Filler effects inspired high performance polyurethane elastomer design: segment arrangement control

Elastomers with high strength and toughness are in great demand. Previous research on elastomers focused mainly on the design of new chemical structures, but their complicated synthesis process and expensive monomers have restricted the practical application of these materials. Inspired by general filler effects, a strategy is proposed to remarkably enhance the mechanical properties of thermoplastic polyurethane (TPU) elastomers by designing the arrangement of hard/soft segments using traditional chemical compositions. By utilizing the synergetic effect of weak hard segments, normal TPU elastomers are upgraded into advanced elastomers. Combining experiments and simulations, it is demonstrated that a suitable sequence length can achieve considerably enhanced strength and toughness by maximizing the relative surface area of hard domains. Mixing the obtained elastomer with an ionic liquid can result in a durable ionogel sensor with balanced mechanical strength and ionic conductivity. This easy-to-implement strategy offers a new dimension for the development of high-performance elastomers.

Satisfactory Tensile Strength and Strain of Recyclable Polyurethane with a Trimaleimide Structure for Thermal Self-Healing and Anticorrosive Coatings

Bismaleimide (BMI) is often used as the cross-linking reagent in Diels-Alder (D-A)-type intrinsic self-healing materials (DISMs) to promote the connectivity of damaged surfaces based on reversible D-A bond formation on the molecular scale. Until now, although DISMs have exhibited great potential in the applications of various sensors, electronic skin, and artificial muscles, it is still difficult to prepare DISMs with satisfactory self-healing abilities and high tensile strengths and strains at the same time, thus largely limiting their applications in self-healing anticorrosive coatings. Herein, symmetrical trimaleimide (TMI) was successfully synthesized, and trimaleimide-structured D-A self-healing polyurethane (TMI-DA-PU) was prepared via the reversible D-A reaction (cycloaddition of furan and maleimide). As a DISM, TMI-DA-PU exhibits apparently higher self-healing efficiency (98.7%), tensile strength (25.4 MPa), and strain (1378%) compared to bismaleimide-structured D-A self-healing polyurethane (BMI-DA-PU) (self-healing efficiency, 90.2%; tensile strength, 19.3 MPa; strain, 1174%). In addition, TMI-DA-PU shows a high recycling efficiency (>95%) after 4 cycles of recycling. A series of characterizations indicate that TMI provides more monoene rings as the self-healing sites, forms denser cross-linked structures compared to BMI, and is, thus, more appropriate to be used for DISM applications. Moreover, the barrier abilities of coatings can be semi-quantitatively expressed by the impedance value at 0.01 Hz (|Z|0.01 Hz). The |Z|0.01 Hz value of the TMI-DA-PU coating is 3.93 × 109 Ω cm2 on day 0, which is significantly higher than that of the BMI-DA-PU coating (6.76 × 108 Ω cm2 on day 0), indicating that the denser rigid cross-linked structure of TMI results in the small porosity in the TMI-DA-PU coating, thus effectively improving the anticorrosion performance. The construction of DISMs with the structure of TMI demonstrates immense potential in self-healing anticorrosive coatings.

3D Printing of Self-Healing Materials

This review article presents a comprehensive overview of the latest advances in the field of 3D printable structures with self-healing properties. Three-dimensional printing (3DP) is a versatile technology that enables the rapid manufacturing of complex geometric structures with precision and functionality not previously attainable. However, the application of 3DP technology is still limited by the availability of materials with customizable properties specifically designed for additive manufacturing. The addition of self-healing properties within 3D printed objects is of high interest as it can improve the performance and lifespan of structural components, and even enable the mimicking of living tissues for biomedical applications, such as organs printing. The review will discuss and analyze the most relevant results reported in recent years in the development of self-healing polymeric materials that can be processed via 3D printing. After introducing the chemical and physical self-healing mechanism that can be exploited, the literature review here reported will focus in particular on printability and repairing performances. At last, actual perspective and possible development field will be critically discussed.

