Vitrimeric Silicone Elastomers Enabled by Dynamic Meldrum's Acid-Derived Cross-Links

Stretchable Electronics Based on PDMS Substrates

Stretchable electronics, which can retain their functions under stretching, have attracted great interest in recent decades. Elastic substrates, which bear the applied strain and regulate the strain distribution in circuits, are indispensable components in stretchable electronics. Moreover, the self-healing property of the substrate is a premise to endow stretchable electronics with the same characteristics, so the device may recover from failure resulting from large and frequent deformations. Therefore, the properties of the elastic substrate are crucial to the overall performance of stretchable devices. Poly(dimethylsiloxane) (PDMS) is widely used as the substrate material for stretchable electronics, not only because of its advantages, which include stable chemical properties, good thermal stability, transparency, and biological compatibility, but also because of its capability of attaining designer functionalities via surface modification and bulk property tailoring. Herein, the strategies for fabricating stretchable electronics on PDMS substrates are summarized, and the influence of the physical and chemical properties of PDMS, including surface chemical status, physical modulus, geometric structures, and self-healing properties, on the performance of stretchable electronics is discussed. Finally, the challenges and future opportunities of stretchable electronics based on PDMS substrates are considered.

Surface Modification of (Non)-Fluorinated Vitrimers through Dynamic Transamination

Surface modifications are typically permanent in shape and chemistry. Herein, vinylogous urethane (VU) chemistry is presented as an easily accessible and versatile platform for rapid, facile, and reworkable surface modification. It is demonstrated that both physical and chemical post-modification of permanent, yet dynamic elastic polymer networks are achieved. Surface patterns with high regularity are created, both via a straightforward replication process using a polydimethylsiloxane stamp (resolution ca. 10-100 µm) as well as using thermally activated nano-imprint lithography (NIL) to form hole, pillar, or line patterns (ca. 300 nm) in elastic VU-based vitrimers. The tunable, rapid exchange allows patterning at 130 °C in less than 15 min, resulting in an increased water contact angle and surface-structure induced light reflection. Moreover, it is also demonstrated that the use of a single dynamic covalent chemistry makes it possible to strongly adhere to fluorinated and non-fluorinated materials based on incompatible matrices, causing cohesive failure in a peel test. In a topography scan, the visibly transparent interface is shown to possess a continuous phase without a gap, while maintaining distinctively separated (non)-fluorinated domains. Finally, this approach allowed for a straightforward coating of a non-fluorinated material with a fluorinated monomer to minimize the overall fluorinated content.

Facile Multicomponent Polymerization and Postpolymerization Modification via an Effective Meldrum's Acid-Based Three-Component Reaction

Providing access to highly diverse polymer structures by multicomponent reactions is highly desirable; efficient Meldrum's acid-based multicomponent reactions, however, have been rarely highlighted in polymer chemistry. Here, the three-component reaction of Meldrum's acid, indole, and aldehyde is introduced into polymer synthesis. Direct multicomponent polymerization of Meldrum's acid, dialdehyde, and diindole can perform under mild conditions, resulting in complex Meldrum's acid-containing polymers with well-defined structures, and high molecular weights. Additionally, nearly quantitative postpolymerization modification can also perform via this Meldrum's acid-based multicomponent reaction. These results indicate that Meldrum's acid-based multicomponent reaction will be a potential tool to prepare novel polymers.

Reprocessable, Reworkable, and Mechanochromic Polyhexahydrotriazine Thermoset with Multiple Stimulus Responsiveness

Developing recyclable, reworkable, and intelligent thermosetting polymers, as a long-standing challenge, is highly desirable for modern manufacturing industries. Herein, we report a polyhexahydrotriazine thermoset (PHT) prepared by a one-pot polycondensation between 4-aminophenyl disulfide and paraformaldehyde. The PHT has a glass transition temperature of 135 °C and good solvent resistance. The incorporation of dual stimuli-responsive groups (disulfide bond and hexahydrotriazine ring) endows the PHT with re-processability, re-workability, and damage monitoring function. The PHT can be repeatedly reprocessed by hot pressing, and a near 100% recovery of flexural strength is achieved. The PHT can also degrade in inorganic acid or organic thiol solutions at room temperature. The thermally reworkable test demonstrates that, after heating the PHT at 200 °C for 1 h, the residuals can be easily wiped off. Finally, the PHT exhibits a reversible mechanochromic behavior when damaged.

