Synthesis and properties of room-temperature self-healing polyurethane elastomers

Two kinds of polyurethane elastomers were synthesized. One containing acylhydrazone bonds was named TPIA. The other containing both acylhydrazone and disulfide bonds was named TPID. Self-repairable ability and reprocessability of these two elastomers were studied. The results show that: The polyurethane elastomer TPIA can automatically repair damage to it under acidic conditions. After self-healing for 24 h, the strength and the elongation value at break recovered to 32% and 55% of the originality, respectively. The polyurethane elastomer TPID can automatically repair damage to it under visible light at room temperature. After 24 h of self-healing time, 75% of the original strength and 100% of the original elongation values at break were obtained. These two polyurethane elastomers can be reprocessed in their cured state by just applying temperature and pressure.     

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 Eu~(3+) and Al~(3+) 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.     

基于多重可逆作用的自修复聚氨酯弹性体的制备及性能

在聚氨酯主链上引入可逆二硫键, 同时使用硼酸构建的硼酸酯键作为可逆交联点, 使聚氨酯内部形成交联网络结构, 制备了一种兼具高强度、 高韧性及高修复效率的自修复聚氨酯弹性体. 红外光谱、 动态力学分析、 力学测试、 电子显微镜及修复测试结果表明, 制备的自修复聚氨酯具有硬而韧的特性, 原样强度高达23.3 MPa, 断裂伸长率可达1177%, 并且修复条件温和, 剪断拼接的试样经60 ℃, 24 h修复后可恢复99%的原样强度, 且该修复过程可重复多次进行. 此外, 该材料还具有多通道修复特性, 通过热修复或水辅助热修复的方式均可实现材料的修复, 并且水辅助热修复速率更快.     

Ultra-robust,self-healable and recyclable polyurethane elastomer via a combination of hydrogen bonds,dynamic chemistry,and microphase separation

Flexibility, robustness, transparency, and recyclability are critical to the application of self-healing polymer materials in the field of flexible electronics. However, integrating all the above properties remains a huge challenge to date. In this work, we put forward a facile strategy to prepare polyurethane (PU) elastomer with ultra-high strength and self-healing performance based on hydrogen bonds, disulfide dynamic chemistry, and microphase separation at the same time. Three different self-healing PUs were obtained by introducing disulfide bonds and different types of hydrogen bonds. A robust, transparent, and recyclable PU with amino-terminated chain extender (PUA) with fast and efficient self-healing performance was prepared. The mechanical and self-healing properties of the PUA were effectively balanced by the synergistic effect of reversible interaction of disulfide bonds and the formation of microphase separated structure. The results indicated that the PUA exhibited high transparency up to 90% and excellent mechanical property, e.g. the tensile strength and elongation at break can reach 37.10 MPa and 1080%, respectively. Meanwhile, it can achieve a high self-healing efficiency of 96.8% at 80 °C for 4 h and maintain 84% of the initial mechanical strength even after four times of recycling. Moreover, the colloid graphite/PUA flexible strain sensor was prepared by the combination of colloid graphite and PUA, which can accurately detect both large and tiny scale deformations.     

Dynamic elastomers based on bio-derived crosslinker

Petroleum-derived monomers are the most common building blocks for ester-based thermosets. Bio-derived thermoset elastomers are becoming viable alternatives to conventional thermosets. Herein, we developed a biobased vitrimer-type thermoset elastomers using abundant and sustainable raspberry ketone as feedstock. We utilize raspberry ketone to create building blocks for dynamic oxime chemistry and crosslinked these through free radical polymerization with poly(ethylene glycol) methyl ether methacrylate as a comonomer. In contrast to other dynamic networks based on ester bonds, which need catalysts, this is undesirable since catalyst deactivation or leaching lowers its effect over time and may impair reuse. This network incorporates catalyst-free bond exchange reactions in catalyst-dependent polyester networks by substituting oxime-esters for typical ester linkages. The elastomer exhibits stress relaxation, a low glass transition temperature (T_g) (−55 to −40.2°C) and tensile strength up to 5.2 ± 3.0 kPa. Furthermore, the dynamic oxime transesterification exchange mechanism allows elastomers to be reprocessed using a hot press at 160°C and 8 × 10³ kPa pressure. After reprocessing, the tensile strength of elastomers can be recovered up to 78.1 ± 10.9%. This work integrates the principles of catalyst-free dynamic exchange process and mechanical recycling coupled with biobased components to provide a rational solution towards conventional elastomers. In the future, these elastomers can be exploited for the development of hydrogels, recyclable elastomers, and commodity plastics.     

