Physical Cross-linking Points Research Articles

High temperature shape memory aliphatic polybenzimidazole

High-temperature shape memory polymers have attracted significant attention in the fields of aerospace and structural components industries. Here, we present a new type of high-performance shape memory polybenzimidazole based on 3,3′-diaminobenzidine and sebacic acid (C8-PBI). Polycondensation in Eaton's reagent allowed obtaining high-strength (97.7 ± 5.1 MPa) C8-PBI-based films with high glass-transition temperature (243 °C) and thermal stability up to 466 °C. The combination of soft aliphatic and hard benzimidazole segments in the macromolecular structure and strong intermolecular hydrogen bonds that serve as physical crosslinking points in the polymer network, thereby imparting shape memory properties to C8-PBI. Shape fixity ratio and shape recovery ratio of C8-PBI reach 99.8% and 97.0%, respectively.

Rheology, crystallization, and enhanced mechanical properties of uniaxially oriented ethylene–octene copolymer/polyolefin elastomer blends

The application of polyethylene in artificial turf promotes research on the processing and properties of uniaxial extension modified polyethylene materials. The effects of comonomer (octene/butene) structure on the properties of pre-stretched linear low density polyethylene (LLDPE)/polyolefin elastomer (POE) blends are investigated. The melt tensile stress and melt stretch ratio of LLDPE ethylene‒octene copolymer (EOC)/POE reach 4.0 × 105 Pa and 12.8, respectively. At the pre-stretching ratio λ = 5, the tensile strength of EOC/POEλ5 reaches 62.5 MPa, the critical length is 5.7 mm, and the strain hardening degree increases. Differential scanning calorimetry, Raman spectroscopy and X-ray diffraction show the crystallinity of EOC/POEλ5 is higher than that of LLDPE ethylene‒butene copolymer (EBC)/POEλ5. Large short-chain branch length EOC is demonstrated to rebuild the “physical crosslinking point” after pre-stretched, thereby enhancing the interaction between molecular chains in EOC/POEλ5. This research provides a new clue for the development of artificial turf materials.

Improving aging resistance of PIM‐1 thin films by nano‐TiO2 filler used for robust solvent permeation

AbstractPolymers of intrinsic microporosity (PIMs) are promising materials for membrane separation because their special rigid and contorted structures contribute to high permeability. However, their chain rearrangement to fill excessive free volume makes the permeability stability a tough challenge. In this work, we report on a new use of rutile nano‐TiO2 to mitigate the physical aging of PIM‐1 (a typical PIM) nanofilms for stable permeability by mixing matrix. It was shown that the PIM‐1 membrane incorporated with nano‐TiO2 displayed remarkably higher aging resistance with a lower swelling degree in long‐term ethanol soaking, having more stable ethanol permeance with only a 5% decrease after 35 days, lower than 25% of the pure one. The mechanism of anti‐aging was revealed by molecular simulation, thermal, tensile mechanical, and dynamic mechanical analysis. It was found that nano‐TiO2 had good compatibility with PIM‐1 due to strong coordination interaction, making its uniform dispersion in polymer. Additional solvent permeation channels were also created to increase solvent permeance without compromising solute rejection. Due to the reliable interaction of nano‐TiO2, which makes particles serve as physical crosslinking points, the movement of PIM‐1 chains was limited partially to mitigate aging, enabling PIM‐1‐based membranes to have robust solvent permeation.

Angiogenesis Promotion by Combined Administration of DFO and Vein Endothelial Cells Using Injectable, Biodegradable, Nanocomposite Hydrogel Scaffolds.

