Cross-linked Elastomers Research Articles

Advancements in structure-property correlation studies of cross-linked citric acid-based elastomers from the perspective of medical application.

Herein, a renewed prominence towards the synthesis of poly(alkylene citrate) (PAC) biomaterials and their detailed chemical, structural and mechanical characterization has been reported. Based on the modifications to the PAC synthesis protocol introduced in this study, the fabrication process was significantly streamlined, the reaction yields were increased, and the homogeneity of the final materials was found to be substantially improved. Comprehensive nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS) studies of the fabricated prepolymers shed light on the mechanism of the PAC cross-linking process and supported the design of materials with enhanced biocompatibility. Therefore, the initial molar ratio of the reagents involved in the synthesis of PAC materials was found to be pivotal to both the biological and mechanical properties of the final products. Moreover, cell viability and proliferation assays revealed enhanced biocompatibility of the materials formulated with a molar ratio of diol over citric acid (3 : 2 mol/mol) in comparison to the most commonly described 1 : 1 analogue without affecting the possibility of further functionalization. Furthermore, this work creates a new paradigm for prospective studies on the properties of modified PAC materials and their application in medicine and tissue engineering.

Design and Characterization of Elastic Artificial Skin Containing Adenosine-Loaded Solid Lipid Nanoparticles for Treating Wrinkles.

Adenosine (AD), which is used for treating wrinkles, exhibits poor skin permeation. The aim of the present study was to develop a cross-linked silicone-based cellulose elastomer as an elastic artificial skin for the treatment of skin wrinkles, a biocompatible lipid-based nano-carrier for enhancing the skin permeation of AD, and a formulation consisting of the lipid-based carrier incorporated in the elastic artificial skin. AD-loaded solid lipid nanoparticles (SLNs) were prepared using a double-emulsion method. Particle characteristics and mechanical properties of SLNs and elastic artificial skin, respectively, were assessed. Skin permeation was evaluated using SkinEthic RHE tissue, a reconstructed human epidermis model. The mean particle size and zeta potential for SLNs ranged from 123.57 to 248.90 nm and −13.23 to −41.23 mV, respectively. The components of neither SLNs nor the elastic artificial skin were cytotoxic, according to cell- and tissue-viability assays and EU classification. SLNs and the elastic artificial skin exhibited sustained drug release for 48 h. The amount of AD released from SLNs and elastic artificial skin was approximately 10 times and 5 times higher, respectively, than that from AD solution. Therefore, elastic artificial skin incorporated with AD-loaded SLNs may serve as a promising topical delivery system for cosmeceutical treatment of skin wrinkles.

Substrate Resistance to Traction Forces Controls Fibroblast Polarization

The mechanics of fibronectin-rich extracellular matrix regulate cell physiology in a number of diseases, prompting efforts to elucidate cell mechanosensing mechanisms at the molecular and cellular scale. Here, the use of fibronectin-functionalized silicone elastomers that exhibit considerable frequency dependence in viscoelastic properties unveiled the presence of two cellular processes that respond discreetly to substrate mechanical properties. Weakly cross-linked elastomers supported efficient focal adhesion maturation and fibroblast spreading because of an apparent stiff surface layer. However, they did not enable cytoskeletal and fibroblast polarization; elastomers with high cross-linking and low deformability were required for polarization. Our results suggest as an underlying reason for this behavior the inability of soft elastomer substrates to resist traction forces rather than a lack of sufficient traction force generation. Accordingly, mild inhibition of actomyosin contractility rescued fibroblast polarization even on the softer elastomers. Our findings demonstrate differential dependence of substrate physical properties on distinct mechanosensitive processes and provide a premise to reconcile previously proposed local and global models of cell mechanosensing.

Tough Double Metal-ion Cross-linked Elastomers with Temperature-adaptable Self-healing and Luminescence Properties

Smart materials with a combination of tough solid-like properties, fast self-healing and optical responsiveness are of interests for the development of new soft machines and wearable electronics. In this work, tough physically cross-linked elastomers that show high mechanical strength, intriguing temperature-adaptable self-healing and fluorochromic response properties are designed using aluminum (Al) and fluorescent europium (Eu) ions as cross-linkers. The ionic Al-COOH binding is incorporated to construct the strong polymer network which mainly contributes to the mechanical robustness of the elastomer consisting of two interpenetrated networks. The Eu-iminodiacetate (IDA) coordination is mainly used to build the weaker but more dynamic network which dominate the elasticity, self-healing and luminescence of the elastomer. Moderate Eu3+ and Al3+ contents give these supramolecular elastomers high toughness. The temperature-sensitive Eu-IDA coordination enables tunable self-healing rate and efficiency along with fast Eu-centered “ON/OFF” switchable red emission. The mechanical, self-healing and luminescence properties of these elastomers can be adjusted by tuning the ratio of the two types of metal ions. This elastomer is potentially applicable for biosensors, wearable optoelectronics and anticounterfeiting materials.