Reprocessable Cross-Linked Polymer Networks: Are Associative Exchange Mechanisms Desirable?

Covalent adaptable networks (CANs) are covalently cross-linked polymers that may be reshaped via cross-linking and/or strand exchange at elevated temperatures. They represent an exciting and rapidly developing frontier in polymer science for their potential as stimuli-responsive materials and to make traditionally nonrecyclable thermosets more sustainable. CANs whose cross-links undergo exchange via associative intermediates rather than dissociating to separate reactive groups are termed vitrimers. Vitrimers were postulated to be an attractive subset of CANs, because associative cross-link exchange mechanisms maintain the original cross-link density of the network throughout the exchange process. As a result, associative CANs demonstrate a gradual, Arrhenius-like reduction in viscosity at elevated temperatures while maintaining mechanical integrity. In contrast, CANs reprocessed by dissociation and reformation of cross-links have been postulated to exhibit a more rapid decrease in viscosity with increasing temperature. Here, we survey the stress relaxation behavior of all dissociative CANs for which variable temperature stress relaxation or viscosity data are reported to date. All exhibit an Arrhenius relationship between temperature and viscosity, as only a small percentage of the cross-links are broken instantaneously under typical reprocessing conditions. As such, dissociative and associative CANs show nearly identical reprocessing behavior over broad temperature ranges typically used for reprocessing. Given that the term vitrimer was coined to highlight an Arrhenius relationship between viscosity and temperature, in analogy to vitreous glasses, we discourage its continued use to describe associative CANs. The realization that the cross-link exchange mechanism does not greatly influence the practical reprocessing behavior of most CANs suggests that exchange chemistries can be considered with fewer constraints, focusing instead on their activation parameters, synthetic convenience, and application-specific considerations.

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

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

Implantation of Recyclability and Healability into Cross-Linked Commercial Polymers by Applying the Vitrimer Concept

Vitrimers are a new class of cross-linked materials that are capable of network topology alternation through the associative dynamic bond-exchange mechanism, which has recently been invented to solve the problem of conventional cross-linked materials, such as poor recyclability and healability. Thus far, the concept of vitrimers has been applied to various commercial polymers, e.g., polyesters, polylactides, polycarbonates, polydimethylsiloxanes, polydienes, polyurethanes, polyolefins, poly(meth)acrylates, and polystyrenes, by utilizing different compatible bond-exchange reactions. In this review article, the concept of vitrimers is described by clarifying the difference from thermoplastics and supramolecular systems; in addition, the term "associative bond-exchange" in vitrimers is explained by comparison with the "dissociative" term. Several useful functions attained by the vitrimer concept (including recyclability and healability) are demonstrated, and recent molecular designs of vitrimers are classified into groups depending on the types of molecular frameworks. This review specifically focuses on the vitrimer molecular designs with commercial polymer-based frameworks, which provide useful hints for the practical application of the vitrimer concept.

Rates of transesterification in epoxy-thiol vitrimers

Vitrimers, an important subset of dynamically crosslinked polymer networks, have many technological applications for their excellent properties, and the ability to be re-processed through plastic flow above the so-called vitrification temperature. We report a simple and efficient method of generating such adaptive crosslinked networks relying on transesterification for their bond exchange by utilising the 'click' chemistry of epoxy and thiols, which also has the advantage of a low glass transition temperature. We vary the chemical structure of thiol spacers to probe the effects of concentration and the local environment of ester groups on the macroscopic elastic-plastic transition. The thermal activation energy of transesterification bond exchange is determined for each chemical structure, and for a varying concentration of catalyst, establishing the conditions for the optimal, and for the suppressed bond exchange. However, we also discover that the temperature of elastic-plastic transition is strongly affected by the stiffness (dynamic rubber modulus) of the network, with softer networks having a much lower vitrification temperature even when their bond-exchange activation energy is higher. This combination of chemical and physical control factors should help optimise the processability of vitrimer plastics.

