the electromechanical properties of MWNT/gelatin hydrogel composites investigated between 30 °C and 90 °C shows three distinct regimes of temperature-dependent storage modulus behavior. In the deflection testing, the effects of electric field on the deflection distance and the dielectrophoresis force of the MWNT/gelatin hydrogel composites were also investigated. MWNT/gelatin hydrogel composites suspended in the silicone oil between electrodes, respond rapidly with a deflection toward the anode site, indicating the attractive force between anode and the polarized carboxyl group as the gelatin structure possesses negative charges.

Characterization and modeling the thermo-mechanical cure-dependent properties of epoxy molding compound

Semi-conductor devices are mostly encapsulated by epoxy molding compound (EMC) materials. During encapsulation stresses are generated due to the curing of the molding compound. Moreover, additional stresses will build up during cooling down from molding to ambient temperature caused by the differences in the coefficient of thermal expansion. These residual stresses add up to the stresses generated during mechanical loading and may lead to product failure. The viscoelastic properties of the encapsulation material depend highly on temperature and degree of cure. In this paper the thermo-mechanical properties of an epoxy molding compound with and without filler are investigated both experimentally and theoretically. The cure dependent properties of the EMCs, likewise, the cure kinetics, coefficient of thermal expansion and cure shrinkage were measured using a Differential Scanning Calorimetry (DSC) and a GNOMIX high pressure dilatometer. In addition using a Dynamic Mechanical Analyzer (DMA) the time and temperature dependent storage modulus was measured and mastercurves were constructed.