Desferrioxamine (DFO) upregulates HIF-1α and stimulates expression of vascular endothelial growth factor (VEGF), thereby accelerating neovascularization. As DFO acts primarily upon surrounding vein endothelial cells to stimulate angiogenesis, the angiogenic efficacy of DFO could be reduced in severely injured tissues lacking a sufficient number of vein endothelial cells. We hypothesized that combined administration of DFO and vein endothelial cells is a promising tissue engineering approach for promoting neovascularization. In this study, we evaluated the applicability of this approach using injectable, biocompatible, biodegradable nanocomposite gels consisting of poly(dl-lactide-co-glycolide)-b-polyethylene glycol-b-poly(dl-lactide-co-glycolide) (PLGA-PEG-PLGA) copolymers and clay nanoparticle LAPONITE. The nanocomposites exhibited irreversible thermo-gelation in the presence of DFO, and the mechanical strength was strongly affected by the amount of DFO. The storage moduli of the gels increased with increasing amount of DFO. These results indicate that the interaction between DFO and LAPONITE works as physical cross-linking points and facilitates the formation of the gel network. The nanocomposite gels achieved sustained slow release of DFO due to interactions between DFO and LAPONITE. Human umbilical vein endothelial cells (HUVECs) cultured on DFO-loaded nanocomposite gels exhibited a higher degree of vascular tube formation than cells cultured on nanocomposite gels without DFO. Moreover, the number of branching points and the diameter of the blood vessels regenerated in the gels significantly increased with increasing DFO amount, indicating that DFO released from the gels facilitates vascular tube-forming capacity. As a proof of concept, we demonstrate that the combined administration of DFO and vein endothelial cells using nanocomposite gels promotes greater angiogenesis than DFO administration alone using the same gels by in vivo experiments, confirming the validity of our hypothesis. Considering the multiple advantages of nanocomposite gels with regard to potential vascularization capacity, certain biocompatibility, biodegradability, and injectable cell- and drug-delivery capacity, we concluded that the nanocomposite gels have potential utility as scaffolding biomaterials for vascularization in tissue engineering applications.

Engineering the shape memory parameters of graphene/polymer nanocomposites through atomistic simulations: On the effect of nanofiller surface treatment

This paper aims to comprehend the mechanisms underlying the shape memory behavior of polylactic acid infused with graphene functionalized by four groups of –OH, –CH3, –NH2, and tethered polymer layer. Applying molecular dynamics simulation, it is revealed that the graphene surface treatment enhances the shape fixity ratio of nanocomposites monotonically by increasing the physical cross-linking points within the polymer matrix. The improvement would be even more pronounced by increasing the coverage degree of small functional groups and grafting density of the covalently bonded polymer chains. Monitoring the key parameters illustrates that contrary to the OH groups, which improve the shape recovery value, the other functional groups degrade it by prohibiting the polymer chains mobility. Attempts to explore the governing mechanism demonstrate that shape fixity is improved when the difference between the potential energy variations in the loading and unloading stages increases. Interestingly, shape recovery is only under the influence of conformational entropy, and it is not affected by the potential energy. As such, we also probe variations of the radius of gyration during the recovery stage to address the role of different functionalization procedures on the reported shape recovery parameter.

3D Printable, ultra-stretchable, Self-healable, and self-adhesive dual cross-linked nanocomposite ionogels as ultra-durable strain sensors for motion detection and wearable human-machine interface

Stretchable and transparent ionic conductive gels are considered as promising wearable strain sensors for motion detection and human–machine interface. However, it is a challenge to develop efficient and convenient method to fabricate high-precision, self-healable, highly stretchable and temperature tolerant ionogels, particularly can work in extreme environments. Herein, a novel 3D printable nanocomposite chemical ionogel has been fabricated by one-step photopolymerization, which POSS nanodots as multifunctional chemical crosslinking points and PAA hydrogen bonds as reversible physical crosslinking points. The special dual cross-linking structures endow it record-ultrahigh stretchability (≈7000%), high conductivity, self-healability, self-adhesiveness, and superior wide tolerant temperatures (-78–235 °C). The nanocomposite ionogels based strain sensors show widely sensing range (1%-1200%), excellent durability (10000 cycles), extreme low (-60 °C) and high (150 °C) working temperatures, and can be assembled as wearable sensors for detecting various human motions. More importantly, these ionogels can also be applied for the wearable human–machine interface which can recognize and distinguish the human gestures and can also control the mechanical hand, particularly can work normally from −60 to 150 °C. Therefore, our superior multifunctional ultra-stretchable nanocomposite ionogel design creates a new concept for development of 3D printing intelligent electronics and artificial intelligence, especially for applications in extreme environments.

