Catalyst-Free Mechanochemical Recycling of Biobased Epoxy with Cellulose Nanocrystals

Catalyst-free readily dual-recyclable acetal-based covalent adaptable cellulose networks

Despite covalent adaptable networks (CANs) imparting the favorable features of crosslinked polymers, as well as the functionality of reprocessing, reshaping and welding, due to exchange reaction enabled topology changes; it is still a huge challenge to design catalyst-free, fast reprocessing, controlled degradation and polymer recyclable biomass base CANs. Herein, for the first time, acetal-based covalent adaptable cellulose networks (ACCs) were utilized to synthesize readily reconstructable cellulose-based thermosets with mechanical tunability. ACCs were synthesized via catalyst-free "click" addition of cellulose and divinyl ether without releasing small molecule byproducts. Different crosslinking densities and crosslinkers were used to explore the structure-property relationship, the mechanical and thermal properties of the ACCs were strongly influenced by these factors. ACCs can obtain enhanced tensile strength or elongation at break by changing the structure of the crosslinker. Furthermore, the reworking, welding and shape memory properties of these ACCs, based on the dynamic exchange reaction of acetal bonds, were investigated. In addition, these ACCs can be degraded under acidic conditions, and closed-loop utilization of polymer was possible. Thus, ACCs can be mechanically and chemically double-cycled, which will contribute to solving the white pollution problem and resource waste as a new class of sustainable plastics.

Interfacial Compatibility of Core-Shell Cellulose Nanocrystals for Improving Dynamic Covalent Adaptable Networks' Fracture Resistance in Nanohybrid Vitrimer Composites

The development of polymeric nanocomposites with dynamic covalent adaptable networks and biobased nanomaterials has been a promising approach toward sustainable advanced materials, enabling reprogramming and recycling capabilities. Herein, a core-shell nanohybrid of functionalized cellulose nanocrystals (CNCs) is explored to provide crucial interfacial compatibility for improving the covalent adaptable networks of epoxy-thiol vitrimers in fracture resistance. The poly(ε-caprolactone) (PCL) shells grafted from CNC surfaces can be cross-linked with the covalent adaptable networks via a hot-pressing transesterification process. According to the additive concentration and annealing temperature, the stress relaxation behavior of nanohybrid vitrimer composites can be effectively regulated by the core-shell PCL-grafted CNC (CNC-PCL) nanohybrids from a dispersed to cross-linked interaction. The addition of 15 wt % of the core-shell CNC-PCLs exhibits the reinforced improvement of nanohybrid vitrimer composites in the average Young's modulus of 2.5×, fracture stress of 5.4×, and fracture strain of 2.0×. The research findings might have profound implications for developing synergistic interfacial compatibility between dynamic vitrimer networks and functional nanoparticles for advanced polymeric nanocomposites.

Fabrication of well-dispersed cellulose nanocrystal reinforced biobased epoxy composites using reversibility of covalent adaptable network

Cellulose nanocrystal (CNC) shows great potential in reinforced composites but it is difficult to disperse in epoxy thermosets due to its poor dispersity in epoxy monomers. Herein, we reported a novel approach to disperse CNC in epoxidized soybean oil (ESO)-derived epoxy thermosets uniformly by using the reversibility of dynamic imine-containing ESO-derived covalent adaptable network (CAN). The crosslinked CAN was deconstructed by an exchange reaction with ethylenediamine (EDA) in dimethyl formamide (DMF), leading to a solution of deconstructed CAN with plenty of hydroxyl and amino groups, which could form strong hydrogen bonds with hydroxyl groups of CNC and thus facilitated and stabilized dispersion of CNC in the deconstructed CAN solution. Epoxy composite with well-dispersed CNC was finally achieved by a reformation of CAN through the removal of DMF and EDA. In this way, the epoxy composites with CNC content up to 30 wt% were successfully prepared and showed drastically reinforced mechanical properties. The tensile strength and Young's modulus of the CAN were improved by up to ~70 % and ~45 times with the incorporation of 20 and 30 wt% CNC, respectively. The composites showed excellent reprocessability without significant loss in mechanical properties after reprocessing.

Upcycling of thermosetting polymers into high-value materials

Thermosetting polymers, a large class of polymers featuring excellent properties, have been widely used and play an irreplaceable role in our life. Nevertheless, they are arduous to be recycled or reused on account of their permanently cross-linked networks, and the main recycling approaches used currently include energy recovery through incineration, utilization as fillers after mechanical grinding, and pyrolysis, which only reclaim a small fraction or partial value of thermosetting polymers and their downstream materials. In this minireview, we provide an overview of the efforts undertaken towards upcycling thermosetting polymers in recent years. The research progress on physical upcycling, carbonization, solvolysis and vitrimerization of thermoset waste to high-value materials, including oil-water separation materials, 3D printable materials, functional carbon materials (supercapacitors, photothermal conversion materials, and catalytic materials), additives, emulsifiers, biolubricants, and vitrimers, are summarized and discussed. Perspectives on the future development of the art of upcycling thermosets are also provided.

