Storage Modulus Curves Research Articles

Effect of SiO2 Particles on the Relaxation Dynamics of Epoxidized Natural Rubber (ENR) in the Melt State by Time-Resolved Mechanical Spectroscopy.

The rheological behavior of an epoxidized natural rubber (ENR) nanocomposite containing 10 wt.% of silica particles was examined by time-resolved mechanical spectroscopy (TRMS), exploiting the unique capability of this technique for monitoring the time-dependent characteristics of unstable polymer melts. The resulting storage modulus curve has revealed a progressive evolution of the elastic component of the composite, associated with slower relaxations of the ENR macromolecular chains. Two major events were identified and quantified: one is associated with the absorption of the epoxidized rubber macromolecules onto the silica surface, which imposes further restrictions on the motions of the chains within the polymer phase; the second is related to gelation and the subsequent changes in rheological behavior resulting from the simultaneous occurrence cross-linking and chain scission reactions within the ENR matrix. These were quantified using two parameters related to changes in the storage and loss modulus components.

Mechanical response of two different molecular weight polycarbonates at varying rates and temperatures

Polymers are widely used for lightweight design in industrial applications, such as helmets and car bumpers, where the most common causes of failure or damage are dynamic impact events. It is well known that the mechanical response of most polymers is highly dependent on the loading rate and temperature, and that it is not sufficient to use properties measured under static loads in the analysis of dynamic events. However, the time-temperature equivalence phenomenon offers the chance to predict high-rate performance using low-rate data. In this study, information about the constitutive behaviour of two different molecular weight polycarbonates, is obtained in low-rate experiments and then compared with the high-rate response. A master curve of storage modulus constructed from Dynamic Mechanical Analysis data is employed to understand the viscoelastic response under small-strain loading at various frequencies and temperatures. For the large-strain constitutive response, experiments at strain rates from 0.001 s-1 to 3000 s-1 are performed using a conventional crosshead device, hydraulic device, and split-Hopkinson pressure bar. Moreover, experiments at strain rates of 0.01 s-1 and temperatures from -60 to 120 °C are also performed, and the results are compared. This approach can distinguish ’constitutive’ rate dependence from the effects of specimen self-heating due to adiabatic heating under high-rate deformation. Meanwhile, the molecular weight effects on the mechanical response at varying rates and temperatures are also noted.

Material characterization of a pultrusion specific and highly reactive polyurethane resin system: Elastic modulus, rheology, and reaction kinetics

This study presents a detailed material characterization study of a pultrusion specific polyurethane resin system. Firstly, the chemical behaviour was characterized by utilizing differential scanning calorimetry. The cure kinetics was fitted well to an autocatalytic cure kinetics model with Arrhenius temperature dependency. The resin system did not show any inhibition time. Then, the viscosity evolution was observed using a rheometer. From these measurements, the gelation point was estimated from the criterion of cross-over point of storage and loss modulus curves. The corresponding cure degree of gelation point was found to be 0.79 using the cure kinetics model. The complex viscosity evolution through dynamic scans was predicted well with a cure degree and temperature dependent viscosity model. The elastic modulus and glass transition temperature of fully and partially cured samples were measured by means of DMA in tension mode. A five step cure hardening instantaneous linear elastic model was fitted well to a wide range of cure degree values after gelation. Lastly, the fitted material models were employed in a case study of a typical pultrusion process to visualize the effects of pulling speed on the material property evolution.

Predict the Phase Angle Master Curve and Study the Viscoelastic Properties of Warm Mix Crumb Rubber-Modified Asphalt Mixture.

To identify the most accurate approach for constructing of the dynamic modulus master curves for warm mix crumb rubber modified asphalt mixtures and assess the feasibility of predicting the phase angle master curves from the dynamic modulus ones. The SM (Sigmoidal model) and GSM (generalized sigmoidal model) were utilized to construct the dynamic modulus master curve, respectively. Subsequently, the master curve of phase angle could be predicted from the master curve of dynamic modulus in term of the K-K (Kramers–Kronig) relations. The results show that both SM and GSM can predict the dynamic modulus very well, except that the GSM shows a slightly higher correlation coefficient than SM. Therefore, it is recommended to construct the dynamic modulus master curve using GSM and obtain the corresponding phase angle master curve in term of the K-K relations. The Black space diagram and Wicket diagram were utilized to verify the predictions were consistent with the LVE (linear viscoelastic) theory. Then the master curve of storage modulus and loss modulus were also obtained. Finally, the creep compliance and relaxation modulus can be used to represent the creep and relaxation properties of warm-mix crumb rubber-modified asphalt mixtures.

