Dissociative Carbamate Exchange Anneals 3D Printed Acrylates.

Relative to other additive manufacturing modalities, vat photopolymerization (VP) offers designers superior surface finish, feature resolution, and throughput. However, poor interlayer network formation can limit a VP-printed part's tensile strength along the build axis. We demonstrate that the incorporation of carbamate bonds capable of undergoing dissociative exchange reactions provides improved interlayer network formation in VP-printed urethane acrylate polymers. In the presence of dibutyltin dilaurate catalyst, the exchange of these carbamate bonds enables rapid stress relaxation with an activation energy of 133 kJ/mol, consistent with a dissociative bond exchange process. Annealed XY tensile samples containing a catalyst demonstrate a 25% decrease in Young's modulus, attributed to statistical changes in network topology, while samples without a catalyst show no observable effect. Annealed ZX tensile samples printed with layers perpendicular to tensile load demonstrate an increase in elongation at break, indicative of self-healing. The strain at break for samples containing a catalyst increases from 33.9 to 56.0% after annealing but decreases from 48.1 to 32.1% after annealing in samples without a catalyst. This thermally activated bond exchange process improves the performance of VP-printed materials via self-healing across layers and provides a means to change Young's modulus after printing. Thus, the incorporation of carbamate bonds and appropriate catalysts in the VP-printing process provides a robust platform for enhancing material properties and performance.

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.

Mechanistic Study of Stress Relaxation in Urethane-Containing Polymer Networks.

Cross-linked polymers are used in many commercial products and are traditionally incapable of recycling via melt reprocessing. Recently, tough and reprocessable cross-linked polymers have been realized by incorporating cross-links that undergo associative exchange reactions, such as transesterification, at elevated temperatures. Here we investigate how cross-linked polymers containing urethane linkages relax stress under similar conditions, which enables their reprocessing. Materials based on hydroxyl-terminated star-shaped poly(ethylene oxide) and poly((±)-lactide) were cross-linked with methylene diphenyldiisocyanate in the presence of stannous octoate catalyst. Polymers with lower plateau moduli exhibit faster rates of relaxation. Reactions of model urethanes suggest that exchange occurs through the tin-mediated exchange of the urethanes that does not require free hydroxyl groups. Furthermore, samples were incapable of elevated-temperature dissolution in a low-polarity solvent (1,2,4-trichlorobenzene) but readily dissolved in a high-polarity aprotic solvent (DMSO, 24 to 48 h). These findings indicate that urethane linkages, which are straightforward to incorporate, impart dynamic character to polymer networks of diverse chemical composition, likely through a urethane reversion mechanism.

Rapidly Reprocessable Cross-Linked Polyhydroxyurethanes Based on Disulfide Exchange.

Polymer networks that are cross-linked by dynamic covalent bonds often sacrifice the robust mechanical properties of traditional thermosets in exchange for rapid and efficient reprocessability. Polyurethanes are attractive materials for reprocessable cross-linked polymers because of their excellent mechanical properties, widespread use, and ease of synthesis, but their syntheses typically rely on harmful isocyanate precursors. Polyhydroxyurethanes (PHUs), derived from amines and cyclic carbonates, are promising alternatives to traditional polyurethanes. PHU networks are reprocessable via transcarbamoylation reactions even in the absence of external catalysts, but this process occurs over hours at temperatures above 150 °C. We have dramatically shortened the reprocessing times of PHU networks by incorporating dynamic disulfide bonds. Using cystamine as a comonomer gives materials with similar thermal stability and mechanical properties to other rigid cross-linked PHUs. Despite their excellent mechanical properties, these materials show rapid stress relaxation and have characteristic relaxation times as low as 30 s at 150 °C. This property enables reprocessing with quantitative recovery of cross-link density as measured by DMTA after only 30 min of elevated-temperature compression molding. Disulfide incorporation is a promising approach to obtain reprocessable, cross-linked PHU resins that are not derived from isocyanates.

Mechanically activated, catalyst-free polyhydroxyurethane vitrimers.

Vitrimers are polymer networks whose cross-links undergo associative exchange processes at elevated temperature, usually in the presence of an embedded catalyst. This design feature enables the reshaping of materials with mechanical properties similar to thermoset resins. Here we report a new class of vitrimers consisting of polyhydroxyurethanes (PHUs) derived from six-membered cyclic carbonates and amines. PHU networks relax stress and may be reprocessed at elevated temperature and pressure in the absence of an external catalyst. The as-synthesized networks exhibit tensile properties comparable to those of leading thermosets and recover ca. 75% of their as-synthesized values following reprocessing. Stress relaxation occurs through an associative process involving nucleophilic addition of free hydroxyl groups to the carbamate linkages and exhibits an Arrhenius activation energy of 111 ± 10 kJ/mol, which is lower than that observed for molecular model compounds (148 ± 7 kJ/mol). These findings suggest that transcarbamoylation is activated by mechanical stress, which we attribute, on the basis of DFT calculations, to the twisting of N lone pairs out of conjugation with the carbonyl π orbitals. PHU vitrimers are a promising new class of repairable networks because of their outstanding mechanical properties, avoidance of toxic isocyanate monomers, and catalyst-free repair processes.

Cation-Dependent Stabilization of Electrogenerated Naphthalene Diimide Dianions in Porous Polymer Thin Films and Their Application to Electrical Energy Storage.

Porous polymer networks (PPNs) are attractive materials for capacitive energy storage because they offer high surface areas for increased double-layer capacitance, open structures for rapid ion transport, and redox-active moieties that enable faradaic (pseudocapacitive) energy storage. Here we demonstrate a new attractive feature of PPNs--the ability of their reduced forms (radical anions and dianions) to interact with small radii cations through synergistic interactions arising from densely packed redox-active groups, only when prepared as thin films. When naphthalene diimides (NDIs) are incorporated into PPN films, the carbonyl groups of adjacent, electrochemically generated, NDI radical anions and dianions bind strongly to K(+), Li(+), and Mg(2+), shifting the formal potentials of NDI's second reduction by 120 and 460 mV for K(+) and Li(+)-based electrolytes, respectively. In the case of Mg(2+), NDI's two redox waves coalesce into a single two-electron process with shifts of 240 and 710 mV, for the first and second reductions, respectively, increasing the energy density by over 20 % without changing the polymer backbone. In contrast, the formal reduction potentials of NDI derivatives in solution are identical for each electrolyte, and this effect has not been reported for NDI previously. This study illustrates the profound influence of the solid-state structure of a polymer on its electrochemical response, which does not simply reflect the solution-phase redox behavior of its monomers.