Thermorheological Complexity Research Articles

Rheological Property Modification of a Molten-State Polyamide through the Addition of an α-Olefin-Maleic Anhydride Copolymer.

The rheological properties of a polyamide (PA) resin with low crystallinity were modified by melt-mixing it with a small amount of an alternative α-olefin-maleic anhydride copolymer as a reactive compound. Because PA has a low melting point, rheological characterization was performed over a wide temperature range. Owing to the reaction between PA and the alternative α-olefin-maleic anhydride copolymer, the blend sample behaved as a long-chain branched polymer in the molten state. The thermo-rheological complexity was obvious owing to large flow activation energy values in the low modulus region, i.e., the rheological time-temperature superposition principle was not applicable. The primary normal stress difference under steady shear was greatly increased in the wide shear rate range, leading to a large swell ratio at the capillary extrusion. Furthermore, strain hardening in the transient elongational viscosity, which is responsible for favorable processability, was clear. Because this is a simple modification method, it will be widely employed to modify the rheological properties of various polyamide resins.

Thermorheological complexity in Nylon12‐Cork composites: Role of interfacial compatibilizer

AbstractHerein, we demonstrate how compatibilizers affect the thermorheological properties and morphology of engineering thermoplastic composites based on nylon and cork. Nylon12 was melt‐mixed with cork treated with four distinct compatibilizers with radically different chemical structures and compositions, respectively: (i) Octadecanamide (ODA), (ii) polyethylene‐graft‐maleic anhydride (PE‐g‐MA), (iii) polypropylene‐graft‐maleic anhydride (PP‐g‐MA), and (iv) (3‐aminopropyl)triethoxysilane (APTS). The rheological behavior was assessed using an oscillatory rheometer and the morphology was visualized using a scanning electron microscope. The rheological data were thoroughly analyzed using different formalisms based on empirical, phenomenological, and molecular origin, including the Carreau‐model, time–temperature superposition (TTS), Han plot, and van Gurp–Palmen (vGP) plot. It is observed that rheological data of neat nylon can be treated as themorheologically simple. However, the inclusion of cork makes the composite thermorheologically complex and increases the relaxation time (τ) by >50‐fold when compared to the neat polymer. As compatibilizers, ODA and PP‐g‐MA significantly lower τ, improve internal flow behavior, and demonstrate good cork distribution and dispersion. However, PE‐g‐MA does not affect the τ of composites significantly. The application of APTS as a compatibilizer enhances the interfacial interaction between Nylon12 and cork most significantly, which leads to a notable rise in τ, and the composite's elastic modulus surges by two orders in magnitude compared to neat Nylon. Additionally, the polymer melt flow becomes elastically dominant. Finally, it is observed that the Han plot offers a better insight into the correlation between changes in the thermorheological behavior and the microstructural modifications to the composites.Highlights Utilizing cork particles as a sustainable raw material for engineering thermoplastics. Quantifying compatibilizer's impact on rheological properties of polymer composite. Illustrating how interfacial interaction affects thermorheological complexity in composites. Visualizing morphological change in nylon‐cork composite with interfacial interaction. Producing lightweight engineering polymer composites using cork as a filler.

Characterization of relaxation behaviour of CF/PEKK aerospace composites using the time-temperature-crystallinity superposition principle

In this study, the Time-Temperature-Crystallinity Superposition Principle (TTCSP) was applied to determine the viscoelastic behavior of Thermo-rheological Complex Materials (TCM), specifically Carbon fibre/Poly-Ether-Ketone-Ketone (CF/PEKK) composites. The study investigated the effects of various parameters on the viscoelastic behavior of the composites, such as the degree of crystallinity after different melting temperatures, relaxation, and crystallization times. The TTCSP was utilized on the relaxation data to generate great-grand master curves for the degree of crystallinity for different laminate lay-ups. Hot press forming was employed to manufacture samples under different processing conditions, including various melting and cold crystallization temperatures. Differential Scanning Calorimetry (DSC) was employed to calculate the degree of crystallinity of CF/PEKK composites, while the Dynamic Mechanical Analyzer (DMA) was used to obtain the relaxation data. The generated great-grand master curves proved effective in predicting the relaxation behavior of the composites consolidated using single and double hold cycles at different melting temperatures and crystallization times, respectively. The great-grand master curves presented in this study can serve as valuable tool to calibrate key viscoelastic and/or thermo-viscoelastic material models for aerospace-grade CF/PEKK composites. These models are crucial for simulations aimed at predicting residual stresses and process-induced deformations during the thermoforming process.