Block Copolymers of Poly(ω-Pentadecalactone) in Segmented Polyurethanes: Novel Biodegradable Shape Memory Polyurethanes

In this report, the synthesis of poly(ω-pentadecalactone) (PPDL) (co)polymers and their incorporation into polyurethanes (PUs) are reported. Optimal conditions for the ring-opening polymerization (ROP) of ω-pentadecalactone (PDL) using dibutyltin dilaurate catalyst were established. For the synthesis of linear and crosslinked PUs, 50 kDa poly(ε-caprolactone) (PCL) and 1,6-hexamethylenediisocyanate (HDI) were used. The obtained polyurethanes were characterized by Attenuated Total Reflectance Fourier-Transform Infrared spectroscopy (AT-FTIR), differential scanning calorimetry (DSC), and dynamical mechanical analysis (DMA). The DMA of the selected sample showed a rubbery plateau on the storage modulus versus temperature curve predicting shape memory behavior. Indeed, good shape memory performances were obtained with shape fixity (Rf) and shape recovery (Rr) ratios.

Preparation, Thermal Analysis, and Mechanical Properties of Basalt Fiber/Epoxy Composites.

In this study, basalt fiber-reinforced polymer (BFRP) composites with epoxy matrix, 20 layers, and volume fraction of fibers Vf = 53.66%, were prepared by a hand lay-up compression molding combined method. The fabric of the basalt fibers is in twill 2/2 weave. Through dynamic mechanical analysis (DMA), their viscoelastic behavior at elevated temperatures and in various frequencies was explored, whereas thermomechanical analysis (TMA) took part in terms of creep recovery and stress-relaxation tests. Moreover, the glass transition temperature (Tg) of the BFRP composites was determined through the peak of the tanδ curves while the decomposition of the BFRP composites and basalt fibers, in air or nitrogen atmosphere, was explored through thermogravimetric analysis (TGA). The mechanical behavior of the BFRP composites was investigated by tensile and three-point bending experiments. The results showed that as the frequency is raised, the BFRP composites can achieve slightly higher Tg while, under the same circumstances, the storage modulus curve obtains a less steep decrease in the middle transition region. Moreover, the hand lay-up compression molding hybrid technique can be characterized as efficient for the preparation of polymer matrix composites with a relatively high Vf of over 50%. Remarkably, through the TGA experiments, the excellent thermal resistance of the basalt fibers, in the temperature range 30–900 °C, was revealed.

Effect of organoclay modifier structure on the viscoelastic and thermal properties of poly(methyl methacrylate)/organoclay nanocomposites

Poly(methyl methacrylate) (PMMA)/clay nanocomposites (NCs) were prepared by suspension polymerization of methyl methacrylate in the presence of two different organoclays (Cloisite 30B, Cloisite 15A) with clay loading ranged from 0.5 to 5 wt%. Increase in molecular weight of the PMMA matrix with addition of the clay was revealed by gel permeation chromatography (GPC) and intrinsic viscosity measurements. As confirmed by X-ray diffraction (XRD), the NCs had an intercalated structure. The organoclays-MMA/PMMA compatibility was investigated by swelling tests and solubility parameter approach. Rheological behavior of PMMA NCs in molten state was analyzed through construction of master curves of complex viscosity, storage, and loss modulus by applying the time–temperature superposition procedure. Melt rheology, scanning electron microscopy (SEM), and UV/Vis spectroscopy results confirmed higher extent of clay dispersion in the NCs with Cloisite 30B. Compared to pure PMMA, all these NCs show increase of glass transition temperature as measured by DSC and improved thermal stability determined by thermogravimetric analysis (TGA). The results obtained by dynamic mechanical analysis showed that the storage modulus of the NCs was higher by incorporation of clay into the PMMA matrix, increasing as the amount of clay increased and that their mechanical performance was significantly enhanced.