Self-healing hydrogels for bone defect repair

Severe bone defects can be caused by various factors, such as tumor resection, severe trauma, and infection. However, bone regeneration capacity is limited up to a critical-size defect, and further intervention is required. Currently, the most common clinical method to repair bone defects is bone grafting, where autografts are the "gold standard." However, the disadvantages of autografts, including inflammation, secondary trauma and chronic disease, limit their application. Bone tissue engineering (BTE) is an attractive strategy for repairing bone defects and has been widely researched. In particular, hydrogels with a three-dimensional network can be used as scaffolds for BTE owing to their hydrophilicity, biocompatibility, and large porosity. Self-healing hydrogels respond rapidly, autonomously, and repeatedly to induced damage and can maintain their original properties (i.e., mechanical properties, fluidity, and biocompatibility) following self-healing. This review focuses on self-healing hydrogels and their applications in bone defect repair. Moreover, we discussed the recent progress in this research field. Despite the significant existing research achievements, there are still challenges that need to be addressed to promote clinical research of self-healing hydrogels in bone defect repair and increase the market penetration.

Triple crosslinking conductive hydrogels with digitally printable and outstanding mechanical stability for high-resolution conformable bioelectronics

Soft, conductive, and stretchable hydrogels offer a broad variety of applications, including skin-interfacing electrodes, biomonitoring patches, and electrostimulation. Despite rapid developments over the last decades, a combination of good electrical and mechanical properties, low-cost fabrication, and biocompatibility is yet to be demonstrated. Also, the current methods for deposition and patterning of these hydrogels are manual, and there is a need toward autonomous and digital fabrication techniques. In this work, we demonstrate a novel Gallium (Ga) embedded sodium-alginate-polyacrylamide-LAPONITE® (Ga-SA-PAAM-La) hydrogel, that is ultra-stretchable (Maximum strain tolerance of∼985%), tough (toughness ∼30 kJ m^-3), bio-adhesive (adhesion energy ∼216 J m^-2), conductive, and digitally printable. Ga nanoparticles are used as radical initiators. By adjusting the sonication parameters, we control the solution viscosity and curing time, thus allowing us to prepare pre-polymers with the desired properties for casting, or digital printing. These hydrogels benefit from a triple-network structure due to the role of Ga droplets as crosslinkers besides BIS (N,N'-methylene-bis-acrylamide) and LAPONITE®, thus resulting in tough composite hydrogels. The inclusion of LAPONITE® into the hydrogel network improved its electrical conductivity, adhesion, digital printability, and its mechanical properties, (>6× compared to the same hydrogel without LAPONITE®). As electrodes in the electrocardiogram, the signal-to-noise ratio was surprisingly higher than the medical-grade Ag/AgCl electrodes, which are applied for monitoring muscles, heart, respiration, and body joint angle through EMG, ECG, and bioimpedance measurements. The results obtained prove that such digitally printed conductive and tough hydrogels can be used as potential electrodes and sensors in practical applications in the next generation of printed wearable computing devices.

Dually-dynamic covalent tetraPEG hydrogels end-linked with boronate ester and acylhydrazone groups

Well-defined dually dynamic hydrogels were prepared by end-linking four-armed poly(ethylene glycol) stars (tetraPEG stars) through two different types of dynamic covalent cross-links, boronates and acylhydrazones, leading to robust, self-healable materials. This required the prior end-functionalization of tetraPEG stars, originally bearing four hydroxyl terminal groups, with glucoronate, acylhydrazide and benzaldehyde groups, resulting in three differently end-functional star polymers. A first type of dually dynamic hydrogel resulted from the combination of the first two differently end-functionalized tetraPEG stars, cross-linked by 4-formylphenyl boronic acid, a small molecule bearing both an aldehyde and a boronic acid group, respectively complementary to the acylhydrazide and glucoronate end-groups of the two above-mentioned tetraPEG stars. For comparison, a singly-dynamic hydrogel cross-linked with only acylhydrazone groups was also prepared, as well as a double-like hydrogel combining the constituents of both of the above-mentioned hydrogels. All three types of hydrogels were prepared at three different pH values, 8.5, 10.5 and 12.5, leading to a total number of nine samples. All nine samples were investigated for their self-healing, mechanical, viscoelastic and aqueous swelling/degradation properties. This study sets the basis for the development of well-defined polymeric dynamic covalent hydrogels where their self-healing and stability can be readily tuned.

Self-Healing Thiolated Pillar[5]arene Films Containing Moxifloxacin Suppress the Development of Bacterial Biofilms

Polymer self-healing films containing fragments of pillar[5]arene were obtained for the first time using thiol/disulfide redox cross-linking. These films were characterized by thermogravimetric analysis and differential scanning calorimetry, FTIR spectroscopy, and electron microscopy. The films demonstrated the ability to self-heal through the action of atmospheric oxygen. Using UV–vis, 2D 1H-1H NOESY, and DOSY NMR spectroscopy, the pillar[5]arene was shown to form complexes with the antimicrobial drug moxifloxacin in a 2:1 composition (logK11 = 2.14 and logK12 = 6.20). Films containing moxifloxacin effectively reduced Staphylococcus aureus and Klebsiella pneumoniae biofilms formation on adhesive surfaces.

