Stress Relaxation Experiments Research Articles

An experimental and theoretical investigation on the hyper-viscoelasticity of polyamide 12 produced by selective laser sintering.

Polyamide 12 (PA12) is vastly utilized in many additive manufacturing methods, such as Selective Laser Sintering (SLS), and a better understanding of its mechanical behaviors promotes available knowledge on the behaviors of 3D-printed parts made from this polymer. In this paper, SLS-produced standard tensile specimens are studied under monotonic and cyclic tension tests, as well as stress relaxation experiments, and the obtained force-displacement responses are shown to be consistent with a hyper-viscoelastic material model. This finding is also observed in typical pantographic structures produced by the same manufacturing parameters. To propose a constitutive model for predicting these behaviors, the convolution integral of a strain-dependent function and a time-dependent function is developed where the material parameters are determined with the use of both short-term and long-term responses of the specimens. Numerical results of the presented model for standard test specimens are shown to be in good agreements with the experimental ones under various loading conditions. To prove the capabilities of the proposed model in studying any SLS-produced part, finite element implementation of the constitutive equations is shown to provide numerical results in agreement with the empirical findings for tensile loading of the 3D-printed pantographic structure.

Polysquaramides.

Thermoplastics, while advantageous for their processability and recyclability, often compromise thermochemical stability and mechanical strength compared to thermosets. Addressing this limitation, we introduce an innovative approach employing reversibly cross-linked polymers, utilizing squaramide moieties to reconcile recyclability and robustness. Herein, we detail the synthesis of supramolecularly cross-linked polysquaramides through the condensation polymerization of diethyl squarate with primary and secondary diamines. This methodology embeds hydrogen-bonding squaramide motifs into the polymer chains, yielding materials with significantly enhanced storage moduli, reaching up to 1.2 GPa. Material characterization via dynamic mechanical analysis, creep-recovery, and stress relaxation experiments delineate a distinctive rubbery plateau across a broad temperature range, excellent creep resistance, and multimodal viscoelastic flow, respectively, attributable to the dynamic nature of the supramolecular cross-links. Additionally, the study showcases the modulation of glass transition temperature (Tg) by altering the monomer composition and stoichiometry, demonstrating the tunability of polymer viscoelastic properties through precise control over hydrogen bonding interactions. Overall, the incorporation of squaramide motifs not only provides the structural integrity and mechanical performance of these thermoplastics but also leads to engineering materials with tailored viscoelastic characteristics.

Nanocarbon architecture-dependent strengthening and deformation in Al matrix composites

The extraordinary strength of inner graphene walls in carbon nanotube (CNT) is barely exerted due to the weak inner-wall shear resistance, which extremely limits its load-bearing capability. To overcome such deficiency, nanocarbon architecture engineering from CNT to graphene nanoribbon (GNR) was performed via longitudinal unzipping of multi-walled CNT, which was utilized to reinforce pure Al. Results show that the activation volume of composites at macroyielding point, evaluated by stress relaxation experiments, monotonically decreases from CNT/Al to GNR/Al, which results in the continuous increase of critical resolved shear stress (CRSS) called for dislocation nucleation/cross-slip at the grain boundaries. Shear-lag model and numerical simulations demonstrate the increased load-transfer effect from CNT/Al to GNR/Al. Meanwhile, the isotropic and kinematic hardening in nanocarbon/Al composites were investigated both by loading-unloading-reloading tests and strain hardening model on basis of dislocation behavior, wherein the effective stress was determined as being larger than back stress in the composites. Detailed analysis further indicates that the nanocarbon architecture from CNT to GNR increases the back stress strengthening due to the enhanced dislocation accumulation at nanocarbon/Al interface. Moreover, as CNT was unfolded to GNR, the failure mode of reinforcements in the composites gradually changed from pull-out to breakage.

