Influence of Isocyanate Structure on Recyclable Shape Memory Poly(thiourethane)

In this study, poly(thiourethane) (PTU) with different structures is synthesized by click chemistry from trimethylolpropane tris(3-mercaptopropionate) (S3) and different diisocyanates (hexamethylene diisocyanate, HDI, isophorone diisocyanate, IPDI and toluene diisocyanate, TDI). Quantitative analysis of the FTIR spectra reveals that the reaction rates between TDI and S3 are the most rapid, resulting from the combined influence of conjugation and spatial site hindrance. Moreover, the homogeneous cross-linked network of the synthesized PTUs facilitates better manageability of the shape memory effect. All three PTUs exhibit excellent shape memory properties (R_r and R_f are over 90%), and an increase in chain rigidity is observed to negatively impact the shape recovery rate and fix rate. Moreover, all three PTUs exhibit satisfactory reprocessability performance, and an increase in chain rigidity is accompanied by a greater decrease in shape memory and a smaller decrease in mechanical performance for recycled PTUs. Contact angle (<90°) and in vitro degradation results (13%/month for HDI-based PTU, 7.5%/month for IPDI-based PTU, and 8.5%/month for TDI-based PTU) indicate that PTUs can be used as long-term or medium-term biodegradable materials. The synthesized PTUs have a high potential for applications in smart response scenarios requiring specific glass transition temperatures, such as artificial muscles, soft robots, and sensors.

Self-Healing Polymeric Soft Actuators

Natural evolution has provided multicellular organisms with sophisticated functionalities and repair mechanisms for surviving and preserve their functions after an injury and/or infection. In this context, biological systems have inspired material scientists over decades to design and fabricate both self-healing polymeric materials and soft actuators with remarkable performance. The latter are capable of modifying their shape in response to environmental changes, such as temperature, pH, light, electrical/magnetic field, chemical additives, etc. In this review, we focus on the fusion of both types of materials, affording new systems with the potential to revolutionize almost every aspect of our modern life, from healthcare to environmental remediation and energy. The integration of stimuli-triggered self-healing properties into polymeric soft actuators endow environmental friendliness, cost-saving, enhanced safety, and lifespan of functional materials. We discuss the details of the most remarkable examples of self-healing soft actuators that display a macroscopic movement under specific stimuli. The discussion includes key experimental data, potential limitations, and mechanistic insights. Finally, we include a general table providing at first glance information about the nature of the external stimuli, conditions for self-healing and actuation, key information about the driving forces behind both phenomena, and the most important features of the achieved movement.

Photocrosslinking of Polyacrylamides Using [2 + 2] Photodimerisation of Monothiomaleimides

The [2 + 2] photocycloaddition of monothiomaleimides (MTMs) has been exploited for the photocrosslinking of polyacrylamides. Polymer scaffolds composed of dimethylacrylamide and varying amounts of d,l-homocysteine thiolactone acrylamide (5, 10, and 20 mol %) were synthesized via free-radical polymerization, whereby the latent thiol functionality was exploited to incorporate MTM motifs. Subsequent exposure to UV light (λ = 365 nm, 15 mW cm^–2) triggered intermolecular crosslinking via the photodimerization of MTM side chains, thus resulting in the formation of polyacrylamide gels. The polymer scaffolds were characterized using Fourier transform infrared spectroscopy, UV–visible spectroscopy, ¹H NMR spectroscopy, and size exclusion chromatography, confirming the occurrence of the [2 + 2] photocycloaddition between the MTM moieties. The mechanical and physical properties of the resulting gels containing various MTM mol % were evaluated by rheology, compression testing, and swelling experiments. In addition, scanning electron microscopy was used to characterize the xerogel morphology of 5 and 10 mol % MTM hydro- and organo-gels. The macro-porous morphology obtained for the hydrogels was attributed to phase separation due to the difference in solubility of the PDMA modified with thiolactone side chains, provided that a more homogeneous morphology was obtained when the photo-gels were prepared in DMF as the solvent.