Viscoelastic analysis of oat grain within linear viscoelastic region by using dynamic mechanical analyzer

Abstract The stress relaxation, creep-recovery, temperature, and frequency sweep tests were performed within the linear viscoelastic region by using a dynamic mechanical analyzer to investigate the viscoelastic characteristic of oat grain. The result showed that 5-element Maxwell and Burgers model were able to describe viscoelastic behaviors better. The relaxation stress decreased with the increasing moisture content from 6.79 to 23.35%, while the creep strain increased as well as the final percentage recovery decreased from 58.61 to 32.50%. In frequency sweep, storage modulus increased with the increasing frequency. In temperature sweep, there was a clear turning point in storage modulus, loss modulus, and tan delta curves with increasing temperature. The turning value of 167.47, 147.44, 134.27, 132.41, 110.28, and 92.62 °C detected in the tan delta were regarded as the best glass transition temperatures. This temperature was found to be lower than gelatinization heating temperature and decrease with the increase of moisture content. The crystalline structure of oat exhibited a typical A-type pattern and corresponding crystallinity increased from 22.03 to 31.86% with increasing moisture content. The scanning electron microscopy (SEM) micrograph of oat section was found that the size and adhesive effect of starch granules increased due to hydration.

Thermorheological properties of asphalt binders

AbstractThe rheological properties of different types of asphalt binders were studied and compared considering their constituents and physical characteristics. The saturate, aromatic, resin, and asphaltene (SARA) analysis and differential scanning calorimetry (DSC) have shown their individual constituents and two distinct glass transition temperatures, indicating the phase changes of the two main components of the asphalt binders, namely asphaltenes and maltenes. Rheological characterization was performed over a wide range of temperatures (−10°C‐60°C) showing that these materials may exhibit viscoelastic solid to viscous liquid behaviour. Master curves of complex viscosity, storage modulus, and loss modulus were constructed by applying the time‐temperature superposition principle, which was found applicable over the temperature range considered. Stress relaxation and steady‐shear test were applied to the samples in order to determine their rheological behaviour in the nonlinear viscoelastic regime (viscosity and nonlinear relaxation modulus). The rheological results were modelled and revealed that the Kaye‐Bernstein‐Kearsley‐Zappas (K‐BKZ) constitutive equation is suitable in representing the rheological behaviour of asphalts. The SARA analysis and rheological measurements were found to be compatible.

Effect of thermal cycling on mechanical and thermal properties of basalt fibre-reinforced epoxy composites

Current study investigated the effect of thermal fatigue on mechanical and thermal properties of basalt fibre-reinforced polymer (BFRP) composites. To this, basalt fibre textiles in 2/2 twill pattern was used to fabricate BFRP composites. Thermal cycling experiment was carried out between $$-40$$ and $$+120^{\circ }$$ C for 20, 40, 60, 80 and 120 cycles. Moreover, dynamic mechanical analyzer (DMA) was used to evaluate the effect of thermal cycling on thermal properties of BFRPs. Moreover, we compared the extracted viscoelastic characteristics, such as storage modulus, loss modulus and loss factor curves with original thermal-treated BFRP specimens. Based on the results, thermal cycling affected the characteristics of composites in the post-curing stage due to an increase in temperature. Finally, the effect of thermal cycling on water absorption properties of BFRP composites was examined by hydrophobicity test. The results showed that tensile strength, flexural modulus and ILSS values increased with the increase in the number of cycles up to 80 cycles. In other words, an increase in the number of cycles increased the hydrophobicity of BFRP composites by decreasing the contact angles. Finally, the mechanical properties of tested composites were significantly decreased when the number of cycles reached 120. This was due to the mismatch of thermal expansion coefficient and long crack formation in the structure of composite.

