Reprocessable Acid-Degradable Polycarbonate Vitrimers

Disulfide exchange assisted self-healing epoxy/PDMS/graphene oxide nanocomposites

Vitrimers, a class of polymeric networks that change their topology above a threshold temperature, have been investigated in recent years. In order to further extend their properties, in this research, we demonstrate disulfide exchange assisted polydimethylsiloxane (PDMS)- and graphene oxide (GO)-involved epoxy vitrimers, which exhibit a reduction in glass transition temperature and storage modulus with increase in flexural strain and low-temperature self-healing. Stress relaxation and Arrhenius study were carried out for the analysis of vitrimeric behavior, where the prepared epoxy material displays self-healing at 80 °C for 5 min, whereas a low-temperature self-healing (60 °C) was observed for epoxy/PDMS/GO nanocomposites.

Reprocessing Postconsumer Polyurethane Foam Using Carbamate Exchange Catalysis and Twin-Screw Extrusion

Cross-linked polyurethane (PU) is extensively used as thermoset foam; however, methods to directly reprocess PU foam waste derived from commercial sources into similar value materials have not been developed. We demonstrate that introducing dibutyltin dilaurate (DBTDL) into cross-linked PU foams and films enables their reprocessing at elevated temperatures via dynamic carbamate exchange reactions. Both model and commercial cross-linked PU foams were continuously reprocessed using twin-screw extrusion to remove gaseous filler and produce PU filaments or films with elastomeric or rigid thermoset mechanical properties. The properties of microcompounded model PU foam were in excellent agreement with PU film synthesized using the same monomers, indicating that this process occurs efficiently. These findings will enable the bulk reprocessing of commercial thermoset PU waste and inspire the further development of reprocessing methods for other thermosets and the compatibilization of chemically distinct cross-linked materials.

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.

Hydrolysis and Solvolysis as Benign Routes for the End-of-Life Management of Thermoset Polymer Waste

The production of thermoset polymers is increasing globally owing to their advantageous properties, particularly when applied as composite materials. Though these materials are traditionally used in more durable, longer-lasting applications, ultimately, they become waste at the end of their usable lifetimes. Current recycling practices are not applicable to traditional thermoset waste, owing to their network structures and lack of processability. Recently, researchers have been developing thermoset polymers with the right functionalities to be chemically degraded under relatively benign conditions postuse, providing a route to future management of thermoset waste. This review presents thermosets containing hydrolytically or solvolytically cleavable bonds, such as esters and acetals. Hydrolysis and solvolysis mechanisms are discussed, and various factors that influence the degradation rates are examined. Degradable thermosets with impressive mechanical, thermal, and adhesion behavior are discussed, illustrating that the design of material end-of-life need not limit material performance.

Triblock polyester thermoplastic elastomers with semi-aromatic polymer end blocks by ring-opening copolymerization

Thermoplastic elastomers benefit from high elasticity and straightforward (re)processability; they are widely used across a multitude of sectors. Currently, the majority derive from oil, do not degrade or undergo chemical recycling. Here a new series of ABA triblock polyesters are synthesized and show high-performances as degradable thermoplastic elastomers; their composition is poly(cyclohexene-alt-phthalate)-b-poly(ε-decalactone)-b-poly(cyclohexene-alt-phthalate) {PE-PDL-PE}. The synthesis is accomplished using a zinc(ii)/magnesium(ii) catalyst, in a one-pot procedure where ε-decalactone ring-opening polymerization yielding dihydroxyl telechelic poly(ε-decalatone) (PDL, soft-block) occurs first and, then, addition of phthalic anhydride/cyclohexene oxide ring-opening copolymerization delivers semi-aromatic polyester (PE, hard-block) end-blocks. The block compositions are straightforward to control, from the initial monomer stoichiometry, and conversions are high (85-98%). Two series of polyesters are prepared: (1) TBPE-1 to TBPE-5 feature an equivalent hard-block volume fraction (f hard = 0.4) and variable molar masses 40-100 kg mol-1; (2) TBPE-5 to TBPE-9 feature equivalent molar masses (∼100 kg mol-1) and variable hard-block volume fractions (0.12 < f hard < 0.4). Polymers are characterized using spectroscopies, size-exclusion chromatography (SEC), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). They are amorphous, with two glass transition temperatures (∼-51 °C for PDL; +138 °C for PE), and block phase separation is confirmed using small angle X-ray scattering (SAXS). Tensile mechanical performances reveal thermoplastic elastomers (f hard < 0.4 and N > 1300) with linear stress-strain relationships, high ultimate tensile strengths (σ b = 1-5 MPa), very high elongations at break (ε b = 1000-1900%) and excellent elastic recoveries (98%). There is a wide operating temperature range (-51 to +138 °C), an operable processing temperature range (+100 to +200 °C) and excellent thermal stability (T d,5% ∼ 300 °C). The polymers are stable in aqueous environments, at room temperature, but are hydrolyzed upon gentle heating (60 °C) and treatment with an organic acid (para-toluene sulfonic acid) or a common lipase (Novozyme® 51032). The new block polyesters show significant potential as sustainable thermoplastic elastomers with better properties than well-known styrenic block copolymers or polylactide-derived elastomers. The straightforward synthesis allows for other commercially available and/or bio-derived lactones, epoxides and anhydrides to be developed in the future.