Stimuli-Responsive Electrochemical Energy Storage Devices

Electrochemical energy storage (EES) devices have been swiftly developed in recent years. Stimuli-responsive EES devices that respond to different external stimuli are considered the most advanced EES devices. The stimuli-responsive EES devices enhanced the performance and applications of the EES devices. The capability of the EES devices to respond to the various external stimuli due to produced advanced EES devices that distinguished the best performance and interactions in different situations. The stimuli-responsive EES devices have responsive behavior to different external stimuli including chemical compounds, electricity, photons, mechanical tensions, and temperature. All of these advanced responsiveness behaviors have originated from the functionality and specific structure of the EES devices. The multi-responsive EES devices have been recognized as the next generation of stimuli-responsive EES devices. There are two main steps in developing stimuli-responsive EES devices in the future. The first step is the combination of the economical, environmental, electrochemical, and multi-responsiveness priorities in an EES device. The second step is obtaining some advanced properties such as biocompatibility, flexibility, stretchability, transparency, and wearability in novel stimuli-responsive EES devices. Future studies on stimuli-responsive EES devices will be allocated to merging these significant two steps to improve the performance of the stimuli-responsive EES devices to challenge complicated situations.

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.

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.

Actuator Behaviour of Tailored Poly(thiourethane) Shape Memory Thermosets

In this work, a new family of poly(thiourethane) shape memory thermosetting actuators was developed and characterized. These materials can be easily prepared from mixtures of two different aliphatic diisocyanates and a trithiol in the presence of a latent catalyst, allowing an easy manipulation of the formulation. Rheological studies of the curing process confirm the latent character of the formulations. The glass transition temperatures and the mechanical properties can be modified by varying the proportion of diisocyanates (hexamethylene diisocyanate, HDI, and isophorone diisocyanate, IPDI) with stoichiometric amounts of trimethylolpropane tris(3-mercaptopropionate). The shape-memory behavior was deeply investigated under three different conditions: unconstrained, partially constrained, and fully constrained. Tests were performed in single cantilever bending mode to simulate conditions closer to real complex mechanics of thermomechanical actuators under flexural performances. The complex recovery process in single cantilever bending mode was compared with that obtained using tensile mode. The results evidenced that the amount of recovery force in fully constrained conditions, or energy released during the recovery process in partially constrained, can be modulated by simply changing the proportion of both diisocyanates. A simple model based on Timoshenko beam theory was used for the prediction of the amount of work performed. The reported results are an important guideline to design shape-memory materials based on poly(thiourethane) networks, establishing criteria for the choice of the material depending on the expected application.

Probing stress relaxation behavior in glassy methacrylate networks containing thio-carbamate additives

The incorporation of thiourethane prepolymer (TU) into either the organic phase or as a surface treatment for filler particles in composites reduces polymerization stress and improves fracture toughness. The aim of this study was to gain insight into the influence of the inclusion of thiourethanes on the resulting network of methacrylate-based materials polymerized via free-radical mechanisms. Dynamic mechanical analysis was used to elucidate network parameters and potential stress relaxation behavior of these networks. TU oligomers were synthesized using a combination of trimethylol-tris-3-mercaptopropionate and dicyclohexylmethane 4,4'-diisocyanate and added into composite formulations at 20 wt% replacing part of the organic matrix and/or as TU-silanes used to functionalize filler particles (TU-matrix, TU-Sil or TU-matrix/sil). Materials not containing any form of TU were used as the control (in those cases, 3-(trimethoxysilyl)propyl methacrylate was used as the silane agent). Filler was added at 50 wt%. Degree of conversion was evaluated by near-IR spectroscopy, mechanical properties by 3-point bending and rotational rheometry. Dynamic mechanical analysis was used to obtain network parameters (glass transition temperature (Tg), storage modulus, cross-link density, and breadth of tan delta a proxy for network homogeneity - temperature sweep experiments) and to evaluate the potential for network relaxation (stress relaxation). TU-containing formulations showed 10% higher DC than the control. The time to reach storage/loss modulus crossover in the rheometer experiments was significantly longer for TU-matrix and TU-matrix/sil in comparison with the control (21.6, 27.9, and 5.1 s, respectively). TU-matrix and TU-matrix/sil presented significant lower Tg than the control (151.5, 153.8, and 161.3 °C, respectively). There were no statistical differences among the groups in terms of shear modulus, cross-link density, breadth of tan delta, flexural strength/modulus, and toughness. For at least one group (TU-matrix/sil), the relaxation time was four times faster than for the control at 105 °C. The addition of TU additives into dental polymers resulted in a stark reduction in the stress relaxation time. This behavior, in tandem with the network characterization and mechanical properties seems to indicate the TU networks undergo a variety of reversible associative and dissociative chemical reactions which facilitate enhanced stress relief.

