Dynamic structures containing polymers are considered a new class of polymeric materials with very attractive properties, since they can behave as thermosets at room temperature but at the same time they can be reprocessed to any shape. Recently, aromatic disulfides have been explored as dynamic bonds because of their good exchange reaction rate at low temperatures. In this work, we explore the utilization of aromatic diselenides on the basis of their lower bond energies in comparison with their disulfide counterparts. By comparing a model exchange reaction between aromatic diselenides and aromatic disulfides, the ability of aromatic diselenides to exchange faster than aromatic disulfides has been demonstrated. These results are supported by molecular quantum chemical calculations, confirming the faster exchange of aromatic diselenides in comparison with disulfides. In addition, a [2 + 1] radical-mediated reaction mechanism is proposed for aromatic diselenides to understand their faster kinetic exchange. An aromatic diselenide has been incorporated into polyurethane networks using a para-substituted amine diphenyl-diselenide. The resulting materials not only exhibit faster self-healing properties than the corresponding disulfide based materials, but also show the ability to be processed at temperatures as low as 100 °C. The fact that aromatic amines are already used as crosslinkers in a wide range of commercial products makes this system very attractive.

Hydrogels have emerged as highly attractive polymer-based materials owing to their unique solid–liquid rheological duality, which allows their use in 3D extrusion and inkjet printing, particularly in the field of biomedical applications. The dynamic moduli (storage modulus, G′, and loss modulus, G′′), relaxation modulus, shear-thinning behaviour, thixotropy, viscoelasticity, and yield stress are the most commonly employed rheological concepts for hydrogel applications in 3D/4D additive manufacturing. Extrusion, inkjet printing, and stereolithography are the most studied manufacturing technologies for hydrogel bioprinting applications. Moreover, hydrogels exhibit a combination of cohesive properties of solids and the transport characteristics of liquids. Their rheological behaviour, however, varies depending on whether they are chemically cross-linked, showing a pure solid elastic behaviour, or physically crosslinked, showing viscoelastic behaviour. While rheology reveals much information about the flow behaviour of liquids or deformation behaviour of solids, it is not as obvious as to anticipate the printability of hydrogels. Therefore, a deep understanding of rheological principles and their correlation with printability is essential. This review begins summarizing various polymer hydrogels. Subsequently, the definition and description of the most employed concepts, namely relaxation modulus, storage, and loss moduli, and many others, are necessary to understand and associate the feasibility of hydrogels for a particular bioprinting process. This review mainly addresses: (i) rheological determination of the processing window for direct ink writing (DIW), (ii) rheological restrictions for printing beyond direct ink writing (DIW), and (iii) vat photopolymerization bioprinting and the biological implications of bioprinting. Finally, all the above concepts are illustrated with a few examples of biomedical applications of 3D/4D printed hydrogels.

Rheology is proposed as a tool to explore plasticized poly(vinyl chloride) (PVC) formulations to be used in the fused filament fabrication (FFF) 3D printing process and so manufactures flexible and ductile objects by this technique. The viscoelastic origin of success/failure in FFF of these materials is investigated. The analysis of buckling of the filament is based on the ratio between compression modulus and viscosity, but for a correct approach the viscosity should be obtained under the conditions established in the nozzle. As demonstrated by small amplitude oscillatory shear (SAOS) measurements, PVC formulations have a crystallites network that provokes clogging in the nozzle. This network restricts printing conditions, because only vanishes at high temperatures, at which thermal degradation is triggered. It is observed that the analysis of the relaxation modulus G(t) is more performing than the G″/G′ ratio to get conclusions on the quality of layers welding. Models printed according to the established conditions show an excellent appearance and flexibility, marking a milestone in the route to obtain flexible objects by FFF.

Polyurethanes are one of the most versatile families of polymers and are one of the most widely used in thermosets. Due to their static cross-linked structure, they have limited capacity to be reprocessed, recycled or reshaped and therefore once these materials suffer any damage, they are typically discarded. One of the key features of polyurethanes is given by the dynamic behavior of the urethane group at high temperatures, especially in the presence of an appropriate catalyst. In this work a detailed study is performed to demonstrate the dynamic nature of aliphatic and aromatic polyurethanes in the presence of different catalysts. Our study shows that the dynamic behavior is highly dependent on the nature of the isocyanate, the urethane/free alcohol ratio and the nature of the catalyst that is used in the polymerization. We found that the stress relaxation of polyurethane thermosets in the absence of a catalyst is related to the degradation of the network and that the catalyst plays a pivotal role in enhancing the dynamic behavior prior to degradation, especially in aromatic polyurethanes. In fact, the organocatalyst p-toluensulfonic acid (PTSA) is shown to have the ability to enhance the dynamic behavior via hydroxyl mediated transcarbamoylation (associative mechanism) whereas dibutyltin dilaurate (DBTDL) catalyzes transcarbamoylation reactions via both associative and dissociative exchange mechanisms at temperatures as low as 120 °C. These results have been confirmed by model reactions performed with low molecular weight compounds. We believe that this work offers a significant insight into the influence of structural parameters and catalyst effects in the intrinsic dynamic behavior of polyurethanes, and moreover, provides new alternatives to selectively enhance the dynamic nature of polyurethanes.