Thermal stress calculation of wax-based warm mix asphalt considering thermorheologically complex behavior

The warm mix asphalts which reduce the environmental pollution always show thermorheologically complex behavior under the realistic working conditions (the long-term loading at the low temperature) in the cold region. However, many scholars ignore the thermorheologically complex behavior when calculating thermal stresses. In this paper, the new thermal stresses calculation method considering thermorheological complexity of four wax-based warm mix additives (Fischer-Tropsch (F-T) based wax, fatty acid amides (FAA) based wax, polyethylene (PE)-based wax, and linear aliphatic hydrocarbon (LAH)-based wax) modified asphalt binders were proposed, which was the process of incremental iterative (Named as ‘the incremental method’). The results show that the warm mix asphalt exhibits the thermorheologically complex behavior under long-term loading condition at the low temperature through the tail of the master curve of stiffness modulus, and the thermal stresses considering thermorheologically complex characteristics calculated by the incremental method are greater than that calculated by the traditional Monismith method. Therefore, the incremental method should be used to calculate the thermal stress of asphalt binders under the long-term loading condition instead of the Monismith method, otherwise the cracking risk of the asphalt binder will be underestimated. The wax-based warm mix additives with longer chain molecules and cold storage time can increase the thermorheological complexity more evidently, and the thermal stresses of asphalts with thermorheologically complex characteristics are more sensitive to cooling rate. All in all, the proposed incremental method will provide a supplement to the calculation method of the thermal stresses in the asphalt binder.

Study of Morphology, Rheology, and Dynamic Properties toward Unveiling the Partial Miscibility in Poly(lactic acid)-Poly(hydroxybutyrate-co-hydroxyvalerate) Blends.

The purpose of the present work was to gain a fundamental understanding of how the composition and physico-chemical properties affect the rheology, morphology, miscibility, and thermal stability of poly(lactic acid) (PLA)-poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) biopolymer blends obtained by melt mixing. First, restricted processing conditions were chosen, due to the inherent thermal degradation of PHBV, as proven by rheological dynamic time sweep (DTS) measurements and size-exclusion chromatography (SEC). Based on this, the composition dependence of the blends was investigated using small-amplitude oscillatory shear rheology (SAOS), and the results were confirmed by scanning electron microscopy (SEM) analysis. Subsequently, the changes in glass transition temperatures (Tgs) from the molten to the solid state, as observed by DMA and DSC, were verified by coupling SAOS to dielectric relaxation spectroscopy (DRS). Herein, the thermo-rheological complexity of PLA/PHBV blends in the melt was revealed, especially for PLA-rich blends. Irregularly structured morphologies, caused by highly mismatched viscoelastic properties, illustrated the degree of partial miscibility. Moreover, the thermo-rheological complexity appeared in the molten state of the asymmetric PLA-rich phases could be correlated to the crystal-amorphous interfacial MWS polarization, because of the locally-induced phase separation and heterogeneity, and owing to the differences in their crystallization properties during cooling. The miscibility also suffered from the lower thermal stability of PLA and the even more unstable PHBV. Nevertheless, the melt-induced degradation process of the PLA/PHBV blends seemed to be responsible for some of the in situ self-compatibilization and plasticization mechanisms. As a result, the miscibility and thermo-rheological simplicity were improved for the intermediate and PHBV-rich compositions at low temperatures, since their properties were, to a large extent, governed by the significant degradation of PHBV. The present findings should increase the understanding of morphological changes in PLA/PHBV blends and help control their micro/nanostructure.