Toward Stimuli-Responsive Dynamic Thermosets through Continuous Development and Improvements in Covalent Adaptable Networks (CANs)

Covalent adaptable networks (CANs), unlike typical thermosets or other covalently crosslinked networks, possess a unique, often dormant ability to activate one or more forms of stimuli-responsive, dynamic covalent chemistries as a means to transition their behavior from that of a viscoelastic solid to a material with fluid-like plastic flow. Upon application of a stimulus, such as light or other irradiation, temperature, or even a distinct chemical signal, the CAN responds by transforming to a state of temporal plasticity through activation of either reversible addition or reversible bond exchange, either of which allows the material to essentially re-equilibrate to an altered set of conditions that are distinct from those in which the original covalently crosslinked network is formed, often simultaneously enabling a new and distinct shape, function, and characteristics. As such, CANs span the divide between thermosets and thermoplastics, thus offering unprecedented possibilities for innovation in polymer and materials science. Without attempting to comprehensively review the literature, recent developments in CANs are discussed here with an emphasis on the most effective dynamic chemistries that render these materials to be stimuli responsive, enabling features that make CANs more broadly applicable.

Sequence-Rearranged Cocrystalline Polymer Network with Shape Reconfigurability and Tunable Switching Temperature

Switching temperature (T_sw) is a key parameter governing the service condition of shape memory polymers (SMPs). However, tuning T_sw of SMPs often requires sophisticated synthesis or intricate processing. Herein, we report a simple yet effective strategy to prepare the SMPs with tunable T_sw and good reconfigurability by using the cocrystalline polyesters as the reversible phase. The cocrystallizable copolyesters with rearranged sequences were prepared by the transesterification of mixed polyester diols and then photo-cross-linked to achieve the SMP networks. Cocrystallization of copolymer blocks endows the SMP networks tunable melting point and relatively high crystallinity, affording the network good shape fixing and recovery ability at body temperature. Besides, the dynamic nature of transesterification, that enables the network to have good shape reconfigurability, allows for the easy processing of SMPs with complicated shapes. The reconfigurable SMPs capable of actuating at the body temperature show great potential for use as biomedical devices.

Dynamic Polymer Network System Mediated by Radically Exchangeable Covalent Bond and Carbolong Complex

It is appealing to develop dynamic polymer systems with multifunctionl properties. Herein, we report a polyurethane elastomer with a dynamic covalent polymer network containing a radically exchangeable 2-arylindane-l,3-dione dimer as thermally sensitive and reversible cross-links. In addition, the carbolong complex, an excellent photothermal agent, is incorporated into the dynamic network backbone. With the irradiation of NIR light, the carbolong complex rapidly generates thermal energy, which subsequently triggers the cleavage of the dynamic covalent bond to generate radicals and activate the polyurethane network. In proof-of-concept experiments, we demonstrate that the utility of a combination of radically exchangeable covalent bond and carbolong moiety brings multiple functional characteristics to the polymer network with a capability of spatiotemporal control, including thermochromism, photochromism, rewritability, malleability, and self-healing. This study holds potentials for exploring more tunable dynamics and improved material properties.

