Description of the tension kinetics of amorphous and amorphous-crystalline polymers under conditions of moderate and deep cooling by the common nonlinear rheological equation

A comprehensive experimental and theoretical study of the regularities of active deformation at a constant rate of an amorphous polymer at room temperature and the influence of moderate and deep cooling on them was performed. The samples of amorphous aromatic polyimide (an analogue of kapton H) that are randomly cut fragments of the industrially produced thermoplastic film with a thickness of 25 μm were the object of the experimental study. The σ–ε diagrams of the tensile test, where σ and ε=ε˙t are the tensile stress and the relative strain, respectively, were recorded for 32 samples at three rates ε˙ = 7⋅10–5, 7⋅10–4, 6⋅10–3 s–1 under three temperatures T = 293, 77, and 4.2 K. In the state of deep cooling at T = 4.2 K, several samples were deformed as brittle glassy bodies – rupture after short elastic deformation. But the majority of the samples at all values of the experimental parameters (T,ε˙) had the rheological properties of rubber-like highly elastic materials (elastomers) with traditional tensile test diagrams: initial stage I of linear elastic deformation σI=Meε with Young’s modulus Me=Me(T); stage III of linear highly elastic deformation σIII=σfe+Mheε with modulus Mhe=Mhe(T) and conditional limit of elasticity σfe=σfe(T,ε˙); intermediate stage II of the relaxation type σII(ε;T,ε˙) with a nonlinear stress-strain dependence. The σ–ɛ diagrams of the individual samples with sufficiently high accuracy coincide with the graph of the function σ(ε;T,ε˙) which is the solution of the previously derived nonlinear rheological equation (V. D. Natsik and H. V. Rusakova, Fiz. Nizk. Temp.48, 281 (2022) [Low Temp. Phys.48, 253 (2022)]; Fiz. Nyzk. Temp.49, 246 (2023) [Low Temp. Phys.49, 228 (2023)]). In its derivation, a molecular-kinetic model was used: an amorphous polymer is considered as a set of statistically independent kinetic units, namely, molecular segments, and the elementary act of deformation is caused by thermomechanical activation of nonlinear excitations of these segments called elastons. The elaston mechanism of transformation of the deformation diagrams of amorphous polyimide samples under their moderate and deep cooling is discussed in detail: the transition between deformation states of warm and frozen elastomer, low-temperature effects of structural-deformation glass transition and deformation melting. Comparing the results of experiments and theory made it possible to obtain the empirical estimates for the macromechanical characteristics of the studied samples and the microparameters of elaston excitations. A significant and unsystematic (random) scatter of the macro- and micromechanical characteristics of the samples was revealed, which indicates a significant and random heterogeneity of the macro- and microstructure of the polyimide film from which they were made.

The effect of temperature variations over a wide range on the rheological properties of amorphous polymers with high rubber-like elasticity (elastomers) is discussed. A theoretical study of the transition from the deformation state of a warm elastomer to the state of a frozen one, the effects of structural-strain glass transition and forced elasticity was done. Two types of mechanical testing of polymer samples are considered in detail: slow tensile deformation at a constant rate and relaxation of the deforming stress after deformation stops. The study was carried out on the basis of the previously proposed molecular-kinetic model of the processes of highly elastic deformation of amorphous polymers and the corresponding nonlinear rheological equation (V. D. Natsik and H. V. Rusakova, Fiz. Nizk. Temp. 48, 281 (2022) [Low Temp. Phys. 48, 253 (2022)]).

Polymer extension flows and instabilities

In recent years, graphene/polymer (GE/polymer) aerogels have been extensively studied and applied in flexible strain sensors. However, for most GE/polymer aerogels, it is difficult to satisfy both high elasticity and excellent conductivity, which restricts its application as wearable strain sensors. Therefore, it still remains a great challenge for fabricating highly elastic, conductive, and stable sensing aerogels. In this work, a highly elastic and conductive GE/carboxymethylcellulose (GE/CMC) aerogel is prepared by a facile solution mixing–freeze-drying process. Thanks to the strong hydrogen-bonding synergistic interactions between GE sheets and flexible CMC chains, high C/O atomic ratio, and inerratic conductive network, our GE/CMC hydrophobic aerogels exhibit stable high elasticity (4000 steady compression cycles at 50% strain), superior electrical conductivity (86.73 S m−1 under 70% compression strain), and excellent compression sensitivity (gauge factor can reach 1.58 under 45–70% strain). With above outstanding performances, our GE/CMC aerogel can be expected to be applied in wearable strain sensors.