Triblock Elastomeric Vitrimers: Preparation, Morphology, Rheology, and Applications

Vitrimer chemistry of boronic transesterification was introduced into a triblock thermoplastic elastomer to allow associative bond exchange in its highly ordered nanoscale domains. The purpose of this localized dynamic cross-linking is to enable reprocessing while also improving creep resistance. Triblock poly(styrene-b-butadiene-b-styrene) (SBS) was used as an elastomeric vitrimer precursor to gain understanding of fundamental structure–property effects of localized dynamical cross-linking. The occurrence of the UV-induced thiol–ene reaction was corroborated by Fourier transform infrared (FTIR) spectroscopy and swelling experiments. Transmission electron microscopy (TEM) images and small-angle X-ray scattering (SAXS) profiles demonstrate a prominently lamellar morphology with domain spacings that decline with the increase of cross-linking degree. Short and long relaxation modes of triblock SBS vitrimers (vSBSs) were found to have opposite temperature dependence, which caused thermorheological complexity in linear viscoelasticity. As compared to its random analog, vSBS shows much slower stress relaxation and higher activation energy, as well as superior resistance to creep under a steady load. The enhanced rheological properties are attributed to the high concentration of localized cross-links in the soft phase and to additional constraints that hinder network strand diffusion in the microphase-separated structure. The mechanical properties of vSBSs at low strain were substantially enhanced by the associative dynamic cross-links between the soft segments, increasing the maximum stress to 7.5 MPa at 100% strain. Its Young’s modulus of 88.1 MPa is about 2.8 times that of neat SBS. Applications of vSBSs as hot-melt adhesives were explored, showing excellent lap shear strength on iron plates. Due to the dynamic exchange of boronic ester bonds, adhesive joints can efficiently rebind at least three times and the lap shear strength remains almost unchanged.

A computational framework for rheologically complex thermo-visco-elastic materials

Fractional calculus has been proved to be very effective in representing the visco-elastic relaxation response of materials with memory such as polymers. Moreover, in modeling the temperature dependency of the material functions in thermo-visco-elasticity, the standard time–temperature superposition principle is known to be ineffective in most of the cases (thermo-rheological complexity). In this work, a novel finite element formulation and numerical implementation is proposed for the simulation of transient thermal analysis in thermo-rheologically complex materials. The parameters of the visco-elastic fractional constitutive law are assumed to be temperature dependent functions and an internal history variable is introduced to track the changes in temperature which are responsible for the phase transition of the material. The numerical approximation of the fractional derivative is employed via the so called Grünwald–Letnikov approximation. The proposed model is used to numerically solve some test cases related to relaxation and creep tests conducted on a real polymer (Ethylene Vinyl Acetate), which is used as the major encapsulant of solar cells in photovoltaics.

Understanding thermal and rheological behaviors of bimodal polymethyl methacrylate (BPMMA) fabricated via solution blending

Abstract In this work, a series of bimodal polymethyl methacrylate (BPMMA) was fabricated via solution-blending two neat PMMA resins. Rheology, DMTA, thermal infrared imager measurements were used in an attempt to probe the internal structure of the as-prepared BPMMA. It was demonstrated that the thermorheological behavior of the BPMMA was heavily dependent on shear rate, temperature as well as blending ratio. In addition, a typical “V-shaped” response, namely, a dip in storage modulus (G′) followed by an upturn in the plot of G′ versus measuring temperature for D4 (with lower weight-average molecular weight) was observed, characteristic of occurrence of thermorheological complexity. Our experimental results of physical–mechanical testings suggested that the BPMMA had better comprehensive properties than those of their neat PMMA counterparts.

Enhanced thermally conductive and thermomechanical properties of polymethyl methacrylate (PMMA)/graphene nanoplatelets (GNPs) nanocomposites for radiator of electronic components

In this work, multi-layer hot pressing (MLHP) method was used to prepare the graphene nanoplatelets (GNPs) filled polymethyl methacrylate (PMMA) composites. Effect of GNP content on the viscoelastic behavior and thermomechanical properties of composites was studied via rheometer, scanning electron microscope (SEM), infrared thermal imaging (ITI) and dynamic mechanical thermal analysis (DMTA), etc. According to dynamical rheological testing, thermorheological complexity was clearly displayed at elevated GNP content, based upon flow activation energy (Ea) and time-temperature superposition (TTS) principle. The infrared thermal imaging (ITI) indicated that a filler conductive network was gradually formed with increasing filler content. The average heating rate of the samples increased from 0.74 to 0.92 °C/s. The Agari model was found to fairly interpret the variation of thermal conductivity as a function of GNP loading, which agreed well with SEM observation and indicative of the formation of a GNP network structure. Dynamic thermomechanical properties of the composites were heavily influenced by the temperature distributions within the samples. The addition of GNP increased the glass transition temperature of PMMA from 116.7 to 125.9 °C, which favored enhanced thermophysical properties of PMMA. The prepared composites in this work showed wonderful mechanical properties even at relatively high temperatures (e.g., having storage modulus of 1.3–1.9 GPa and yielding strength more than 58.0 MPa at 50 °C). The current work is practically significant for further expanding the application range of PMMA/GNP nanocomposites.