A framework for linear viscoelastic characterization of asphalt mixtures

The master curve of a viscoelastic variable is of significance due to its capability of characterizing the linear viscoelastic (LVE) property in an extended time or frequency range. However, the master curves constructed using the traditional approach fail to strictly comply with the LVE theory, leading to inaccurate predictions in the extended range. In order to address this issue, a framework was developed for the LVE characterization of asphalt mixtures. The generalized logistic sigmoidal model was adopted as the master curve model of storage modulus. A numerical model of loss modulus was established in relation to the continuous relaxation spectrum, whose mathematical model was derived in light of its relationship with the storage modulus. The model parameters determined using the storage modulus and loss modulus test data were employed to construct the master curves of storage modulus, loss modulus, dynamic modulus and phase angle. Then the relaxation modulus master curve was generated by establishing a numerical model. Afterwards, the continuous retardation spectrum was solved numerically based on its relationship with the continuous relaxation spectrum. The master curves of storage compliance, loss compliance and creep compliance were obtained using the corresponding numerical models that were established with respect to the continuous retardation spectrum. The interrelationship among the viscoelastic variables was then employed to obtain the dynamic compliance and phase angle master curves. It was demonstrated that the developed framework ensured the master curves of all viscoelastic variables complied with the LVE theory.

Physical and rheological properties of nano-[formula omitted] and nanocomposite modified bitumens

Usage of nano-TiO2 and nano-TiO2/styrene-butadiene-styrene (SBS) nano-composite as a modifier in bitumen was studied. To this end, conventional test methods including softening point and penetration tests were applied to binders to examine the physical changes after nano-TiO2 modification. The rheological characterizations of base and nano modified binder were performed by means of constructing master curves of complex viscosity (η∗), storage (G′) and loss modulus (G″) by time-temperature superposition principle. Short-term aging behavior of binders were investigated by means of roll thin film oven test (RTFOT). Dynamic shear rheometer (DSR) was employed to determinate high temperature performance grade (PG) of bitumens by using Superpave specification. Multiple stress creep recovery (MSCR) test, up-to-date method proposed for high temperature performance grades, was applied to short-term aged bitumen. The performance grades of base and nano modified bitumens determined by Superpave specification and MSCR method were also compared. The results suggest that addition of nano-TiO2 has a positive influence on stiffness of bitumen whereas the nanocomposite modifier formed by means of nano-TiO2 and SBS polymer could satisfactorily be employed as modifier in bitumen as better rheological parameters were obtained compared to pure SBS modification.

Blends of rABS and SEBS: Influence of In-Situ Compatibilization on the Mechanical Properties.

In this study, the in-situ compatibilization reaction between recycled acrylonitrile–butadiene–styrene copolymer (rABS) and functional styrene–ethylene–butylene–styrene block maleic anhydride (SEBS-g-MAH) was confirmed, which contributed to the toughening phenomenon of rABS, especially the notched impact strength. As mechanical test that manifested, the rABS/SEBS-g-MAH blends are stronger and more ductile than the rABS/SEBS blends. Prominently, the former has great advantage over the latter in terms of improving the impact performance. Scanning electron microscope (SEM) images showed that the compatible segments that were generated by reaction not only improve the interface adhesion of rABS/SEBS-g-MAH blends but also promote the evolution of co-continuous structures, which can be evidently observed after etching. Furthermore, the SEM micrographs of tensile fracture surfaces indicated that the formation of the co-continuous phase and the improvement of interface adhesion are the most profound reasons for the excellent tensile properties of the rABS/SEBS-g-MAH blends. The impact fracture surface revealed that two-phase interface affects crack propagation and shear yielding absorbs more impact energy than simple interface debonding does at higher deformation rates. Meanwhile, rheological analysis demonstrated that the complex viscosity of the rABS/SEBS-g-MAH (80/20 wt%) blend with a co-continuous structure exhibits a maximum positive deviation at low frequencies from the theoretical value calculated using the rule of logarithmic sum, which indicated a connection between co-continuous structure and complex viscosity. In addition, the storage modulus vs. loss modulus curves of the blends revealed that the viscoelastic behavior of rABS/SEBS-g-MAH blends is very similar to that of rABS.