Tough and Resilient Hydrogels Enabled by a Multifunctional Initiating and Cross-Linking Agent.

Many high-strength hydrogels have been developed in recent years; however, few of them are both tough and resilient, and their intrinsic paradoxical nature makes designing a gel with both high toughness and high resilience a great challenge. To address this problem, we introduced both N,N,N,N-pentamethyldiethylenetriamine (PA) and N,N-methylenebisacrylamide (MBA) into polyacrylamide hydrogel networks to construct an entangled network that contains chemically cross-linked chains and branched chains simultaneously. The entanglements of branched chains can act as a physical cross-linking point to uniformly disperse stress on molecular chains, and chemical cross-linking ensures the stability of the hydrogel network. The increase in the number and length of branched chains is able to achieve an enhancement in strength while the slip of the entangled polymer chains can effectively achieve energy dissipation and can improve the toughness of the gel. Moreover, the resultant hydrogels exhibit an excellent resilience (>98%). Therefore, high toughness and resilience are achieved simultaneously. In addition, we also investigated the initiation mechanism of PA. This strategy creates a new way for the preparation of next-generation high toughness and high resilience hydrogel-based materials, which have promising applications in wearable, flexible strain/pressure sensors.

MoS2-Embedded, Interpenetrating Network Composite Hydrogels that Show Controlled Release of Dyes and Tunable Strength.

This paper describes a conceptual design of hierarchical composite hydrogels. The hydrogel materials comprise MoS2 flakes and interpenetrating polymer networks, and further exhibit controlled release and tunable strength that are caused by the synergistic combination of select components. In terms of design, MoS2 flakes initiate radical polymerization of chosen monomers and simultaneously provide physical cross-linking points, both of which afford a primary composite network. Then, the sequential formation of additional networks results in functional, hierarchical, composite hydrogels. Therefore, we were able to demonstrate double-network hydrogels as a stimuli-responsive vector for programmed release of cargo molecules in response to heat or light or to form triple-network hydrogels showing tunable mechanical strength owing to intermolecular interaction between charged monomers and MoS2 flakes. The design concept would be expanded by incorporating other chalcogenides or functional monomers, which advance the properties and functionalities of materials and broadens the versatility of nanocomposite hydrogels.

Super tough and stable solid–solid phase change material based on π-π stacking

The solid–solid phase change materials (SSPCMs) have become the preferred materials in thermal energy storage via absorbing latent heat from ambient environment. However, the trade-off between the mechanical properties, stability and recyclability is still the obstacle and barrier for development of SSPCMs. Herein, we proposed a facile and novel strategy to prepare SSPCMs for address above issues by introducing π-π stacking to form physical crosslinking points in linear polyethylene glycol (PEG). The strong intermolecular forces formed by π-π stacking not only prevented leakage of the PEG even at 130 °C but imparted high temperature stability and excellent toughness (172.44 MJ/m3) to fabricated SSPCMs. It is worth mentioning that the prepared PCMs can be added with CNT in a simple process way to improve the photo-thermal conversion ability and thermal conductivity of SSPCMs. Besides, the as-prepared SSPCMs exhibited excellent flexibility, and were expected to be excellent thermal/photo energy storage materials for human thermal management and wearable devices.

Study on UV aging resistance of nano-TiO2/montmorillonite/styrene-butadiene rubber composite modified asphalt based on rheological and microscopic properties

