3D observations provide striking findings in rubber elasticity.

The mechanical response of rubbers has been ubiquitously assumed to be only a function of the imposed strain. Using innovative X-ray measurements capturing the three-dimensional spatial volumetric strain fields, we demonstrate that rubbers and indeed many common engineering polymers undergo significant local volume changes. But remarkably, the overall specimen volume remains constant regardless of the imposed loading. This strange behavior which also leads to apparent negative local bulk moduli is due to the presence of a mobile phase within these materials. Combining X-ray tomographic observations with high-speed radiography to track the motion of the mobile phase, we have revised classical thermodynamic frameworks of rubber elasticity. The work opens broad avenues to understand not only the mechanical behavior of rubbers but a large class of widely used engineering polymers.

Thermodynamic Parameters of Crosslinked Elastomers (BR, SBR and NBR) and Their Blends.

Herein, a methodology is employed based on the Flory-Rehner equation for estimating the Flory-Huggins interaction parameter (χ12*) of crosslinked elastomer blends. For this purpose, binary elastomer blends containing polybutadiene rubber (BR), styrene-butadiene rubber (SBR) and nitrile-butadiene rubber (NBR), were prepared in a mixing chamber at a temperature below the activation of the crosslinking agent. Swelling tests with benzene were employed to determine the crosslinked fraction, finding that after 20 min of thermal annealing, the BR and NBR were almost completely crosslinked, while the SBR only reached 60%. Additionally, the BR-SBR blend increased by 2-3 times its volume than its pure components; this could be explained based on the crosslink density. From the mechanical tests, a negative deviation from the rule of mixtures was observed, which suggested that the crosslinking was preferably carried out in the phases and not at the interface. Furthermore, tensile tests and swelling fraction (ϕsw) results were employed to determine the average molecular weight between two crosslinking points (Mc), and subsequently χ12*. Calculated χ12* values were slightly higher than those reported in the literature. The calculated thermodynamic parameters for the blends showed positive ΔGmix values and endothermic behavior, suggesting their immiscible nature.

Recent Developments in Synthesis, Properties, Applications and Recycling of Bio-Based Elastomers.

In 2021, global plastics production was 390.7 Mt; in 2022, it was 400.3 Mt, showing an increase of 2.4%, and this rising tendency will increase yearly. Of this data, less than 2% correspond to bio-based plastics. Currently, polymers, including elastomers, are non-recyclable and come from non-renewable sources. Additionally, most elastomers are thermosets, making them complex to recycle and reuse. It takes hundreds to thousands of years to decompose or biodegrade, contributing to plastic waste accumulation, nano and microplastic formation, and environmental pollution. Due to this, the synthesis of elastomers from natural and renewable resources has attracted the attention of researchers and industries. In this review paper, new methods and strategies are proposed for the preparation of bio-based elastomers. The main goals are the advances and improvements in the synthesis, properties, and applications of bio-based elastomers from natural and industrial rubbers, polyurethanes, polyesters, and polyethers, and an approach to their circular economy and sustainability. Olefin metathesis is proposed as a novel and sustainable method for the synthesis of bio-based elastomers, which allows for the depolymerization or degradation of rubbers with the use of essential oils, terpenes, fatty acids, and fatty alcohols from natural resources such as chain transfer agents (CTA) or donors of the terminal groups in the main chain, which allow for control of the molecular weights and functional groups, obtaining new compounds, oligomers, and bio-based elastomers with an added value for the application of new polymers and materials. This tendency contributes to the development of bio-based elastomers that can reduce carbon emissions, avoid cross-contamination from fossil fuels, and obtain a greener material with biodegradable and/or compostable behavior.

Crosslinking of Epoxidized Natural Rubber with Borax for Self-Healing and Self-Repairing Properties: pH Dependence.