Influence of Temperature on Mechanical Properties of P(BAMO-r-THF) Elastomer.

The relationship between temperature and the mechanical properties of an end cross-linked equal molar random copolyether elastomer of 3,3-bis(azidomethyl)oxetane and tetrahydrofuran (P(BAMO-r-THF)) was investigated. During this investigation, the performances of two P(BAMO-r-THF) elastomers with different thermal histories were compared at different temperatures. The elastomer as prepared at 20 °C (denoted as S0) exhibited semi-crystallization morphology. Wide angle X-ray diffraction analysis indicated that the crystal grains within elastomer S0 result from the crystallization of BAMO micro-blocks embedded in P(BAMO-r-THF) polymeric chains, and the crystallinity is temperature irreversible under static conditions. After undergoing a heating-cooling cycle, this elastomer became an amorphous elastomer (denoted as S1). Regarding mechanical properties, at 20 °C, break strains and stresses of 315 ± 22% and 0.46 ± 0.01 MPa were obtained for elastomer S0; corresponding values of 294 ± 6% and 0.32 ± 0.02 MPa were obtained for elastomer S1. At −40 °C, these strains and stresses simultaneously increased to 1085 ± 21% and 8.90 ± 0.72 MPa (S0) and 1181 ± 25% and 10.23 ± 0.44 MPa (S1), respectively, owing to the strain-induced crystallization of BAMO micro-blocks within the P(BAMO-r-THF) polymeric chains.

Supramolecular and Physically Double-Cross-Linked Network Strategy toward Strong and Tough Elastic Fibers.

The strength and toughness are two trade-off properties of a material, yet Nature can achieve strong and tough materials by introducing sacrificial bonds into a system. Here, we present a four-component multiblock copolymer (mBCP) approach toward strong and tough elastic fibers, by introducing terpyridine moieties into poly(ether ester) mBCP elastomers. After coordination with Fe(II), supramolecular cross-links are formed within the physically cross-linked thermoplastic elastomers. The toughening elastic fibers with a double-cross-linked network structure show high tensile strength (ca. 300 MPa) and toughness (ca. 100 MJ m-3). In addition, they display excellent resilience with enhanced self-healing properties. Our strategy provides a promising way for the development of strong and tough elastomers by introducing metal-ligand sacrificial bonds into mBCPs elastomers.

Self-Healing and Reconfigurable Actuators Based on Synergistically Cross-Linked Supramolecular Elastomer.

Stimulus-responsive soft actuators show great potential in intelligent robot systems for their various virtues, such as arbitrary shape morphing, outstanding adaptability to environment, and multidegrees of freedom. However, it is extremely challenging to achieve a combination of excellent actuating performance and robust mechanical strength as well as self-healing property. Herein we report a near-infrared light-responsive soft actuator based on the synergistic effects of a crystalline physical cross-linked network and a hydrogen bonding supramolecular network. The actuator exhibits outstanding comprehensive performance including fast and reliable light-responsive behavior (bending angle over 90° within 1.6 s), robust mechanical strength (12.52 MPa), superfast self-healing speed (2 s), and satisfactory self-healing efficiency in both mechanical (87.68%) and actuating (99.50%) performance. In addition, it is convenient to fabricate and reconfigure the actuators by a mild-temperature molding strategy to acquire various three-dimensional structures, thus achieving diverse actuating locomotion. This work provides a powerful and facile strategy to prepare soft actuators with intriguing performance, allowing significant progress in broadening their practical application.