Probing the remarkable vitrimeric performance of Poly(dithiourethanes): A comprehensive investigation into the dynamic behavior

Vitrimers and vitrimer-like systems are renowned for their exceptional mechanical and thermal characteristics, coupled with their inherent recyclability. In this study, a novel class of vitrimer-like systems is introduced, wherein a tetrathiol and diisothiocyanate are blended to create poly(dithiourethanes), catalyzed by a basic catalyst. An innovative Covalent Adaptable Network (CAN) is synthesized, featuring a glass transition temperature (Tg) of 332 K as determined by calorimetry, with a tensile modulus of 2240 MPa. The material’s properties are investigated through solubility, stress-relaxation, and creep experiments, revealing relaxation times as short as 0.7 min at 363 K, indicative of its robust dynamic behavior. Dielectric spectroscopy is employed to analyze the various relaxation processes characteristic of such materials, particularly when the topology freezing transition (Tv) occurs below the Tg. This technique complements the findings of dynamic thermomechanical analysis (DMA), enhancing our understanding of the material’s behavior. Furthermore, the materials exhibit recyclability through a grinding and compression molding cycle without significant mechanical damage. Their dielectric constant and low loss factor, combined with the reversibility of their chemical structures, position them as promising candidates for energy-related applications.

Biobased partially crosslinked polyβ-hydroxyesters synthesized from low-melt carboxyl-polyamides and a dicyclocarbonate

A new kind of biobased partially crosslinked polyβ-hydroxyesters (cPHEs) was synthesized through the ring-opening polymerization of a five-membered dicyclocarbonate and carboxyl-terminated dimer acid (DA) oligopolyamides (DAPAcs). The DAPAcs provided carboxyl groups and attacked the cyclocarbonate groups to form β-hydroxyester units, which was confirmed by a model reaction. DAPAcs were synthesized from hexamethylenediamine and excess DA. Their structure and properties were confirmed by FTIR, 1H NMR, and DSC. cPHEs exhibited semicrystalline characters with good thermal properties evidenced by wide-angle X-ray scattering, DSC, and TGA. Introducing DAPA sequences improved intermolecular interactions and tensile strength. The attack of pendent hydroxy groups to remaining cyclocarbonate groups led to crosslinked cPHEs. Dynamic β-hydroxylester segments impart the reprocessability of cPHEs, and stress relaxation experiments were conducted at different temperatures to evaluate the exchange kinetics. A new synthetic route different from normal epoxy-carboxyl route was established. This method broadens the research of polyβ-hydroxylesters and their application range.

Cationic UV-curing of bio-based epoxidized castor oil vitrimers with electrically conductive properties

The growing appeal of carbon nanotube composites in the contemporary market derives from their exceptional thermal and chemical stability, coupled with electrical conductivity. In this study, we combined these salient features with a biobased epoxy matrix having vitrimeric properties, hence being reprocessable and resheapable, to obtain a biobased conductive coating. Epoxidised castor oil (ECO) was chosen as a monomer precursor for the straightforward synthesis. The synthesis relied on a cationic UV-curing process, embedding the conductive carbon nanotubes in the matrix. Photo DSC and transmission FTIR analysis were conducted to determine the final conversion of the epoxy rings in the cationic photocuring process. Thermo-mechanical properties were evaluated by tensile tests, and DMTA. Thermal stability was assessed by TGA. Dielectric spectroscopy confirmed increased electrical conductivity in the presence of increasing CNT content, reaching a percolation threshold at 0.5 phr of CNTs. Vitrimeric properties were proved by stress relaxation experiments, and the UV-cured composite underwent a thermo-activated transesterification reaction starting from 70 °C, catalysed by dibutyl phosphate. Overall, the ECO-CNT composite showed high thermal resistance (up to 400 °C) electrical conductivity with 0.5 phr CNT concentration, and vitrimeric properties. The study can be, therefore, considered a promising starting point to obtain sustainable biobased and electrically conductive vitrimers.

A Mechanical Model for Stress Relaxation of Polylactic Acid/Thermoplastic Polyurethane Blends

Polylactic acid (PLA) is considered a promising biodegradable polymer alternative. Due to its high brittleness, composite materials made by melt blending thermoplastic polyurethane (TPU) with PLA can enhance the toughness of PLA. To understand the forced aging caused by stress relaxation in polymer materials, this study explains the stress relaxation experiments of PLA/TPU blends with different mass ratios under applied strain through mechanical model simulations. The Kelvin representation of the standard linear solid model (SLSM) is used to analyze the stress relaxation data of TPU/PLA blends, successfully explaining that the Young’s moduli (E1 and E2) of springs decrease with increasing temperature and TPU content. The viscosity coefficient of the PLA/TPU blends decreases with increasing temperature, and its reciprocal follows the Arrhenius law. For TPU/PLA blends with increased concentration of TPU, the activation energy for stress relaxation shows a linear decrease, confirmed by the glass transition point measured by DMA, indicating that it does not involve chemical reactions.