In order to improve the high-temperature performance and anti-ultraviolet aging performance of butadiene styrene rubber (SBR) modified asphalt, nano-TiO2 and Montmorillonite (MMT) were used for composite modification, and the test scheme was determined by Box Behnken Design (BBD). The high-speed shearing method is used to prepare nano-TiO2/MMT/SBR composite modified asphalt. Meanwhile, the dynamic shear rheometer (DSR) test and the bending beam rheometer (BBR) test were used to evaluate the rheological properties of the composite modified asphalt before and after aging to evaluate its anti-ultraviolet aging performance. Finally, the interaction and modification mechanism of the modifiers were revealed by scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR). The results show that the optimal dosage of nano-TiO2, nano-MMT and SBR latex (solid content 60%) are 1.5%, 4%, and 6%, respectively. The synergistic effect of nano-TiO2 and nano-MMT can significantly delay the conversion process of light components to asphaltenes during asphalt aging, and makes CMI value of SBR modified asphalt significantly reduced and almost stabilized at about 2, and the average stiffness modulus change rate decreases from 0.27 to 0.13, and the carbonyl index dropped from 5.16 to 0.28. SEM test results show that nano-TiO2 particles can be used as physical cross-linking points to promote the combination of nano-MMT/SBR modifiers and asphalt, making the microstructure of nano-TiO2/MMT/SBR modified asphalt more stable and uniform. The modification of Nano-TiO2/MMT/SBR to asphalt is a physical modification process, more Si-O-Si functional groups were generated during the aging process of MMT, which further promote the formation of MMT and asphalt network structure. In conclusion, the study of nano-TiO2/MMT/SBR composite modified asphalt in this paper provided a reference for the selection of modified asphalt in cold and ultraviolet regions.

Dielectric elastomer sensor with high dielectric constant and capacitive strain sensing properties by designing polar-nonpolar fluorosilicone multiblock copolymers and introducing poly(dopamine) modified CNTs

The application of silicone rubber as dielectric elastomer sensor is limited by its poor strain sensing performance because of low dielectric constant (ε′). In this study, we designed and fabricated a fluorosilicone dielectric composite with high ε′ and capacitive strain sensing performance by designing polar-nonpolar multiblock fluorosilicone copolymer and introducing poly(dopamine)-modified nanotubes (CNT-PDA). Firstly, a polar (bis(3-aminopropyl)-terminated poly(3,3,3-trifluoropropyl) methylsiloxane (H2N-PFMS-NH2)) and a nonpolar (bis(3-aminopropyl)-terminated poly(dimethylsiloxane) (H2N-PDMS-NH2)) silicone oligomers were synthesized by anionic ring opening polymerization. Then, a series of PFMS/PDMS multiblock copolymer were synthesized through a one-pot polycondensation reaction between isocyanate groups from methylene-bis(4-cyclohexylisocyanate) and amino groups from the as prepared H2N-PFMS-NH2 and H2N-PDMS-NH2. The hydrogen bonds between urea groups act as physical crosslinking points on the multiblock copolymer. The H2N-PFMS-NH2 chains with polar CF3 groups contribute to high dipole polarizability and thus a high ε’. A strong interfacial adhesion between CNT-PDA and the multiblock copolymer was obtained by hydrogen bonding between the urea groups of the multiblock copolymer and CNT-PDA, resulting in strong interfacial polarizability. The as-prepared composite exhibits a ε′ of about 86.7 at 103 Hz and a capacitance change (ΔC) of about 0.91 nF at 200% strain at only 0.3 vol% of CNT-PDA, which was increased by 23.8 and 8.1 times, respectively, compared with the CNT-PDA/commercial nonpolar reference SiR composites. The largely improved ε′ and capacitive strain sensing properties are mainly attributed to the synergistic polarization effect, which was discussed based on impedance analysis and calculating the contribution of each composition on ε’ and ΔC.

Templating assembly of NIR light-actuated TPU/SCNT-C60 flexible structures with high conductivity and controllable recovery behavior

Advanced flexible thermoplastic polyurethane-based films via different response have attracted great attentions for shape memory and other potential applications. However, the accurate and repeatable response control with high mechanical and thermal properties still remains a great challenge. Here, we proposed the flexible photo-actuated thermoplastic polyurethane/carbon nanotube/fullerene composite film (TPU/SCNT-C60) via nano-templated assembly strategy, in which a petal-like SCNT-C60 nanostructure can be obtained by tuning the constituent of carbon nano-templates. This special carbon framework can increase physical cross-linking points and served as photo-thermal conversion reagent due to the enhanced intermolecular interactions and synergistic effect, resulting in an obvious increase of the thermal conductivity and mechanical properties. Especially, the flexible TPU/SCNT-C60 1:3 film exhibited a high thermal conductivity of 1.21 W m − 1K−1 and a high tensile strength over 23 MPa with a large workable strain range about 500%. Moreover, it presented controllable NIR photo-actuated shape change and recovery behavior with excellent repeatability during complete 15 cycles. Our strategy opens a new way for photo-triggered device and show tremendous potential for various applications.