Dynamic Covalent Polyurethane Network Materials: Synthesis and Self-Healability

Polyurethane (PU) has not only been widely used in the daily lives, but also extensively explored as an important class of the essential polymers for various applications. In recent years, significant efforts have been made on the development of self-healable PU materials that possess high performance, extended lifetime, great reliability, and recyclability. A promising approach is the incorporation of covalent dynamic bonds into the design of PU covalently crosslinked polymers and thermoplastic elastomers that can dissociate and reform indefinitely in response to external stimuli or autonomously. This review summarizes various strategies to synthesize self-healable, reprocessable, and recyclable PU materials integrated with dynamic (reversible) Diels-Alder cycloadduct, disulfide, diselenide, imine, boronic ester, and hindered urea bond. Furthermore, various approaches utilizing the combination of dynamic covalent chemistries with nanofiller surface chemistries are described for the fabrication of dynamic heterogeneous PU composites.

Biobased Nonisocyanate Polyurethanes as Recyclable and Intrinsic Self-Healing Coating with Triple Healing Sites

Polymer coatings having high amounts of renewable carbon and self-healing properties are highly sought after in a sustainability perspective. We report here the development of bio-/CO₂-derived nonisocyanate polyurethane (NIPU) coatings which are recyclable and healable via three different types of healing mechanisms. These NIPUs contain furan rings in their main chain which after cross-linking with bismaleimides form organogels having a thermo-reversible sol-gel transition and solvent-borne coatings with improved properties. Judicial selection of the bismaleimide cross-linker structure enabled us to produce recyclable and intrinsic healable coatings mediated by heat (thermo-healing), moisture (moisture-healing), and, more interestingly, dry conditions at room temperature (self-healing). The intrinsic moisture-healing property of NIPU-based coatings is unprecedented and is mainly due to the presence of hydroxyl functionalities in the NIPU structure. The uniqueness of these cross-linked biobased NIPU as recyclable coatings having triple healing sites present in their structure gives these materials potential for sustainable and functional applications.

Macromolecularly Engineered Thermoreversible Heterogeneous Self-Healable Networks Encapsulating Reactive Multidentate Block Copolymer-Stabilized Carbon Nanotubes

The development of heterogeneous covalent adaptable networks (CANs) embedded with carbon nanotubes (CNTs) that undergo reversible dissociation/recombination through thermoreversibility has been significantly explored. However, the carbon nanotube (CNT)-incorporation methods based on physical mixing and chemical modification could result in either phase separation due to structural incompatibility or degrading conjugation due to a disruption of π-network, thus lowering their intrinsic charge transport properties. To address this issue, the versatility of a macromolecular engineering approach through thermoreversibility by physical modification of CNT surfaces with reactive multidentate block copolymers (rMDBCs) is demonstrated. The formed CNTs stabilized with rMDBCs (termed rMDBC/CNT colloids) bearing reactive furfuryl groups is functioned as a multicrosslinker that reacts with a polymaleimide to fabricate robust heterogeneous polyurethane (PU) networks crosslinked through dynamic Diels-Alder (DA)/retro-DA chemistry. Promisingly, the fabricated PU network gels in which CNTs through rMDBC covalently embedded are flexible and robust to be bendable as well as exhibit self-healing elasticity and enhanced conductivity.

A novel hydrogel with self-healing property and bactericidal activity

In this study, we have designed and synthesized a novel poly (4 - vinyl benzene boronic acid - co - N - vinyl pyrrolidone - co - 1 - vinyl - 3 - butylimidazolium bromide) hydrogel (VNV hydrogel) dressing with good self-healing properties and bactericidal activity. The gelation and self-healing of this hydrogel are mainly achieved by the formation of a dynamic B-O-B bond between the polymer chains, which is fractured by external forces and subsequently reformed. This self-healing mechanism is studied in detail through the molecular design of the hydrogel. The introduction of hydrophilic chemical groups can effectively improve the porous structures, water absorption and molecular migration. These properties have a positive effect on improving self-healing properties of dynamic crosslinked hydrogels. Furthermore, this VNV hydrogel dressing displays good antibacterial activity against E. coli, S. aureus, and C. albicans. The application of VNV hydrogel dressing on rat wound surface can effectively accelerate wound healing. These results indicate that this novel VNV hydrogel dressing with good self-healing properties and bactericidal activity has potential applications in wound dressings.