Determination of thermal transitions of gluten-free chestnut flour doughs enriched with brown seaweed powders and antioxidant properties of baked cookies

A protocol for determining the characteristic temperatures of thermomechanical transitions on gluten-free flour doughs is proposed. This protocol is based on the mathematical analysis of experimental curve of storage modulus (G′) vs temperature obtained by means of Dynamic Mechanical Thermal Analysis (DMTA) technique. Doughs at constant consistency of chestnut flour with different levels (3, 6 and 9% flour basis, f.b.) of brown seaweed (Bifurcaria bifurcata, Fucus vesiculosus and Ascophyllum nodosum) powders addition, 2% f.b. of guar gum and 1.8% f.b. of salt with different water absorption were used to test the proposed protocol. The ranges of temperatures corresponding to starch gelatinization (59–97 °C), amylopectin crystallites melting (82–101 °C), reversible dissociation of lipid-amylose complexes (107–128 °C) and amylose melting (133–171 °C) showed a strong dependence with water absorption of samples. Doughs with the same water absorption submitted to starch gelatinization during mixing were also analysed to corroborate the protocol suitability. Total polyphenols content and radical scavenging activity of extracts from chestnut flour-seaweed powder blends and seaweed-enriched chestnut cookies baked at 180 °C were determined. Extraction assisted with ultrasounds was carried out employing acetone-water (70:30 v/v) solution as solvent during 4 min with a liquid/solid ratio of 30 w/w. Seaweed powder addition had a positive effect on antioxidant properties of doughs before baking. However, the seaweed powder addition effect on baked products (cookies) is not clear due to antioxidant activity is overlapped by Maillard's products generated during baking.

Characterization and phase‐transition behavior of thermoresponsive PVME nanogels in the presence of cellulose nanowhiskers: Rheology, morphology, and FTIR studies

The effects of cellulose nanowhiskers (CNWs) and irradiation dose on phase separation behavior as well as rheological and physical properties of Poly vinyl methyl ether (PVME) nanogels are investigated. Interactions between CNWs and polymer chains lead to the dehydration of PVME chains. Increasing the irradiation dose or CNW weight fraction enhances the VPTT, due to the decreased chain mobility. The phase separation behaviors of hybrid and nonhybrid nanogels are investigated at two different quench depths. At the shallow quench depth, the samples phase separate through nucleation and growth (NG) mechanism. However in the case of hybrid nanogels with a higher temperature, as a result of the enhanced dynamic asymmetry induced by the increased quench depth and the interaction of PVME chains with CNWs, the phase separation mechanism changes from NG to viscoelastic phase separation (VPS). The linear rheological behavior of nanogels was well correlated with the evolution of corresponding phase‐separating morphologies. At 40°C, a shoulder appeared in the storage modulus curve in the intermediate frequency range, due to shape relaxation of the droplets formed by NG mechanism. However, a solid‐like behavior was observed due to the existence of a percolated network structure induced by VPS at 60°C. POLYM. ENG. SCI., 59:899–912, 2019. © 2018 Society of Plastics Engineers

Dynamic Mechanical Analysis of Waste Polyethylene Terephthalate Bottle

Polyethylene terephthalate (PET) led to the disposing and managing issue due to it extensively consumptions. Hence, its application in construction field particular on concrete production has become general. However, the preliminary study on PET itself is very limited. This study was focused on the investigation on mechanical properties of PET film using dynamic mechanical analysis (DMA) over a temperature range of 40-180°C at fixed frequency of 1 Hz. In the paper, glass transition temperature were analyzed with the curves of storage modulus (E’), loss modulus (E”) and tan delta. The results show decreasing curve of storage modulus, meanwhile loss storage and tan delta present same curve which is increases with temperature as glassy phase and radically decreases due to rubbery phase. From these curves, the glass transition temperature (Tg) of PET have been characterized which the value has been found to be 114°C. Generally, this study could provide better characterization of a material in order to examine the ability of a material to store or lose energy.