A 3D printable and self-healing polydimethylsiloxane elastomer

Silicone elastomers are broadly used in various fields because of their unique properties, such as flexibility, durable dielectric insulation, and excellent stability in hash environments. As a result, three-dimensional (3D) printing of silicone elastomers is frequently required to construct personalized structures. However, existing 3D-printing of silicone elastomers are less accurate, difficult to maintain shape, or require doping modification with thixotropic agents. Moreover, common 3D-printable silicone elastomers do not have self-healing capability, so they have to be discarded upon damaging. Herein, by introducing hydrogen bonds to improve the shape retention ability and induce network reversibility, we have developed a self-healing polydimethylsiloxane elastomer, which can be readily 3D-printed by fused deposition modeling (FDM) technology. We believe that this new silicone elastomer would be useful in the field of biomedical materials, flexible electronics, medical inserts, soft robots and so on.     

A Robust Self-healing Polyurethane Elastomer Enabled by Tuning the Molecular Mobility and Phase Morphology through Disulfide Bonds

Elastomers with outstanding strength, toughness and healing efficiency are highly promising for many emerging fields. However, it is still a challenge to integrate all these beneficial features in one elastomer. Herein, an asymmetric alicyclic structure adjacent to aromatic disulfide was tactfully introduced into the backbone of polyurethane(PU) elastomer. Specifically, such elastomer(PU-HPS) was fabricated by polycondensing polytetramethylene ether glycol(PTMEG), isophorone diisocyanate(IPDI) and p-hydroxydiphenyl disulfide(HPS) via one-pot method. The molecular mobility and phase morphology of PU-HPS can be tuned by adjusting the HPS content. Consequently, the dynamic exchange of hydrogen and disulfide bonds in the hard segment domains can also be tailored. The optimized sample manifests outstanding tensile strength(46.4 MPa), high toughness(109.1 MJ/m3), high self-healing efficiency after fracture(90.3%), complete scratch recovery(100%)and good puncture resistance. Therefore, this work provides a facile strategy for developing robust self-healing polymers.     

Quadruple H-Bonding and Polyrotaxanes Dual Cross-linking Supramolecular Elastomer for High Toughness and Self-healing Conductors

Tough and self-healable substrates can enable stretchable electronics long service life. However, for substrates, it still remains a challenge to achieve both high toughness and autonomous self-healing ability at room temperature. Herein, a strategy by using the combined effects between quadruple H-bonding and slidable cross-links is proposed to solve the above issues in the elastomer. The elastomer exhibits high toughness (77.3 MJ m⁻³), fracture energy (≈127.2 kJ m⁻²), and good healing efficiency (91 %) at room temperature. The superior performance is ascribed to the inter and intra crosslinking structures of quadruple H-bonding and polyrotaxanes in the dual crosslinking system. Strain-induced crystallization of PEG in polyrotaxanes also contributes to the high fracture energy of the elastomers. Furthermore, based on the dual cross-linked supramolecular elastomer, a highly stretchable and self-healable electrode containing liquid metal is also fabricated, retaining resistance stability (0.16–0.26 Ω) even at the strain of 1600 %.     