Intrinsic Self-Healing Epoxies in Polymer Matrix Composites (PMCs) for Aerospace Applications

This article reviews some of the intrinsic self-healing epoxy materials that have been investigated throughout the course of the last twenty years. Emphasis is placed on those formulations suitable for the design of high-performance composites to be employed in the aerospace field. A brief introduction is given on the advantages of intrinsic self-healing polymers over extrinsic counterparts and of epoxies over other thermosetting systems. After a general description of the testing procedures adopted for the evaluation of the healing efficiency and the required features for a smooth implementation of such materials in the industry, different self-healing mechanisms, arising from either physical or chemical interactions, are detailed. The presented formulations are critically reviewed, comparing major strengths and weaknesses of their healing mechanisms, underlining the inherent structural polymer properties that may affect the healing phenomena. As many self-healing chemistries already provide the fundamental aspects for recyclability and reprocessability of thermosets, which have been historically thought as a critical issue, perspective trends of a circular economy for self-healing polymers are discussed along with their possible advances and challenges. This may open up the opportunity for a totally reconfigured landscape in composite manufacturing, with the net benefits of overall cost reduction and less waste. Some general drawbacks are also laid out along with some potential countermeasures to overcome or limit their impact. Finally, present and future applications in the aviation and space fields are portrayed.

Preparation, characterization and properties of intrinsic self-healing elastomers

Significant advances have been made in the development of self-healing synthetic polymer materials in recent years. This review article discusses the recent progress in preparation, characterization and properties of different kinds of intrinsic self-healing elastomers based on reversible covalent bonds and dynamic supramolecular chemistry. Healing conditions, mechanical property recovery and healing efficiency are the main discussion topics. Potential applications, challenges and future prospects in self-healing elastomer fields are also discussed in the last part of this review.

Three-Dimensional Self-Healable Touch Sensing Artificial Skin Device

Human skin is a unique functional material that perfectly covers body parts having various complicated shapes, spontaneously heals mechanical damage, and senses a touch. E-skin devices have been actively researched, focusing on the sensing functionality of skin. However, most e-skin devices still have limitations in their shapes, and it is a challenging issue of interest to realize multiple functionalities in one device as human skin does. Here, new artificial skin devices are demonstrated in application-oriented three-dimensional (3D) shapes, which can sense exact touch location and heal mechanical damage spontaneously. Beyond the conventional film-type e-skin devices, the artificial skin devices are fabricated in optimal three-dimensional structures, via systematic material design and characterization of ion-conductive self-healing hydrogel system and its extrusion-based 3D printing. The ring-shaped and fingertip-shaped artificial skin devices are successfully fabricated to fit perfectly on finger models, and shows large electronic signal contrast, ∼5.4 times increase in current, upon a human finger contact. Furthermore, like human skin, the device provides the exact positional information of an arbitrary touch location on a three-dimensional artificial skin device without complicated device fabrication or data processing.

Recyclable and Mendable Cellulose-Reinforced Composites Crosslinked with Diels⁻Alder Adducts

Owing to their natural abundance and exceptional mechanical properties, cellulose fibers (CFs) have been used for reinforcing polymers. Despite these merits, dispersing hydrophilic CFs in a hydrophobic polymer matrix is challenging. To address this, an amphiphilic ammonium salt was employed as the dispersant for CFs in this study. The hydrophobic CFs were mixed with a healable polymer to produce CF-reinforced composites. As the thermosetting polymer was crosslinked with Diels–Alder (DA) adducts, it was mended and recycled via a retro DA reaction at 120 °C. Interestingly, the CF-reinforced polymer composites were mended and recycled as well. When 5 wt % of the hydrophobic CFs was added to the polymer, maximum tensile strength, elongation at break, Young’s modulus, and toughness increased by 70%, 183%, 75%, and 420%, respectively. After recycling, the CF-reinforced composites still featured better mechanical properties than recycled polymer.

Progress in self-healing hydrogels assembled by host–guest interactions: preparation and biomedical applications

Preparation and biomedical applications of self-healing hydrogels assembled from hosts of cyclodextrins and cucurbit[n]urils with various guests were reviewed.