Dynamic reaction-induced phase separation in tunable, adaptive covalent networks

A series of catalyst-free, room temperature dynamic bonds derived from a reversible thia-Michael reaction are utilized to access mechanically robust dynamic covalent network films. The equilibrium of the thiol addition to benzalcyanoacetate-based Michael-acceptors can be directly tuned by controlling the electron-donating/withdrawing nature of the Michael-acceptor. By modulating the composition of different Michael-acceptors in a dynamic covalent network, a wide range of mechanical properties and thermal responses can be realized. Additionally, the reported systems phase-separate in a process, coined dynamic reaction-induced phase separation (DRIPS), that yields reconfigurable phase morphologies and reprogrammable shape-memory behaviour as highlighted by the heat-induced folding of a predetermined structure.

Siloxane crosslinks with dynamic bond exchange enable shape programming in liquid-crystalline elastomers

Liquid crystalline elastomers (LCE) undergo reversible shape changes in response to stimuli, which enables a wide range of smart applications, in soft robotics, adhesive systems or biomedical medical devices. In this study, we introduce a new dynamic covalent chemistry based on siloxane equilibrium exchange into the LCE to enable processing (director alignment, remolding, and welding). Unlike the traditional siloxane based LCE, which were produced by reaction schemes with irreversible bonds (e.g. hydrosilylation), here we use a much more robust reaction (thiol-acrylate/thiol-ene 'double-click' chemistry) to obtain highly uniform dynamically crosslinked networks. Combining the siloxane crosslinker with click chemistry produces exchangeable LCE (xLCE) with tunable properties, low glass transition (-30 °C), controllable nematic to isotropic transition (33 to 70 °C), and a very high vitrification temperature (up to 250 °C). Accordingly, this class of dynamically crosslinked xLCE shows unprecedented thermal stability within the working temperature range (-50 to 140 °C), over many thermal actuation cycles without any creep. Finally, multiple xLCE sharing the same siloxane exchangeable bonds can be welded into single continuous structures to allow for composite materials that sequentially and reversibly undergo multiple phase transformations in different sections of the sample.

Dynamic Bottlebrush Polymer Networks: Self-Healing in Super-Soft Materials

We introduce a design strategy to expand the range of accessible mechanical properties in covalent adaptable networks (CANs) using bottlebrush polymer building blocks. Well-defined bottlebrush polymers with rubbery poly(4-methylcaprolactone) side chains were cross-linked in formulations that include a bislactone and strong Lewis acid (tin ethylhexanoate). The resulting materials exhibit tunable stress-relaxation rates at elevated temperatures (160-180 °C) due to dynamic ester cross-links that undergo transesterification with residual hydroxy groups. Varying the cross-linker loading or bottlebrush backbone degree of polymerization yields predictable low-frequency shear moduli ca. 10-100 kPa, well below values typical of linear polymer CANs (1 MPa). These extensible networks can be stretched to strains as large as 350% before failure and undergo efficient self-healing to recover >85% of their original toughness upon repeated fracture and melt processing. In summary, molecular architecture creates new opportunities to tailor the mechanical properties of CANs in ways that are otherwise difficult to achieve.

Highly Stable Meldrum's Acid Derivatives for Irreversible Aqueous Covalent Modification of Amines

This work describes the development of a highly stable and pH-responsive probe for lysine modification. The scaffold has marked stability in the presence of several biological nucleophiles and across a wide pH range (2-12). Several functional analogs showed robust labeling of a protein at pH > 9. Taken together, our system displays versatility and can be easily adapted for variety of applications, while demonstrating stability suitable for a wide range of biologically compatible systems.