Epoxidized natural rubber (ENR) crosslinked using borax, which exhibits self-healing and self-repairing properties, is successfully developed. The crosslink formation of ENR by using borax under neutral and alkaline conditions is investigated. Fourier transform infrared spectroscopy (FTIR) shows that the borate-ester bond is formed in ENR prepared under both neutral and alkaline conditions, whereas boron nuclear magnetic resonance (11 B-NMR) results exhibit that the ENR prepared under alkaline conditions more actively forms crosslink networks with borax. Moreover, the crosslink density and gel content increase significantly with the presence of borax in alkaline conditions. The crosslink density and gel content of ENR with 10 phr borax are higher by 155% and 36%, respectively, than those of neat ENR. Furthermore, the formation of the crosslinking ENR by borax enhances self-healing and self-repairing properties. The healing efficiency significantly increases from 1.09% to 85.90%, when ENR is developed under alkaline conditions with 30 phr borax. These results represent the first successful demonstration of the efficient use of borax as a crosslinker in ENR, which exhibits its promising self-healing and self-repairing properties under atmospheric conditions without the need for external stimuli. The ENR prepared in this work holds great promise for various self-healing rubber applications.

Elastomer Nanocomposites: Effect of Filler-Matrix and Filler-Filler Interactions.

The reinforcement of elastomers is essential in the rubber industry in order to obtain the properties required for commercial applications. The addition of active fillers in an elastomer usually leads to an improvement in the mechanical properties such as the elastic modulus and the rupture properties. Filled rubbers are also characterized by two specific behaviors related to energy dissipation known as the Payne and the Mullins effects. The Payne effect is related to non-linear viscoelastic behavior of the storage modulus while the Mullins or stress-softening effect is characterized by a lowering in the stress when the vulcanizate is extended a second time. Both effects are shown to strongly depend on the interfacial adhesion and filler dispersion. The basic mechanisms of reinforcement are first discussed in the case of conventional rubber composites filled with carbon black or silica usually present in the host matrix in the form of aggregates and agglomerates. The use of nanoscale fillers with isotropic or anisotropic morphologies is expected to yield much more improvement than that imparted by micron-scale fillers owing to the very large polymer-filler interface. This work reports some results obtained with three types of nanoparticles that can reinforce rubbery matrices: spherical, rod-shaped and layered fillers. Each type of particle is shown to impart to the host medium a specific reinforcement on account of its own structure and geometry. The novelty of this work is to emphasize the particular mechanical behavior of some systems filled with nanospherical particles such as in situ silica-filled poly(dimethylsiloxane) networks that display a strong polymer-filler interface and whose mechanical response is typical of double network elastomers. Additionally, the potential of carbon dots as a reinforcing filler for elastomeric materials is highlighted. Different results are reported on the reinforcement imparted by carbon nanotubes and graphenic materials that is far below their expected capability despite the development of various techniques intended to reduce particle aggregation and improve interfacial bonding with the host matrix.

Probing the Free Volume in Polymers by Means of Positron Annihilation Lifetime Spectroscopy.

Positron annihilation lifetime spectroscopy (PALS) is a valuable technique to investigate defects in solids, such as vacancy clusters and grain boundaries in metals and alloys, as well as lattice imperfections in semiconductors. Positron spectroscopy is able to reveal the size, structure and concentration of vacancies with a sensitivity of 10-7. In the field of porous and amorphous systems, PALS can probe cavities in the range from a few tenths up to several tens of nm. In the case of polymers, PALS is one of the few techniques able to give information on the holes forming the free volume. This quantity, which cannot be measured with macroscopic techniques, is correlated to important mechanical, thermal, and transport properties of polymers. It can be deduced theoretically by applying suitable equations of state derived by cell models, and PALS supplies a quantitative measure of the free volume by probing the corresponding sub-nanometric holes. The system used is positronium (Ps), an unstable atom formed by a positron and an electron, whose lifetime can be related to the typical size of the holes. When analyzed in terms of continuous lifetimes, the positron annihilation spectrum allows one to gain insight into the distribution of the free volume holes, an almost unique feature of this technique. The present paper is an overview of PALS, addressed in particular to readers not familiar with this technique, with emphasis on the experimental aspects. After a general introduction on free volume, positronium, and the experimental apparatus needed to acquire the corresponding lifetime, some of the recent results obtained by various groups will be shown, highlighting the connections between the free volume as probed by PALS and structural properties of the investigated materials.