Environmentally Adaptable and Temperature-Selective Self-Healing Polymers

Development of polymeric materials capable of self-healing at low temperatures is an important issue since their mechanical strength and self-healing performance are often in conflict with each other. Herein, random copolymers with self-healing capability in a wide temperature range prepared from 2-(dimethylamino)ethyl methacrylate (DMAEMA), glyceryl monomethacrylate (GlyMA), and butyl methacrylate monomers via free-radical polymerization and subsequent cross-linking with hexamethylene diisocyanate are reported. Wound closure is facilitated by swelling below the lower critical solution temperature or by heating above the glass transition temperature (T _g ) of the polymer. GlyMA units form metal-ligand coordination complexes with dibutyltin dilaurate, leading to the formation of new carbonate bonds under ambient CO₂ and H₂ O conditions. Although swelling/heating reduces the polymer's mechanical strength, it is fully restored following chemical re-bonding/drying at room temperature. The swelling and degree of scratch healing are affected by pH, temperature, and the DMAEMA content.

Electrically-Responsive Reversible Polyketone/MWCNT Network through Diels-Alder Chemistry

This study examines the preparation of electrically conductive polymer networks based on furan-functionalised polyketone (PK-Fu) doped with multi-walled carbon nanotubes (MWCNTs) and reversibly crosslinked with bis-maleimide (B-Ma) via Diels-Alder (DA) cycloaddition. Notably, the incorporation of 5 wt.% of MWCNTs results in an increased modulus of the material, and makes it thermally and electrically conductive. Analysis by X-ray photoelectron spectroscopy indicates that MWCNTs, due to their diene/dienophile character, covalently interact with the matrix via DA reaction, leading to effective interfacial adhesion between the components. Raman spectroscopy points to a more effective graphitic ordering of MWCNTs after reaction with PK-Fu and B-Ma. After crosslinking the obtained composite via the DA reaction, the softening point (tan(δ) in dynamic mechanical analysis measurements) increases up to 155 °C, as compared to the value of 130 °C for the PK-Fu crosslinked with B-Ma and that of 140 °C for the PK-Fu/B-Ma/MWCNT nanocomposite before resistive heating (responsible for crosslinking). After grinding the composite, compression moulding (150 °C/40 bar) activates the retro-DA process that disrupts the network, allowing it to be reshaped as a thermoplastic. A subsequent process of annealing via resistive heating demonstrates the possibility of reconnecting the decoupled DA linkages, thus providing the PK networks with the same thermal, mechanical, and electrical properties as the crosslinked pristine systems.

Multiple Stimuli-Responsive Fluorescent Sensor from Citric Acid and 1-(2-Aminoethyl)piperazine

Multiresponsive fluorescent supramolecular materials are quite interesting, for they combine the multiresponsiveness of supramolecules and the high sensitivity of fluorescent materials. Different from the multiresponsive supramolecular materials based on host-guest interactions, in this report, a supramolecular ionic network was fabricated by 1-(2-aminoethyl)piperazine and citric acids via ionic interactions. Despite the fact that there are no conventional chromophores, the obtained supramolecular ionic material can emit strong fluorescence. Most interestingly, the thin film of this supramolecular ionic material can change its fluorescent intensity in response to four external stimuli, including humidity, triethylamine, acetic acid, and temperature. Beneficial to the supramolecular ionic structure, this multiresponsive fluorescent sensor is self-healable. It is found that a new route has been opened to prepare the multiresponsive fluorescent sensors.

Supramolecular Nested Microbeads as Building Blocks for Macroscopic Self-Healing Scaffolds

The ability to construct self‐healing scaffolds that are injectable and capable of forming a designed morphology offers the possibility to engineer sustainable materials. Herein, we introduce supramolecular nested microbeads that can be used as building blocks to construct macroscopic self‐healing scaffolds. The core–shell microbeads remain in an “inert” state owing to the isolation of a pair of complementary polymers in a form that can be stored as an aqueous suspension. An annealing process after injection effectively induces the re‐construction of the microbead units, leading to supramolecular gelation in a preconfigured shape. The resulting macroscopic scaffold is dynamically stable, displaying self‐recovery in a self‐healing electronic conductor. This strategy of using the supramolecular assembled nested microbeads as building blocks represents an alternative to injectable hydrogel systems, and shows promise in the field of structural biomaterials and flexible electronics.