Grain structure tailoring strategy for heterogeneous lamella SiCp/2024Al composites with exceptional strength-ductility synergy

In this work, a novel grain structure tailoring strategy using one-stepped planetary ball milling, different from conventional powder metallurgy routes that pre-prepared components with different microstructures and then mixed them up, was implemented to fabricate micro- and nano-sized SiC particles (m- & n-SiCp)/2024Al composites with heterogeneous lamella structure. The controllable transformation from homogeneous to heterogeneous grain structures is achieved via nonuniformly distributed n-SiCps induced local grain refinement. Heterogeneous lamella composites exhibit a superior modulus-strength-ductility synergy of 95.3 GPa in Young's modulus, 750.7 MPa in tensile strength and 4.9 % in uniform elongation, particularly with no less than 145 % and 175 % improvements in ductility and toughness, compared with homogeneous composites. Compressive stress relaxation experiments were conducted to reveal the structural dependence of plastic deformation behaviors. Sustainedly rising hetero-deformation induced (HDI) stress produced by extra geometrically necessary dislocation (GND) accumulation at heterogeneous soft/hard domain interfaces and sequential activation of multiple dislocation mediated mechanisms based on load transfer in heterogeneous lamella composites contribute to the enhanced strain hardening capacity, which offers intrinsic toughening. Also, extrinsic toughening originates from enhanced microcrack multiplication and crack-tip blunting.

Self-Healing Transparent Poly(dimethyl)siloxane with Tunable Mechanical Properties: Toward Enhanced Aging Materials for Space Applications.

When exposed to the geostationary orbit, polymeric materials tend to degrade on their surface due to the appearance of cracks. Implementing the self-healing concept in polymers going to space is a new approach to enhancing the lifespan of materials that cannot be replaced once launched. In this study, the elaboration of autonomous self-healing transparent poly(dimethylsiloxane) (PDMS) materials resistant to proton particles is presented. The PDMS materials are functionalized with various compositions of urea and imine moieties, forming dynamic covalent and/or supramolecular networks. The hydrogen bonds induced by the urea ensure the formation of a supramolecular network, while the dynamic covalent imine bonds are capable of undergoing exchange reactions. Materials with a broad range of mechanical properties were obtained depending on the composition and structure of the PDMS networks. As coating applications in a spatial environment were mainly targeted, the surface properties of the polymer are essential. Thus, percentages of scratch recovery were determined by AFM. From these data, self-healing kinetics were extracted and rationalized based on the polymer structures. The obtained data were in good agreement with the relaxation times determined by rheology in stress relaxation experiments. Moreover, the accelerated aging of materials under proton irradiation, simulating a major part of the geostationary environment, revealed a strong limitation or disappearance of cracks while keeping the transparency of the PDMS. These promising results open routes to prepare new flexible autonomous polymeric materials for space applications.

Mechanical Behaviors of Hybrid Composites with Orthogonal Spiral Wire Mesh and Polyurethane Elastomer

This work aims to significantly improve the mechanical properties of conventional rigid lattice structures under repeatable large deformations. A novel hybrid material is proposed based on the concept of interpenetrating composite materials. The material utilizes a woven TC4 orthogonal spiral wire mesh as the skeleton and PU elastomer (OSWM‐PU) as the matrix. The uniaxial tensile tests demonstrate that OSWM‐PU possesses the excellent load‐bearing capacity, allowing for large deformations (≥60%) while maintaining partial integrity even after matrix fracture. Optical measurements and simulation analysis reveal that Poisson's ratio can be adjusted within a certain range by manipulating the microscopic parameters (p, d) of the longitudinal helical filaments. Cyclic tensile experiments further demonstrate that OSWM‐PU exhibits exceptional energy absorption performance, multiple energy dissipation modes, and a more pronounced Mullins effect. The stress relaxation experiment reveals the significant influence of the volume fraction of the skeleton on long‐term loading conditions. The orthogonal spiral wire skeleton exhibits a superior hooking effect without dividing the matrix, enabling OSWM‐PU to possess enhanced collaborative deformation capability and inherent designability in the orthogonal direction. These characteristics make it highly promising for applications in various robot joints and as flexible aircraft skin, offering excellent prospects for utilization.