Chemically Triggered Synthesis, Remodeling, and Degradation of Soft Materials

Polymer topology dictates dynamic and mechanical properties of materials. For most polymers, topology is a static characteristic. In this article, we present a strategy to chemically trigger dynamic topology changes in polymers in response to a specific chemical stimulus. Starting with a dimerized PEG and hydrophobic linear materials, a lightly cross-linked polymer, and a cross-linked hydrogel, transformations into an amphiphilic linear polymer, lightly cross-linked and linear random copolymers, a cross-linked polymer, and three different hydrogel matrices were achieved via two controllable cross-linking reactions: reversible conjugate additions and thiol-disulfide exchange. Significantly, all the polymers, before or after topological changes, can be triggered to degrade into thiol- or amine-terminated small molecules. The controllable transformations of polymeric morphologies and their degradation herald a new generation of smart materials.

Thermoresponsive self-healable and recyclable polymer networks based on a dynamic quinone methide–thiol chemistry

A new type of thermoresponsive dynamic covalent polymer network was developed with excellent self-healable and recyclable properties based on a new thermoresponsive dynamic covalent chemistry between a para-quinone methide and thiol nucleophiles.

Cross-linker control of vitrimer flow

The rate of stress relaxation in a vitrimer can be modulated by changing solely the structure of the cross-linker electrophile.

Structural tuning of polycaprolactone based thermadapt shape memory polymer

Polycaprolactone based thermadapt shape memory polymers with precisely controlled structures allow tunable shape reconfigurability.

Functionalization of polyisoprene and polystyrene via reactive processing using azidoformate grafting agents, and its application to the synthesis of dioxaborolane-based polyisoprene vitrimers

Azidoformates carrying dioxaborolane functions were used to functionalize PI and PS, and to prepare PI vitrimers by reactive processing.

Mechanically robust and reprocessable imine exchange networks from modular polyester pre-polymers

Covalent adaptable networks (CANs) containing dynamic imine cross-links impart recyclability to thermoset materials, and the distribution of these cross-links greatly affects their observed thermomechanical properties.

Influence of the polymer matrix on the viscoelastic behaviour of vitrimers

Vitrimers, an emerging field of research, in which still many fundamental aspects of material design remain to be explored. Here, we systematically explore the effect of the choice of the matrix on a dynamic exchange reaction in a polymer network.

Block Copolymer Vitrimers

In this report, we merge block copolymers with vitrimers in an effort to realize the prospect of higher-order, nanoscale control over associative cross-link exchange and flow. We show the use of controlled polymerization as a vital tool to understand fundamental structure-property effects through the precise control of polymer architecture and molecular weight. Vitrimers derived from self-assembling block copolymers exhibit superior resistance to macroscopic deformation in comparison to their analogs generated from statistical copolymers. Our results suggest that the enhanced creep resistance achieved by control over chain topology in block vitrimers can be used to tune viscoelastic properties. The resistance to macroscopic deformation that arises from a microphase-separated structure in this new class of materials differentiates block vitrimers from their statistical counterparts and introduces the potential of topology-control over viscoelastic flow.

Catalyst-Free Dynamic Networks for Recyclable, Self-Healing Solid Polymer Electrolytes

Polymer networks with dynamic covalent cross-links act as solids but can flow at high temperatures. They have been widely explored as reprocessable and self-healing materials, but their use as solid electrolytes is limited. Here we report poly(ethylene oxide)-based networks with varying amounts of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to understand the impact of a salt on the ion transport and network dynamics. We observed that the conductivity of our dynamic networks reached a maximum of 3.5 × 10^-4 S/cm at an optimal LiTFSI concentration. Rheological measurements showed that the amount of LiTFSI significantly affects the mechanical properties, as the shear modulus varies between 1 and 10 MPa and the stress relaxation by 2 orders of magnitude. Additionally, we found that these networks can efficiently dissolve back to pure monomers and heal to recover their conductivity after damage, showing the potential of dynamic networks as sustainable solid electrolytes.