Influence of treating parameters on thermomechanical properties of recycled epoxy-acid vitrimers

Vitrimers have the characteristics of shape-reforming and surface-welding, and have the same excellent mechanical properties as thermosets; so vitrimers hold the promise of a broad alternative to traditional plastics. Since their initial introduction in 2011, vitrimers have been applied to many unique applications such as reworkable composites and liquid crystal elastomer actuators. A series of experiments have investigated the effects of reprocessing conditions (such as temperature, time, and pressure) on recycled materials. However, the effect of particle size on the mechanical properties of recycled materials has not been reported. In this paper, we conducted an experimental study on the recovery of epoxy-acid vitrimers of different particle sizes. Epoxy-acid vitrimer powders with different particle size distributions were prepared and characterized. The effects of particle size on the mechanical properties of regenerated epoxy-acid vitrimers were investigated by dynamic mechanical analysis and uniaxial tensile tests. In addition, other processing parameters such as temperature, time, and pressure are discussed, as well as their interaction with particle size. This study helped to refine the vitrimer reprocessing condition parameter toolbox, providing experimental support for the easy and reliable control of the kinetics of the bond exchange reaction.

Recyclable and Dual Cross-Linked High-Performance Polymer with an Amplified Strength-Toughness Combination

Supramolecular chemistry has provided versatile and affordable solutions for the design of tough, flexible polymers. However, application of supramolecular chemistry has been limited to the production of rigid, high-performance polymers due to weak segment mobility. This paper describes a new method of toughening rigid high-performance polymers using the synergistic effect between dual Cu²⁺ -coordination bonds as a crosslink. These dual Cu²⁺ -coordination cross-linked high-performance polymers are a class of rigid polymers with an outstanding combination of strength and toughness. The distinct lifetimes and binding strengths of the dual Cu²⁺ -coordination bonds in a rigid polymer network elicit different dynamic behaviors to improve its energy dissipation and mechanical properties. Moreover, the reformation and removal of Cu²⁺ -coordination bonds by pyrophosphoric acid endows these cross-linked high-performance polymers with recyclability.

Waterborne Methacrylate-Based Vitrimers

We demonstrate waterborne, unimolecularly dissolved vitrimer prepolymer systems that can be transferred into a vitrimer material using catalytic transesterification. The one-component prepolymer system can be processed via film casting and subsequent heat-induced cross-linking. A variation of the density of side chain hydroxy groups over ester and amide groups in the methacrylate/methacrylamide backbone, as well as of the Lewis acid catalyst loading, allow control of the extent of cross-linking and exchange rates. The increase of the amount of both catalyst and hydroxy groups leads to an acceleration of the relaxation times and a decrease of the activation energy of the transesterification reactions. The system features elastomeric properties, and the tensile properties are maintained after two recycling steps. Thus far, vitrimers have been limited largely to hydrophobic polymers; this system is a step forward toward waterborne, one-component materials, and we demonstrate its use in waterborne bioinspired nanocomposites.

Dicarboxylic acid-epoxy vitrimers: influence of the off-stoichiometric acid content on cure reactions and thermo-mechanical properties

Vitrimers with 1 : 1 to 2 : 1 epoxy/acid ratio and TBD show increased stiffness and gradual transition from an exchangeable to non-exchangeable network.

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.

Solvent processable and recyclable covalent adaptable organogels based on dynamic trans-esterification chemistry: separation of toluene from azeotropic mixtures

New covalent adaptable networks (CANs) possessing processability and recyclability to monomers are desirable as an alternative to traditional plastics to address plastic waste-related issues.

A comprehensive review of the structures and properties of ionic polymeric materials

This review focuses on the mechanistic approach, the structure–property relationship and applications of ionic polymeric materials.

Inherently degradable cross-linked polyesters and polycarbonates: resins to be cheerful

We summarise the most recent advances in the synthesis and characterisation of degradable thermosetting polyester and polycarbonates, including partially degradable systems derived from itaconic acid and isosorbide.

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.