Polyester networks based on Tulipalin A and ε‐caprolactone and activation of the Thioether groups by incorporation of trialkylsulfonium salts

AbstractLinear and star‐shaped poly(ε‐caprolactone‐co‐α‐methylene‐γ‐butyrolactone) P(CL‐co‐MBL) copolyesters with a pendant functional double bond of α‐methylene‐γ‐butyrolactone (MBL) comonomer were used as polymeric precursor for organo‐gels formation. Crosslinking was carried out by light‐initiated thia‐Michael reaction using 1,5‐pentanedithiol (PDT) and pentaerythritol tetrakis(3‐mercaptopropionate) (PETK). Double bond conversion and the content of free thiol groups in the obtained networks was followed and rationally estimated using confocal Raman spectroscopy. The gel content and the crosslinking density varied based on MBL comonomer and crosslinker content and were highest for slight thiol to double bond excess (SH/vinyl; 2/1 molar ratio) while decreasing with further thiol excess. These parameters were more pronounced by the replacement of linear PDT with the star‐shaped PETK crosslinker. The thermal and rheological properties investigation of the obtained materials was performed employing DSC, TGA and using frequency and temperature sweeps rheological measurements. Shear moduli in the range 0.14 up to 0.93 MPa were obtained due to different content of MBL in pre‐polymer and crosslinker in network formation. The strength was caused by the crystalline phase of the PCL segments up to a temperature of 50°C. Formed thioether bond within the network were not reversible up to 150°C as it was found based on temperature sweep rheology. Following, the activation and conversion of thioether bonds within the obtained materials into trialkylsulfonium salts was attained through the alkylation using butyl brosylate. The dynamic nature of the transalkylation at elevated temperature 150°C allowed network rearrangement, which was proved by stress relaxation experiment and investigation of the self‐healing capabilities.

Sampling Points-Independent Identification of the Fractional Maxwell Model of Viscoelastic Materials Based on Stress Relaxation Experiment Data.

Considerable development has been observed in the area of applying fractional-order rheological models to describe the viscoelastic properties of miscellaneous materials in the last few decades together with the increasingly stronger adoption of fractional calculus. The fractional Maxwell model is the best-known non-integer-order rheological model. A weighted least-square approximation problem of the relaxation modulus by the fractional Maxwell model is considered when only the time measurements of the relaxation modulus corrupted by additive noises are accessible for identification. This study was dedicated to the determination of the model, optimal in the sense of the integral square weighted model quality index, which does not depend on the particular sampling points applied in the stress relaxation experiment. It is proved that even when the real description of the material relaxation modulus is entirely unknown, the optimal fractional Maxwell model parameters can be recovered from the relaxation modulus measurements recorded for sampling time points selected randomly according to respective randomization. The identified model is a strongly consistent estimate of the desired optimal model. The exponential convergence rate is demonstrated both by the stochastic convergence analysis and by the numerical studies. A simple scheme for the optimal model identification is given. Numerical studies are presented for the materials described by the short relaxation times of the unimodal Gauss-like relaxation spectrum and the long relaxation times of the Baumgaertel, Schausberger and Winter spectrum. These studies have shown that the appropriate randomization introduced in the selection of sampling points guarantees that the sequence of the optimal fractional Maxwell model parameters asymptotically converge to parameters independent of these sampling points. The robustness of the identified model to the measurement disturbances was demonstrated by analytical analysis and numerical studies.