Parameter Identification and Validation of Shape-Memory Polymers within the Framework of Finite Strain Viscoelasticity.

Shape-Memory Polymers (SMPs) can be stretched to large deformations and recover induced strains when exposed to an appropriate stimulus, such as heat. This emerging class of functional polymers has attracted much interest and found applications in medicine and engineering. Nevertheless, prior to any application, their physical and mechanical properties must be thoroughly studied and understood in order to make predictions or to design structures thereof. In this contribution, the viscoelastic behavior of a polyether-based polyurethane (Estane) and its rate- and temperature-dependent behavior have been studied experimentally and by the mean of simulations. The model-inherent material parameters are identified with the assumption of the thermo-rheological complexity. Here, the numerical results of uni-axial stress relaxations were compared with the associated experiments in conjucation with the Levenberg-Marquard optimization method to determine the parameters of the Prony equation. The ability of the model to simulate the thermo-mechanical properties of Estane was evaluated by data-rich experimental observations on tension and torsion in various temperature ranges. Heterogeneous tests are included into the experimental program to cover a broader spectrum of loading scenarios.

Thermorheological evidence and structure of heterogeneity in syndiotactic polypropylene melts with strong memory effects

The memory effect of semi-crystalline polymers is usually related to the heterogeneity in self-nucleated (SN) melts. In this study, the strong memory effects of syndiotactic polypropylene (sPP) and the heterogeneity in SN melts were systematically investigated in a wide temperature range even above its equilibrium melting temperature (Tmo). The rheological measurements revealed that the SN melts exhibited thermorheologically complex behaviors, indicating that the presence of the heterogeneity in the SN melts. From the rheological plots of phase angle (δ) with the complex modulus (G*), the parameter Δ ln G* δ=45° values were determined and used to compare the change of the degree of thermorheological complexity in the SN melts during the heating process. It was found that the memory effect's efficiency to promote the crystallization process improved as the increase of Δ ln G* δ=45° values, which suggested that the memory effect depended on the heterogeneity in the SN melts. After purifying the sample, the strong memory effect still existed, which indicated that it was not caused by the stabilization effect of impurities. The results of wide/small-angle X-ray scattering and infrared spectroscopy revealed that there were no partial residual crystallites or regular chain conformation in our investigated SN melts. Based on the complex chain structure of our investigated sPP, we speculated that the heterogeneity in sPP melts was caused by the separation between the long crystallizable sequence and the atactic sequence.

Rheological Behavior of Blends of Metallocene Catalyzed Long-Chain Branched Polyethylenes. Part I: Shear Rheological and Thermorheological Behavior.

Long-chain branched metallocene-catalyzed high-density polyethylenes (LCB-mHDPE) were solution blended to obtain blends with varying degrees of branching. A high molecular LCB-mHDPE was mixed with low molecular LCB-mHDPE at varying concentrations. The rheological behavior of those low molecular LCB-mHDPE is similar but their molar mass and molar mass distribution are significantly different. Those blends were characterized rheologically to study the effects of concentration, molar mass distribution, and long-chain branching level of the low molecular LCB-mHDPE. Owing to the ultra-long relaxation times of the high molecular LCB-mHDPE, the blends exhibited a clearly more long-chain branched behavior than the base materials. The thermorheological complexity analysis showed an apparent increase in the activation energies Ea determined from G′, G″, and especially δ. Ea(δ), which for LCB-mHDPE is a peak function, turned out to produce even more pronounced peaks than observed for LCB-mPE with narrow molar mass distribution and also LCB-mPE with broader molar mass distribution. Thus, it is possible to estimate the molar mass distribution from the details of the thermorheological complexity.