One-Pot Synthesis of Supertough, Sustainable Polyester Thermoplastic Elastomers Using Block-Like, Gradient Copolymer as Soft Midblock

It remains challenging to synthesize supertough thermoplastic elastomers (TPEs) since the stretchability and tensile strength are mutually exclusive. Here, we report a one-pot strategy for the prep...

Strain induced strengthening of soft thermoplastic polyurethanes under cyclic deformation

AbstractWe investigate the cyclic mechanical behavior in uniaxial tension of three different commercial thermoplastic polyurethane elastomers (TPU) often considered as a sustainable replacement for common filled elastomers. All TPU have similar hard segment contents and linear moduli but sensibly different large strain properties as shown by X‐ray analysis. Despite these differences, we found a stiffening effect after conditioning in step cyclic loading which greatly differs from the common softening (also referred as Mullins effect) observed in chemically crosslinked filled rubbers. We propose that this self‐reinforcement is related to the fragmentation of hard domains, naturally present in TPU, in smaller but more numerous sub‐units that may act as new physical crosslinking points. The proposed stiffening mechanism is not dissimilar to the strain‐induced crystallization observed in stretched natural rubber, but it presents a persistent nature. In particular, it may cause a local reinforcement where an inhomogeneous strain field is present, as is the case of a crack propagating in cyclic fatigue, providing a potential explanation for the well‐known toughness and wear resistance of TPU.

Effect of AC stressed aging on partial discharge, thermal and tensile performance of silicone rubber-based composites

In this paper, partial discharge characteristics of the multi-stressed aged and un-aged high temperature vulcanized silicone rubber (HTV-SR) samples with several concentrations of alumina trihydrate (ATH) and silica are studied. This is to elucidate whether the addition of nano/micro fillers of silica and ATH in silicone rubber enhances the resistance against partial discharges and aging behavior under different weathering conditions. Further, performance assessment is made by thermogravimetric analysis, differential scanning calorimetry and through measurement of tensile strength. Using IEC (b) electrode system, partial discharge measurements are performed according to IEC 60270. From these experimental measurements, it has been observed that by adding filler contents, the multi-stress aging performance improved and partial discharge resistance is enhanced up to 40% as compared to the HTV-SR without any filler. Moreover, through analysis of data obtained by thermogravimetric, indicate that nano doping increases the physical and chemical cross-linking points which confine the motion at the particle-matrix interface. As a result, the enhanced thermal stability of the composites. Considerable improvement in the stress-strain behavior of test samples is also observed with the addition of silica and ATH.

Ultra-stretchable and healable hydrogel-based triboelectric nanogenerators for energy harvesting and self-powered sensing.

The next-generation multifunctional soft electronic devices require the development of energy devices possessing comparable functions. In this work, an ultra-stretchable and healable hydrogel-based triboelectric nanogenerator (TENG) is prepared for mechanical energy harvesting and self-powered sensing. An ionic conductive hydrogel was developed with graphene oxide and Laponite. as the physical cross-linking points, exhibiting high stretchability (∼1356%) and healable capability. When using the hydrogel as the electrode, the TENG can operate normally at 900% tensile strain, while the electrical output of the TENG can fully recover to the initial value after healing the damage. This hydrogel-based TENG is demonstrated to power wearable electronics, and is used as a self-powered sensor for human motion monitoring and pressure sensing. Our work shows opportunities for multifunctional power sources and potential applications in wearable electronics.