Biobased, reprocessable, self-blown non-isocyanate polyurethane foams: Influence of blowing agent structure and functionality

The ever-increasing scrutiny of polymer environmental issues calls for advancements in polymers derived from sustainable sources, which can include biobased precursors. However, biobased polymers often exhibit properties inferior to petrochemical-based materials. Here, we address this issue relevant in the development of polyhydroxyurethane (PHU) foams made from the tandem aminolysis-decarboxylation of dimer-acid-derived cyclic carbonates by probing the effects of thiol structure and functionality on foam properties. Incorporating difunctional, trifunctional, and tetrafunctional thiols, we observed that the foam morphology is independent of thiol structure. Using dynamic mechanical analysis, compressive testing, and hysteresis testing, we demonstrated enhancements of mechanical properties due to increase in cross-link density. Additionally, with the glass transition temperature well below room temperature, the foams are categorized as high-resiliency flexible foams. Then, we reprocessed the foams into bulk materials via hydroxyurethane dynamic chemistry, showing full recovery of cross-link density and demonstrating added sustainability and recyclability benefits of these materials. Moreover, the utility of the reprocessed films as dimensionally stable elastomers is demonstrated by the elevated-temperature creep arrest. Notably, cyclic carbonates derived from dimer acid also contain ester groups that can exchange during reprocessing. We performed stress relaxation experiments and demonstrated that transesterification plays a key role in tuning the relaxation phenomena. This study elucidates the modular control of PHU foam mechanical properties and demonstrates the tunable design and relaxation behaviors of biobased PHU foams.

Effect of Zr on static recrystallization of deformed austenite and strain-induced precipitation in Ti microalloyed steel

The static recrystallization and strain-induced precipitation behavior of Ti and Ti-Zr low carbon microalloyed steels were studied through isothermal stress relaxation experiments at different deformation temperatures (875–1025 °C). The precipitation kinetic curves of microalloyed carbides were obtained, and the effect of Zr on the precipitation kinetics of microalloyed carbides was analyzed. Two static recrystallization kinetic models were established, and the activation energies for static recrystallization of Ti steel and Ti-Zr steel were calculated to be 312.44 and 246.59 kJ/mol, respectively. The results indicate that the addition of Zr lowers the nose point temperature of the precipitation–time–temperature curve, shortens the incubation period for the nucleation of precipitates, promotes the nucleation of strain-induced precipitates in Ti microalloyed steel, and reduces the coarsening rate of precipitates particles by an order of magnitude. In addition, the addition of Zr also advances the end time of static recrystallization of deformed austenite, inhibits the growth of static recrystallization austenite grains in Ti steel during the isothermal process, and makes the austenite grains finer and more uniform.

Rheological behavior of Pluronic/Pluronic diacrylate hydrogels used for bacteria encapsulation in engineered living materials.

Pluronic (Plu) hydrogels mixed with variable fractions of Pluronic diacrylate (PluDA) have become popular matrices to encapsulate bacteria and control their growth in engineered living materials. Here we study the rheological response of 30 wt% Plu/PluDA hydrogels with PluDA fraction between 0 and 1. We quantify the range of viscoelastic properties that can be covered in this system by varying in the PluDA fraction. We present stress relaxation and creep-recovery experiments and describe the variation of the critical yield strain/stress, relaxation and recovery parameters of Plu/PluDA hydrogels as function of the covalent crosslinking degree using the Burgers and Weilbull models. The analyzed hydrogels present two stress relaxations with different timescales which can be tuned with the covalent crosslinking degree. We expect this study to help users of Plu/PluDA hydrogels to estimate the mechanical properties of their systems, and to correlate them with the behaviour of bacteria in future Plu/PluDA devices of similar composition.

Creep and stress relaxation experiments of 1960-grade high-strength steel wire at elevated temperatures