The danger of the toxicity and inefficacy of alcohol-based hand rubs in Iran during COVID-19: a cross-sectional study.

BACKGROUND: The use of disinfectants and alcohol-based hand rubs (ABHR) to prevent COVID-19 transmission increased in the first wave of the infection. To meet the increased demand, the Iranian Ministry of Health issued an emergency use authorization allowing new manufacturers to enter the market, despite the limited capacity for surveillance of these products during COVID-19. Methanol poisoning outbreaks spread rapidly, and more people died from methanol poisoning than COVID-19 in some cities. The aim of this study was to analyze some ABHRs in the Iranian market to see if (a) ABHRs are standard and suitable for hand antisepsis and (b) contained potentially dangerous toxic alcohols. METHOD: Between February and March 2020, 64 brands of ABHR were conveniently collected from pharmacies, supermarkets, and shops selling hygienic products and analyzed using Gas Chromatography. World Health Organization and Food and Drug Administration guidelines were used to define minimum requirements for ABHR. For estimating the risk for acute methanol poisoning, we assumed a serum methanol concentration of 200 mg/L following ABHR ingestion was sufficient to cause intoxication. This threshold concentration would be achieved in an average 75-kg adult after consuming 8000 mg (or eight grams) methanol in 1-2 h. RESULTS: The median [IQR] (range) concentration of ethanol, isopropanol, and methanol were 59% v/v [32.2, 68] (0, 99), 0 mg/L [0, 0] (0, 197,961), and 0 mg/L [0, 0] (0, 680,100), respectively. There was a strong negative correlation between methanol and ethanol contents of hand rubbers (r= -0.617, p < 0.001). Almost 47% of ABHRs complied with minimum standards. In 12.5% of ABHRs, high concentrations of methanol were observed, which have no antiseptic properties but could cause acute methanol poisoning if ingested. CONCLUSION: COVID-19 initiated a policy for distribution and use of ABHR with little control. As ABHR and masks are still accepted preventive measures of the disease, non-standard ABHR compositions may increase the population's risk to both COVID-19 infection and methanol poisoning.

In-situ and real-time nano/microplastic coatings and dynamics in water using nano-DIHM: A novel capability for the plastic life cycle research.

A novel nano-digital inline holographic microscope (nano-DIHM) was used to advance in-situ and real-time nano/microplastic physicochemical research, such as particle coatings and dynamic processes in water. Nano-DIHM data provided evidence of distinct coating patterns on nano/microplastic particles by oleic acid, magnetite, and phytoplankton, representing organic, inorganic, and biological coatings widely present in the natural surroundings. A high-resolution scanning transmission electron microscopy confirmed nano-DIHM data, demonstrating its nano/microplastic research capabilities. The sedimentation of two plastic size categories was examined: (a) ∼10 to 700 µm, and (b) ∼ 1 to 5 mm. Particle size was the primary factor affecting the sedimentation for studied (a) microplastics and (b) pellets. Two types of silicone rubbers exhibited different sedimentation processes. We also demonstrated that inorganic ions in seawater and oleic acid organic coatings altered the sedimentation velocity of studied plastics by 9 - 13% and 5 - 9%, respectively. Semi-empirical probability functions were developed and incorporated into a numerical model (CaMPSim-3D) to simulate the transport of studied microplastics and pellets in the Saint John River estuary. Water dynamics was the driving force of plastic transport, yet the accumulation of plastics was selectively dependant on particle physicochemical properties such as size and density by ∼ 7%. The usage of nano-DIHM for targeted identification of nano/microplastic hotspots and aquatic plastic wastes remediation were discussed.