Adaptable Crosslinks in Polymeric Materials: Resolving the Intersection of Thermoplastics and Thermosets

The classical division of polymeric materials into thermoplastics and thermosets based on covalent network structure often implies that these categories are distinct and irreconcilable. Yet, the past two decades have seen extensive development of materials that bridge this gap through incorporation of dynamic crosslinks, enabling them to behave as both robust networks and moldable plastics. Although their potential utility is significant, the growth of covalent adaptable networks (CANs) has obscured the line between "thermoplastic" and "thermoset" and erected a conceptual barrier to the growing number of new researchers entering this discipline. This Perspective aims to both outline the fundamental theory of CANs and provide a critical assessment of their current status. We emphasize throughout that the unique properties of CANs emerge from the network chemistry, and particularly highlight the role that the crosslink exchange mechanism (i.e., dissociative exchange or associative exchange) plays in the resultant material properties under processing conditions. Predominant focus will be on thermally induced dynamic behavior, as the majority of presently employed exchange chemistries rely on thermal stimulus, and it is simple to apply to bulk materials. Lastly, this Perspective aims to identify current issues and address possible solutions for better fundamental understanding within this field.

Detecting topology freezing transition temperature of vitrimers by AIE luminogens

Vitrimers are one kind of covalently crosslinked polymers that can be reprocessed. Topology freezing transition temperature (Tv) is vitrimer's upper limit temperature for service and lower temperature for recycle. However, there has been no proper method to detect the intrinsic Tv till now. Even worse, current testing methods may lead to a misunderstanding of vitrimers. Here we provide a sensitive and universal method by doping or swelling aggregation-induced-emission (AIE) luminogens into vitrimers. The fluorescence of AIE-luminogens changes dramatically below and over Tv, providing an accurate method to measure Tv without the interference of external force. Moreover, according to this method, Tv is independent of catalyst loading. The opposite idea has been kept for a long time. This method not only is helpful for the practical application of vitrimers so as to reduce white wastes, but also may facilitate deep understanding of vitrimers and further development of functional polymer materials.

Anilinium Salts in Polymer Networks for Materials with Mechanical Stability and Mild Thermally Induced Dynamic Properties

Dynamic nucleophilic exchange of quaternary anilinium salts has been incorporated into rehealable and malleable polymeric materials that can be activated under mild (60 °C) thermal stimulus. The mechanism of dynamic exchange between quaternary anilinium salt and free aniline was assessed in small-molecule model experiments. The dynamic exchange was found to be dissociative in nature, due to the indirect S_N2 mechanism, where initially the bromide anion attacks the anilinium salt to generate an alkyl bromide which undergoes subsequent attack by a free aniline group. A quaternary anilinium-based cross-linker was synthesized to act as dynamic linkages in the polymer network. Cross-linked polymeric materials showed thermoresponsive rehealing and malleability properties at 60 °C along with being resistant to irreversible creep under ambient conditions. The use of anilinium salts enables dynamic exchange to occur with significantly milder thermal stimulus than other comparable materials, while maintaining mechanical stability.

Reprocessable Polyhydroxyurethane Network Composites: Effect of Filler Surface Functionality on Cross-link Density Recovery and Stress Relaxation

Conventional polymer network composites cannot be recycled for high-value applications because of the presence of permanent covalent cross-links. We have developed reprocessable polyhydroxyurethane network nanocomposites using silica nanoparticles with different surface functionalities as reinforcing fillers. The property recovery after reprocessing is a function of the interaction between the filler surface and the network matrix during the network rearrangement process. When nonreactive silica nanoparticles lacking significant levels of surface functional groups are used at 4 wt % (2 vol %) loading, the resulting network composite exhibits substantial enhancement in mechanical properties relative to the neat network and based on values of rubbery plateau modulus is able to fully recover its cross-link density after a reprocessing step. When nanoparticles have surface functional groups that can participate in dynamic chemistries with the reprocessable network matrix, reprocessing leads to losses in mechanical properties associated with cross-link density at potential use temperatures, along with faster rates and lower apparent activation energies of stress relaxation at elevated temperature. This work reveals the importance of appropriate filler selection when polymer network composites are designed with dynamic covalent bonds to achieve both mechanical reinforcement and excellent reprocessability, which are needed for the development of recyclable polymer network composites for advanced applications.

Transformation of polyethylene into a vitrimer by nitroxide radical coupling of a bis-dioxaborolane

This work reports the synthesis and characterization of HDPE vitrimers obtained via reactive extrusion in the presence of bis-nitroxide dioxaborolanes.