Vitrimer-Cellulose Paper Composites: A New Class of Strong, Smart, Green, and Sustainable Materials

Vitrimer, the third category of polymer materials, combines the properties of traditional thermosets and thermoplastics and has gained much interest from industry since the first report in 2011. Currently, many researchers focus on the exploration of new chemistry for novel vitrimer synthesis but pay less attention to the fabrication of vitrimer composites based on known vitrimer systems. The latter can not only largely decrease the cost of vitrimers but also provide a facile way to increase the variety of vitrimer-based materials and extend the applications of vitrimers in different fields. In this study, we developed a new class of vitrimer composite using polycarbonate as a matrix and natural cellulose paper as the reinforcing framework for the first time. The resultant materials possess exceptional mechanical properties and great thermal/chemical stability, simultaneously exhibiting a series of smart properties, such as shape-memory, reshaping, self-healing, and reprocessing. Noteworthily, the two main components of the resultant materials, polycarbonate and natural cellulose, can be easily recycled under mild conditions; thus, these new vitrimer composites qualify as novel green and sustainable materials.

Stress relaxation and thermally adaptable properties in vitrimer-like elastomers from HXNBR rubber with covalent bonds

Widening the scope of skeletons in the chemistry of vitrimer(-like) high molecular weight rubbers, the present study highlights the preparation of vitrimer-like elastomers based on a technically relevant rubber that is characterised by high thermal and oxidation stability. In particular, we prepared covalently crosslinked hydrogenated carboxylated nitrile butadiene rubber (HXNBR) networks that can rearrange their topology due to the exchangeable nature of the crosslinks. By crosslinking with a di-functional epoxide, β-hydroxyl ester linkages are incorporated into the rubber, enabling thermo-activated transesterifications in the presence of the catalyst triazabicyclodecene. At moderate temperatures, the covalent linkages ensure good mechanical properties as well as chemical and thermal stability of the rubber, which is essential for most applications. In addition, bond exchange reactions allow for fast and distinctive stress relaxation at elevated temperatures. Due to the enhanced network mobility above the vitrification transition temperature, the materials exhibit thermally adaptable properties. A comparative study throughout all experiments with catalyst-free samples serving as a reference is made. Shape change experiments reveal a certain malleability of the HXNBR elastomers and improved adhesion properties are shown by means of lap shear tests. In the presence of catalyst, the failure mechanism changes from adhesive to cohesive failure proving the weldability of the material. Furthermore, the samples exhibit thermally triggered repair capabilities as demonstrated by stress-rupture tests. In general, it is shown that already low quantities of exchangeable crosslinks of associative nature impart a promising thermal adaptability into high molecular weight HXNBR rubber.

Vitrimerization: A Novel Concept to Reprocess and Recycle Thermoset Waste via Dynamic Chemistry

A new approach for reprocessing of existing thermoset waste is presented. This work demonstrates that unrecyclable thermoset materials can be reprocessed using the concept of associative dynamic bonding, vitrimers. The developed recycling methodology relies on swelling the thermoset network into a solution of a catalyst, which enables transesterification reactions allowing dynamic bond exchange between ester and hydroxyl groups within the thermoset network. Thermal and mechanical properties for recycled polyurethane and epoxy networks are studied and a strategy to maintain the properties of recycled materials is discussed. The developed methodology promises recycling and even upcycling and reprocessing of previously thought intractable materials. Moreover, processability of vitrimerized thermosets with common thermoplastic manufacturing methods opens up the possibility of tuning recycled networks by adding nanoparticles. This flexibility keeps the application window of recycled thermosets very broad.

Unraveling the Mechanistic Origins of Epoxy Degradation in Acids

Water diffusion into polymers like thermosetting epoxies is well-studied; however, comparably little has been reported thus far on the related but very different mechanism of acid diffusion and the corresponding influence on material degradation. The diffusion of hydrochloric acid into an amine-cured epoxy system was studied in this work using gravimetric analysis and dielectric monitoring concurrently, and the mass uptake behavior was observed to differ significantly compared with water diffusion, faster by an order of magnitude. A unique 3-stage diffusion of acid into epoxy was observed due to the influence of Coulombic interactions between oppositely charged ionic species diffusing at different rates. Material characterization studies have revealed that the dominant degradation mechanism is physical in nature, with the formation of surface cracks driven by the swelling stresses due to the core-shell swelling behavior in highly concentrated hydrochloric acid, leading to an erosion-type degradation phenomenon. The insights gained from understanding acid electrolyte diffusion could serve to design a more effective and efficient process to enable thermoset recycling by facilitating rapid material breakdown or the design of acid-resistant materials for various applications in chemical storage tanks, batteries, and protective coatings in a corrosive environment.