Effects of Amine and Silane Antistripping Agents on Rheological Behavior of Crumb Rubber Modified Binder at High and Low Temperature

Abstract This study focused on evaluating the effect of amine and silane antistripping agents on the rheological behavior of crumb rubber modified binder (CRMB60). In accordance with the manufacturer’s recommendation, 0.5 % and 0.1 % of amine and silane agents (by weight), respectively, were added to CRMB60. The effect of amine and silane antistripping agents on CRMB60 was studied by using various rheological tests. Amine reduced and silane caused a significant rise in the high failure temperature. However, both the antistrips raised the low failure temperature. All the tested binders (without and with antistripping agents) exhibited thermorheological complexity. Complex modulus (G*) master curves indicated that amine decreased and silane enhanced the elastic behavior of CRMB60 in high-temperature regime. Creep stiffness master curves depicted an increased stiffness at low temperature for silane modified CRMB60. Further, the stress relaxation master curve indicated that both amine and silane antistripping agents compromise the relaxation properties at low temperatures. Fourier transform infrared spectroscopy revealed that amine and silane antistripping agents can cause chemical changes in CRMB60. Overall, this study concluded that the antistripping agents affect the rheological behavior of CRMB60 and that detailed rheological investigation should be conducted prior to adopting an antistripping agent.

Viscoelastic and Morphological Evaluation of Aged Polymer Modified Asphalt Binders

Asphalt binder is a complex viscoelastic material. It implies that based on temperature and loading, its behaviour changes from a viscous material to an elastic material. Modification and ageing further add to this complexity. Over the years, a number of studies have examined all the aspects of polymer-modified binders (PMB), but there is still work needed to correlate the viscoelastic behaviour of the PMBs to their morphological behaviour. So, in the present study PMBs are prepared by modifying VG-10 base binder with the help of styrene butadiene styrene and ethylene vinyl acetate. Binders were aged using rolling thin-film oven test and pressure ageing vessel. Viscoelastic and morphological evaluation of binders was carried out by using dynamic shear rheometer and scanning electron microscopy (SEM), respectively. The test results showed that polymer modification enhances the thermorheological complexity of the base binders, while ageing does not affect it. Linear viscoelastic limits of the binders were found to be decreasing after ageing and modification. Ageing degrades the polymer network so after ageing PMBs were found to be behaving similar to base binder. The SEM results support these results.

Filler network structure in graphene nanoplatelet (GNP)-filled polymethyl methacrylate (PMMA) composites: From thermorheology to electrically and thermally conductive properties

In this work, graphene nanoplatelet (GNP) filled polymethyl methacrylate (PMMA) composites were prepared using solution method via a specially designed route and relatively high thermal conductivities of the composites were achieved at a low GNP loading. The effect of GNP content on rheological behavior, thermal and electrical conductivity of the composites was intensively investigated. Thermorheological complexity was displayed at elevated GNP loading, and the rheological percolation threshold of GNP in PMMA decreased from 7.96 wt% at 220 °C to 4.02 wt% at 260 °C according to Winter-Chambon method, suggesting that GNP was more likely to form a seepage network at higher temperature. The DMTA results showed that the increase in moduli of the composites should be ascribed to the formation of the GNP-GNP network structure. The electrical conductivity of the composites underwent a sudden jump by seven orders of magnitude, which also indicated the formation of a GNP conductive pathway in the matrix with an electrical percolation threshold of 2–4 wt%. The results of transient temperature measurement were in good consistent with the thermal conductivity versus GNP loading, which was compared with various thermal conduction models with a modified Agari model presenting an acceptable evaluation of the dispersion status of GNP in the matrix. The experimental electrical and thermal conductivities as a function of GNP content could well be interpreted by the filler network structure as observed in morphological studies.

Rethinking the Analysis of the Linear Viscoelastic Behavior of an Epoxy Polymer near and above the Glass Transition

In this communication, we propose a different approach for analyzing linear viscoelastic relaxation data that is more faithful to the underlying physics and naturally accommodates the thermorheological complexity that is observed in glass-forming polymers. Specifically, the linear viscoelastic behavior was evaluated for a diglycidyl ether of bisphenol-A epoxy cured with 4,4′-methylenedianaline with a glass transition temperature (Tg) of 101.5 °C. The dynamic storage and loss moduli were measured from 10–2 to 101.7 Hz for 19 temperatures between 90 and 180 °C. The experimental window was extended by two orders of magnitude using stress relaxation experiments for temperatures between 90 and 112.5 °C. The G′ and G″responses for this single-phase polymer are thermorheologically complex, thus precluding the construction of master curves via time–temperature superposition. The traditional method of determining the relaxation spectrum implicitly assumes a constant spectral density where the spectral strength cha...