With the increasing spacing of large-span structures, the application of ultra-high-strength steel wire with a strength approaching or exceeding 2000 MPa is rapidly expanding. However, the related research on its creep and stress relaxation behaviour is still limited. In this paper, a total of 30 high-temperature creep tests and 31 high-temperature stress relaxation tests were conducted on 1960-grade high-strength steel wire (1960-HSSW) with a diameter of 5.0 mm. The full process creep strain curves were obtained by reinserting and resetting the high-temperature extensometer. The maximum creep strain measured was 50.67 %. The creep and stress relaxation curves of different high-strength steel wires (steel strands) were compared, revealing significant variations in mechanical properties among different steel wires. The steady-state creep rate and high-temperature creep rupture critical stress ratio of the 1960-HSSW were given. Especially, the critical stress ratio became lower than the yield strength reduction coefficient at temperatures exceeding 400 °C and it indicated that the 1960-HSSW could undergo creep rupture before yield. The stress relaxation coefficient was introduced to consider the stress relaxation of 1960-HSSW. Stress relaxation curves for the 1960-HSSW were provided at temperatures ranging from 100 to 550 °C. Moreover, based on the microstructure observed under transmission electron microscopy, it was found that damage occurred in the 1960-HSSW after exposure to 300 °C. Therefore, a critical temperature of 300 °C was recommended for the fire protection of 1960-HSSW, which could be the theoretical support for its fire protection design. Additionally, a high-temperature creep prediction model, which includes expressions for the termination time, was established for the 1960-HSSW. As a result, a conservative prediction of the high-temperature creep for the 1960-HSSW was provided. Moreover, A stress relaxation coefficient model was also provided. The predicted results of the aforementioned models were in good agreement with experimental results.

Processing of Polymers Stress Relaxation Curves Using Machine Learning Methods

Currently, one of the topical areas of application of machine learning methods is the prediction of material characteristics. The aim of this work is to develop machine learning models for determining the rheological properties of polymers from experimental stress relaxation curves. The paper presents an overview of the main directions of metaheuristic approaches (local search, evolutionary algorithms) to solving combinatorial optimization problems. Metaheuristic algorithms for solving some important combinatorial optimization problems are described, with special emphasis on the construction of decision trees. A comparative analysis of algorithms for solving the regression problem in CatBoost Regressor has been carried out. The object of the study is the generated data sets obtained on the basis of theoretical stress relaxation curves. Tables of initial data for training models for all samples are presented, a statistical analysis of the characteristics of the initial data sets is carried out. The total number of numerical experiments for all samples was 346020 variations. When developing the models, CatBoost artificial intelligence methods were used, regularization methods (Weight Decay, Decoupled Weight Decay Regularization, Augmentation) were used to improve the accuracy of the model, and the Z-Score method was used to normalize the data. As a result of the study, intelligent models were developed to determine the rheological parameters of polymers included in the generalized non-linear Maxwell-Gurevich equation (initial relaxation viscosity, velocity modulus) using generated data sets for the EDT-10 epoxy binder as an example. Based on the results of testing the models, the quality of the models was assessed, graphs of forecasts for trainees and test samples, graphs of forecast errors were plotted. Intelligent models are based on the CatBoost algorithm and implemented in the Jupyter Notebook environment in Python. The constructed models have passed the quality assessment according to the following metrics: MAE, MSE, RMSE, MAPE. The maximum value of model error predictions was 0.86 for the MAPE metric, and the minimum value of model error predictions was 0.001 for the MSE metric. Model performance estimates obtained during testing are valid.

How to Make the Stress Relaxation Experiment for Polymers More Informative.

Different viscoelastic models and characteristics are commonly used to describe, analyze, compare and improve the mechanical properties of polymers. A time-dependent linear relaxation modulus next to frequency-domain storage and loss moduli are the basic rheological material functions of polymers. The exponential Maxwell model and the exponential stretched Kohlrausch-Williams-Watts model are, probably, the most known linear rheological models of polymers. There are different identification methods for such models, some of which are dedicated to specific models, while others are general in nature. However, the identification result, i.e., the best model, always depends on the specific experimental data on the basis of which it was determined. When the rheological stress relaxation test is performed, the data are composed of the sampling instants used in the test and on the measurements of the relaxation modulus of the real material. To build a relaxation modulus model that does not depend on sampling instants is a fundamental concern. The problem of weighted least-squares approximation of the real relaxation modulus is discussed when only the noise-corrupted time-measurements of the relaxation modulus are accessible for identification. A wide class of models, that are continuous, differentiable and Lipschitz with respect to parameters, is considered for the relaxation modulus approximation. The main results concern the models that are selected asymptotically as the number of measurements tends to infinity. It is shown that even when the true relaxation modulus description is completely unknown, the approximate optimal model parameters can be derived from the measurement data that are obtained for sampling instants that are selected randomly due to the appropriate randomization introduced whenever certain conditions regarding the adopted class of models are satisfied. It is shown that the most commonly used stress relaxation models, the Maxwell and Kohlrausch-Williams-Watts models, satisfy these conditions. Since the practical problems of the identification of relaxation modulus models are usually ill posed, Tikhonov regularization is applied to guarantee the stability of the regularized solutions. The approximate optimal model is a strongly consistent estimate of the regularized model that is optimal in the sense of the deterministic integral weighted square error. An identification algorithm leading to the best regularized model is presented. The stochastic-type convergence analysis is conducted for noise-corrupted relaxation modulus measurements, and the exponential convergence rate is proved. Numerical studies for different models of the relaxation modulus used in the polymer rheology are presented for the material described by a bimodal Gauss-like relaxation spectrum. Numerical studies have shown that if appropriate randomization is introduced in the selection of sampling instants, then optimal regularized models of the relaxation modulus being asymptotically independent of these time instants can be recovered from the stress relaxation experiment data. The robustness of the identification algorithm to measurement noises was demonstrated both by analytical and numerical analyses.