Larger reversible deformations during the flow of mono- and poly-disperse elastomers

Cell printing is becoming a common technique to fabricate cellularized printed scaffold for biomedical application. There are still significant challenges in soft tissue bioprinting using hydrogels, which requires live cells inside the hydrogels. Moreover, the resilient mechanical properties from hydrogels are also required to mechanically mimic the native soft tissues. Herein, we developed a visible-light cross-linked, single-network, biodegradable hydrogel with high elasticity and flexibility for cell printing, which is different from previous highly elastic hydrogel with double-network and two components. The single-network hydrogel using only one stimulus (visible light) to trigger gelation can greatly simplify the cell printing process. The obtained hydrogels possessed high elasticity, and their mechanical properties can be tuned to match various native soft tissues. The hydrogels had good cell compatibility to support fibroblast growth in vitro. Various human cells were bioprinted with the hydrogels to form cell–gel constructs, in which the cells exhibited high viability after 7 days of culture. Complex patterns were printed by the hydrogels, suggesting the hydrogel feasibility for cell printing. We believe that this highly elastic, single-network hydrogel can be simply printed with different cell types, and it may provide a new material platform and a new way of thinking for hydrogel-based bioprinting research.

One of the important problems of the modern rheology of polymer materials, namely, the possibility of describing the deformation of amorphous polymers within the framework of linear rheological relationships between relative deformation and deforming stress or the need to use nonlinear rheological equations is considered. The criteria for distinguishing these approaches, namely, the determination of the corresponding critical values of the macro- and microphysical characteristics of the material and the conditions for carrying out the mechanical tests are also discussed. In particular, the difference between the influence of kinetic and thermodynamic nonlinear effects on the regularities of deformation processes of amorphous polymers in warm and frozen states was noted. The influence of nonlinear effects on the general shape and characteristics of individual stages of the “relative strain - deforming stress” diagram at deformation of polymer samples with specified values of strain rate and temperature is analyzed in detail. The results of the theoretical analysis were used for the physical interpretation of the general form and features of individual stages of the tensile test diagrams of amorphous polyimide films (V. D. Natsik, H. V. Rusakova, S. V. Lubenets, V. A. Lototskaya, and L. F. Yakovenko, Fiz. Nyzk. Temp. 49, 569 (2023) [Low Temp. Phys. 49, 521 (2023)]), empirical estimates for the rheological characteristics of this polymer were obtained.

Using the experimental method of highly sensitive thermoactivation spectroscopy, we studied the spectra of thermally stimulated luminescence (TSL) of purified crystals of NaCl and NaCl-Li under low-temperature (95 K) elastic deformation ( ) in a wide range of the spectrum (200¸850 nm). In the TSL of a NaCl crystal, the dominant peak is an -center, whose intensity doubles during low-temperature deformation and has a maximum of thermal damage at 165–170 K, scanning of which corresponds to the maximum of the TSL spectrum at 3.5 eV. The spectrum of the TSL -peak and the X-ray luminescence coincide and have the same nature – the radiative relaxation of self-trapped excitons upon recombination of mobile holes with electrons. In NaCl-Li TSL, the dominant peaks are F/ and HA (Li) centers, the intensity of which increases 10-fold during low-temperature deformation and having maximum thermal destruction at 110 K and 125 K, respectively. When scanning the TSL spectra at peaks 110K and 125K, we observed radiation with maxima at 2.72 eV and 2.69 eV. An analysis shows that a light lithium cation in the NaCl-Li lattice creates a local deformation as a result of which HA (Li) centers appear. Elastic deformation further stimulates the formation of HA (Li) -centers, evidenced by a 13-fold increase in the intensity of the TSL peak at 125K.