Enhanced self-healing driving force in polymer materials by regulating molecular structure

For self-healing polymers, obtaining excellent healing ability and mechanical properties usually need complex chemical structure, external healing conditions, and high manufacturing difficulty. Therefore, self-healing efficiency and rate, mechanical strength, and simple structure design as well as no additional healing conditions of the material are contradictory properties and are difficult to optimize simultaneously. Herein, self-healable thermoplastic poly (urethane urea) elastomers driven by surface energy were fabricated by the introduction of asymmetric alicyclic structures and the healing properties in polymers were optimized by regulating surface energy. The results showed that with the increasing of isophorone diamine contents, the surface energy driving force increased from 36 kPa to 149 kPa, the healing time decreased from 30d to 5d, and healing efficiency, and tensile strength reached 100.9% and 4.04 MPa at room temperature. At the same time, polymers also obtained a high healing efficiency under high-temperature healing conditions. The healing mechanism is that asymmetric alicyclic structures with steric hindrance and ring flip promote the dissociation of hydrogen bonds, provide sufficient chain mobility, decrease the junction density, and improve the surface energy as well as the dissociation and reconstruction of hydrogen bonds. Energetic polymer composites using thermoplastic poly (urethane urea) elastomers as matrix obtained excellent healing properties. This study will offer a novel healing approach for developing advanced self-healing polymer materials.     

Synthesis of fullerenol-derived elastomers and conductive elastomers

Utilization of polyhydroxylated C₆₀ in a condensation reaction with diisocyanated oligo(tetramethylene oxide) led to the successful fabrication of elastomeric poly(urethane-ether) networks. These polymer networks exhibit interesting thermal behavior at low temperatures, improved tensile strength and elongation at ambient temperatures, and enhanced thermal mechanical stability at high temperatures. Design of conducting elastomers was made by carrying out an in situ polymerization of conductive polymer precursors in an interpenetrating fashion at the near-surface of polyhydroxylated C₆₀-hypercrosslinked elastomers. Results demonstrated that elastomers with an appreciable conductivity while retaining desirable elastic properties of the network can be achieved. The room-temperature conductivity of polyaniline interpenetrated (IPN) conducting elastomer was found to be 2.0 Scm⁻¹. The tensile strength and elongation at break of one conductive IPN elastomer was found to be 20 MPa and 480%, respectively. Interestingly, the strain dependent conductivity of these conducting elastomers was found to increase progressively above 200% of elongation. These results demonstrated, for the first instance, conductivity measurements of organic conducting elastomers at an elongation length of higher than 300%, showing a r.t. conductivity of >4.0 Scm⁻¹.     

Self‐healing polyurethane based on disulfide bond and hydrogen bond

Tough and transparent polyurethane networks with self‐healing capability at mild temperature conditions were successfully prepared in a 1‐pot procedure. The self‐healing ability of synthesized polyurethane comes from the covalent disulfide metathesis and non‐covalent H‐bonding. The mechanical testing indicates that disulfide metathesis reforms the covalent bonds on a longer time scale, while H‐bonding gives rise to a healing efficiency of around 46% in the early healing processing. The compromise between mechanical performance and healing capability is reached by tailoring the concentration of disulfide. The tensile strength of the sample with 100% self‐heal efficiency can get to 5.01 MPa, which can be explained by higher mobility of polymer chain under ambient temperature from creep testing.     

Recyclable polybutadiene elastomer based on dynamic imine bond

Catalyst‐free recyclable polybutadiene (PB) elastomer cross‐linked by dynamic imine bonds is prepared by the reaction between amine functionalized PB and aldehyde cross‐linkers. The dynamic nature of imine bond is investigated by rheometry and creep‐recovery experiments. The cross‐linking degrees are regulated by incorporating different amount of aldehyde, and their influence on the cross‐linked elastomers is investigated in detail. The temperature‐induced imine exchange reactions enable recycling of the cross‐linked PB elastomers and their mechanical properties are almost unchanged after several cycles. It is important to note that the elastomers also show excellent solvent resistance even at high temperature. The good mechanical properties, solvent resistance and recycling ability of the resultant PB elastomer demonstrate the superiority of the imine bonds in the design of recyclable polymers. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2011–2018     