Sol–gel-derived hierarchically wrinkled mesoporous ceramics for enhancement of cell alignment

Hierarchical ceramic structures offer great potential for multitudinous industrial applications (e.g., cell alignment) due to their intrinsic hydrophilic and environmental resistance properties. However, to date, the fabrication of hierarchical ceramic structures requires specific equipment (e.g., high vacuum systems) and time-consuming processes; such requirements limit the practical utilization of hierarchical ceramics. In this study, we combined ceramic thin films produced from interfacial sol–gel reactions and surface wrinkling from elastomer cross-linking contraction; we conducted a dynamic interfacial release process to fabricate hierarchically wrinkled mesoporous ceramic surfaces on soft elastomers. The parameters of sol–gel reactions, including precursors, acid/base-catalysts, aging time, and temperature, were systematically controlled to demonstrate the feasibility of sol–gel reactions for fabrication of hierarchically mesoporous ceramic wrinkled surfaces (i.e., mesopores on a microsized wrinkled surface). Furthermore, the wrinkled surfaces demonstrated great enhancement of growth alignment of human blood vessel cells. Thus, we developed a simple (yet cost-effective), mold-free, large-area, and hierarchical ceramic wrinkle formation process; it can be used in many practical applications, such as cell culture, microfluidic chips, antiskidding surfaces, antiglare structures, and photoelectric devices.

Physical Parameters of Polymer Composite Materials Created on the Basis of Low and High Molecular Weight Rubbers

ABSTRACT. The theoretical foundations of the structural and mechanical behavior of filled three-dimensional cross-linked elastomers are supplemented. Numerical experiments are carried out, based on the data of numerical experiments, three dimensional cross linked, filled with dispersed particles frost-resistant elastomers based on low and high molecular weight rubbers are developed. Theoretical and experimental data are compared and their good convergence is shown. Comparison of the physical parameters of the composites created by the authors on the basis of low and high molecular weight rubbers showed that the deformation characteristics of the composite based on high molecular weight rubbers are many times superior to composites based on low molecular weight, as well as their glass transition temperature is also very different. The created composites are recommended as a structural material in relation to the engineering problem of creating wear-resistant parts and components of road and air transport operated in a wide temperature range, including the Far North and the Arctic.

Preparation of Degradable Polymenthide and Its Elastomers from Biobased Menthide via Organocatalyzed Ring-opening Polymerization and UV Curing

Development of degradable polyester elastomers plays an important role in the applications of soft mateirals. Noncrystalline polymenthides (PMs) from menthol derived lactone monomers are excellent soft segments for preparing degradable polyester elastomers. By using cyclic trimeric phosphazene base (CTPB) as an organocatalyst, we successfully synthesized PMs with different molecular weights (8.2 kDa to 100.7 kDa) in high yields via ring-opening polymerization (ROP) of menthide. When a CTPB/urea binary catalytic system was adopted, the polymerizations proceeded in a more controlled manner. Using glycerol as initiator, star shaped PMs with well-defined structure were synthesized and subsequently end-capped by acrylate. UV irradiation of the terminal acrylate groups in the star-shaped PMs resulted in formation of chemically cross-linked polyester elastomers without heat or other stimuli. The obtained polyester elastomers exhibit matched modulus (3.8–5.5 MPa), tensile strength (0.56–0.68 MPa), and strain at break (280%–320%) with soft body tissues, displaying great potential in biomedical applications.

Tough Supramolecular Elastomer via Entropy-Driven Hydrogen Bonds between Vicinal Diols

Multidentate hydrogen bonds (H-bonds) play a pivotal role in determining the structure, dynamicity, and function of biological macromolecules, motivating many researchers to design artificial H-bonded functional polymers. However, it is still challenging to achieve mechanical robustness without sacrificing dynamicity because of the rigid and aggregating nature of conventional strong H-bonding motifs. Here, we show that extremely simple aliphatic vicinal diols (VDs) form an unexpectedly strong yet flexible dimer, yielding a mechanically robust and highly dynamic, recyclable, and self-healable elastomer simply by embedding VDs into polymer backbones. Density functional theory calculation revealed that the VDs could dimerize into multiple stable forms through multidentate H-bonds, and the dimerization was favored not only enthalpically but also entropically because of the wide variety of dimer modes. These entropy-driven strong H-bonds endowed the VD-functionalized polymer with mechanical robustness similar to that of covalently cross-linked elastomers while retaining functionalities based on the dynamicity of the H-bonds. The flexible nature of VD dimers also suppressed their aggregation. This study demonstrates the new concept of entropy-driven H-bonds in polymeric materials that realizes both mechanical robustness and dynamicity.