Formation mechanism of high-strain bands in commercially pure titanium

Sir: As mentioned by the author in his publication,1 to resolve the nasolabial fold (NLF), filler injection is most commonly used. In the study, fillers were injected in multiple layers of the NLF. However, we would like to discuss some complications associated with this technique. First, the facial artery is one of the largest arteries in the face and it runs near the NLF. Lee et al2 described variations in the course and depth of the facial artery and reported that more than 70% of the facial arteries were found at the NLF. The author of this article described injecting the filler at the supraperiosteal, deep and superficial fatty layers, and subdermal layer,1 but as seen in another study,2 filler injection in the deep and superficial fatty layers can be dangerous. Ultrasound shows that the facial artery can be detected at the NLF.3 In addition, it was found that 29% of the facial arteries run in the subcutaneous layer.3 Ultrasonographic image of the facial artery at the NLF area is shown in Figure 1. This means that filler injection in multiple layers is a very dangerous procedure as vascular injuries can occur, resulting in tragic complications such as skin necrosis.Fig. 1.: Doppler ultrasound image of facial artery located in the subcutaneous layer at the NLF.Second, the author did not mention the diameter of the cannula and compared to the puncturing needle, the cannula seemed to be smaller than 25 G. The outer diameter of a 25 G cannula is 0.51 mm, and the facial artery diameter is described to be 1.7–3.6 mm.4 This shows that a cannula can perforate the facial artery anytime, resulting in vascular complications. Clinicians should not be overly confident when using a cannula for injecting the filler. A recent article shows that a 27-G cannula can be as dangerous as a 27-G needle.5 Third, to describe specific fillers as being highly elastic or cohesive, rheological data should be provided. The author described Yvoire Y solution 720° as having high particle elasticity and stated that this is characteristic of a biphasic filler. However, he described Yvoire Y solution 540° as having low particle elasticity. As far as we know, Yvoire Y solution 540° also has characteristics of a biphasic filler. Usually, hyaluronic acid filler has rheological properties like high elastic modulus (characteristic of a biphasic filler) or high cohesiveness (characteristic of a monophasic filler). “High particle elasticity” was unclear when the nature of the filler was described. Furthermore, the particle size and rheological data of the filler used in the research were not provided. We would prefer it if hyaluronic acid was described as being volumizing at deep to mid-levels rather than having a high or low particle elasticity. Filler injection is a relatively easy esthetic procedure; nevertheless, surgeons must be aware of the possible vascular complications associated with this procedure. DISCLOSURE The authors have no financial interest to declare in relation to the content of this article.

Liquid-free ionic conductive elastomers (ICEs) are ideal materials for wearable strain sensors in increasingly flexible electronic devices. However, developing recyclable ICEs with high elasticity, self-healability, and recyclability is still a great challenge. In this study, we fabricated a series of novel ICEs by in situ polymerization of lipoic acid (LA) in poly(acrylic acid) (PAA) solution and cross-linking by coordination bonding and hydrogen bonding. One of the obtained dynamically cross-linked interlocking double-network ICEs, PLA-PAA4-1% ICE, showed excellent mechanical properties, with high elasticity (90%) and stretchability (610%), as well as rapid self-healability (mechanical self-healing within 2 h and electrical recovery within 0.3 s). The PLA-PAA4-1% ICE was used as a strain sensor and possessed excellent linear sensitivity and highly cyclic stability, effectively monitoring diverse human motions with both stretched and compressed deformations. Notably, the PLA-PAA4-1% ICE can be fully recycled and reused as a new strain sensor without any structure change or degradation in performance. This work provided a viable path to fabricate conductive materials by solving the two contradictions of high mechanical property and self-healability, and structure stability and recyclability. We believe that the superior overall performance and feasible fabrication make the developed PLA-PAA4-1% ICE hold great promise as a multifunctional strain sensor for practical applications in flexible wearable electronic devices and humanoid robotics.

