Controlling the Thermomechanical and Viscoelastic Properties of Polymer Networks by Combining Associative and Dissociative Dynamic Covalent Bonds

Introducing dynamic bonds into polymer networks can produce materials that can be more easily processed, applied, and recycled than their permanent counterparts. Moreover, they show improved toughness, stretchability, and the ability to heal physical damage. This dynamicity can be introduced into polymers by means of supramolecular interactions, dissociative dynamic covalent bonding, or associative dynamic covalent bonding (vitrimers).<br/>The behavior of these polymer networks heavily depends on the kinetics and thermodynamics of the dynamic bonds, which are entirely different for each mechanism. On the one hand, the viscosity of vitrimers show a linear behavior (Arrhenius behavior) as a function of inverse temperature. On the other hand, dissociative networks can exhibit abrupt changes in viscosity above a critical temperature. Both systems have their strengths and weaknesses, but both suffer from the same compromise. Having to choose whether to be able to self-heal/reprocess at mild conditions or to not creep at operation temperatures.<br/>Here, we tackled this intrinsic problem by developing a strategy to engineer the relaxation time regime in dynamic polymer materials. We achieve this control by combining, in a single network, Diels-Alder and β-hydroxy ester bonds. Diels-Alder bonds based on furan and maleimide are dissociative dynamic covalent bonds and show an abrupt change in viscosity above a critical temperature. β-hydroxy ester bonds are associative dynamic covalent bonds and show an Arrhenius behavior. By tuning the ratio between Diels-Alder bonds and β-hydroxy ester bonds it is possible to access the complete range of relaxation times between the two mechanisms. The polymer network that is crosslinked exclusively by Diels-Alder bonds becomes liquid above its gel transition temperature of around 100 °C, with viscosity as low as 0.45 Pa.s at 125 °C. As the proportion of associative bonds increases, the gel transition temperature of the system increases until a point where it no longer degel, which means that the β-hydroxy ester bonds have a concentration high enough to create a network. For the network crosslinked only by β-hydroxy ester bonds, it remains solid and doesn’t relax a measurable amount of stress at a time scale of &gt;750 minutes at 130 °C. Even after the point where the material doesn’t degel, healing of completely cut tensile specimens remains possible at a reasonable temperature of 90 °C for 30 minutes, without any external pressure. These conditions are notably milder conditions than the ones normally needed to self-heal materials based on transesterification.<br/>The materials are also designed following the 12 principles of green chemistry. They are synthesized using 100% bio-based raw materials, based on fatty acids, without using any solvent, and are potentially (bio)degradable. As all the components have a similar backbone, the mechanical properties of the materials don’t vary much between materials with different ratios of associative and dissociative bonds. The Young’s modulus of the whole range of materials sits around 1.5 ± 0.5 MPa at room temperature.<br/>With this strategy, the design of the mechanical properties is decoupled from the relaxation dynamics. Combining two types of dynamic bonds with very dissimilar relaxation dynamics empowers a range of applications where the materials can be tailored according to their application, processing, as well as the timescale of their use. Additionally, this approach is not exclusive to our system but can be generalized to any compatible pair of dissociative and associative dynamic covalent bonds.