Hybrid polyhydroxyurethanes: How to overcome limitations and reach cutting edge properties?

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.

Polymeric foams can be found virtually everywhere due to their advantageous properties compared with counterparts materials. Possibly the most important class of polymeric foams are polyurethane foams (PUFs), as their low density and thermal conductivity combined with their interesting mechanical properties make them excellent thermal and sound insulators, as well as structural and comfort materials. Despite the broad range of applications, the production of PUFs is still highly petroleum-dependent, so this industry must adapt to ever more strict regulations and rigorous consumers. In that sense, the well-established raw materials and process technologies can face a turning point in the near future, due to the need of using renewable raw materials and new process technologies, such as three-dimensional (3D) printing. In this work, the fundamental aspects of the production of PUFs are reviewed, the new challenges that the PUFs industry are expected to confront regarding process methodologies in the near future are outlined, and some alternatives are also presented. Then, the strategies for the improvement of PUFs sustainability, including recycling, and the enhancement of their properties are discussed.