Control of hard segment size in polyurethane formation

In general, segmented polyurethane elastomers are prepared by reacting an isocyanate-capped polyol prepolymer with a short-chain diol chain extender, yielding an elastomer with hard segments of uniform size. However, the hard segment size will not be uniform if the polyurethane polymer is prepared by forming the hard segment first, followed by soft segment formation. Because the mechanical properties of polyurethane elastomers depend on the relative ratio of the hard to soft segments as well as the effectiveness of the hard segment as a physical crosslinker, the control of the size distribution of the hard segment is a key factor in designing polyurethane elastomers. It was found that reaction conditions can affect the size distribution of hard segments derived from an aliphatic diisocyanate with differential reactivity between the two isocyanate groups. Lower reaction temperatures and simultaneous mixing of all reactants gave the preferred size distribution of hard segments. © 1995 John Wiley & Sons, Inc.     

兼具导热和自修复功能的聚合物复合材料

针对聚合物复合材料存在的结构受损导致导热和力学强度降低的问题,提出利用导热填料增强自修复聚合物,实现导热性能和力学强度的快速修复.通过对双(3-氨丙基)封端的聚二甲基硅氧烷(H2N-PDMS-NH2)进行端基改性,得到脲基嘧啶酮(UPy)双封端的聚二甲基硅氧烷(UPy-PDMS-UPy),于60°C下20 h后拉伸强度修复效率可达86.6%.进一步填充羟基化氮化硼(mBN)制备兼具自修复功能的导热复合材料,研究发现mBN的填充导致复合材料强度提高但韧性降低,对导热性能和自修复功能分别起积极和不利影响.当mBN含量为30 wt%时,热导率高达2.579 W·m-1·K-1,于60°C下40 h后拉伸强度修复效率达82.0%.红外热像仪显示,损伤处接触10 h后,m BN-30/UPy-PDMS-UPy上表面温度接近初始温度,展现出导热通路的修复特征,实现导热与自修复功能的兼备.     

基于金属配位键的自修复、可重塑聚丁二烯橡胶的合成及性能

橡胶材料通常因经过硫化及补强等工艺处理而呈现出热固性,因而难以被回收处理,容易造成严重的资源浪费和环境污染.本文通过在聚丁二烯上修饰羧酸基团,再加入锌离子(Zn2+)与羧酸配位,制备了基于金属配位键交联的自修复橡胶(PB-COOH/Zn2+).该橡胶具有良好的机械性能和优秀的自修复及重塑性能,在70℃下修复3 h,其韧性可以恢复到初始强度,修复效率可达100%. PB-COOH/Zn2+较高的聚合物链段运动能力及配位键交联网络良好的动态性不仅赋予其优异的修复性能,还使得其在较温和的条件下可以进行多次重塑,在70℃及5 MPa的条件下重塑3次仍能保持原有的机械性能.此外,通过在PB-COOH/Zn2+中掺杂适量的碳纳米管,不仅增强了其机械性能,还使其具备了电致修复及传感能力,扩宽了PB-COOH/Zn2+作为环境友好型材料的应用前景.     

Studies of alkylthio-substituted aromatic diamines as curatives for polyurethane cast elastomers

A series of alkylthio-substituted aromatic diamines was synthesized using a convenient high yield procedure. The method consisted of heating a mixture of dialkyl disulfide and aromatic diamine in the presence of cuprous iodide or other Lewis acid catalyst. Dialkyl disulfide was continually replenished as consumed, throughout the reaction, to maintain the desired reaction temperature. Compounds so prepared were isolated by first precipitating the catalyst with solid caustic and then vacuum flashing the crude products. When desired, the final product purity could be increased by washing with acid to remove starting material or reaction intermediates. The final products were often liquids or low melting solids and showed utility as curatives for polyurethane cast elastomers. Alkylthio substitution of the aromatic diamines lowered reactivity toward isocyanates and generally provided for facile processing during molding. The series of derivatives allowed for great flexibility in controlling both diamine reactivity and the physical properties of the final elastomers. These benefits arose from the diverse electronic, steric and isomeric properties of the derivatives. Polymers were prepared from the alkylthio-substituted compounds and commercially available TDI-based prepolymers using conventional cast elastomer techniques. The physical properties of the polymers were determined and their relation to alkylthiodiamine structure examined.     