Repeatedly Intrinsic Self-Healing of Millimeter-Scale Wounds in Polymer through Rapid Volume Expansion Aided Host-Guest Interaction

Implantable and wearable materials, which are usually used in/on a biological body, are mostly needed with biomimetic self-healing function. To enable repeatable large-wound self-healing and volume/structure recovery, we verified a proof-of-concept approach in this work. We design a polymer hydrogel that combines temperature responsiveness with an intrinsic self-healing ability through host-guest orthogonal self-assembly between two types of poly(N-isopropylacrylamide) (PNIPAM) oligomers. The result is thermosensitive, capable of fast self-repair of microcracks based on reversible host-guest assembly. More importantly, when a large open wound appears, the hydrogel can first close the wound via volume swelling and then completely self-repair the damage in terms of intrinsic self-healing. Meanwhile, its original volume can be easily recovered by subsequent contraction. As demonstrated by the experimental data, such millimeter-level wound self-healing and volume recovery can be repeatedly carried out in response to the short-term cooling stimulus. With low cytotoxicity and good biocompatibility, moreover, this highly intelligent hydrogel is greatly promising for practical large-wound self-healing in wound dressing, electronic skins, wearable biosensors, and humanoid robotics, which can tolerate large-scale human motions.

Achievement of Both Mechanical Properties and Intrinsic Self-Healing under Body Temperature in Polyurethane Elastomers: A Synthesis Strategy from Waterborne Polymers

Inspired by the growing demand for smart and environmentally friendly polymer materials, we employed 2,2'-disulfanediyldianiline (22DTDA) as a chain extender to synthesize a waterborne polyurethane (WPUR). Due to the ortho-substituted structure of the aromatic disulfide, the urea moieties formed a unique microphase structure in the WPUR, its mechanical strength was enhanced more 180 times relative to that of the material prepared without 22DTDA, and excellent self-healing abilities at body temperature in air or under ultrasound in water were obtained. If the self-healing process was carried out at 37 °C, 50 °C or under ultrasound, the ultimate tensile strength and elongation at break of the healed film could reach 13.8 MPa and 1150%, 15.4 MPa and 1215%, or 16 MPa and 1056%, respectively. Moreover, the WPUR films could be re-healed at the same fracture location over three cutting-healing cycles, and the recovery rates of the tensile strength and elongation at break remained almost constant throughout these cycles.

Reversible networks of degradable polyesters containing weak covalent bonds

The synthesis of reversible polyester networks based mainly on the Diels–Alder chemistry, alkene [2 + 2] cycloaddition or transesterification reactions and studies of their reversibility and its consequences are reviewed.

Repeated Intrinsic Self-Healing of Wider Cracks in Polymer via Dynamic Reversible Covalent Bonding Molecularly Combined with a Two-Way Shape Memory Effect

To enable repeated intrinsic self-healing of wider cracks in polymers, a proof-of-concept approach is verified in the present work. It operates through two-way shape memory effect (SME)-aided intrinsic self-healing. Accordingly, a reversible C-ON bond is introduced into the main chain of crosslinked polyurethane (PU) containing an elastomeric dispersed phase (styrene-butadiene-styrene block copolymer, SBS). The PU/SBS blend was developed by the authors recently, and proved to possess an external stress-free two-way SME after programming. As a result, the thermal retractility offered by the SME coupled with the reversible C-ON bonds can be used for successive crack closure and remending based on synchronous fission/radical recombination of C-ON bonds. Moreover, multiwalled carbon nanotubes are incorporated to impart electrical conductivity to the insulating polymer. Repeated autonomic healing of wider cracks is thus achieved through narrowing of cracks followed by chemical rebonding under self-regulating Joule heating. No additional programming is needed after each healing event, which is superior to one-way SME-assisted self-healing. The outcomes set an example of integrating different stimuli-responsivities into single materials.