Rheological analysis of thermo-responsive alginate/PNIPAAm graft copolymers synthesized by gamma radiation

Focused on biomedical applications of thermo-responsive polymers, low-doses of gamma radiation from a 60Co source were applied in a simple one-pot method to synthesize graft copolymers of alginate and poly(N-isopropylacrylamide) (PNIPAAm) with different compositions. The molar percentage of grafted NIPAAm (% molar NIPAAm) was determined by thermogravimetric analysis (TGA) and elemental analysis (EA), being the copolymer structure-property relationship studied in terms of thermo-associative and rheological behavior in aqueous solutions.The addition of more NIPAAm monomer in the initial mixture of reaction, as well as, increasing absorbed dose lead to a greater grafting. However, increasing radiation dose produces copolymers with diminished viscoelastic properties caused by the alginate backbone scission.From rheological curves, two transition temperatures, Ta and Tgel, were determined as a consequence of the thermo-responsiveness of PNIPAAm side chain. Storage (G′) and loss (G″) modulus curves undergo a slope inversion at Ta temperature, where both moduli begin to increase caused by an associative behavior of PNIPAAm domains. While, Tgel temperature is related to the onset of the gelation process at the G′/G″ crossover.In order to design samples with liquid-gel transitions close to the human body, able to form gels in situ once inoculated, it was possible to tailor both transition temperatures selecting copolymers with an appropriate PNIPAAm content and optimizing the copolymer concentration in the aqueous solution. A good agreement between transition temperatures and viscoelastic properties was achieved for 5 wt% aqueous solutions of copolymers with low NIPAAm content synthesized at the lowest absorbed dose (0.5 kGy).

Loading rate and temperature dependence of flexural behavior in injection-molded glass fiber reinforced polypropylene composites

This paper is concerned with the influence of loading rate/temperature and frequency/temperature on static flexural behavior and dynamic mechanical properties of injection molded glass fiber reinforced polypropylene (GFPP) composites, respectively. Furthermore, the mechanism of long-term durability of PP and GFPP was investigated by the master curve of storage modulus constructed based on the Time-Temperature-Superposition (TTSP) method. Three-point bending tests at various loading rates/temperatures and dynamic mechanical analysis (DMA) at various frequencies/temperatures are performed. The time-temperature dependence of static flexural properties (modulus, E, and strength, σ) and dynamic properties (storage modulus, E’) in PP and GFPP is discussed. The results show that both static flexural behavior and dynamic mechanical behavior of PP and GFPP have significant time-temperature dependent phenomenon, which conforms to the time-temperature equivalence principle. The long-term durability of GFPP depends on the matrix resin rather than the GF reinforcement and its interface at temperatures above the glass transition temperature (Tg).

Recycled clay/PET nanocomposites evaluated by novel rheological analysis approach

Clay-polymer nanocomposites exhibit complex rheological behavior due to physical and also possibly chemical interactions between individual phases. Up to now, rheology of clay-polymer nanocomposites has been usually described by evaluation of complex viscosity curve (shear thinning phenomenon), storage modulus curve (G´ secondary plateau) or plotting parameters characterizing damping behavior (e.g. Van Gurp-Palmen-plot, Cole-Cole plot). On the contrary to evaluation of damping behavior, new approach – based on evaluation of rigidity behavior – was tested, where the values of cot δ were calculated and called as „storage factor“, analogically to broadly used loss factor. Afterwards, values of storage factor were integrated over measured frequency range and called as “cumulative storage factor”. In this contribution, clay-PET nanocomposites with different organoclays have been prepared and characterized by both conventional as well as novel analysis approach. Rheological results have been supported by AFM micrographs.

Experimental analysis and prediction of viscoelastic creep properties of PP/EVA/LDH nanocomposites using master curves based on time–temperature superposition

ABSTRACTPrediction of viscoelastic behavior of polymers over a long‐term period is of vital importance for engineering applications. An attempt was made to uncover the interplay between the morphology and viscoelastic behavior of compatibilized polypropylene/ethylene vinyl acetate (EVA) copolymer blends in the presence of layered double hydroxide (LDH) nanoplatelets. The time–temperature superposition (TTS) principle and WLF equations were merged to obtain master curves of storage modulus at defined reference temperatures enabling prediction of storage modulus at high frequency ranges which are not experimentally measureable. Moreover, the creep compliance master curves were acquired for different reference temperatures to predict the creep compliance of nanocomposites over long period of times. It was found that the presence of LDH decreases the creep compliance at long period of times while it decreases the unrecoverable deformation of EVA domains. A simple mechanism was proposed to explain the creep and recovery behavior of samples blend at different temperatures. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46725.