Smart hydrogels, or stimuli-responsive hydrogels, are three-dimensional networks composed of crosslinked hydrophilic polymer chains that are able to dramatically change their volume and other properties in response to environmental stimuli such as temperature, pH and certain chemicals. Rapid and significant response to environmental stimuli and high elasticity are critical for the versatility of such smart hydrogels. Here we report the synthesis of smart hydrogels which are rapidly responsive, highly swellable and stretchable, by constructing a nano-structured architecture with activated nanogels as nano-crosslinkers. The nano-structured smart hydrogels show very significant and rapid stimuli-responsive characteristics, as well as highly elastic properties to sustain high compressions, resist slicing and withstand high level of deformation, such as bending, twisting and extensive stretching. Because of the concurrent rapid and significant stimuli-response and high elasticity, these nano-structured smart hydrogels may expand the scope of hydrogel applications, and provide enhanced performance in their applications.

For a fiber‐based strain sensor to be used as a wearable device, its conductivity and sensing characteristics should be stably maintained even during repeated mechanical movements. Additionally, the sensing characteristics should remain unaffected by external contaminants, such as water or sweat, as the sensor is expected to be in contact with the human body. In this study, a superhydrophobic and highly elastic strain‐sensing fiber with durability against continuous tension and contraction while maintaining a stable sensing performance even when in contact with water and sweat is developed. A carbon nanotube is embedded, which is a highly conductive material, inside a spandex fiber with high elasticity and shape recovery rate, enabling the stable measurement of repetitive joint movements under various strain conditions. Furthermore, a superhydrophobic silica aerogel is embedded inside the spandex fiber to facilitate stable sensing without malfunction even when exposed to external contaminants. The proposed strain‐sensing fiber can monitor joints of the human body during various movements, such as dumbbell pressing, squatting, walking, and running. Therefore, the study findings can contribute to the development of wearable healthcare devices that warrant reliable sensing.

Pressure sensors with high elasticity are in great demand for the realization of intelligent sensing, but there is a need to develope a simple, inexpensive, and scalable method for the manufacture of the sensors. Here, we reported an efficient, simple, facile, and repeatable "dipping and coating" process to manufacture a piezoresistive sensor with high elasticity, based on homogeneous 3D hybrid network of carbon nanotubes@silver nanoparticles (CNTs@Ag NPs) anchored on a skeleton sponge. Highly elastic, sensitive, and wearable sensors are obtained using the porous structure of sponge and the synergy effect of CNTs/Ag NPs. Our sensor was also tested for over 2000 compression-release cycles, exhibiting excellent elasticity and cycling stability. Sensors with high performance and a simple fabrication process are promising devices for commercial production in various electronic devices, for example, sport performance monitoring and man-machine interfaces.

Scaffold-based tissue engineering is a promising strategy to address the rapidly growing demand for bone implants, but developing scaffolds with bone extracellular matrix-like structures, suitable mechanical properties, and multiple biological activities remains a huge challenge. Here, it is aimed to develop a wood-derived composite scaffold with an anisotropic porous structure, high elasticity, and good antibacterial, osteogenic, and angiogenic activities. First, natural wood is treated with an alkaline solution to obtain a wood-derived scaffold with an oriented cellulose skeleton and high elasticity, which can not only simulate collagen fiber skeleton in bone tissue but also greatly improve the convenience of clinical implantation. Subsequently, chitosan quaternary ammonium salt (CQS) and dimethyloxalylglycine (DMOG) are further modified on the wood-derived elastic scaffold through a polydopamine layer. Among them, CQS endows the scaffold with good antibacterial activity, while DMOG significantly improves the scaffold's osteogenic and angiogenic activities. Interestingly, the mechanical characteristics of the scaffolds and the modified DMOG can synergistically enhance the expression of yes-associated protein/transcriptional co-activator with PDZ binding motif signaling pathway, thereby effectively promoting osteogenic differentiation. Therefore, this wood-derived composite scaffold is expected to have potential application in the treatment of bone defects.