Thermomechanical devulcanization of butyl rubber using twin-screw extruder: Process parameters, viscoelastic and compatibility properties

Thermomechanical devulcanization of butyl rubber (BR1), bromobutyl rubber (BR2) vulcanizates, both prepared in laboratory, and a waste commercial butyl rubber (BR3) using a twin-screw extruder is reported. The commercial butyl rubber vulcanizate (BR3) was used as a tire curing bladder and the waste rubber received from a tire manufacturing company. The devulcanization process was carried out at various barrel temperatures and screw speeds (40, 80 and 120 r/min). The lowest sol fraction was obtained for devulcanized waste commercial butyl rubber (D-BR3). According to Horikx theory, devulcanized butyl rubber (D-BR1) presented mainly crosslink breakup while devulcanized bromobutyl rubber (D-BR2) and devulcanized waste commercial butyl rubber (D-BR3) showed a mixture of main chain and crosslink scission. As cure rheographs at 190°C indicated, the addition of devulcanized rubbers to the corresponding virgin ones influenced both scorch and optimal cure times. The tensile properties of devulcanized/virgin rubber blends changed depending on the blend type and virgin rubber. The stress relaxation experiment revealed that the addition of devulcanized rubber to virgin one mostly influenced short relaxation times and also decreased G0. BR1/D-BR3 blends exhibited higher elastic component than BR1/D-BR1 and BR2/D-BR2 samples. The Han plots revealed higher compatibility for BR2/D-BR2 blends.

Fabrication of recyclable and biodegradable PBAT vitrimer via construction of highly dynamic cross-linked network

The emergence of biodegradable materials has not satisfactorily addressed the issue of plastic waste management, owing to their high cost and poor mechanical performance and durability that impair their potential to replace traditional plastics. In this study, a dynamic cross-linked network with robust remoldability was fabricated in PBAT. Transesterification catalysts were employed to catalyze the reaction of β-hydroxyl groups produced from the branching reaction of polyester end groups and epoxy branching agents with the abundant ester groups of PBAT to produce biodegradable PBAT vitrimers. Swelling experiments, DMA tests and stress relaxation experiments confirmed the presence of the dynamic cross-linking structure, while rheological analysis demonstrated that the highly catalyst-loaded PBAT vitrimer dynamic cross-linked network could achieve chain relaxation by exchanging bonds quickly at physical entanglement and chemical cross-linking points. Multiple recycling experiments further demonstrated that the robust dynamic cross-linked network of PBAT vitrimers possessed both strong stress relaxation ability and remoldability, thus conferring excellent recyclability to PBAT. Upon recycling 4 times, the elongation at break of the PBAT vitrimer remained at 93.46 % (with a tensile strength retention of 85.99 %), whereas the raw PBAT could only maintain an elongation at break of 28.01 % (with a tensile strength retention of 56.09 %). TGA and hydrolysis experiments revealed that PBAT vitrimers exhibited better degradation performance relative to raw PBAT. PBAT vitrimers, featuring excellent mechanical performance and durability, can be recycled several times in easily recyclable settings, thus reducing production costs, and finally biodegrade after disposal, thereby serving as a viable replacement for traditional plastics. This straightforward and practicable approach provides a solution for plastic waste management.