Self-Healing of Polymers and Polymer Composites

This review is devoted to the description of methods for the self-healing of polymers, polymer composites, and coatings. The self-healing of damages that occur during the operation of the corresponding structures makes it possible to extend the service life of the latter, and in this case, the problem of saving non-renewable resources is simultaneously solved. Two strategies are considered: (a) creating reversible crosslinks in the thermoplastic and (b) introducing a healing agent into cracks. Bond exchange reactions in network polymers (a) proceed as a dissociative process, in which crosslinks are split into their constituent reactive fragments with subsequent regeneration, or as an associative process, the limiting stage of which is the interaction of the reactive end group and the crosslink. The latter process is implemented in vitrimers. Strategy (b) is associated with the use of containers (hollow glass fibers, capsules, microvessels) that burst under the action of a crack. Particular attention is paid to self-healing processes in metallopolymer systems.

Vitrimerization of Crosslinked Unsaturated Polyester Resins: A Mechanochemical Approach to Recycle and Reprocess Thermosets

Unsaturated polyester resins (UPRs) are expansively used in different applications and recycling the significant amounts of UPR waste is still a universal problem. Vitrimerization is a feasible, environmental-friendly, cost effective, and operative method, which can be used for recycling the crosslinked UPRs. In this method, the thermoset permanent network is changed into a dynamic network similar to the vitrimer-type polymers. The results show that the existence of a transesterification catalyst in the system significantly enhances the efficiency of vitrimerization. The vitrimerized UPR thermosets can be reprocessed three times with mechanical properties comparable to the initial UPR. The results show that the excess of external hydroxyl groups in the system can prevent the formation of zinc ligand complexes in the network and consequently reduce the crosslinked density and mechanical properties of vitrimerized samples. The vitrimerized thermoset powder can be reprocessed through injection molding, extrusion, and compression molding which are conventional thermoplastic processing techniques. The unrecyclable UPR thermoset wastes can be recycled and reused through vitrimerization with the least loss in mechanical properties.

Simple and Rapid Mechanochemical Synthesis of Lactide and 3S-(Isobutyl)morpholine-2,5-dione-Based Random Copolymers Using DBU and Thiourea

There is a growing interest surrounding morpholine-2,5-dione-based materials due to their impressive biocompatibility as well as their capacity to break down by hydrolytic and enzymatic pathways. In this study, the ring-opening (co)polymerization of leucine-derived 3S-(isobutyl)morpholine-2,5-dione (MD) and lactide (LA) was performed via ball-milling using a catalytic system composed of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 3-[3,5-bis(trifluoromethyl)phenyl]-1-cyclohexylthiourea (TU). Once the homopolymerizations of MD and LA optimized and numerous parameters were studied, the mechanochemical ring-opening copolymerization of these monomers was explored. The feasibility of ring-opening copolymerizations in mechanochemical systems was demonstrated and a range of P(MD-co-LA) copolymers were produced with varying proportions of MD (23%, 48%, and 69%). Furthermore, the beneficial cocatalytic effects of TU with regards to ROP control were found to be operative within mechanochemical systems. Further parallels were observed between solution- and mechanochemical-based ROPs.

Unmodified Silica Nanoparticles Enhance Mechanical Properties and Welding Ability of Epoxy Thermosets with Tunable Vitrimer Matrix

Epoxy/silica thermosets with tunable matrix (vitrimers) were prepared by thermal curing of diglycidyl ether of bisphenol A (DGEBA) in the presence of a hardener-4-methylhexahydrophthalic anhydride (MHHPA), a transesterification catalyst-zinc acetylacetonate (ZAA), and 10-15 nm spherical silica nanoparticles. The properties of the resulting material were studied by tensile testing, thermomechanical and dynamic mechanical analysis. It is shown that at room temperature the introduction of 5-10 wt% of silica nanoparticles in the vitrimer matrix strengthens the material leading to the increase of the elastic modulus by 44% and the tensile stress by 25%. Simultaneously, nanoparticles enhance the dimensional stability of the material since they reduce the coefficient of thermal expansion. At the same time, the transesterification catalyst provides the thermoset with the welding ability at heating, when the chain exchange reactions are accelerated. For the first time, it was shown that the silica nanoparticles strengthen welding joints in vitrimers, which is extremely important, since it allows to repeatedly use products made of thermosets and heal defects in them. Such materials hold great promise for use in durable protective coatings, adhesives, sealants and many other applications.