Analysis of Thermoplastic Copolymers by Mild Thermal Degradation Coupled to Ion Mobility Mass Spectrometry.

Thermal desorption/degradation with an atmospheric solids analysis probe (ASAP) and ion mobility (IM) separation are coupled with mass spectrometry (MS) analysis and tandem mass spectrometry (MS/MS) fragmentation to characterize thermoplastic elastomers. The compounds investigated, which are used in the manufacture of a wide variety of packaging materials, are mainly composed of thermoplastic copolymers, but also contain additional chemicals ("additives"), like antioxidants and UV stabilizers, for enhancement of their properties or protection from degradation. The traditional method for analyzing such complex mixtures is vacuum pyrolysis followed by electron or chemical ionization mass spectrometry, often after gas chromatography separation. Here, an alternative, faster approach, involving mild degradation at atmospheric pressure (ASAP) and subsequent characterization of the desorbates and pyrolyzates by IM-MS, and if needed, MS/MS is presented. Such multidimensional dispersion considerably simplifies the resulting spectra, permitting the conclusive separation, characterization, and classification of the multicomponent materials examined.

Modifications of Textile Materials with Functional Silanes, Liquid Silicone Softeners, and Silicone Rubbers-A Review.

General information concerning different kinds of chemical additives used in the textile industry has been described in this paper. The properties and applications of organofunctional silanes and polysiloxanes (silicones) for chemical and physical modifications of textile materials have been reviewed, with a focus on silicone softeners, silane, and silicones-based superhydrophobic finishes and coatings on textiles composed of silicone elastomers and rubbers. The properties of textile materials modified with silanes and silicones and their practical and potential applications, mainly in the textile industry, have been discussed.

Realizing Vat-Photocycloaddition 3D Printing with Recyclable Synthetic Photorheological Silicone Fluids.

Synthetic silicone rubbers are finding a broad spectrum of applications, yet there is a demand for developing greener silicone rubbers with processability, recyclability, and reversible tunability in their mechanical properties. Here, a recyclable photorheological silicone fluid (RPSF) is developed, which realizes completely reversible wavelength-selective liquid-rubber conversion upon photoirradiation, relying on the reversible photocycloaddition of coumarin upon alternating irradiation of light with wavelengths of 365 (UV365 ) and 254 nm (UV254 ). Rheological studies demonstrate that the storage modulus of the developed RPSF increases by a factor of more than 100 000 upon UV365 irradiation to reach 20-50 kPa, while it decreases to ≈0.01 kPa upon UV254 irradiation. The reversibility of the photocycloaddition of coumarin enables the application of RPSF as a photodismantlable adhesive. Furthermore, unprecedented vat-photocycloaddition 3D printing of silicone rubber is realized by taking advantage of the excellent photocurability, that is, the dramatic increase in viscoelasticity upon UV365 irradiation.

Organosilicone Compounds in Supercritical Carbon Dioxide.

This review considers the key advantages of using supercritical carbon dioxide as a solvent for systems with organosilicon compounds. Organosilicon polymeric materials synthesis as well as the creation and modification of composites based on them are discussed. Polydimethylsiloxane and analogues used as polymerization stabilizers and nucleation promoters in pore formation processes are analyzed as well.

Highly stretchable and robust transparent conductive polymer composites for multifunctional healthcare monitoring.