Design of melt-recyclable poly(ε-caprolactone)-based supramolecular shape-memory nanocomposites

A novel poly(epsilon-caprolactone) (PCL) supramolecular network exhibiting shape-memory behavior was successfully constructed with pendant UPy units that are highly able to dimerize. The dynamic network was obtained by a simple and versatile strategy consisting of chain-extension reaction between α,ω-dihydroxyoligoPCL and hydroxylated UPy units in the presence of hexamethylene diisocyanate as a coupling agent and further intermolecular dimerization of the UPy along the polyurethane backbone. 1H NMR analyses confirmed the dynamic features of the system, and DMTA in tensile mode was investigated to assess the SMP properties. Recyclability was also assessed by taking advantage of these supramolecular networks. Further addition of cellulose nanocrystals into the polymer network enabled adjustment of the extent of the net-points and therefore the SMP features. As confirmed by dispersion tests in solution and SEM observations, these bio-based nanofillers were homogeneously distributed in the network via supramolecular interaction between the hydroxyl groups present on their surface and UPy moieties along the polyurethane backbone. Thus, the here developed nanomaterials might reveal applicability in areas where a combination of SMP and biocompatibility is needed.

Cucurbit[8]uril-Based Giant Supramolecular Vesicles: Highly Stable, Versatile Carriers for Photoresponsive and Targeted Drug Delivery

Highly stable giant supramolecular vesicles were constructed by hierarchical self-assembly of cucurbit[8]uril (CB[8])-based supra-amphiphiles for photoresponsive and targeted intracellular drug delivery. These smart vesicles can encapsulate the model drugs with high loading efficiencies and then release them by manipulating photoswitchable CB[8] heteroternary complexation to regulate the formation and dissociation of supra-amphiphiles that cause dramatic morphological changes of the assemblies to achieve remote optically controlled drug delivery. More importantly, the confocal microscopy analysis, cellular uptake experiment, and cell viability assay have shown that the giant vesicles are able to maintain the structural integrity and stability within actual cellular environments and exhibit obvious advantages for intracellular drug delivery such as low toxicity, easy surface modification for tumor-targeting selectivity, and rapid internalization into different human cancer cell lines. A synergistic mechanism that integrates multiple pathways including energy-dependent endocytosis, macropinocytosis, cholesterol-dependent endocytosis, and microtubule-related endocytosis was determined to facilitate the internalization process. Moreover, cytotoxicity experiments and flow cytometric analysis have demonstrated that the doxorubicin hydrochloride-loaded vesicles exhibited a significant therapeutic effect for tumor cells upon UV light irradiation, which makes the photoresponsive system more promising for potential applications in pharmaceutically relevant fields.

Healable and flexible transparent heaters

Challenges associated with the mechanical fracture of electrical conductors have hindered the realization of truly flexible high performance wearable electronics. Here, transparent healable electrodes have been developed and examined to alleviate these problems. The composite electrode features a layer of an interconnecting AgNW network on a polyurethane film modified with Diels-Alder adducts (PU-DA). Surface modification using hydrophilic molecules improved adhesion of the AgNW network and resulted in mechanically robust flexible electrodes with a figure of merit sheet resistance of 13.3 Ω □^-1 and 77% transmittance at 550 nm. Transparent and flexible healable heaters (TFHH) with good mechanical and thermal stability were fabricated using these electrodes for potential applications in thermochromics, electrically driven displays and defrosters. The PU-DA TFHHs exhibited high Joule heating temperatures of 102 °C with a low operation voltage (6 V), fast thermal response (150 s) and enhanced robustness to endure large repeated mechanical strain for over 500 bending cycles with a small variance in resistance (<10%). After deliberate damage by a knife cut, the electrodes were healed and recovered to their original conductivity via a simple heat treatment at 120 °C. Uniquely, the healing process can also be triggered by utilising electrical power.

Ambient temperature induced Diels–Alder crosslinked networks based on controlled methacrylate copolymers for enhanced thermoreversibility and self-healability

An effective thermoreversible crosslinked network fabricated at ambient temperature from a new, controlled methacrylate copolymer having reactive maleimide pendants and a trifunctional furan (TFu) exhibiting effective self-healability.