Storage Moduli and Porosity of Soft PDMS PolyMIPEs Can Be Controlled Independently Using Thiol–Ene Click Chemistry

Porous elastomeric polymers have been used in a wide range of applications because of their unique characteristics such as biocompatibility, gas permeability, thermal stability, and hydrophobic and dielectric properties. Poly(dimethyl siloxane) (PDMS), a commercially available elastomer, has also been shown to exhibit specific acoustic properties. However, the material properties were limited because of a lack of control over the chemistry used to prepare the cross-linked PDMS elastomer. Here, the synthesis of PDMS-based polymerized medium internal-phase emulsions (polyMIPEs) with tunable storage shear moduli (G′) has been performed using macromolecular thiol−ene reactions. Storage shear modulus values from ∼38 to ∼330 kPa were achieved by changing the stoichiometric ratio of the thiol-to-ene-functionalized PDMS, whereas the porosity of the polyMIPEs was controlled by the volume of the aqueous phase used in the emulsion formulation. Very low sound velocities (∼40 m/s) through the porous materials were recorded using acoustic characterization. Therefore, this work provides an example of the synthesis of soft polyMIPEs with possible applications as acoustic materials.

Stretchable and High-performance Sensor films Based on Nanocomposite of Polypyrrole/SWCNT/Silver Nanowire.

We report the fabrication of stretchable sensor films (SSF) using a composite of functionalized polypyrrole- single-walled carbon nanotube (SWCNT)-silver nanowire hybrid networks embedded into a cross-linked polydimethylsiloxane elastomer. The SSF exhibited low resistivity of 30 Ω/sq and an outstanding mechanical elasticity of up to 25% (no visible change in the sheet resistance after 100 cycle at stretching-release test of 25%). These SSFs were responsive to 1 ppm ammonia gas even at a low temperature of 40 °C with 20% relative humidity and also maintained reproducibility and reversibility when repeatedly exposed to ammonia gas more than 100 times. In addition, it was confirmed that the sensor film was hardly affected even at a relative humidity range of 20% to 80%.

Preparation, Characterization of Propargyl Terminal Polybutadiene and Its Crosslinked Elastomers Properties.

Propargyl terminal Polybutadiene (PTPB) was successfully prepared through hydroxyl terminal polybutadiene (HTPB) end-capping modification. The FTIR and 13C NMR results indicated that the HTPB terminal hydroxyl was thoroughly replaced and yielded the target product, PTPB, with a theoretical propargyl content of 0.66 mmol g−1. In comparison with HTPB, PTPB has a lower viscosity. Using 1,6-diazide hexane as a curing agent, polytriazole crosslinked polybutadiene (PTriPB) elastomers with various functional molar ratios (R) were prepared by CuAAC reaction, and the glass transition temperatures of the resultant PTriPB elastomers were approximately −75 °C, measured by differential scanning calorimetry (DSC), nearly independent of elastomer R values. Mechanical tests indicated, that with the increase in R, the mechanical properties of PTriPB elastomers exhibit a parabolic dependence on R. In addition, the thermal stability of PTriPB elastomers were also studied. The findings revealed some fundamental features of polytriazole crosslinking elastomer prepared by CuAAC reaction.

Design of next-generation cross-linking structure for elastomers toward green process and a real recycling loop

Currently adopted cross-linking methods in rubber industry are suffering from variable persistent issues, including the utilization of toxic curing packages, release of volatile organic compounds (VOCs) and difficulties in the recycling of end-of-life materials. It is of great importance to explore a green cross-linking strategy in the area. Herein, we report a new “green” strategy based on hydrolyzable ester cross-links for cross-linking diene-typed elastomers. As a proof of concept, a commercial carboxylated nitrile rubber (XNBR) is efficiently cross-linked by a bio-based agent, epoxidized soybean oil (ESO), without any toxic additives. ESO exhibits an excellent plasticization effect and excellent scorch safety for XNBR. The cross-linking density and mechanical properties of the ESO-cured XNBR can be manipulated in a wide range by changing simply varying the content of ESO. In addition, zinc oxide (ZnO) performs as a catalyst to accelerate the epoxide opening reaction and improve the cross-linking efficiency, serving as reinforcement points to enhance the overall mechanical properties of the ESO-cured XNBR. Furthermore, the end-of-life elastomer materials demonstrate a closed-loop recovery by selectively cleaving the ester bonds, resulting in very high recovery of the mechanical performance of the recycled composites. This strategy provides an unprecedented green avenue to cross-link diene elastomers and a cost-effective approach to further recycle the obtained cross-linked elastomers at high efficiency.