Soft, stretchable, conductive thin films have propelled to the forefront of applications in stretchable sensors for on-skin health monitoring. Stretchable conductive films require high conformability, stretchability, and mechanical/chemical stability when integrated into the skin. Here, we present a highly stretchable, conductive, and transparent natural rubber/silver nanowire (AgNW)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composite film. Overcoating the PEDOT:PSS layer results in outstanding mechanical robustness and chemical stability by suppressing the mechanical and chemical degradation of the nanowire networks. Moreover, the introduction of the organic surface modifier enhances the bonding strength between the natural rubber substrate and AgNW at the interface. The highly conformable composite films are integrated into multifunctional on-skin sensors for monitoring various human motions and biological signals with low-power consumption. We believe that the highly stretchable, robust, and conformable natural rubber/AgNW/PEDOT:PSS composite film can offer new opportunities for next-generation wearable sensors for body motion and physiological monitoring.

Ecological Impact of End-of-Life-Tire (ELT)-Derived Rubbers: Acute and Chronic Effects at Organism and Population Levels.

Considering the large amount of tires that reach the end of life every year, the aim of this study was the evaluation of both acute and chronic effects of end-of-life-tire (ELT)-derived rubber granules (ELT-dg) and powder (ELT-dp) on a freshwater trophic chain represented by the green alga Pseudokirchneriella subcapitata, the crustacean Daphnia magna and the teleost Danio rerio (zebrafish). Adverse effects were evaluated at the organism and population levels through the classical ecotoxicological tests. Acute tests on D. magna and D. rerio revealed a 50% effect concentration (EC50) > 100.0 mg/L for both ELT-dg and ELT-dp. Chronic exposures had a lowest observed effect concentration (LOEC) of 100.0 mg/L for both ELT-dg and ELT-dp on P. subcapitata grow rate and yield. LOEC decreased in the other model organisms, with a value of 9.8 mg/L for D. magna, referring to the number of living offspring, exposed to ELT-dg suspension. Similarly, in D. rerio, the main results highlighted a LOEC of 10.0 mg/L regarding the survival and juvenile weight parameters for ELT-dg and a LOEC of 10.0 mg/L concerning the survival and abnormal behavior in specimens exposed to ELT-dp. Tested materials exhibited a threshold of toxicity of 9.8 mg/L, probably a non-environmental concentration, although further investigations are needed to clarify the potential ecological impact of these emerging contaminants.

Fracture Toughness of Moldable Low-Temperature Carbonized Elastomer-Based Composites Filled with Shungite and Short Carbon Fibers.

This work evaluated the fracture toughness of the low-temperature carbonized elastomer-based composites filled with shungite and short carbon fibers. The effects of the carbonization temperature and filler content on the critical stress intensity factor (K1c) were examined. The K1c parameter was obtained using three-point bending tests for specimens with different l/b ratio (notch depth to sample thickness) ranging from 0.2 to 0.4. Reliable detection of the initiation and propagation of cracks was achieved using an acoustic sensor was attached to the samples during the bending test. The critical stress intensity factor was found to decrease linearly with increasing carbonization temperature. As the temperature increased from 280 to 380 °C, the K1c parameter was drastically reduced from about 5 to 1 MPa·m1/2 and was associated with intense outgassing during the carbonization step that resulted in sample porosity. The carbon fiber addition led to some incremental toughening; however, it reduced the statistical dispersion of the K1c values.

The Final Frontier of Sustainable Materials: Current Developments in Self-Healing Elastomers.

It is impossible to describe the recent progress of our society without considering the role of polymers; however, for a broad audience, "polymer" is usually related to environmental pollution. The poor disposal and management of polymeric waste has led to an important environmental crisis, and, within polymers, plastics have attracted bad press despite being easily reprocessable. Nonetheless, there is a group of polymeric materials that is particularly more complex to reprocess, rubbers. These macromolecules are formed by irreversible crosslinked networks that give them their characteristic elastic behavior, but at the same time avoid their reprocessing. Conferring them a self-healing capacity stands out as a decisive approach for overcoming this limitation. By this mean, rubbers would be able to repair or restore their damage automatically, autonomously, or by applying an external stimulus, increasing their lifetime, and making them compatible with the circular economy model. Spain is a reference country in the implementation of this strategy in rubbery materials, achieving successful self-healable elastomers with high healing efficiency and outstanding mechanical performance. This article presents an exhaustive summary of the developments reported in the previous 10 years, which demonstrates that this property is the last frontier in search of truly sustainable materials.