Rheological and thermorheological assessment of polyethylene in multiple extrusion process

The relationship between rheological properties and structural changes in polyethylene chains due to the recycling of two linear low-density polyethylenes (LL-A and LL-B) has been investigated. These samples were subjected to multiple extrusion processes (9 extrusion steps). According to the results of gel content experiments, crosslinking did not occur. The results of the gel permeation chromatography indicated that the chain scission was slightly predominant during the re-extrusion, while the rheometry results clearly show the effects of long-chain branching during degradation. Therefore, the degradation proceeded through both paths. Based on the rheological results and the Cole-Cole method, LL-B exhibited a higher degradation rate during the extrusion process (because of the low antioxidant content), compared to LL-A. The results of thermorheological methods including van-Gurp-Palmen and activation energy spectra indicated that the extent of thermorheological complexity was intensified with increasing the number of extrusion cycles and consequently increasing the number of long-chain branches.

Thermo-Rheological Complexity of Novel Branch Polyethylene Synthesized by High Performance Bulky α-Diimine Nickel (II) Catalysts

Thermo-Rheological Complexity of Novel Branch Polyethylene Synthesized by High Performance Bulky <i>α</i>-Diimine Nickel (II) Catalysts

Low-temperature rheological and morphological characterization of SBS modified bitumen

Polymer modification is widely used to improve the engineering properties of bitumen, the most commonly used polymer modifier being styrene-butadiene-styrene (SBS) block copolymer. Although extensive studies have been performed on polymer modified bitumen (PMB), no reliable data is currently available on the effect of polymer modification on the dynamic rheological properties at low temperatures. In this study, we focus on the rheology of SBS modified bitumen near and below the glass transition temperature (Tg) using the 4-mm DSR technique. In addition, fluorescence microscopy and temperature-modulated differential scanning calorimetry are used to study the phase behavior and interactions in the SBS-bitumen blends. At high SBS concentrations, thermorheological complexity is observed in the investigated temperature range, attributable to the formation of a continuous SBS-rich network structure. In the case of compatible SBS-bitumen blends, a linear correlation is established between the flexural creep stiffness measured by bending beam rheometry (BBR) and the complex shear modulus measured by 4-mm DSR. Deviations from this linear trend are shown to result from the macro-phase separation induced by the poor compatibility of SBS and bitumen.

Abstract of: Investigations on the Thermorheologically Complex Material Behaviour of the Laminated Safety Glass Interlayer Ethylene-Vinyl-Acetate

Laminated safety glass has become an indispensable component in building construction, automotive and solar industry. It consists of at least two glass panes, that are laminated together with a polymeric interlayer. Mechanically speaking, the polymeric interlayer enables a shear transfer between the two glass panes. Here, the difficulty lies in the understanding of the real shear transmission. On the one hand, polymeric interlayers show a time dependent material behaviour, which can be described with a ‘Prony-series’ in the linear viscoelastic area. On the other hand, polymeric interlayers show a temperature dependent material behaviour. Hence, a Prony-series is only valid for one specific temperature. However, since relaxation is based on molecular movements and rearrangement processes, which can be thermally activated, an increase in temperature leads to an acceleration of the relaxation process. The time-temperature correlation can be taken into account by means of a ‘Time-Temperature-Superposition-Principle’ (TTSP). The relaxation curve of a thermorheologically simple material shifts solely horizontal along the time axis due to temperature changes, while its shape remains constant. Mathematically, this means, that all relaxation times of the Prony-series are multiplied by the same shift factor aT. Recent research of the authors shows, that some polymeric interlayers don’t follow a simple TTSP. The experimental identification through ‘Dynamical-Mechanical-Thermal-Analysis’ as well as ‘Differential Scanning Caliometry’ and numerical incorporation of this thermorheologically complex material behaviour into state-of-the-art Finite-Element-Software will be investigated on the example of ‘Ethylene-vinyl acetate’ in the following paper.