Auswahl der wissenschaftlichen Literatur zum Thema „Covalent adaptable networks“

The ability to behave in a fluidlike manner fundamentally separates thermoset and thermoplastic polymers. Bridging this divide, covalent adaptable networks (CANs) structurally resemble thermosets with permanent covalent crosslinks but are able to flow in a manner that resembles thermoplastic behavior only when a dynamic chemical reaction is active. As a consequence, the rheological behavior of CANs becomes intrinsically tied to the dynamic reaction kinetics and the stimuli that are used to trigger those, including temperature, light, and chemical stimuli, providing unprecedented control over viscoelastic properties. CANs represent a highly capable material that serves as a powerful tool to improve mechanical properties and processing in a wide variety of polymer applications, including composites, hydrogels, and shape-memory polymers. This review aims to highlight the enabling material properties of CANs and the applied fields where the CAN concept has been embraced.

A new type of tetraphenylborate salts derived from highly basic and nucleophilic amines, namely 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) and triazabicyclodecene (TBD), was applied to the preparation of networked poly(thiourethane)s (PTUs), which showed a vitrimer-like behavior, with higher stress-relaxation rates than PTUs prepared by using dibutyl thin dilaurate (DBTDL) as the catalyst. The use of these salts, which release the amines when heated, instead of the pure amines, allows the formulation to be easily manipulated to prepare any type of samples. The materials prepared from stoichiometric mixtures of hexamethylene diisocyanate (HDI), trithiol (S3) and with a 10% of molar excess of isocyanate or thiol were characterized by FTIR, thermomechanical analysis, thermogravimetry, stress-relaxation tests and tensile tests, thus obtaining a complete thermal and mechanical characterization of the materials. The recycled materials obtained by grinding the original PTUs and hot-pressing the small pieces in the optimized time and temperature conditions were fully characterized by mechanical, thermomechanical and FTIR studies. This allowed us to confirm their recyclability, without appreciable changes in the network structure and performance. From several observations, the dissociative interchange trans-thiocarbamoylation mechanism was evidenced as the main responsible of the topological rearrangements at high temperature, resulting in a vitrimeric-like behavior.

Introducing dynamic covalent bonding into thermoset polymers has received considerable attention because they can repair or recover when damaged, thereby minimizing waste and extending the service life of thermoset polymers. However, most of the yielded dynamic covalent bonds require an extra catalyst, high temperature and high-pressure conditions to trigger their self-healing properties. Herein, we report on a catalyst-free bis-dynamic covalent polymer network containing vinylogous urethane and disulfide bonds. It is revealed that the introduction of disulfide bonds significantly reduces the activation energy (reduced from 94 kJ/mol to 51 kJ/mol) of the polymer system for exchanging and promotes the self-healing efficiency (with a high efficiency of 86.92% after being heated at 100 °C for 20 h) of the material. More importantly, the mechanical properties of the healed materials are comparable to those of the initial ones due to the special bis-dynamic covalent polymer network. These results suggest that the bis-dynamic covalent polymer network made of disulfide and inter-vinyl ester bonds opens a new strategy for developing high-performance vitrimer polymers.

Les réseaux dynamiques doubles (RDD) sont des matériaux polymères qui combinent deux agents de réticulation distincts (ou plus) dans un seul système. En couplant différentes stratégies de réticulation, des matériaux sur mesure peuvent être conçus. Cette thèse explore la mise en œuvre du concept de vitrimère dans les RDD. Des élastomères vitrimères constitués de deux réseaux dynamiques interpénétrés reposant respectivement sur la métathèse des esters boroniques et sur l’échange imine-aldéhyde ont été conçus dans ce but. Les deux réactions procèdent selon un mécanisme dégénéré et sont orthogonales l’une à l’autre. Par l’utilisation de deux réticulations dynamiques différentes, deux dynamiques distinctes sont établies dans le réseau. Pour obtenir et évaluer le RDD final, les sous-réseaux respectifs ont été synthétisés au préalable et caractérisés en tant que réseaux simples. Les caractéristiques des réseaux simples ont été adaptés individuellement pour répondre à leurs besoins spécifiques en termes de comportement dynamique, de mise en œuvre et de stabilité dimensionnelle. Ces propriétés ont été ajustées en modifiant la masse molaire des précurseurs thermoplastiques, leur taux de fonctionnalité, leur densité de réticulation ou le temps de vie des liens dynamiques. Les deux réseaux ont été réunis avec succès en un RDD. Une étude comparative a permis de comprendre comment les sous-réseaux individuels contribuent aux propriétés du RDD, et si des effets synergiques sont obtenus. Dans les faits, la structure interpénétrée du vitrimère RDD permet d’augmenter à la fois la résistance au fluage et l’allongement à la rupture, ce qui est particulièrement difficile à réaliser, mais hautement souhaitable pour la plupart des élastomères. Au-delà, les matériaux obtenus ont montré un très fort potentiel pour le recyclage mécanique et chimique
Dual dynamic networks (DDNs) are polymeric materials that combine two (or more) distinct crosslinkers in one system. By coupling different crosslinking strategies, precisely tailored materials can be designed. This thesis explores the implementation of the vitrimer concept into DDNs. Elastomeric vitrimers consisting of two interpenetrated dynamic networks that rely on boronic ester metathesis and on imine-aldehyde exchange, respectively, were designed to this aim. Both reactions proceed via a degenerate mechanism and are orthogonal to each other. By the engagement of two types of dynamic covalent crosslinks, two distinct dynamics are established in each subnetwork. To obtain and evaluate the final DDN, the respective subnetworks were synthesized beforehand, and characterized as single networks. The characteristics of the single networks were tailored individually to fulfill their specific needs in terms of dynamic behavior, processability and dimensional stability. These properties were adjusted by changing the molar mass of the thermoplastic precursors, their degree of functionality, their crosslinking density, or the lifetime of the dynamic bonds. The two networks were successfully united into a DDN. In a comparative study, insights were obtained how the individual subnetworks contribute to the DDN’s properties, and whether synergetic effects arise. In fact, the interpenetrated nature of the vitrimer DDN allows increasing at the time creep resistance and elongation at break, which is really challenging to achieve, yet highly desirable for most elastomers. Over and beyond, the obtained materials show great potential for mechanical and chemical recycling