Synthesis and Vulcanization of Polymyrcene and Polyfarnesene Bio-Based Rubbers: Influence of the Chemical Structure over the Vulcanization Process and Mechanical Properties.

The overuse of fossil-based resources to produce thermoplastic materials and rubbers is dramatically affecting the environment, reflected in its clearest way as global warming. As a way of reducing this, multiple efforts are being undertaken including the use of more sustainable alternatives, for instance, those of natural origin as the main feedstock alternative, therefore having a lower carbon footprint. Contributing to this goal, the synthesis of bio-based rubbers based on ß-myrcene and trans-ß-farnesene was addressed in this work. Polymyrcene (PM) and polyfarnesene (PF) were synthesized via coordination polymerization using a neodymium-based catalytic system, and their properties were compared to the conventional polybutadiene (PB) and polyisoprene (PI) also obtained via coordination polymerization. Moreover, different average molecular weights were also tested to elucidate the influence over the materials' properties. The crosslinking of the rubbers was carried out via conventional and efficient vulcanization routes, comparing the final properties of the crosslinking network of bio-based PM and PF with the conventional fossil-based PB and PI. Though the mechanical properties of the crosslinked rubbers improved as a function of molecular weight, the chemical structure of PM and PF (with 2 and 3 unsaturated double bonds, respectively) produced a crosslinking network with lower mechanical properties than those obtained by PB and PI (with 1 unsaturated double bond). The current work contributes to the understanding of improvements (in terms of crosslinking parameters) that are required to produce competitive rubber with good sustainability/performance balance.

Shape-Memory Composites Based on Ionic Elastomers.

Shape-memory polymers tend to present rigid behavior at ambient temperature, being unable to deform in this state. To obtain soft shape-memory elastomers, composites based on a commercial rubber crosslinked by both ionic and covalent bonds were developed, as these materials do not lose their elastomeric behavior below their transition (or activation) temperature (using ionic transition for such a purpose). The introduction of fillers, such as carbon black and multiwalled carbon nanotubes (MWCNTs), was studied and compared with the unfilled matrix. By adding contents above 10 phr of MWCNT, shape-memory properties were enhanced by 10%, achieving fixing and recovery ratios above 90% and a faster response. Moreover, by adding these fillers, the conductivity of the materials increased from ~10-11 to ~10-4 S·cm-1, allowing the possibility to activate the shape-memory effect with an electric current, based on the heating of the material by the Joule effect, achieving a fast and clean stimulus requiring only a current source of 50 V.

Controllable Design and Preparation of Hydroxyl-Terminated Solution-Polymerized Styrene Butadiene for Polyurethane Elastomers with High-Damping Properties.

Vibration and noise are ubiquitous in social life, which severely damage machinery and adversely affect human health. Thus, the development of materials with high-damping performance is of great importance. Rubbers are typically used as damping materials because of their unique viscoelasticity. However, they do not satisfy the requirements of different applications with various working conditions. In this study, the advantages of the high loss factor of styrene butadiene rubber (SBR) are combined with the strong designability of polyurethane. Hydroxyl-terminated solution-polymerized styrene butadiene rubbers (HTSSBRs) with different structures are prepared using anionic polymerization. HTSSBRs are then used as the soft segment during the synthesis of temperature-tunable high-damping performance polyurethanes (HTSSBR-polyurethanes (PUs)). The prepared HTSSBR-PUs with different structures exhibit excellent loss performance, a maximum loss factor (tan δmax ) of above 1.60, and an effective damping performance over a wide temperature range compared to traditional SBR and polyurethane. Therefore, this work offers an effective method for the design of damping materials with adjustable properties.