Mechano-switchable, luminescent gels derived from salts of a long-chained, fatty-acid gelator

Stimulus-responsive molecular gel systems, based on metal salts of a luminescent gelator, 9,10-dioxooctadecanoic acid (DODA), are reported. These salts are structurally the simplest metallo-gelators of which we are aware that exhibit controllable mechano-responsive and luminescent properties. Aggregation is more favored by the metal salts than for DODA itself. However, gelation ability differs dramatically depending on the metal ion: whereas the salts with zinc(ii) and calcium(ii) are inefficient gelators, those with nickel(ii) and copper(ii) can gelate various aromatic liquids, alkanes, and long-chained alcohols. Unlike the DODA gels, no aggregation-induced shift in the positions of the emission spectra of the metal salts could be observed as the sols were transformed to their gel phases. Gels of both nickel(ii) and copper(ii) salts in benzonitrile are among the few known examples with crystalline networks and exhibiting thixotropic behavior. However, there are significant differences in their abilities to recover the initial viscoelastic properties. Structural data for the solid and gel states lead us to conclude that differences among the gelating abilities can be attributed principally to the specific nature of interactions of the salts at their head groups. They appear to control the mechanical and emissive properties of the gels as well as whether the initial aggregation of the salts in the sol phases will support the growth of 1D objects that are capable of maintaining strong contacts, leading to 3D networks and gel formation. Overall, the results provide a facile strategy for the design of luminescent materials with controllable mechano-responsiveness by modifying the metal ions within fibrillar assemblies.

The underlying mechanisms for self-healing of poly(disulfide)s

Recently, self-healing polymers based on disulfide compounds have gained attention due to the versatile chemistry of disulfide bonds and easy implementation into polymeric materials. However, the underlying mechanisms of disulfide exchange which induce the self-healing effect in poly(disulfide)s remain unclear. In this work, we elucidate the process of disulfide exchange using a variety of spectroscopic techniques. Comparing a model exchange reaction of 4-aminophenyl disulfide and diphenyl disulfide with modified reactions in the presence of additional radical traps or radical sources confirmed that the exchange reaction between disulfide compounds occurred via a radical-mediated mechanism. Furthermore, when investigating the effect of catalysts on the model exchange reaction, it could be concluded that catalysts enhance the disulfide exchange reaction through the formation of S-based anions in addition to the radical-mediated mechanism.

One-Pot Preparation of Autonomously Self-Healable Elastomeric Hydrogel from Boric Acid and Random Copolymer Bearing Hydroxyl Groups

Self-healable hydrogels based on the dynamically reversible boronate ester or borate ester bonds are usually prepared by reacting boronic acid or boric acid with diol compounds or polymer-like poly(vinyl alcohol) bearing a hydroxyl group in each monomer unit. Herein, we report a finding that not only facilitates the preparation but also extends the range of self-healable hydrogels of this kind. By simply copolymerizing commercially available N,N-dimethylacrylamide and 2-hydroxyethyl acrylate (8:2 weight ratio) in the presence of boric acid in a one-pot fashion, the resulting random copolymer can gel in aqueous solution at pH = 9, giving rise to a solid hydrogel (tensile strength >0.5 MPa at water content of 30%) that, on the one hand, can autonomously self-heal (near 100% fracture stress recovery within 48 h in air at room temperature) and, on the other hand, shows the characteristics of elastomer (little stress relaxation under loading and small residual deformation after unloading upon repeated 300% elongation cycles). The results reveal that it can be sufficient to have a random copolymer with comonomer units bearing hydroxyl groups for reacting with boric acid to generate dynamically reversible borate ester bonds. This finding thus points out a general, facile, and cost-effective method to obtain and explore new borate ester bond-based self-healable hydrogels.

Water-Reprocessable, Reformable, and Ecofriendly Sustainable Material Based on Disulfide-Cross-Linked Polyethyleneimine

A reformable polymer gel material has been developed based on the disulfide cross-linking of low-molecular-weight polyethylenimine (PEI) that can be synthesized through a facile thiolation method and reprocessed through an aqueous method without the use of solvents or additional chemicals. Despite being made with water-soluble PEI, the cross-linked gel shows good mechanical integrity and its properties can be controlled through the fabrication parameters, maintaining the hydrophilic nature of PEI while being sufficiently robust to form a free-standing film that does not dissolve in water. The properties of the gel have been characterized by Fourier transform infrared spectroscopy, thermogravimetry, and dynamic mechanical analyses, showing the effect of parameters such as the degree of thiolation and thermal curing. The reformability of the gel comes from the disulfide cross-links, which can be disrupted and reformed through a simple, aqueous processing method utilizing ultrasonication, creating an aqueous dispersion, which can be recast multiple times with minimal loss in physical properties.