The preparation and characterization of a heterocyclic meta-aramid fiber with outstanding thermal stability

A heterocyclic meta-aramid fiber containing benzimidazole structure was prepared by the low-temperature polycondensation of m-phenylenediamine, 2-(4-aminophenyl)-5-benzimidazole, and isophthaloyl dichloride, followed by dry-jet wet spinning. The structure and properties of the heterocyclic modified meta-aramid (h-aramid for short) were explored and compared with poly(isophthaloylmetaphenylene diamine) (m-aramid for short). H-aramid exhibits excellent spinnability. The surface of the fiber is smooth and even, with a partially ordered structure. Mechanical properties, thermal resistance, and flame retardant of h-aramid are significantly enhanced as the introduction of benzimidazole can not only increase the rigidity of molecular chains but also strengthen hydrogen bonds and act as physical crosslinking points. The strength and modulus of h-aramid are up to 0.84 GPa and 16.66 GPa, respectively, with an elongation at break of 20%. The glass transition temperature and melting temperature of h-aramid reach 302°C and 356°C, respectively, about 30°C and 40°C higher than that of m-aramid. Temperature of 10 wt% weight loss under nitrogen and air atmosphere is 17°C and 39°C higher than those of m-aramid. Besides, the thermal degradation behavior of h-aramid was studied, and its degradation process and mechanism was proposed.

Oligoether grafting on cellulose microfibers for dispersion in poly(propylene glycol) and fabrication of reinforced polyurethane composite

Application of cellulose microfibers as a reinforcing agent for high-performance polyurethane composites is rarely mentioned because the inherent hydrophilicity combined with the adverse aspect ratio of the fibers cause difficulties in dispersion within and compatibility with the polymer matrix. Here, we propose a feasible approach to graft oligoethers synthesized by ring-opening polymerization in a heterogeneous reaction to the cellulose microfiber surface. The oligomers composed of ether linkages and flexible hydroxyl groups provide cellulose fibers with good dispersity in poly(propylene glycol) and potential physical and chemical cross-linking points in reaction with isocyanates to form good interfacial adhesion between the fibers and the polyurethane matrix. Consequently, the tensile properties and viscoelasticity of the oligomer-grafted cellulose-filled polyurethane composite were enhanced compared with those of the neat elastomer and composite containing pristine cellulose microfibers. This reinforcement effect is attributed to the natural stiffness of the fiber bundles and the cross-linking network generated by the oligomers grafted on the fiber surface.

Hydrogen Bonding-Derived Healable Polyacrylate Elastomers via On-demand Copolymerization of n-Butyl Acrylate and tert-Butyl Acrylate.

Achieving a desirable combination of good mechanical properties and healing efficiency is a great challenge in the development of self-healing elastomers. Herein, a class of tough and strong self-healing polyacrylate elastomers (denoted as HPs) was developed simply by free-radical copolymerization of n-butyl acrylate (nBA) and tert-butyl acrylate (tBA) and a subsequent hydrolysis reaction rather than direct copolymerization of nBA and acrylic acid (AA). The tiny difference in reactivity between nBA and tBA makes the structural units of the copolymer easy to control. Precise regulation of molecular composition can be realized just by varying the relative monomer content, making its mechanical properties to vary from ductile to robust. Strikingly, when HP samples are cut off within the gauge length, they can heal into coherent and smooth samples and recover at least 79% of the original strength. Hydrogen bond interactions serve as physical cross-linking points, contributing to the high mechanical performance (fracture energy of up to 73.78 MJ·m-3 and tensile strength of up to 17.80 MPa) as well as shape memory function. Moreover, the HP samples emit strong fluorescence when exposed to a 365 nm UV lamp and exhibit an aggregation-enhanced emission effect in the state of dissolution.

Effects of Polyhedral Oligomeric Silsesquioxane (POSS) on Thermal and Mechanical Properties of Polysiloxane Foam.

The thermal and mechanical properties of polysiloxane foam are greatly improved by the addition of acrylolsobutyl polyhedral oligomeric silsesquioxane (MA0701, hereinafter referred to as MAPOSS), which has double bonds. The morphologies and properties of the polysiloxane composite foam were characterized. The average cell diameter of the composite foams decreased, while the cell density increased with increasing MAPOSS. Meanwhile, MAPOSS can enhance thermal conductivity and thermal stability. Thermal conductivity increased by 25%, and the temperature at the maximum weight loss rate increased from 556 °C to 599 °C. In addition, MAPOSS also promoted heterogeneous nucleation by functioning as a nucleating agent, which can increase cell density to improve the mechanical properties. The compressive strength of the composite foam increased by 170% compared with that of pure foam. In the composite, MAPOSS increased the cross-linking density by acting as a physical cross-linking point and limited the movement of the segments.