Biodegradable aliphatic thermoplastic polyurethane based on poly(ε‐caprolactone) and L‐lysine diisocyanate

A biodegradable aliphatic thermoplastic polyurethane based on L ‐lysine diisocyanate and 1,4‐butanediol hard block segments, and 2000 g/mol poly(ε‐caprolactone) diol soft block segments was synthesized. The resulting polymer was a tough thermoplastic with ultimate tensile strength of 33 MPa and elongation of 1000%. The polymer displayed classic segmented thermoplastic elastomer morphology with distinct hard block and soft block phases. Thermal and dynamic mechanical analyses determined that the material has a useful service temperature range of around ?40 °C to +40 °C, making it an excellent candidate for low‐temperature elastomer and film applications, and potentially as a material for use in temporary orthopedic implant devices. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2990–3000, 2006     

Influence of Material Properties on the Damage-Reporting and Self-Healing Performance of a Mechanically Active Dynamic Network Polymer in Coating Applications

We conducted a detailed investigation of the influence of the material properties of dynamic polymer network coatings on their self-healing and damage-reporting performance. A series of reversible polyacrylate urethane networks containing the damage-reporting diarylbibenzofuranone unit were synthesized, and their material properties (e.g., indentation modulus, hardness modulus, and glass-transition temperature) were measured conducting nanoindentation and differential scanning calorimetry experiments. The damage-reporting and self-healing performances of the dynamic polymer network coatings exhibited opposite tendencies with respect to the material properties of the polymer network coatings. Soft polymer network coatings with low glass-transition temperature (~10 °C) and indentation hardness (20 MPa) exhibited better self-healing performance (almost 100%) but two times worse damage-reporting properties than hard polymer network coatings with high glass-transition temperature (35~50 °C) and indentation hardness (150~200 MPa). These features of the dynamic polymer network coatings are unique; they are not observed in elastomers, films, and hydrogels, whereby the polymer networks are bound to the substrate surface. Evidence indicates that controlling the polymer’s physical properties is a key factor in designing high-performance self-healing and damage-reporting polymer coatings based on mechanophores.     

Synthesis, characterization and mechanical properties of polyester-based aliphatic polyurethane elastomers containing hyperbranched polyester segments

Aliphatic polyester-based polyurethane (PU) elastomers with hyperbranched polyester segments were synthesized from polyester diol, hydroxyl-terminated hyperbranched polyester (HB-20), isophorone diisocyanate (PDI) and 1,4-butanediol. The crosslinking density of the PU elastomer was calculated by using Flory-Rehner equation. The degree of hydrogen bonding, the microstructure and the morphologies of these PU materials were characterized by means of FT-IR, WAXD and DSC, respectively. The experimental results showed that the PU elastomers containing small amount of HB-20 exhibited the enhanced hydrogen bonding and mechanical properties. As compared with the comparable PU specimen, the tensile strength of the polyester-based aliphatic PU containing 6 wt% HB-20 increased by 71.2 times, up to 36.1 MPa, and the elongation at break was still as high as 333.1%, resulting from the dual effects of the hydrogen bonding and the crosslinking density in the PU system.     

Toughening a Self-Healable Supramolecular Polymer by Ionic Cluster-Enhanced Iron-Carboxylate Complexes

Supramolecular polymers that can heal themselves automatically usually exhibit weakness in mechanical toughness and stretchability. Here we exploit a toughening strategy for a dynamic dry supramolecular network by introducing ionic cluster-enhanced iron-carboxylate complexes. The resulting dry supramolecular network simultaneous exhibits tough mechanical strength, high stretchability, self-healing ability, and processability at room temperature. The excellent performance of these distinct supramolecular polymers is attributed to the hierarchical existence of four types of dynamic combinations in the high-density dry network, including dynamic covalent disulfide bonds, noncovalent H-bonds, iron-carboxylate complexes and ionic clustering interactions. The extremely facile preparation method of this self-healing polymer offers prospects for high-performance low-cost material among others for coatings and wearable devices.