Solvent-Free Plasticity and Programmable Mechanical Behaviors of Engineered Proteins.

Biopolymeric networks with plasticity show great competences in diverse fields owing to the combined biocompatible and mechanical characteristics. However, to realize such plasticity external complicated treatments, e.g., UV or organic solvent have to be applied, which in turn impair the biological nature and even mechanical properties of those systems. To address this challenge, one new type of anhydrous protein liquid crystalline (LC) gels, which exhibit flexible morphological plasticity and mechanical programmability is demonstrated. Supramolecular interactions in the smectic biogels play an important role for their high plasticity. Remarkably, the samples exhibit outstanding mechanical behaviors. The tensile strength and Young's modulus at MPa levels are comparable or even higher than chemically cross-linked hydrogels and LC elastomers. More importantly, mechanical programmability of the LC gels is achieved by genetically tuning the charge density of protein backbones. Consequently, the mechanical performance is manipulated in the range of one order of magnitude. Thus, this type of anhydrous protein LC gels offers great opportunities for load-bearing high-tech applications.

Movable cross-linked elastomer with aligned carbon nanotube/nanofiber as high thermally conductive tough flexible composite

Thermally conductive flexible materials must be mechanically compatible with flexible electronic devices. They must have a low Young's modulus, high tensile strength, and toughness, in addition to a high thermal conductivity. These requirements have motivated researchers to develop composites of flexible rubber combined with a large amount of high thermally conductive carbon fibers/nanotubes. However, such composites become brittle because of the poor affinity between the rubber and carbon fibers/nanotube and local stress concentrations in the polymer matrix around the carbon fibers/nanotubes. Herein, to reduce this brittleness and achieve a high thermal conductivity, composite materials containing a slide-ring (SR) material and plasma-surface-modified carbon nanofiber (CNF)/carbon nanotube (CNT) aligned via the application of an electric field are described. The addition of a small amount of plasma-surface-modified CNTs to plasma-surface-modified CNF composites and the electric field alignment of these carbon materials produces a clear synergistic effect, facilitating an increase in the thermal conductivity. Furthermore, although this composite contains 45 wt% CNF and 5 wt% CNT, the toughness and tensile strength are no lower than in the case of raw SR. This composite material has a thermal conductivity similar to that of some metals, a low Young's modulus typical of elastomers, and a high tensile strength.

Thermal stability and flame-retardant characteristic of irradiated LDPE and composites

Magnesium hydroxide (MH) and alumina trihydrate (ATH) are extensively consumed as fillers in polyolefins to fabricate naturally responsive cables. Halogen-free wires, and especially cross-linked or thermoplastic elastomers, are further prevalent in worldwide use. The limited oxygen index (LOI), cone calorimeter, smoke density and thermogravimetry (TGA) experiments explain the thermal decomposition, flame-retardant and physical characteristics of samples. Results from studies of TGA, smoke emission and LOI testers showed that the thermal strength and flame-retardant characteristics of the samples containing MH compared with corresponding specimens containing ATH have superior thermal stability. The flame-retardant and heat resistance characteristics of samples were improved due to irradiation and the development of cross-linking bonds in the polymer framework. The results of the smoke density experiment show that adding MH to low-density polyethylene (LDPE) results in the lowest smoke density associated with LDPE and LDPE/ATH compounds. This study proved that the inclusion of MH and irradiation of specimens caused a greater thermal strength and also a superior flame-retardant polymeric product compared with the inclusion of ATH to similar irradiated specimens. These successes are useful and appropriate specifically for cable companies to deliver halogen-free flame-retardant cable materials.

Facile construction of a double network cross-linked luminescent supramolecular elastomer by hydrosilylation and pillar[5]arene host-guest recognition.

Reticulated copolymer host pillar[5]arene cross-linked with poly(dimethylsiloxane) (PDMS) was synthesized for the facile construction of a double network cross-linked elastomer upon noncovalently cross-linking with tetraphenyethylene (TPE)-based tetratopic guests through host-guest interactions. The obtained sample strips had better mechanical properties and luminescence capabilities.

Highly efficient self-healing materials with excellent shape memory and unprecedented mechanical properties

Novel dual-physical cross-linked elastomers with unprecedented mechanical, highly efficient self-healing, and excellent shape memory properties are designed, which will offer new guild for future body-sensors, stimulating reactive and wound healing.